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CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAMLevel 3 Line H: Install Electrical Equipment

LEARNING GUIDE H-4INSTALL DC MOTORS AND GENERATORS

H-4

ForewordThe Industry Training Authority (ITA) is pleased to release this major update of learning resources to support the delivery of the BC Electrician Apprenticeship Program. It was made possible by the dedicated efforts of the Electrical Articulation Committee of BC (EAC).

The EAC is a working group of electrical instructors from institutions across the province and is one of the key stakeholder groups that supports and strengthens industry training in BC. It was the driving force behind the update of the Electrician Apprenticeship Program Learning Guides, supplying the specialized expertise required to incorporate technological, procedural and industry-driven changes. The EAC plays an important role in the province’s post-secondary public institutions. As discipline specialists the committee’s members share information and engage in discussions of curriculum matters, particularly those affecting student mobility.

ITA would also like to acknowledge the Construction Industry Training Organization (CITO) which provides direction for improving industry training in the construction sector. CITO is responsible for organizing industry and instructor representatives within BC to consult and provide changes related to the BC Construction Electrician Training Program.

We are grateful to EAC for their contributions to the ongoing development of BC Construction Electrician Training Program Learning Guides (materials whose ownership and copyright are maintained by the Province of British Columbia through ITA).

Industry Training AuthorityJanuary 2011

DisclaimerThe materials in these Learning Guides are for use by students and instructional staff and have been compiled from sources believed to be reliable and to represent best current opinions on these subjects. These manuals are intended to serve as a starting point for good practices and may not specify all minimum legal standards. No warranty, guarantee or representation is made by the British Columbia Electrical Articulation Committee, the British Columbia Industry Training Authority or the Queen’s Printer of British Columbia as to the accuracy or sufficiency of the information contained in these publications. These manuals are intended to provide basic guidelines for electrical trade practices. Do not assume, therefore, that all necessary warnings and safety precautionary measures are contained in this module and that other or additional measures may not be required.

Acknowledgements and CopyrightCopyright © 2011, 2014 Industry Training Authority

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The Industry Training Authority of British Columbia would like to acknowledge the Electrical Articulation Committee and Open School BC, the Ministry of Education, as well as the following individuals and organizations for their contributions in updating the Electrician Apprenticeship Program Learning Guides:

Electrical Articulation Committee (EAC) Curriculum SubcommitteePeter Poeschek (Thompson Rivers University)Ken Holland (Camosun College)Alain Lavoie (College of New Caledonia)Don Gillingham (North Island University)Jim Gamble (Okanagan College)John Todrick (University of the Fraser Valley) Ted Simmons (British Columbia Institute of Technology)

Members of the Curriculum Subcommittee have assumed roles as writers, reviewers, and subject matter experts throughout the development and revision of materials for the Electrician Apprenticeship Program.

Open School BCOpen School BC provided project management and design expertise in updating the Electrician Apprenticeship Program print materials:

Adrian Hill, Project ManagerEleanor Liddy, Director/SupervisorBeverly Carstensen, Dennis Evans, Laurie Lozoway, Production Technician (print layout, graphics)Christine Ramkeesoon, Graphics Media CoordinatorKeith Learmonth, EditorMargaret Kernaghan, Graphic Artist

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Intellectual Property Program Ilona Ugro, Copyright Officer, Ministry of Citizens’ Services, Province of British Columbia

To order copies of any of the Electrician Apprenticeship Program Learning Guide, please contact us:

Crown Publications, Queen’s PrinterPO Box 9452 Stn Prov Govt563 Superior Street 2nd FlrVictoria, BC V8W 9V7Phone: 250-387-6409Toll Free: 1-800-663-6105Fax: 250-387-1120Email: [email protected]: www.crownpub.bc.ca

Version 1Corrected, March 2016 Revised, April 2014 Corrected, January 2014 New, October 2012

CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3 5

LEVEL 3, LEARNING GUIDE H-4:

INSTALL DC MOTORS AND GENERATORSLearning Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Learning Task 1: Describe the constructional features of DC machines . . . . . . . . . . . . . . . 9Self-Test 1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Learning Task 2: Describe the operating principles of generators . . . . . . . . . . . . . . . . . 17Self-Test 2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Learning Task 3: Describe the characteristics of the various types of DC generators . . . . . . 31Self-Test 3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

Learning Task 4: Describe the operating principles of DC motors. . . . . . . . . . . . . . . . . . 43Self-Test 4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

Learning Task 5: Describe the features and operating characteristics of the shunt motor . . . 61Self-Test 5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

Learning Task 6: Describe the features and operating characteristics of the series motor . . . 69Self-Test 6. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

Learning Task 7: Describe the features and operating characteristics of the compound motor 75Self-Test 7. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

Learning Task 8: Describe the features of DC motor controllers . . . . . . . . . . . . . . . . . . . 81Self-Test 8. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87

Learning Task 9: Describe the operation of magnetic DC motor controllers . . . . . . . . . . . 91Self-Test 9. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .101

Learning Task 10: Describe methods of deceleration for DC motors . . . . . . . . . . . . . . . . .103Self-Test 10 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .107

Learning Task 11: Describe basic maintenance and troubleshooting procedures for DC motor controls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .109Self-Test 11 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .113

Learning Task 12: Describe basic troubleshooting and maintenance procedures for DC motors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .115Self-Test 12 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .122

Answer Key . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .125

6 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3

LEARNING ObjECTIVES H-4


Learning Objectives• Describe the operating principles of DC machines.

• Connect and maintain DC machines.

Activities• Read and study the topics in Learning Guide H-4: Install Electronic Motor Controls.

• Complete Self-Tests 1 through 12. Check your answers with the Answer Key provided at the end of this Learning Guide.

Resources

You are encouraged to obtain the following texts, which provide supplemental learning information:

• Rosenberg, Robert, Electric Motor Repair (3rd edition), Cengage Learning.

• Herman, Stephen L., Direct Current Fundamentals, Delmar Publishers Inc.

• Herman, Stephen L., Delmar’s Standard Textbook of Electricity, Delmar Publishers Inc.

LEARNING ObjECTIVES H-4


BC Trades Moduleswww.bctradesmodules.ca

We want your feedback! Please go the BC Trades Modules website to enter comments about specific section(s) that require correction or modification. All submissions will be reviewed and considered for inclusion in the next revision.

SAFETY ADVISORYBe advised that references to the Workers’ Compensation Board of British Columbia safety regulations contained within these materials do not/may not reflect the most recent Occupational Health and Safety Regulation. The current Standards and Regulation in BC can be obtained at the following website: http://www.worksafebc.com.

Please note that it is always the responsibility of any person using these materials to inform him/herself about the Occupational Health and Safety Regulation pertaining to his/her area of work.

Industry Training Authority January 2011


Learning Task 1:

Describe the constructional features of DC machinesDynamo is a generic term originally given to a rotating machine that converts mechanical energy into electrical energy, primarily in the form of direct current. Although DC generators are sometimes still called dynamos, they are more commonly placed in the family of DC machines that includes both motors and generators.

DC generators are similar to DC motors in both appearance and construction. The main difference between the two machines is the type of surrounding enclosure. Generators tend to have open frames, but motors in a working environment are usually totally enclosed.

To understand DC machines, you must be familiar with the various parts. Following is a brief description of the key parts in any DC machine.

ArmatureThe armature is the rotating part or rotor of the machine. The armature core consists of a stack of soft iron laminations that are slotted to house a set of insulated coil windings (Figure 1).

Figure 1—Armature core

The high-strength steel shaft is supported by bearings to allow uniform rotation of the complete armature assembly within the air gap.

The armature coils are generally wound with varnish-insulated magnet wire, and are protected on all sides with slot-cell insulation (Figure 2). A wedge is driven the full length of the slot to hold the coils tightly in place. The ends of each coil are connected to commutator bars.

There are three basic types of armature windings: frog leg, lap and wave. The coil leads for each type of winding connect to the commutator bars differently. In general:

• Frog leg windings are the most common winding used and are for moderate voltage and current applications.

• Lap windings are used in high-current, low-voltage applications.

• Wave windings are used in low-current, high-voltage applications.

LEARNING TASk 1 H-4


Figure 2—Complete armature

CommutatorThe commutator is an assembly of individually insulated copper bars or segments. They are insulated from each other and the shaft by mica insulation. The brushes and commutator act together as a rotary selector switch between the armature coils and the external load. In a generator, the commutator converts the AC induced within the armature coils to a unidirectional current in the external circuit. In a motor, the direction of thrust caused by motor effect must change as the coils cross the neutral plane. To do this the commutator ensures that the current in each armature coil changes direction as the coil crosses the neutral plane.

Figure 3—Cutaway of a typical commutator assembly

BrushesThe brushes conduct the current to or from the armature coils via the commutator from or to the external circuit. Most DC machines have brushes made of one or more of the following materials:

• Carbon

• Electro-graphite

LEARNING TASk 1 H-4


• Graphite

• Copper graphite

The choice of brush material is based on application and cost.

The brush holder (Figure 4) is designed to support the brush and provide suitable pressure against the commutator surface (usually between 1 and 2 psi).

Figure 4—Typical brush holder

Field polesField poles provide the magnetic field in which the armature will rotate. In a DC machine this field is provided by permanent magnets or by direct current flowing through field coils. Electromagnetic field poles (Figure 5) consist of a soft iron, laminated core and field coils wound with varnish-insulated magnet wire.

Figure 5—Cutaway of the field pole of a compound DC machine

LEARNING TASk 1 H-4


Frame and end shieldsThe frame or yoke (Figure 6) provides mechanical support for the machine and forms part of the magnetic circuit between the field poles. The end shields are mounted at the ends of the frame and contain recesses for the armature shaft bearings.

Figure 6—End shields and frame

BearingsThere are two common classes of bearings used in motors and generators: sleeve bearings and ball bearings (Figure 6).

LEARNING TASk 1 H-4


Figure 7—Cutaway showing mounted sleeve and ball bearings

Sleeve bearings are lubricated with oil applied with an oil ring or oil-soaked wick. New sleeve bearings are usually machined under-size and require reaming for proper fit. Ball bearings are lubricated with grease and may be open or sealed. Sealed bearings are factory lubricated for the life of the bearing. Open bearings require grease nipples in the end shields. Proper lubrication will greatly extend the operating life of a bearing.

Take care not to over-grease the bearings of rotating machines. This can produce heat and shorten bearing life.

Now do Self-Test 1 and check your answers.

LEARNING TASk 1 H-4


Self-Test 1

1. What is the main difference in the construction of a DC generator and a DC motor?

2. The rotating part of a DC generator is called the .

3. What are the three basic types of armature windings?

4. Which armature winding is used in high-current, low-voltage applications?

5. What is the electrical function of the commutator?

6. List the common materials used in making brushes.

7. Suitable brush pressure is usually between and psi.

LEARNING TASk 1 H-4


8. Describe the function of field poles.

9. DC machines use or bearings.

10. Grease nipples in the end shields indicate that the machine contains bearings.

Go to the Answer Key at the end of the Learning Guide to check your answers.


Learning Task 2:

Describe the operating principles of generatorsGenerators are machines that transform mechanical energy into electrical energy. They work under the principle of Faraday’s law of electromagnetic induction (E = βlv). The term field excitation refers to various methods of establishing and controlling the magnetic field so that induction occurs.

Permanent-magnet field polesSmall generators called magnetos use permanent magnets to provide the magnetic field. The voltage generated by magnetos is comparatively small due to the low flux density. In addition, it is not possible to vary the generated voltage by varying the strength of the magnetic field. These factors limit the application of magnetos. Common uses include ignition circuits for small gas engines, non-digital tachometers and meggers.

Electromagnetic field polesElectromagnetic field poles provide a much higher and more controllable flux density than permanent magnets. The current required to excite the field windings or coils may be obtained in two ways:

• From a DC source external to the machine (separately excited).

• From the armature of the generator itself (self-excited). In self-excited generators, the field coils may be connected either in series or in parallel (shunt) with the armature. Series and shunt windings differ considerably in construction.

Series field windingsSeries field windings are connected in series with the armature. Therefore, the winding must be of a sufficient gauge to carry full armature current. For this reason, series windings are made of a few turns of large-gauge wire (Figure 1).

Field poleSN

CW

To external load

+

–

S1 S2A2 A1

Figure 1—Series generator

LEARNING TASk 2 H-4


Shunt field windingsA shunt winding does not need to carry high current and is made of a large number of turns of small-gauge wire. The high coil resistance results in much lower current in the field circuit. This low current through a large number of turns is sufficient to provide a magneto-motive force (ampere-turns) capable of creating the necessary magnetic field. The lower current makes it possible to control generator voltage with a rheostat in the shunt field circuit (Figure 2).

Armature

Field pole

Field rheostat

SN

CW

To external load

+

–

F1 F2A2 A1

Figure 2—Shunt generator with field rheostat

Series and shunt windingsDC machines often have both a series winding and a shunt winding, wound in the same direction on the same field core (Figure 3).

SN

CW

To external loadDiverter rheostat

Field rheostat+

–

F1

F2S2A2

A1S1

Field pole

Armature

Figure 3—Compound generator with series and shunt windings

LEARNING TASk 2 H-4


Separately excited fieldsThe field current for separately excited generators is derived from an external source such as a battery, rectifier circuit or separate DC generator. Therefore, construction is the same as for shunt field windings.

Self-excited fieldsMost DC generators derive their field current from the armature of the machine itself and so are called self-excited generators (Figure 2). At the moment that the generator starts to rotate, no voltage is available to produce field current. Residual magnetism must be present in the field core to start the process of building up voltage.

Requirements for voltage buildupEarlier you learned that Faraday’s law of electromagnetic induction is expressed by the formula E = βlv. (This relationship derives from the fact that 1 volt will be induced in a conductor if it is cutting a magnetic field at a rate of 1 weber per second.)

Since the voltage induced in a conductor depends upon the rate at which it is cutting flux, it is possible to determine the total emf generated in an armature. This generated voltage is a function of the following:

• Number of poles

• Flux per pole

• Rotational speed of the armature

• Number of paths

• Total number of active conductors on the armature

For any given machine, the number of poles, paths and active conductors is fixed. Their influence on the generated voltage can be represented by a constant (k). The numerical value of this constant will change from machine to machine. The generated voltage can now be expressed by the formula

Eg = kΦN

where:

Eg = generated voltage in volts k = constant Φ = flux (excitation) in webers N = rotational speed in revolutions per minute (rpm)

In practice, the factors represented by the constant (k) are not known. However, the formula shows that the generated voltage is directly proportional to the product of flux and speed.

Example: A generator running at 900 rpm has a generated voltage of 100 V. If the excitation remains the same and the speed is increased to 1800 rpm, what is the generated voltage?

LEARNING TASk 2 H-4


Since Eg is proportional to the product of flux and speed, and flux did not change, then the voltage is directly proportional to speed only.

The speed doubled to 1800 rpm, therefore the voltage doubled to 200 V.

Residual magnetismAt the instant of start-up, the speed of a generator is zero, and so the generated voltage is also zero. Because a self-excited generator requires armature voltage to produce field current, there would be no flux for the conductors to cut and no voltage buildup. This problem does not exist, however, if residual magnetism is present on the field poles.

As the armature starts to turn, the following series of events occurs:

• The conductors cut the weak residual field, generating a low voltage.

• In turn, this voltage causes a low current to flow in the field windings, which increases the field strength of the poles.

• The increase in flux causes an increase in voltage, resulting in higher field current and even stronger field strength.

• The process continues until the generator reaches its no-load voltage (Eg).

A generator that has been out of service for an extended length of time may lose its residual magnetism. If there is no residual magnetism, a process called flashing the field can be used for start-up. In this process, a separate DC source (connected with the proper polarity) is used to cause a brief current surge in the field coils. This re-establishes the residual magnetism in the field core. Before flashing the field, disconnect a shunt field from the low-resistance armature.

Direction of rotationThe direction of the induced emf in the armature conductors is a function of the direction of the magnetic field and the direction of rotation of the armature (Fleming’s left-hand rule). It is possible that the direction of rotation may be incorrect for the polarity of the residual magnetism. In this case, the generated voltage produces a field current that strips away the residual magnetism on the poles. As a result, the voltage fails to build up.

It is not usually possible to reverse the direction of the prime mover, which could be an internal combustion engine or a turbine. The simplest solutions are to re-flash the field for the opposite magnetic polarity or to reverse the connections between all field coils and the armature.

Critical field resistanceTo achieve voltage buildup, the residual magnetism must generate enough voltage to increase the strength of the magnetic field poles. The field current depends on the voltage across the field coils and on the resistance of the field circuit. There exists a critical value of field resistance, which limits field current and prevents voltage buildup.

Figure 4 shows these relationships. If field resistance is held constant, a plot of field current versus terminal voltage produces a straight line. Flux produced by field current is not linear,

LEARNING TASk 2 H-4


as shown by the field saturation curve. For buildup to occur, the field resistance curve must lie below the saturation curve.

It is common to control generated voltage with a rheostat in the field circuit. As resistance in the field circuit is increased, the slope or steepness of the field resistance curve increases and may move to the left of the saturation curve. When this happens, there is not enough field current for voltage to build up.

To ensure that voltage builds up during the start-up process, set field rheostats to their minimum value of resistance.

Figure 4—Field saturation and resistance curves

Prime mover speedGenerated voltage is proportional to the product of field flux and armature speed (Eg = kΦN). If the speed is too low, voltage buildup may fail.

Armature reactionWhen an armature is delivering current to a load, a magnetic field surrounds the armature conductors. If uncorrected, the armature field will distort the main field created by the field poles. This armature reaction is one significant factor that influences the terminal voltage of a DC generator. It also impairs commutation by severely affecting the physical condition of the brushes and commutator.

To understand the interaction of the field around the armature conductors with the main field, it is useful to examine each separately.

Figure 5 shows a cross section of the armature field flux in a two-pole generator that is rotating clockwise. To simplify the illustration, the commutator, brushes and connected load are not shown.

LEARNING TASk 2 H-4


Figure 5—Armature field flux in a two-pole generator

If we apply Fleming’s left-hand rule, the induced emf drives current:

• Out of the page in the conductors on the left of the neutral plane (N)

• Into the page in the conductors on the right of the neutral plane

The resultant magnetic field of the armature appears as if a coil is situated on the neutral plane at a right angle to the main field. Applying the left-hand rule for a coil shows that a south pole exists at the top of the armature and a north pole at the bottom.

If we compare the field distributions shown in Figures 5 and 6, we can see that the magnetic fields at the upper tip of the north pole and the lower tip of the south pole are in the same direction. This strengthens the main field in these areas. The magnetic fields are in opposite directions at the lower tip of the north pole and the upper tip of the south pole. This weakens the main field in these areas.

Figure 6—Main field flux

LEARNING TASk 2 H-4


The original position of the neutral plane between the two field poles is called the mechanical neutral plane. In Figure 7, the distortion of the main field shows that the position at which conductors will not cut flux has shifted in the direction of rotation. The net effect is a shift in the position of the neutral plane. The shift is proportional to armature current. The shifted position at which the conductors do not cut flux is called the electrical or magnetic neutral plane.

Figure 7—Main field distortion

Ideally, the brush position is set so that the armature coils will short as they cross the mechanical neutral plane. The shift of the electrical neutral plane with armature current causes an emf to be induced in coils crossing the mechanical neutral plane. High current flows in the shorted coils, limited only by the resistance of the coil and the brush. The result is excessive sparking at the contact point of the brush and the commutator.

Sparking can be eliminated if the position of the brushes is shifted in the direction of rotation, toward the electrical neutral plane. In early applications of DC generators, the brushes were mounted on rotational collars to enable an operator to shift the brushes in response to changing load conditions.

InterpolesBrush shifting is a problem because electrical loads are rarely constant. This means that you must continually adjust the brush position.

Armature reaction is caused by the magnetic field produced by armature current. Introducing additional fields to oppose the armature field can minimize distortion of the main field. This is done by positioning additional field poles on the mechanical neutral plane. These new poles are between the main poles, and are commonly called interpoles. Because they improve commutation, they are also called commutating poles. Figure 5 shows that a south interpole would be required at the top of the armature and a north interpole at the bottom. For generators, interpole polarity is the same as the main pole directly ahead in the direction of rotation.

LEARNING TASk 2 H-4


Figure 8—Interpole connection

The strength of the armature field depends on the armature current. The interpole winding is connected in series with the armature. Therefore, the strength of the interpole field also varies with armature current. To ensure correct polarity, the interpole is usually connected to the armature inside the machine. In Figure 8, the leads A1 and A2 include both the armature and the interpoles.

The desired effect of interpoles is concentrated at the mechanical neutral plane. Because of this and the limited physical space between the main poles, interpole field cores are very narrow. Interpole windings consist of a few turns of large-gauge wire similar to series field windings.

Compensating windingsInterpoles do not eliminate all the field distortion caused by armature reaction. In large, high-performance DC generators, compensating windings (Figure 9) are placed in the main pole faces and connected in series with the armature to eliminate field distortion. Due to their high cost, compensating windings are far less common than interpoles.

LEARNING TASk 2 H-4


Figure 9—Compensating windings

Losses and efficiencyEnergy that is drawn from the source but is not available at the output of the machine is considered lost. There are three types of power loss in DC generators: mechanical, electrical and magnetic (core).

Mechanical lossesMechanical losses associated with rotation of the armature are commonly called rotational losses. These losses are due to bearing friction and the movement of air within the machine, called windage losses. Air movement is an important factor in preventing heat buildup within the machine.

Electrical lossesElectrical losses due to resistance occur whenever current flows in a conductor. Heat is produced that is proportional to the resistance times the square of the current (I2R). Because the armature and series fields carry full load current, considerable heat is generated in the copper conductors.

Core lossesCore losses result from the action of magnetic fields on the materials in the magnetic paths through the machine. These paths include the frame, the field poles and the armature. Core losses include the heating effects of both hysteresis and eddy currents.

Calculating efficiencyFor any device or machine, efficiency is defined as the ratio of the power out to the power in. Input power is always greater than output power because of various losses; therefore, this ratio is between 0 and 1. It is common to multiply the ratio by 100 and express efficiency as a percentage.

In the case of DC generators, the output power is the electrical power available at the output terminals. The input power is the mechanical energy drawn from the shaft rotation of the prime

LEARNING TASk 2 H-4


mover. Machine ratings always describe what is available from the machine. For example, a 100 kW DC generator can deliver 100 kW to an external electrical load. The additional power necessary to overcome the losses within the generator must also come from the prime mover, which must therefore deliver more than 100 kW. The efficiency of the prime mover is considered separately.

Example: A 100 kW DC generator is operating at full load. If the prime mover is delivering 125 kW to the shaft, determine the efficiency of the generator.

Efficiencypower outpower in

kWkW

= ×

= ×

100

100125

100

%

%

== 80%


LEARNING TASk 2 H-4


Self-Test 2

1. Small generators that use permanent magnets for field excitation are called .

2. Describe the difference between separately excited and self-excited fields.

3. List the two ways in which field coils are connected in self-excited generators.

4. Which field windings are made of a low number of turns of large-gauge wire?

5. What device in the shunt field circuit is used to control generator voltage?

6. The buildup of voltage in a self-excited generator requires .

7. List five factors that influence generated voltage.

8. Which of the factors listed in Question 7 can be varied in an operating generator?

LEARNING TASk 2 H-4


9. What is the effect on generated voltage if the speed of the prime mover is decreased by one third?

10. In a self-excited generator, voltage buildup requires the presence of .

11. Voltage buildup occurs until the generator reaches its .

12. Appropriate residual magnetism can be established on the field poles by a process

commonly called .

13. The value of field resistance that will limit field current to a value insufficient for voltage

buildup is called .

14. What should be done at generator start-up to avoid the problem described in Question 13?

15. Define armature reaction.

16. List two effects of armature reaction on the operation of a generator.

17. Interpole coils consist of a:

a. large number of turns of small-gauge wire

b. small number of turns of small-gauge wire

c. large number of turns of large-gauge wire

d. small number of turns of large-gauge wire

LEARNING TASk 2 H-4


18. Interpole coils are connected:

a. across the armature

b. in series with the armature

c. across the series field

d. in series with the shunt field

19. The effects of armature reaction can also be minimized by placing windings called

in the main pole faces.

20. List the three types of power loss in a generator.

21. Rotational losses include and .

22. How is the effect of eddy currents minimized in the construction of DC machines?

23. Heat produced by current flowing through the armature and field coils is called

.

24. Define efficiency.

LEARNING TASk 2 H-4


25. A DC generator delivers 150 kW to a load at an efficiency of 75%. Determine the power delivered to the generator by the prime mover.



Learning Task 3:

Describe the characteristics of the various types of DC generatorsThere are three types of DC generators. Each type is categorized according to its field winding connections:

• The series generator (where the field windings are connected in series with the armature)

• The shunt generator (where the field windings are connected in parallel with the armature)

• The compound generator (where two sets of field windings are used—one in series, and one in parallel with the armature)

In order to obtain the desired performance, it is essential that the field coils be properly connected to the armature and to each other. To standardize and simplify this process, the National Equipment Manufacturers’ Association (NEMA) developed a standard numbering system for lead identification.

The NEMA system uses the following letters to identify the leads of a DC machine:

• A for armature

• F for shunt field coils

• S for series field coils

Numerical subscripts are used to indicate current direction in the windings. Electrons normally flow through the field windings from even- to odd-numbered subscripts (that is, from F2 to F1 and from S2 to S1).

Throughout this Learning Guide and the next, we will use the following convention:

• For clockwise rotation observed from the non-drive end of the machine, A1 will be negative.

• Since the armature is a voltage source, electrons will flow through the armature internally from A2 to A1, making A1 negative with respect to A2 for the external circuit.

In most DC generators, the drive end is at the opposite end to the commutator and is mechanically coupled to the prime mover. This mechanical arrangement facilitates maintenance of the commutator and brushes.

Shunt generatorThe schematic in Figure 1 shows the shunt field connected in parallel with the armature, hence the term shunt. It is common to include a rheostat in the field circuit to control the no load terminal voltage.

LEARNING TASk 3 H-4


In this schematic:

• Electron flow through the armature (the voltage source) is from A2 to A1.

• Electron flow through the shunt field (which is a load across the armature) is from F2 to F1.

Figure 1—Shunt generator

If the polarity of the residual magnetism is reversed by re-flashing the field, the voltage will build up with opposite polarity and A1 will now be positive.

When a shunt generator is operating at no load, the generated voltage (Eg) appears across the line terminals, L1 and L2. When an external load is connected across the output terminals, current flows through the armature. Its value is determined by the impedance of the load.

Three factors associated with armature current can cause a reduction in terminal voltage with the load:

• Although the resistance of the armature is very low (in the order of a few ohms), current through the armature results in an internal voltage drop. This internal voltage drop (often called the IR drop of the armature) results in a slightly lower terminal voltage.

• With any increase in load current, the field-weakening effects of armature reaction also increase. This results in a lower generated voltage.

• Since the shunt field is connected across the armature, any reduction in armature voltage will result in a lower field current and a corresponding reduction in field strength. This results in a further reduction of generated voltage. (Separately excited generators are not affected by this third factor.)

LEARNING TASk 3 H-4


Figure 2 shows the reduction in terminal voltage as load current increases in a typical shunt generator.

Figure 2—Shunt generator characteristic curve

The change in terminal voltage from no load to full load is called voltage regulation. It is commonly expressed as a percentage. It is calculated using the following formula:

% voltage regulation =−E E

Eno load full load

full load

×× 100

Series generatorAs you will see from its voltage characteristics, the series generator is not a practical machine. However, knowing the characteristics of series field coils is important if you are to understand the compound generator, which we will describe later.

As previously discussed, A1 and A2 indicate the armature leads, and S1 and S2 indicate the leads of the series field coils. For clockwise rotation, A1 is negative and current flows through the series field from S2 to S1.

Note that a load must be connected across the line terminals of a series generator in order to allow field current. Otherwise, the field circuit is “open.”

LEARNING TASk 3 H-4


Figure 3 shows the series field connected to the armature circuit.

Figure 3—Series field connected to armature circuit

With armature lead A1 connected to field lead S2, electrons flow internally from A2 to A1 and from S2 to S1.

Figure 4 shows the effect of connected load on terminal voltage. Without an external load, armature current is zero and no current flows through the series field. The only source of excitation is residual magnetism, so the no-load terminal voltage is extremely low. As resistance of the external load is decreased, armature current increases and thereby increases the strength of the series field. Since generated voltage is proportional to speed and flux, the terminal voltage rises steeply as a function of load current. Applications for a generator having this voltage characteristic are extremely limited.

The rise in generated voltage with increasing armature current can be useful, however, in offsetting the decrease in terminal voltage associated with the shunt generator. The compound generator uses the voltage characteristics of both series and shunt fields.

Armature current

Term

inal

vol

tage

Figure 4—Effect of connected load on terminal voltage

LEARNING TASk 3 H-4


Compound generatorThe compound generator contains both shunt and series field coils placed on the same field core. Incorrect connection of the field winding terminals will have a serious effect on the performance characteristics of the machine. For example, current flowing from F2 to F1 in the shunt coils and from S2 to S1 in the series coils will produce magnetic fields that add a condition called cumulative compound. If, however, the connection of the series field is reversed, causing current to flow from S1 to S2, the resulting fields would oppose or subtract. This latter connection is called differential compound.

When connecting and operating compound generators, the shunt field should be considered the main field, with the series field either aiding or opposing (bucking) the magnetic polarity of the shunt field.

There are two possible ways to connect the shunt field coils to the armature, long shunt and short shunt:

• In long shunt, the shunt field is connected across the series combination of both the armature and series field. That is, across the line.

• In short shunt, the shunt field is connected across the armature only.

Figures 5 and 6 show long and short shunt connections for cumulative and differential compound DC generators. When you study these figures, note the direction of the current in the series field with respect to the shunt field.

Figure 5—Cumulative compound generator

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Long shunt Short shunt

Figure 6—Differential compound generator

Cumulative compoundThe cumulative compound connection is far more common in practical application than the differential compound. The degree to which the series field aids the shunt field is an important consideration in designing the generator for a specific application. Three variations are possible:

Flat-compoundThe increasing flux produced by armature current flowing in the series winding offsets the drooping voltage characteristic associated with the shunt winding. The result is that the terminal voltage is relatively flat or constant with respect to load current.

Over-compoundAdding additional turns on the series field provides a rising voltage characteristic that is useful in supplying DC loads at the end of a long transmission line. The rise in voltage at the terminals of the generator compensates for the line drop to the load.

Under-compoundDecreasing the number of series field turns results in a voltage characteristic that lies somewhere between the flat-compound and the shunt generator.

In Figure 7, the field rheostat is used to adjust the no-load voltage of the generator and a diverter rheostat is used to control the compounding and adjust the full-load voltage of the generator.

LEARNING TASk 3 H-4


Figure 7—Diverter and field rheostats

Differential compoundTo appreciate the strange characteristic curve of the differential generator, it is important to recall that the shunt field is the main source of excitation. The effect of the series field becomes more pronounced with increasing armature current. The “droop” in the voltage characteristic (Figure 8) is more severe than that of the shunt generator. As armature current increases, a point is reached where the series winding flux begins to overpower the shunt winding flux, causing a rapid decrease in terminal voltage.

The main application for the differential compound generators is DC arc welding. To start the arc, a high voltage is required to overcome contact resistance. A much lower voltage is then required to maintain the arc.

Term

inal

vol

tage

Armature current

Figure 8—Characteristic curves for DC generators

LEARNING TASk 3 H-4


The effect of long shunt versus short shunt on terminal voltage is relatively minor. However, for maximum shunt field excitation when connecting DC generators (or motors), a general rule is to connect the shunt field across the source of emf. For a generator, this would be the short shunt connection, since the armature is the source of emf. In this case, the IR drop of the series field has no effect on shunt field current.


LEARNING TASk 3 H-4


Self-Test 3

1. A generator characteristic curve shows the effect of on

.

2. Describe the conditions that must be satisfied to obtain the desired performance when connecting a generator.

3. List the letters that are used in the NEMA system to identify the armature, shunt field and series field.

4. The terminal voltage of a shunt generator is controlled by .

5. What determines the amount of current that flows through an armature?

6. List the factors that cause a reduction in terminal voltage with increasing load.

7. Is the series generator a practical machine?

LEARNING TASk 3 H-4


8. Why would it not be practical to control the output of a series generator with a field rheostat?

9. Why is the no-load voltage of a series generator extremely low?

10. In a series generator, why does the terminal voltage rise with increasing load?

11. Describe how the voltage characteristic of series field coils could be useful.

12. A compound generator contains both and fields.

13. The condition under which the series field adds to the shunt field is called

.

14. The condition under which the magnetic flux of the series and shunt fields oppose each

other is called .

LEARNING TASk 3 H-4


15. Describe the difference between long shunt and short shunt connections.

16. A compound generator that has a relatively constant terminal voltage characteristic is called

.

17. Line drop can be compensated for by using a generator that is .

18. Which of the following generators has the most severe drop in terminal voltage with load?

a. series

b. shunt

c. cumulative compound

d. differential compound

19. Which generator would be the most suitable for arc welding?

a. series

b. shunt

c. cumulative compound

d. differential compound

20. With respect to long versus short shunt connections, what is a general rule?

LEARNING TASk 3 H-4


21. Draw the connection diagram for a cumulative compound short shunt DC generator with diverter rheostat and field rheostat. The prime mover is turning the generator in a clockwise direction. Label all components according to NEMA standards. Show direction of currents and indicate line polarity.

22. Connect the terminal box shown as a cumulatively compounded, short-shunt generator driven in a CW direction by the prime mover.

A2 A1

F1 F2

S1S2

L1 L2



Learning Task 4:

Describe the operating principles of DC motorsWhenever a conductor carries current in the presence of a magnetic field, a force will act on the conductor at a right angle to the field. This principle of magnetism is called the motor effect. Figure 1 shows a single conductor between two field poles and the resulting magnetic field.

Figure 1—Motor effect

For the current direction shown, the flux above the conductor opposes or cancels the main field. Below the conductor, the fields are in the same direction and reinforce the main field. Magnetic lines in the same direction repel, and an upward force is exerted on the conductor. Note that the electron flow convention for current is used throughout this Learning Guide.

Right-hand motor ruleThe right-hand motor rule provides a simple way to determine the direction of force acting on a conductor within a magnetic field. See Figure 2.

1. Point the first finger of your right hand in the direction of the magnetic flux of the field.

2. Point the middle finger in the direction of electron current flow.

3. The thumb now points in the direction of the force exerted on the conductor.

LEARNING TASk 4 H-4


Figure 2—Right-hand motor rule

Neutral planeAs discussed earlier with DC generators, brushes are positioned to short circuit the armature coils as they cross the neutral plane (Figure 3).

The force acting on the conductor as it crosses the neutral plane is zero. If you apply the right-hand motor rule to the armature conductors shown in Figure 3, you will see that the forces act in opposite directions on each side of the neutral plane. However, with respect to the centre of the armature, all forces act to produce clockwise rotation.

Figure 3—Neutral plane

LEARNING TASk 4 H-4


Commutator actionThe main difference between DC motors and generators lies in the operation of the armature. For the generator, the armature is a source of emf that delivers current to a load. For the motor, the armature is a load that draws current from an external source. Figure 4 shows a generator and a motor that both rotate in the same direction. Notice that the direction of the motor armature current is opposite to the direction of the current in the generator.

In a generator, the commutator and brushes keep the current flowing in one direction with respect to an external load. In a motor, the direction of thrust caused by motor effect must change as the coils cross the neutral plane. To do this the commutator ensures that the current in each armature coil changes direction as the coil crosses the neutral plane.

Figure 4—Armature current

Reversing rotationAccording to the right-hand motor rule, the direction of the force acting on a conductor is a function of the direction of the magnetic flux and the direction of current in the armature. The direction of rotation can be changed by doing one of the following:

• Reversing the direction of the armature current by reversing the connection of the armature leads (A1 and A2)

• Reversing the magnetic polarity of the field poles by reversing the connection of both the series field (S1 and S2) and the shunt field (F1 and F2)

TorqueTorque is defined as a twisting or turning force that is capable of producing rotation about an axis. In a motor, torque results from forces acting on current-carrying armature conductors. The force on each armature conductor results from the interaction between the main magnetic field and the field produced by current flowing in the armature.

LEARNING TASk 4 H-4


The following formula shows that torque is directly proportional to the product of flux and armature current:

T = kΦI

where:

T = torque in newton-metres Φ = flux (excitation) in webers I = armature current in amperes k = constant of proportionality that includes factors such as:

• The number of poles

• The number of paths

• The total number of conductors on the armature

When studying motors, it is important to understand that the torque developed by a motor is determined by the mechanical load. At no load, the motor will draw sufficient current to overcome the losses of the machine.

Action of counter emfFaraday’s law of electromagnetic induction is based on the principle that when a conductor is moved within a magnetic field, a voltage is induced in the conductor. This principle encompasses all conductors that cut magnetic lines of flux, including the armature conductors of a DC motor.

Figure 4 shows how the right-hand rule establishes clockwise rotation for the motor. If you apply the left-hand rule to the generator, you can see that clockwise rotation will induce an emf in the opposite direction. A motor and a generator are essentially the same machine, and it is apparent that:

The motion of the armature conductors cutting the field in a DC motor results in an induced emf that opposes the applied emf that is producing the armature current.

The fact that the induced emf opposes the applied emf gives rise to the terms counter emf and back emf. The existence of the counter emf suggests that not all of the applied emf will produce current as determined by Ohm’s law. For example, consider a 10-hp, 240-volt DC motor with an armature resistance of 2 ohms. Ohm’s law would indicate an armature current of 120 amps. In fact, the full-load current rating for this motor is 38 amps.

You can determine the value of the counter emf from the generated voltage (Eg) formula introduced in Learning Task 2 of this Learning Guide:

Eg = kΦN

LEARNING TASk 4 H-4


The term effective voltage is used for the product of armature current and armature resistance (IR), and represents the difference between the applied voltage and the counter emf. This relationship can be summarized with the following formula:

IARA = Ea – Eg

where:

IA = armature current RA = armature resistance Ea = applied voltage Eg = counter emf

Motor starting currentThe foregoing formula for effective voltage can be used to examine the starting conditions of a DC motor. By cross-multiplication, armature current (IA) is:

IE E

RAa g

A

=−

At standstill, the motor speed (N) is zero. Therefore, counter emf (Eg = kΦN) equals zero. In the absence of counter emf, the full value of applied voltage results in extremely high current because it is limited only by the resistance of the armature. In practice, DC motors up to 2 hp have sufficient resistance to withstand starting current, and can be started across the line.

If full line voltage is applied to a large DC motor, the starting current may damage the motor, trip the overcurrent device or subject the load to excessive mechanical stress. When starting large motors, resistance is placed in series with the armature to limit the current to a safe value. Typically the value of this current is 1.5 times the full-load current. As the motor accelerates, the counter emf builds up and the starting resistance is gradually reduced.

The starting resistors and associated control equipment are called reduced voltage starters. The starting resistance is usually reduced in fixed increments, which may be achieved by manual or automatic circuits.

Locked rotor conditionsAlthough electrical problems such as incorrect connections or an open circuit could prevent a motor from starting, a locked rotor in a DC motor usually indicates mechanical problems. The motor fails to start because it is unable to provide sufficient torque to accelerate the load.

Mechanical causes include excessive load or badly worn or seized bearings. Improper starting voltage for the load conditions would also prevent the motor from developing sufficient torque.

The locked rotor condition causes the high starting current to continue. This may damage the motor through overheating.

LEARNING TASk 4 H-4


Mechanical loadingKnowing the significance of counter emf is essential for you to understand the process of mechanical loading of a DC motor. Consider a DC motor operating at rated speed under less than full-load conditions. When mechanical load is increased on the shaft of the motor, the following conditions occur:

• An increase in mechanical load causes the motor speed to decrease.

• The decrease in speed causes a reduction in counter emf (Eg = kΦN).

• The decrease in counter emf causes an increase in effective voltage (IARA = Ea – Eg).

• The increase in effective voltage causes an increase in armature current.

• The increase in armature current causes an increase in torque, to balance the increased mechanical load on the shaft (T = kΦI).

If the increase in mechanical load exceeds the motor’s ability to develop additional torque, the motor will stall and draw excessive current. The overcurrent protection should disconnect the motor from the line.

The reverse effect occurs when mechanical load is decreased. As load is decreased, the motor speeds up. The increase in speed causes the counter emf to increase, thereby decreasing effective voltage and armature current. The resulting torque again matches the new conditions of mechanical load.

Torque and horsepowerThe torque (T) developed by a motor can be considered as an equivalent force (F) acting through the radial distance (R) of the armature. The work done in one revolution would be the equivalent force acting through the circumference (2πR) of the armature.

Work π

π

= ×

=

F R

T

2

2

Since power is the rate of doing work, we can use the rotational speed of the motor to determine power.

Power = work per second

In one second, the armature turns N/60 revolutions, where N is the rpm of the motor. Therefore:

WorkπT

πTN

sec=

=

260

260

N

Power

LEARNING TASk 4 H-4


In the SI system, power is in watts, torque is in newton-metres, and speed is in rpm. It is still common in North America to rate motors in horsepower, and most texts express the relationship between torque, speed and power in imperial units. As one horsepower is equivalent to 550 foot-pounds per second, the power formula is as follows:

hp =πTN

60 550

TN33000

TN5252

2

2 3 14

×

=× ×

=

.

where:

T = torque in pounds-feet N = rotational speed in revolutions per minute

1 hp, the imperial measurement for power, is equal to approximately 746 watts, the SI measurement for power.

As a rule of thumb, torque in lb/ft × 746/550, equals torque in newton-metres.

Speed regulationWhen selecting a DC motor for a specific application, it is important to consider how the motor speed is affected by changes in mechanical load. This variation in speed with load is called speed regulation. The change in speed is expressed as a percentage of the rated or full-load speed:

% regulation =−

×N N

NNL FL

FL

100%

where:

NNL = no-load speed in rpm

NFL = full-load speed in rpm

DC motors are well suited to applications that require smooth speed control under a wide range of load conditions. The specific characteristics of series, shunt and compound motors will be discussed later in this Learning Guide.

Base speedDC motors have a normal or base speed at which they will operate without any speed control device in operation. The base speed will vary with mechanical load conditions.

LEARNING TASk 4 H-4


Power is proportional to the product of torque and speed. Operating a DC motor at other than its base speed will affect its ability to deliver rated horsepower at torque levels required to drive the mechanical load.

Above base speedThe current drawn by a DC motor is a function of the counter emf developed by the armature. If a rheostat is placed in the shunt field circuit, the field current will decrease. The decrease in flux will cause the counter emf to decrease and the armature current to increase.

Contrary to what you would expect, the increase in armature current is greater than the decrease in excitation. As a result, the torque (T = kΦI) increases and the motor speeds up. Inserting resistance in the shunt field circuit causes a DC motor to operate above base speed.

Below base speedTo operate a motor below its base speed, it is necessary to reduce armature current by adding resistance in series with the armature. The high levels of armature current would require physically large (and expensive) rheostats that result in very high heat loss.

A more practical solution to operating below base speed involves electronic devices that are capable of controlling the voltage applied to the armature.

Armature reactionIn both DC generators and DC motors, armature current produces a magnetic field that distorts the main field of the machine. The field distortion caused by armature current is called armature reaction. Adverse effects include a weakening of the main field and excessive sparking at the brushes.

For the same main field polarity and direction of rotation, the direction of armature current in a motor is opposite to that in a generator. The magnetic polarity of the armature flux will also be opposite, producing the field distortion shown in Figure 5. The motor armature current causes the electrical neutral plane to shift in the direction opposite to the direction of rotation. Sparking can be reduced by shifting the brushes opposite to the direction of rotation.

Figure 5—Main field distortion

LEARNING TASk 4 H-4


InterpolesIn practice, the mechanical load conditions on a DC motor would necessitate continual shifts in brush position to reduce sparking. Interpoles placed between the main field poles (Figure 6) provide a more satisfactory way of minimizing the distorting effect of armature flux.

Figure 6—Interpoles

To correct for the shift in the neutral plane, a motor interpole must have the same magnetic polarity as the main pole directly behind it with respect to the direction of rotation. Interpoles are connected in series with the armature so that the strength of the interpole matches the strength of the armature field.

Interpoles have narrow field cores and consist of a few turns of heavy-gauge wire. To minimize the possibility of an incorrect connection, the interpole windings are usually included with the armature between leads A1 and A2. They are found in most shunt and compound DC machines above ½ hp.

Compensating windingsCompensating windings, embedded in the face of the main field poles, provide the most effective method of countering the effects of armature reaction. Due to their high cost, however, compensating windings are seldom used in DC motors.

Applications that require compensating windings involve large DC motors subjected to sudden variations in load conditions. In this instance, compensating windings are used in conjunction with interpoles. Both compensating windings and interpoles are series-connected to the armature.

Calculations involving DC motorsThe main tools for making calculations involving voltage and current are Ohm’s law and Kirchhoff’s law. An understanding of effective voltage and the role of counter emf is essential.

LEARNING TASk 4 H-4


Example 1: A 240-volt motor operating at full load develops a counter emf of 222 volts. If the resistance of the armature is 1.5 ohms, determine the armature current.

IE E

R

V

A

Aa g

A

=−

=−

=

( ).

240 2221 5

12

Ω

For compound motors, a more complete problem would include the current drawn by the shunt field and the voltage drop of the series field. You may wish to read further in this Learning Guide before reviewing Example 2.

Example 2: A long-shunt 240-volt compound motor draws 20 amps from the line (Figure 7). Resistance values are:

Armature resistance RA = 1.2 ohms Series field resistance RS = 0.6 ohms Shunt field resistance RF = 120 ohms

Figure 7—Compound motor

Using the given resistance values, determine (a) armature current, (b) voltage across the armature and (c) counter emf.

Solution:

(a) Armature current

Determine current in the armature path by using Kirchhoff’s current law. The line current must be the sum of the currents in the armature circuit and the shunt field.

IL = IA + IF

LEARNING TASk 4 H-4


where:

IL = line current IA = armature current IF = shunt field current

Use Ohm’s law to find the shunt field current:

IER

V

A

Fa

F

=

=

=

240120

2

Ω

By transposing the equation derived from the current law, you can isolate armature current.

IA = IL – IF

= 20A – 2A = 18A

(b) Voltage across armature

The voltage across the armature is the applied voltage minus the IR drop of the series field. You can determine it using Kirchhoff’s voltage law:

EA = Ea – ES

where:

EA = voltage across the armature Ea = applied voltage ES = IR drop of the series field ES = 18 A (0.6 Ω) = 10.8 V

EA = Ea – ES

= 240 V – 10.8 V = 229.2 V

This armature voltage becomes the applied voltage when determining the counter emf in part (c) of the calculation.

(c) Counter emf

The counter emf will be determined from the equation:

Effective voltage IARA = EA – Eg

LEARNING TASk 4 H-4


Remember that the voltage applied to the armature has been reduced by the IR drop of the series field. Transposing this equation:

Eg = EA – IARA

= 229.2 V – 18 A (1.2 Ω)

= 207.6 V

Example 3: A DC motor operating at 1800 rpm develops a torque of 40 newton-metres. Calculate the power in kilowatts delivered to the mechanical load.

Power2π TN60

2 3.14 40 180060

7540 W or

7.54 kW

=

=× × ×

=

=

Example 4: Calculate the torque developed on the shaft of a motor that is delivering 15 kW to a mechanical load at 1200 rpm.

By cross multiplying we obtain:

TP 602 N

15000* 602 3.14 1200

newton-metres

1

=×

=×

× ×

=

≠

220Nm

* remember to convert kilowatts to watts

Example 5: Calculate the horsepower delivered by a motor that develops a torque of 75 pounds-feet at 1750 rpm.

hpTN

=

=×

=

5252

75 17505252

25

LEARNING TASk 4 H-4


Example 6: Calculate the speed of a motor that is developing a torque of 45 pounds-feet when delivering 10 hp to a mechanical load.

By rearranging the hp formula we obtain:

Nhp

T

rpm

=×

=×

=

5252

5252 1045

1167

Example 7: A DC motor has a rated speed of 1800 rpm. Find the speed regulation if the no-load speed is 1850 rpm.

% regulation =N N

NNL FL

FL

−×

=−

×

100

1850 18001800

100

%

%%

. %= 2 78

Example 8: Find the no-load speed (NNL) of a DC motor that has a rated full-load speed NFL =1800 rpm with a speed regulation of 2%.

% regulation =−

×

=−

×

N NN

N NN

N

NL FL

FL

NL FL

FL

100

2 100

%

% %

NNL FL

FL

NL FL FL

NL FL

NN

N N N

N N

−=

− =

=

= ×

0 02

0 2

1 02

1 02 1

.

.

.

. 8800

1836= rpm


LEARNING TASk 4 H-4


Self-Test 4

1. Define torque.

2. Define the motor effect principle of magnetism.

3. Fleming’s right-hand motor rule is used to determine .

4. How does the operation of the armature differ between a DC generator and a DC motor?

5. State two ways in which the direction of rotation of a DC motor can be reversed.

6. Torque is directly proportional to the product of and

.

7. The motion of armature conductors cutting a field results in an induced emf that

the applied emf.

8. The emf induced in armature conductors as they cut the magnetic field is called

.

9. The counter emf of a motor is proportional to and

.

LEARNING TASk 4 H-4


10. Effective armature voltage is the product of and .

11. The difference between applied voltage and counter emf is called .

12. At standstill, the counter emf of a motor is .

13. At start-up, the current drawn by a DC motor is limited only by .

14. How is the starting current limited to a safe value in large DC motors?

15. A locked rotor usually means that the motor will .

16. Why is a locked rotor condition undesirable?

17. An increase in mechanical load on the shaft of a motor causes motor speed to .

18. When the speed of a motor decreases due to an increase in mechanical load, the counter

emf .

19. A reduction in counter emf causes armature current to .

20. What would happen if an increase in mechanical load exceeded the ability of the motor to develop additional torque?

21. In a motor, power is proportional to and .

22. The variation in motor speed with load is called .

23. Determine the percent speed regulation of a 1200-rpm DC motor with a no-load speed of 1260 rpm.

LEARNING TASk 4 H-4


24. The speed at which a DC motor operates without speed control devices is called

.

25. Why does increasing the resistance in the field circuit of a DC motor cause the speed to increase?

26. To operate a motor below base speed, it is necessary to decrease .

27. A practical method of operating below base speed involves controlling

.

28. List two adverse effects of armature reaction if it remains uncorrected.

29. The most common solution for minimizing the effects of armature reaction involves the use

of .

30. Why are interpoles connected in series with the armature?

31. Calculate the armature current of a 120-volt DC motor that is developing a counter emf of 112 volts with an armature resistance of 2 ohms.

LEARNING TASk 4 H-4


32. A 480-volt long-shunt compound motor draws 51 amps from the line when operating under load. The armature resistance is 1.8 ohms, the series field resistance is 1.2 ohms, and the shunt field resistance is 160 ohms. Determine each of the following:

• Shunt field current

• Armature current

• Voltage across the armature

• Counter emf developed by the armature

33. Calculate the torque developed by a DC motor when delivering 10 hp at 1200 rpm.

34. Calculate the percent speed regulation of a DC motor that has a full-load speed of 1800 rpm and a no-load speed of 1840 rpm.

35. Calculate the no-load speed of a DC motor that has a full-load speed of 2400 rpm and a speed regulation of 3%.



Learning Task 5:

Describe the features and operating characteristics of the shunt motorAs discussed earlier, shunt field windings consist of a large number of turns of small-gauge wire. In a DC shunt motor, the field excitation is provided by shunt field windings connected in parallel with the armature.

Connection diagramsNEMA subscripts are used to establish standard field connections with respect to the armature. Earlier we discussed the convention of A1 negative for clockwise rotation of DC generators. The direction of armature current for a shunt generator and a motor with the same direction of rotation are exactly opposite to one another. Essentially, they are the same machine. Figure 1 shows the direction of armature current and the electrical connections for both machines with the same direction of rotation.

Figure 1—Comparison of generator and motor connections

In the generator, the armature is a source of emf with electron current flowing from A2 to A1, making A1 negative. The armature of the motor, however, is a load. With motor lead A1 connected to the negative terminal of the supply, electron current will flow from A1 to A2 (opposite to the generator armature as shown at the top of Figure 1). In both cases, the shunt fields are connected across the same voltage polarity with current flowing from F2 to F1. The magnetic polarity of the fields is the same in both machines. No change in the connections would be required to operate the generator as a motor, and the direction of rotation would remain the same.

Reversing rotationTo reverse the direction of the force acting on the armature conductors, it is necessary to reverse the current direction in either the armature or the field. Unlike in the generator, residual magnetism does not play a role in the operation of a DC motor. Figure 2 shows the connections

LEARNING TASk 5 H-4


for changing the direction of rotation to CCW (counter-clockwise). Compare this standard convention to the connection shown in Figure 1.

Figure 2—Reversing rotation of a shunt motor

In practice, the direction of motor rotation is changed by reversing the armature leads.

Figure 3 shows the effect of changing only the polarity of the line terminals. In this case, the current changes direction in both the armature and the shunt field, and the direction of rotation remains unchanged.

Figure 3—Reversing line connections

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Speed regulation and controlThe field of a shunt motor is connected across the line (Figure 2). In the absence of a field rheostat, the field current and flux are constant. From the equations IARA = Ea – Eg and Eg = kΦN, speed in rpm can be expressed as follows:

NE I R

ka A A=−Φ

where:

Ea, RA, k and Φ are constant and I is the only variable.

The change in current is such that the full-load speed is roughly 95% of the no-load speed. Shunt motors are commonly called constant-speed motors. Figure 4 shows the variation of speed with load.

Figure 4—Speed characteristics of the shunt motor

Review the principles of speed control explained in Learning Task 4 of this Learning Guide.

You can enable a DC motor to operate above base speed by inserting a rheostat in the field circuit (Figure 5). If you need limited speed control, use a shunt motor with a base speed equal to the lowest value of speed required by the application. As the ohmic value of the rheostat is increased, the operating speed of the motor will also increase.

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Figure 5—Shunt motor with field rheostat

Because the speed of a shunt motor increases with resistance in the field circuit, you must take extreme care to ensure that the field circuit does not become open during operation. For this reason the field is never fused. With drastically reduced counter emf, the motor draws very high current. Residual magnetism provides sufficient excitation for the motor to accelerate to unsafe speeds. This condition is called runaway. Over-speed sensors or loss-of-field relays are commonly used to detect over-speed conditions and initiate a safe shutdown of the motor.

Never allow the field circuit to open during operation of a DC shunt motor.

Most applications for DC shunt motors require operation below base speed. The very high power losses associated with the insertion of resistance in the armature circuit have led to the development of electronic circuits that vary the voltage applied to the armature in order to control the speed below base speed.

Torque characteristicsIf the excitation of a shunt motor is held constant, the torque varies directly with armature current (Figure 6). The starting torque of a shunt motor is typically 2.5 to 3 times the full-load torque and is not sufficient for starting heavy loads.

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Figure 6—Torque characteristics of a shunt motor

ApplicationsDC shunt motors are most commonly used in constant-speed applications where a large variation in speed with load is undesirable. Typical uses include drives for paper machines, printing presses, drill presses, lathes, blower, and motor-generator sets.

Shunt motors are not suitable for starting under heavy-load applications such as traction (locomotives) and lifting (cranes) equipment.


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Self-Test 5

1. Shunt field windings consist of a:

a. large number of turns of large-gauge wire

b. small number of turns of large-gauge wire

c. large number of turns of small-gauge wire

d. small number of turns of small-gauge wire

2. Excitation of a shunt motor is provided by field coils connected in series with the armature.

a. True

b. False

3. When using NEMA subscripts, the convention of A1 positive indicates (CCW or CW) rotation.

4. When a DC generator and a DC motor are operated with the same line voltage, polarity and direction of rotation, the respective armature currents will be in opposite directions.

a. True

b. False

5. State two ways to reverse the direction of the force acting on the armature conductors of a shunt motor.

6. What is the most practical method of reversing the direction of rotation of a shunt motor?

a. Reverse the armature leads.

b. Reverse the shunt field leads.

c. Reverse both the armature and shunt field leads.

d. Reverse the line connections.

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7. What happens to the direction of rotation if the polarity of the line terminals is reversed?

8. A shunt motor is commonly called a motor.

9. Why must the field circuit of a shunt motor never be allowed to become open while the motor is in operation?

10. To prevent runaway in a shunt motor, an is used.

11. List three typical uses for a shunt motor.

12. Draw the connection diagram for a DC shunt motor with field rheostat. The motor is to rotate in a clockwise direction. Label all components according to NEMA standards and indicate line polarity. Connect the terminal box according to your diagram.



Learning Task 6:

Describe the features and operating characteristics of the series motorSeries field windings consist of a few turns of large-gauge wire and carry the full armature current. Unlike shunt field coils, the magnetic strength of the series field coils varies with load current.

Connection diagramsFigure 1 shows the electrical connection of a series motor for counter-clockwise rotation.

Figure 1—Connection diagram

Reversing rotationThe direction of rotation is determined by the direction of the force acting on the armature conductors. Rotation can be reversed by interchanging the leads of either the armature (A1–A2) or the series field (S1–S2), as shown in Figure 2. Compare the two diagrams in Figure 2 to the motor diagram in Figure 1.

Figure 2—Reversing rotation of a series motor

Changing the polarity of the line leads reverses the current direction in the armature and also changes the magnetic polarity of the series field poles. In this instance, the direction of rotation is as shown in Figure 3. Compare this to the motor connection in Figure 1.

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– +Line

A1 A2 S1 S2

DC series motor

Figure 3—Reversing line connections

Speed regulation and controlIn the shunt motor, the strength of the field poles is relatively independent of armature current. This results in very small variations in speed with load. In the series motor, the strength of the magnetic field increases with armature current. Every change in load (armature current) produces a change in both current and excitation.

Torque is proportional to the product of flux and current (T = kΦI). If we assume a linear relationship between current and excitation, torque varies with the square of the armature current (Figure 4). As mentioned in Learning Task 4, it is the counter emf developed by the motor (Eg = kΦN) that limits the current to a value consistent with the required torque. The increase in excitation (armature current) is accompanied by a corresponding decrease in speed (Figure 5).

Figure 4—Torque characteristics of a series motor

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Figure 5—Speed characteristics of a series motor

The wide variation in speed with load makes the series motor unsuitable for constant-speed applications.

Speed control of a series motor can only be achieved by limiting armature current. The high power losses associated with resistance in series with the armature make this method of speed control impractical for large motors. In practice, solid-state drives are used to reduce the voltage applied to the motor.

Torque characteristicsWhen starting a series motor, the absence of counter emf results in very high current. This high current produces very high excitation as it passes through the series field. The combination of high current and flux produces extremely high starting torque—as much as five times the full-load value.

A major concern when operating series motors is the effect of the sudden removal of load when the machine is operating. Since torque is proportional to the square of the current, the series motor rapidly accelerates to unsafe operating speeds if the load is suddenly removed. For this reason, belt drives that can slip or break should not be used with series motors.

Always mechanically couple a series motor directly to its load.

ApplicationsThe main characteristics of the series motor are its very high starting torque and very poor speed regulation. In traction and lifting applications, speed variation is not objectionable and high starting torque is used to advantage. Common uses include cranes, hoists, locomotives, mine haulage trucks and automobile starters.


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Self-Test 6

1. Series field windings consist of a:

a. large number of turns of large-gauge wire

b. small number of turns of large-gauge wire

c. large number of turns of small-gauge wire

d. small number of turns of small-gauge wire

2. What is the main difference between series and shunt excitation?

3. State two ways to reverse the direction of rotation of a series motor.

4. Will changing the polarity of the line leads change the direction of rotation of a series motor? Why?

5. Why is the series motor unsuitable for constant-speed applications?

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6. In practice, speed control of series motors is accomplished using .

7. The main advantage of the series motor is its very high starting torque.

a. True

b. False

8. Why should a series motor always be mechanically coupled to the driven load?

9. The main characteristics of the series motor are and

.

10. List three common uses for series motors.



Learning Task 7:

Describe the features and operating characteristics of the compound motorMany applications require a motor that has higher starting and running torque, while still retaining the speed regulation and control available from the shunt motor. These characteristics are available in the compound motor, which has both shunt and series field coils. In most compound motors the characteristics are predominantly shunt, with a few turns of series field to improve torque.

Connection diagramsWhen connecting compound motors, the direction of current in the field windings is critical for proper operation. NEMA subscripts are used to determine whether the series field will aid or oppose the shunt field (by convention, A1 is shown positive with counter-clockwise rotation). As discussed previously, you must consider cumulative versus differential compounding, and long versus short shunt connection.

For DC generators, the general rule is to connect the shunt field across the source of emf. For motors, the long shunt connection is preferred since it eliminates the effect of the series field IR drop from the shunt field winding voltage.

Most applications require cumulative compounding, where the flux of the series field adds to the flux of the shunt field. Figure 1 shows both long and short shunt connections for a cumulative compound motor. For consistency, all connections are shown with electron current flowing from F2 to F1 and from S2 to S1. This way, the shunt and series windings establish the same magnetic polarity on the field poles that they share.

Figure 1—Cumulative compound motor

In extremely rare instances, the series field is connected to produce a flux that will oppose the flux established by the shunt field. This connection is called differential compound. Figure 2 shows the connection of a differential compound motor. Note that the current directions in the field coils are from F2 to F1 and from S1 to S2.

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Figure 2—Differential compound motor

Reversing rotationReversing the direction of rotation of a compound motor can be accomplished by reversing the connection of the armature leads, or both the series and shunt coil leads (Figure 3).

When reversing field connections, take care to ensure that both series and shunt fields are correct with respect to each other. An incorrect connection will change the motor from cumulative to differential, seriously impairing the machine’s operating characteristics. In practice, it is simpler to reverse the rotation by reversing the armature leads only.

Figure 3—Clockwise rotation

Speed regulation and controlFigure 4 compares the speed curves of the series and shunt motors on the same graph. The speed characteristic of the cumulative compound motor lies between the series and shunt curves. The actual position of the intermediate curve will depend on the degree to which the motor is compounded.

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Figure 4—Speed characteristics

As discussed earlier, the speed of a shunt motor can be changed by inserting a rheostat in the shunt field circuit. This method of speed control can also be applied to the compound motor (Figure 5). Electronic adjustable speed drives give the greatest range of speed control.

Figure 5—Compound motor with field rheostat

The flux developed by the series field helps to limit the excessive speed increase resulting from an open circuit in the shunt field. In most applications, the flux developed by the series field is considerably less than the flux developed by the shunt field, and dangerous over-speed may still occur. For this reason, to prevent runaway of compound and shunt motors, you should take the same precautions (such as over-speed centrifugal devices or loss-of-field relays).

Unlike in series motors, the presence of the shunt field provides the compound motor with a fixed no-load speed. For this reason, it is acceptable to belt-drive the mechanical load.

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Torque characteristicsFigure 6 compares the torque curves of the series, shunt and compound motors on the same graph. As you would expect, the torque curve for the compound motor lies between the curves for the series and shunt motors. The degree of compounding determines the actual position of this curve.

Figure 6—Torque characteristics

In the design of compound motors, the degree of compounding can produce torque characteristics ranging from series to shunt. You must be careful to use a motor with torque and speed characteristics that match the specific requirements of the application.

ApplicationsCompound motors are commonly used in applications that require relatively constant speed under varying load conditions. Compound motors are more suitable than shunt motors for handling sudden increases in mechanical load. Applications include presses, shears, compressors and elevators.


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Self-Test 7

1. A compound motor has both and field coils.

2. In most compound motors, the motor characteristics are predominantly like the

motor.

3. When the flux of the series field aids the flux of the shunt field, the connection is called a

compound.

4. When the flux of the series field opposes the flux of the shunt field, the connection is called

a compound.

5. Explain the difference between long shunt and short shunt connections.

6. State two ways of reversing the direction of rotation of a compound motor.

7. Which of the two methods of reversing rotation in Question 6 is preferred?

8. Figure 4 in Learning Task 7 shows the speed characteristics of series, shunt and compound motors. What determines the position of the compound characteristic curve with respect to the series and shunt curves?

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9. The speed of a compound motor can be increased by increasing resistance in the shunt field circuit.

a. True

b. False

10. Is it necessary to take precautions to prevent runaway of a compound motor?

11. Is it acceptable to have belt-driven mechanical loads with a compound motor?

12. When selecting a compound motor, the and characteristics must be matched to the specific application.

13. Compound motors are more suitable than shunt motors for handling .

14. List three applications for compound motors.

15. Draw the connection diagram for a compound DC motor, with a field rheostat for speed control. The motor is to rotate in a counter-clockwise direction. Label all components according to NEMA standards. Show direction of current flow and indicate line polarity. Connect the terminal box according to your diagram



Learning Task 8:

Describe the features of DC motor controllersMotor controllers are devices for controlling the operation of electric motors. DC motor controllers provide the means to:

• Start, stop and reverse DC motors

• Control motor starting current and torque

• Provide the correct operating sequence and speed control of the driven mechanical system

Manual startersThe function of a starter is to start and accelerate a motor. Manual starters require human operation of a mechanical switching device to control the supply of electrical energy to a DC motor. Switch and relay contacts must be DC-rated because of the arcing that occurs when the current is interrupted. Remember that the current in an AC circuit drops to zero twice per cycle (at 180° and 360°), greatly reducing the persistence of contact arcs.

Many small DC motors are controlled by simple, two-pole switches. Overload devices may be incorporated into the switch or separately connected. Switch contacts must be horsepower-rated to handle the inrush current associated with DC motors.

In larger DC motors, the inertia of the armature causes a slower rate of acceleration and a longer interval of inrush current. In the past, starting requirements for these motors were handled with manual devices such as drum controllers and faceplate starters. The brief descriptions of a drum controller and a faceplate starter that follow will familiarize you with their operation. Note, however, that these devices have largely been replaced by magnetic contactors and solid-state devices.

Drum controllerA drum controller (Figure 1) consists of a series of copper contacts mounted on a cylinder that is insulated from a central shaft. When the cylinder is rotated by a handle attached to the shaft, the cylinder contacts make connection to stationary contacts surrounding the cylinder. Rotating the handle to different positions gives the operator a variety of switching operations, including start, stop and reverse of the DC motor.

Figure 2 shows a six-terminal drum controller and the internal contact connections for each of the three positions, forward, stop or reverse. Figure 3 illustrates a drum controller connected to provide forward/reverse operation of a shunt DC motor. Figure 4 illustrates a connection for a series DC motor. Figure 5 illustrates a connection for a compound DC motor.

Figure 1—Drum controller

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F1A2

A1

F2

Figure 2—Internal drum switch connections Figure 3—Shunt DC motor

A2

S2

A1S1

F1A2

A1S1

S2

F2

Figure 4—Series DC motor Figure 5—Compound DC motor

Faceplate starterManual faceplate starters were used when it was necessary to limit the starting current by placing resistance in series with the armature. Faceplate starters were once widely available in two basic configurations commonly called three-terminal and four-terminal starters.

Figure 6 is the schematic of a typical three-terminal starter. As the handle is moved to the first contact, current flows to the armature through all of the starting resistors. Current also flows to the shunt field through a holding coil that will hold the handle in the ON position once it has passed through all of the starting contacts. This arrangement is necessary to prevent runaway should an open circuit occur in the shunt field circuit. The loss of field current will de-energize the holding coil and cause the spring-loaded handle to return to the OFF position. This arrangement provides no-field protection.

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F1

L1

L1

L2

F2

A1

A1F1

A2

Figure 6—Three-terminal faceplate starter

If the motor is operated above base speed by inserting a rheostat in series with the shunt field, the reduction in field current may cause the holding coil to de-energize. If speed control is required, a four-terminal starter is used. The schematic in Figure 7 shows that the holding coil is connected across the line and is not affected by reduction in shunt field current. Since the four-terminal starter does not provide no-field protection, a mechanical over-speed centrifugal device should be used.

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L1

L1

L2

L2

F1

F1

A1

A1

A2

F2

Figure 7—Four-terminal faceplate starter

Magnetic startersThe operation of DC motors is more commonly controlled by magnetically operated switches called magnetic starters or contactors. In a magnetic starter, the motor starter contacts are closed by energizing an electromagnetic coil. Use of the magnetic starter allows fully automatic control of DC motors under a broad range of operating conditions.

The magnetic contactor (Figure 8) consists of the following parts:

• An electromagnet consisting of a coil of insulated wire mounted on a laminated core

• A movable part of the switch called the armature

• One or more movable contacts mounted on the armature

• One or more fixed contacts that will be engaged by the movable contacts

• A spring assembly that will return the armature to the normal position when the coil is de-energized

• A magnetic blowout coil (in high-current applications) that may be built into the device to aid in extinguishing the arc

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Figure 8—Typical magnetic contactor (single pole)

When the coil is energized by the control circuit, the armature is attracted and operates the associated contacts. For various operations, both normally open (NO) and normally closed (NC) contacts may be equipped. At the instant the coil is first energized, there is a large air gap that requires relatively large current to “pull in” the armature. Once the armature has pulled in, a much smaller current is required. This is because of a decrease in the reluctance of the magnetic circuit.

To prevent overheating of the coil, an auxiliary contact on the starter is sometimes used to insert a resistor in series with the coil once the armature has made contact. Some DC starters are equipped with dual coils, one of which is de-energized by an auxiliary contact once the armature makes contact. When the control circuit is de-energized, the armature returns to the normal or OFF position.

Operation of the blowout coil is based upon a principle of magnetism called the motor effect. Whenever a conductor carries current in the presence of a magnetic field, a force will act on the conductor at a right angle to the field. In the case of the blowout coil, the coil is arranged so that a similar force acts on the arc to force (“blow”) it away from the contacts.

Solid-state startersThe torque and variable speed characteristics of DC motors have made them well suited to a wide range of industrial applications. In the past, the DC power supply was a motor-generator set that was capable of producing DC from the available AC source. The relatively high cost of such an installation was greatly reduced with the development of solid-state rectifier systems. The schematic of a three-phase rectifier is shown in Level 3, Learning Guide D-6: Analyze Electronic Circuits, Learning Task 7 and Level 3, Learning Guide J-3: Install Electronic Motor Controls, Learning Task 2.

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In traction applications, such as locomotives and mine haulage trucks, the solid-state controller varies the voltage applied to the armature and series field of a DC series motor, providing extremely high torque at very low speeds. DC shunt motors are used in applications requiring continuously variable, well-regulated speed. To optimize performance, DC shunt motor controllers provide separate DC supplies for the armature and shunt fields.

The heart of the solid-state DC controller is the silicon-controlled rectifier (SCR), which allows control of the output voltage. As you will see in Level 3, Learning Guide J-3: Install Electronic Motor Controls, Learning Task 2, firing of the SCR produces pulsating voltages with distorted waveforms that may adversely affect motor commutation and performance. To optimize performance, the characteristics of the DC motor and the solid-state controller must be matched.


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Self-Test 8

1. State the function of a starter.

2. Why is it essential that switch and relay contacts be DC-rated?

3. Why does inrush current persist longer in large DC motors than in small DC motors?

4. Describe the basic construction of a drum controller.

5. Name the two basic configurations of manual faceplate starters.

6. Which faceplate starter provides no-field protection?

7. Which faceplate starter allows speed control?

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8. Name the six main parts of a DC magnetic contactor.

9. Why is the holding current of a DC contactor much less than the pull-in current?

10. Give two methods that are used to prevent overheating of the coil of a DC contactor.

11. The operation of a blowout coil is based upon what principle?

12. What device is at the heart of the solid-state DC controller?

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13. Connect the DC supply lines, the drum switch and the DC compound motor in the diagram below.

14. Connect the four-terminal faceplate starter in the diagram for a clockwise rotation. Use NEMA standards to identify the armature and field terminals.

F

L1

L2

A

L1 L2



Learning Task 9:

Describe the operation of magnetic DC motor controllersWhen power is applied to a small DC motor, rapid acceleration causes the counter emf to build up rapidly and limit inrush current. Connecting a motor directly across the line is the simplest method of starting, and may be accomplished either manually or with a magnetic starter.

Across-the-line startingControl circuits for DC across-the-line starting are similar to two-wire and three-wire AC control. As under-voltage protection is normally required to prevent automatic restarting of equipment, three-wire control is most frequently used (Figure 1).

When the start pushbutton is depressed, the contactor coil M is energized, closing the power contacts and the sealing contact. The sealing contact is connected across the start pushbutton and maintains a current path to the coil when the start pushbutton is released. Both lines L1 and L2 must be switched by the magnetic starter. Any of the following will de-energize the contactor coil and stop the motor:

• Loss of voltage, including the supply voltage

• Operation of the stop pushbutton

• Opening of the overload contacts

F1

L1 L2

A1 A2

F2

Figure 1—Three-wire control of a DC shunt motor

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Current-limit accelerationWhen the armature of a DC motor is at standstill, no counter emf is generated. Current drawn by the motor is limited only by the resistance of the armature, which is in the order of a few ohms or less. The resulting inrush current can result in overheating of the armature conductors, excessive starting torque, or an IR (voltage) drop in the supply conductors. On large machines, the value of inrush current must be limited during the starting period.

Current-limit acceleration refers to the insertion and removal of external resistance to the armature circuit during the acceleration of the motor from standstill to normal operating speed. As counter emf is a function of field excitation and speed, voltage buildup across the armature can be used to indicate the degree of acceleration achieved by the motor. Armature current will be decreasing as the counter emf builds up. The counter-emf controller employs this principle to provide current-limit acceleration.

Figure 2 is the schematic of a counter-emf controller. In this example, a single stage of acceleration control is used.

L1 L2

F1

A1 S1 S2A2

R1

F2

Figure 2—Counter-emf controller

When the coil of the contactor is energized by the three-wire control circuit, the main contacts M apply voltage across the motor. Current in the armature path is limited by starting resistor R1. To ensure rapid buildup of the counter emf, the shunt field is connected ahead of the starting resistor. The coil of the accelerating relay AC is connected across armature leads A1 and A2.

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Current limit requirements depend on motor design and load conditions. For this reason, accelerating relays are usually adjustable for pickup and dropout voltages. As the motor accelerates, the counter emf will build up across the armature and accelerating relay coil. When the counter emf reaches a pre-set pickup value, contact AC will close, shorting out the starting resistance R1. The dropout voltage is adjusted to prevent the accelerating relay from re-energizing when heavy loads are applied to the motor.

When driving high-inertia mechanical loads, you may have to provide several stages of acceleration. In this application, the accelerating relay may contain two or more separate contacts that are adjusted to cut in at different values of counter emf. Figure 3 shows a two-stage, counter emf controller.

L1

S2S1A2A1

R1 R2

AC1

AC1

AC2

AC2

L2

F2F1

Figure 3—Two-stage, counter-emf controller

When the first pre-set value of counter emf is reached, contact AC1 will close and short out resistor R1. At the second pre-set value of counter emf, which is higher than the first, contact AC2 will close and short out resistor R2.

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Definite-time accelerationDefinite-time acceleration uses timing relays to adjust the time delay between voltage being applied to the coil and the operation of the contacts by the coil. Timers are available that give both instantaneous and delayed operation of the contacts in NO and NC configurations. Figure 4 shows schematic symbols. The timing cycle may be controlled by mechanical, electrical or electronic means. The starting requirements necessitate a time delay when the control circuit is energized. We will therefore have a brief look at the “on delay” characteristics of timers.

Figure 4—Schematic symbols for timing relays

Dashpot timersDashpot timers (Figure 5) operate by forcing a fluid, such as oil, to flow through a small opening or orifice in a piston-cylinder assembly. When the coil is energized, the moving part of the timer is restrained by the fluid on top of the piston. By controlling the rate at which the fluid flows through the orifice, it is possible to control the time delay in the operation of the contacts. A check valve is used to enable the relay to return to the de-energized state without appreciable time delay.

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Figure 5—Dashpot timer

Pneumatic timersPneumatic timers use air rather than oil as the fluid. The piston and cylinder are replaced by a bellows-and-spring assembly (Figure 6).

Figure 6—Pneumatic timer

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Electronic timersMost electronic timers use the R-C time-constant principle for a charging capacitor. In a DC circuit, the length of time it takes to charge a capacitor to a predetermined voltage depends on the amount of resistance connected in series with it. When the capacitor in an electronic timer has charged to the proper voltage, a solid-state switch (such as a transistor or thyristor) activates a power relay. The time delay can be changed by varying the amount of resistance in the capacitor-charging circuit. Modern electronic timing relays offer many modes of operation, including on delay, off delay, one-shot, recycle and watchdog modes, to name but a few. Figure 7 shows an electronic timer with octal socket base.

Figure 7—Electronic timer

Because dashpot, electronic and pneumatic timers operate similarly, the control circuit in Figure 8 is applicable to each of them. For simplicity, a single stage of acceleration is shown.

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L1 L2

F1

A1

R1

TR1

TR1

A2S2S1

F2

Figure 8—Definite-time acceleration

The coil of the timing relay (TR1) is connected across the coil of the contactor M in such a way that the timing cycle starts when the control circuit is initiated. At the end of the timing cycle, contact TR1 will close, removing the starting resistance from the armature circuit.

Take care in setting the time delay to a value that is suitable for the rate of acceleration of the motor. Starting resistors are rated for intermittent duty and may overheat if left in the circuit for too long.

The capacitor-timing starter uses the R-C time constant of a resistor and capacitor to provide the required time delay. Figure 9 shows a capacitor-timing starter with single-stage acceleration.

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R2

R3

R1

F1

A1 A2 S1 S2

F2

L2L1

Figure 9—Capacitor-timing starter

When the main disconnect is closed, relay AC is energized, holding the NC contact across the starting resistor R1 open. Capacitor C is charged through resistor R3, which is used to limit the charge rate. When the main contactor M is energized by operation of the start pushbutton, contacts M1 and M2 will open. Capacitor C will discharge through resistors R2 and R3 and the coil of accelerating relay AC. Relay AC will remain energized until the voltage across the capacitor reaches the dropout value of the relay coil, allowing contact AC to close and short out starting resistor R1. Using a rheostat for resistor R2 will give limited control of the starting time.

Solid-state timers have the advantage of no moving parts and require less maintenance than dashpot and pneumatic timers. These timers provide a broad range of operating modes and very accurate timing, and so are used today instead of the more expensive mechanical timers.

Always ensure that the timer is designed for operation in DC circuits.

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Field loss protectionEarlier you learned that the shunt field circuit must never be allowed to open during operation of a DC shunt motor. The reduction in counter emf may cause the motor to accelerate to unsafe speed. Although over-speed protection can be provided by mechanical sensing devices attached to the motor shaft, the common approach is to include a field loss relay (FLR) in the shunt field circuit (Figure 10).

F1 F2

S1A1 A2

L1 L2

S2

Figure 10—Field loss protection

The FLR consists of a current coil connected in series with the shunt field, and a contact connected in series with the sealing contact. A loss of current in the shunt field circuit will cause the main contactor to de-energize, safely shutting down the motor. To ensure that the motor receives proper excitation and that the field loss protection meets such operating requirements as speed control, the design of the field loss relay coil must match the current requirements of the shunt field.

ReversingThe direction of rotation of a DC motor may be reversed by:

• Reversing the direction of current in the armature

• Changing the magnetic polarity of the field poles

As reversing the polarity of the field poles would involve changing connections to both the series and shunt fields, the common approach is to reverse the connection of the armature leads (A1 and A2).

LEARNING TASk 9 H-4


The use of a drum controller, with sufficient contacts for reversing the armature connections, provides a simple method for reversing the direction of rotation. The drum controller can be used in a manual application or together with a magnetic starter.

Speed controlIn shunt and compound motors, speed control above base speed is achieved by varying field excitation with a rheostat connected in series with the shunt field. Figure 11 shows the control circuit for a compound motor with a single stage of time-limit acceleration and speed control.

When speed control and field loss protection are used together, the rheostat must be selected in such a way that its highest ohms value does not reduce the field current to the point where the FLR drops out.

F1 F2

S1A1

R1

TR1

TR1

A2

L1 L2

S2

Figure 11—Compound motor with time-limit acceleration and speed control


LEARNING TASk 9 H-4


Self-Test 9

1. What is the function of a sealing contact?

2. List three conditions that will de-energize the contactor coil.

3. Why does a DC motor draw heavy current at the moment of starting?

4. List three unwanted conditions that a large inrush current could cause.

5. Define current-limit acceleration.

6. Armature current as counter emf increases.

7. In a counter-emf controller, where is the coil of the accelerating relay connected?

LEARNING TASk 9 H-4


8. What control device is used to provide definite-time acceleration?

9. timers operate by forcing a fluid through the orifice of a piston-cylinder assembly.

10. Pneumatic timers employ as the fluid.

11. Why is it important that starting resistors not remain in circuit for too long?

12. What principle does the capacitor-timing starter use to provide time delay?

13. On an octal socket base used with an electronic timer, the AC supply will go on Pin #

and Pin # .

14. Why is field loss protection required?

15. Field loss protection is most commonly provided by using .

16. The coil of a field loss relay is connected .

17. Connecting a rheostat in series with the shunt field will enable a DC motor to operate

base speed.

18. Draw a diagram that shows the control and power schematics for three-wire starting of a compound DC motor, with field loss protection, configured to turn in a counter-clockwise direction. Label all components, and show line polarity.



Learning Task 10:

Describe methods of deceleration for DC motorsMany applications require mechanical braking of a motor and its associated mechanical load. For example, if a conveyor belt continues to move after the stop button is pressed, the entire system may be negatively affected. On cranes and hoists, brakes are used to ensure that the load stays in place when the motor is stopped.

Electromechanical brakingOn highway vehicles such as cars and trucks, mechanical brakes are operated by hydraulic systems. By contrast, mechanical brakes for use with most electric motors are operated electrically. Although the basic construction of both types of mechanical braking devices is similar, an electromechanical system uses a solenoid rather than the hydraulic piston-and-cylinder assembly used in a vehicle (Figure 1).

F1 F2

S1A1 A2

L1 L2

S2

Figure 1—DC compound motor with electromechanical braking

In electric-motor applications, the braking system is designed to “fail safe.” For safety reasons, the mechanical braking system is designed so that the brake is applied when the solenoid is de-energized. The resulting loss of voltage triggers automatic braking of the motor and the

LEARNING TASk 10 H-4


mechanical load. The brake is released when the solenoid is energized. Figure 1 is a schematic of a DC compound motor with electromechanical braking. In this example, the braking solenoid is connected across the coil of the main contactor.

Dynamic brakingDynamic braking is a method of deceleration that involves reconnecting a DC motor to function as a DC generator. When the armature of a DC generator is delivering current, the armature conductors will experience a force acting opposite, or counter, to the direction of rotation. The direction of this force follows the right-hand motor rule. Armature current produces a counter-torque that opposes the rotation of the machine. When dynamic braking a DC motor, the counter-torque developed by the armature is used for deceleration. Figure 2 is a schematic for a DC motor starter with dynamic braking.

F1 F2

A1 A2

L1 L2

Figure 2—DC shunt motor with dynamic braking

When the start button is depressed, the coil of the dynamic braking (DB) relay is energized. NO contact DB closes and coil M will be energized. The start pushbutton is sealed by a contact on the magnetic contactor M to ensure that the dynamic braking relay does not “seal in” the start pushbutton if a fault occurs in the operation of the magnetic contactor. Power is applied to the armature by closure of the main power contacts on the contactor. NC contact DB is held open



during operation of the motor so that the braking resistor (R), which is connected across the armature, is out of the circuit.

When the stop button is depressed, line voltage is removed from the motor, with the exception of the shunt field. Shunt field excitation is necessary for the motor to function as a generator during the braking cycle. When the DB relay is de-energized, the braking resistor is connected across the armature through NC contact DB. Armature current flowing through the resistor produces a braking counter-torque.

The counter-torque is directly proportional to the rotational speed of the armature. Maximum braking will occur when the stop button is depressed but will decrease as the motor slows down. For this reason, mechanical brakes are often used for the final stage of deceleration.

In some applications, the shunt field is disconnected after the braking operation by the use of an “off-delay” timing relay (Figure 3).

F1 F2

A1 A2

L1 L2

Figure 3—Dynamic braking with off-delay disconnection of the shunt field



Regenerative brakingThe main disadvantage of dynamic braking is the loss of energy that results from converting the rotational energy of the motor and load into heat that is then dissipated by the braking resistor. An alternative is a regenerative braking system, used with a magnetic controller, that stores the energy for future use. A regenerative braking system requires a storage medium, such as batteries, which may not normally be available. Regenerative braking is far less common than mechanical and dynamic braking—though to improve both the range and efficiency of electric vehicles, regenerative braking has been incorporated into their development.

The technology employed in solid-state controllers makes it possible to return power to the source during regenerative braking, eliminating the need for a storage battery.




Self-Test 10

1. When electromechanical braking is used with an electric motor, the braking system is

operated by a .

2. Braking systems used in electric motor applications are designed to .

3. How is the requirement in Question 2 achieved?

4. Define dynamic braking as applied to the operation of a DC motor.

5. During dynamic braking of a DC motor, the armature conductors experience a force acting

.

6. Why does the shunt field remain energized during dynamic braking?

7. Why are mechanical brakes often used in conjunction with dynamic braking?



8. What device is used to disconnect the shunt field after the dynamic braking operation?

9. What is the main disadvantage of dynamic braking?

10. Which braking system stores, or returns to the source, the energy released during the braking of a DC motor?



Learning Task 11:

Describe basic maintenance and troubleshooting procedures for DC motor controlsWhen performing maintenance and troubleshooting functions, you must treat the control circuitry and the controlled DC motor as a complete system. A thorough knowledge of the troubleshooting and maintenance techniques for DC motors is important in performing those tasks for the control circuitry.

DC motor control devices share many of the operational aspects of AC motor controls. The procedures covered in this Learning Task are based on the troubleshooting techniques for AC motor controls, which will be covered in Level 3, Line J-2: Install Magnetic Motor Controls.

The two fundamental steps in troubleshooting are:

• Visual inspection

• Circuit testing and measurement

The majority of faults in motor control involve loose connections and failed components. A visual inspection can quickly locate the source of a fault or the need for maintenance. During this preliminary inspection, use your other senses of smell and hearing. This examination can reveal:

• Tripped circuit breaker, switch or overload relay

• Burned or heat-discoloured components

• Loose connections

• Mechanical problems such as binding or excessive friction

Remember that since the control voltage will likely be DC, there will be no grounded circuit conductor. Test the circuitry to ensure there are no fault paths to ground. For this de-energized circuit test, you will obtain a more reliable result using a megger rather than an ohmmeter.

Mechanical checksElectromagnetic control devices, such as contactors, relays and timers, use mechanical movement to open and close the electrical contacts. These devices can usually be operated manually to ensure that the actuating device travels freely without binding or excessive friction. The travel distance must also be checked to ensure proper operation of the contacts.

Take care when doing manual tests with either the control or power circuits energized. Manual operation brings the hand dangerously close to connection points and contacts, and can result in serious electric shock, causing death or loss of limb.



Closing a contactor manually in an energized circuit will start the associated motor and, in a three-wire control circuit, seal the contactor on. This unexpected operation of the motor may present a danger to other personnel working in the plant. Before doing any manual tests, remove the overload heaters or use other methods to ensure that the motor will not start.

Electrical checksDC electromagnetic control devices differ from their AC counterparts in two main ways:

• The chatter that can occur in an AC device due to the sinusoidal waveform is not present with a filtered DC supply. For this reason, the shading coil found in AC contactors is not present in DC contactors.

• The persistence of arcing when breaking a DC current is far more severe than the arcing that occurs in an AC circuit. DC contactors in larger sizes incorporate blowout coils to minimize the effects of arcing on contact life. Under severe conditions, excessive arcing can result in welding of the contacts, which prevents shutdown of a motor. Blowout coils and main contacts must be inspected and tested regularly to prevent this dangerous condition. DC control devices are often designed with a greater travel distance. This is to ensure that the contact opening is wide enough to avoid arcing across the contact air gap.

All devices used for the control of DC motors must be DC-rated.



Table 1: Troubleshooting guide for DC motor controls

Contacts

Trouble Possible cause Remedy

Welding or freezing • Abnormal current inrush • Use larger contactor or check for grounds, shorts or excessive motor load current.

• Rapid jogging • Install larger device rated for jogging service, or caution the operator.

• Insufficient tip pressure • Replace contact springs. Check contact carrier for damage.

• Low voltage preventing contacts from closing

• Correct the voltage; check for momentary voltage during starting.

• Foreign matter preventing contacts from closing

• Clean contacts with approved solvent.

• Short circuit • Remove short fault and ensure that fuse or breaker size is correct.

• Blowout coil • Check connections or replace blowout coil.

Short contact life or overheating of tips

• Filling or dressing • Do not file silver-faced contacts. Rough spots and discoloration will not harm them.

• Interrupting too-high currents • Install larger device or check for grounds, shorts or excessive motor controls. Use silver-faced contacts.

• Excessive jogging • Install larger device rated for jogging service, or caution the operator.

• Weak contact pressure • Adjust or replace contact springs.

• Dirt or foreign matter on contact surface

• Clean contacts with approved solvent.

• Short circuit • Remove short fault and ensure that fuse or breaker size is correct.

• Loose connection • Clean and tighten the connection.

• Sustained overload • Install larger device or check for excessive load current.

Coil

Overheating • Overvoltage or high ambient temperature.

• Check application and circuit.

• Incorrect coil • Check coil rating. If incorrect, replace.

• Shorted turns due to mechanical damage or corrosion

• Replace coil.

• Undervoltage; failure of magnet to seal-in

• Correct system voltage.

• Dirt or rust on pole faces increasing the air gap

• Clean the pole faces.



Overload relays

Trouble Possible cause Remedy

Tripping • Sustained overload • Check for grounds, shorts or excessive motor currents.

• Loose connection on load wires • Clean and tighten.

• Incorrect heater • Replace relay with correct size heater unit.

Failure to trip (causing motor burnout)

• Mechanical binding, dirt, corrosion

• Clean or replace.

• Wrong heater, or heater omitted and jumper wires used

• Check ratings; apply proper heater.

• Motor and relay in different temperatures

• Adjust relay rating accordingly, or make temperature same for both.

• Wrong calibration or improper calibration adjustment

• Consult factory.

Magnetic and mechanical parts

Noisy magnet • Magnet facing not mating • Replace or realign magnet assembly.

• Dirt or rust on magnet faces • Clean or realign.

• Low voltage • Check system voltage and voltage dips during starting.

Failure to pick up and seal

• Low voltage • Check system voltage and voltage dips during starting.

• Coil open or shorted • Replace coil.

• Wrong coil • Check coil number and replace.

• Mechanical obstruction • With power off, check for free movement of contact and armature assembly.

• Sealing contact • Check and replace, if necessary.

Failure to drop out • Gummy substance on pole faces

• Clean with solvent.

• Voltage not removed • Check coil circuit.

• Worn or rusted parts, causing binding

• Replace defective parts.

• Welded contacts • Determine cause and replace.




Self-Test 11

1. What are the two fundamental steps in troubleshooting DC motor controls?

2. The majority of faults in motor control involve and

.

3. Give four examples of faults that a visual inspection may reveal.

4. What two mechanical conditions can be checked by manually operating a DC contactor?

5. What precautions must be taken before manually operating an electromechanical control device?

6. What are the two main differences between DC and AC electromagnetic control devices?



7. List three conditions that could result in contact welding.

8. List three conditions that can shorten contact life.

9. Can rust or dirt on the pole faces of a contactor result in overheating of the coil?

10. List three conditions that could be responsible for excessive tripping of an overload relay.

11. List two conditions that could prevent a contactor from picking up and sealing.



Learning Task 12:

Describe basic troubleshooting and maintenance procedures for DC motorsOperational difficulties with DC motors can be categorized under two general headings: electrical faults and mechanical faults. This Learning Task introduces some basic procedures to follow in troubleshooting and performing maintenance on DC motors.

Electrical faultsIn general, electrical faults result from an open circuit, a short circuit, a ground, incorrect connection or improper voltage. For proper operation, keep a motor clean and dry. Conduct insulation tests regularly.

If a DC motor does not perform as it should:

1. Check that the supply voltage is correct and ensure that the armature and field coils are properly connected.

2. Locate open circuits using continuity tests of the field and armature circuits.

3. Use a megger to determine whether there are grounds and insulation breakdown.

Short circuits are most likely to occur in the armature itself. First remove the armature from the machine. Then perform the following tests:

Grounded coilConnect a 120 V supply and a test lamp between any commutator bar and the shaft. If the test lamp lights, the armature is grounded.

Open coilFor lap wound armatures, apply a low voltage to two bars on opposite sides of the commutator. Measure the voltage between adjacent bars while moving the leads of the voltmeter around the commutator. You should observe equal voltages. If there is an open coil or bar, you will observe the full applied voltage between the bars connected to the faulty coil. Excessive sparking at the commutator under normal load and failure to come up to nameplate speed are often indicators of an open armature coil. Often the commutator bars to which the open coil is attached will have burn marks.

Shorted coilUse a growler (Figure 1) to test for grounded, open or shorted coils. A growler is a device that produces a strong alternating magnetic field. When placed in a growler, the armature conductors are cut by the magnetic field. If a coil is shorted, high current flows in the coil. A steel strip such as a hacksaw blade will vibrate excessively when passed over a slot containing a shorted coil. Often on larger armatures, the growler is placed on the armature rather than the other way around. Smoke or a distinct burned odour coming from a motor almost always indicates that a coil is shorted.



Figure 1—Using a growler to test for shorted coils

Mechanical faultsMechanical faults can result in overheating or excessive vibration. Common mechanical faults include:

• Worn bearings

• Armature imbalance

• Improper alignment of the motor and the mechanical load

• Loose coupling between the motor and load

• Loose mounting bolts

• Unbalanced mechanical load

Regular maintenance includes cleaning, lubricating and routine inspection of the mechanical condition of the motor and drive system.

Shaft alignmentShaft alignment is critical for minimizing vibration and extending bearing life. Alignment is checked using a device called a dial indicator. If the armature shaft can be moved up and down, the bearings are worn.



Brush/commutator maintenanceWhen operated correctly, a DC motor should not overheat or spark excessively at the brushes.

BrushesBrushes must be constantly checked during operation. Pay special attention to the following:

• Make sure all brushes are of the same type and grade.

• Make sure all brushes are of the same length, neither too short nor too long to allow free movement in the brush holder.

• Check for free movement of brushes in the holder. Any rough particles in the face should be removed and the brush reseated.

• Check for suitable pressure against the commutator surface (usually between 1 and 2 psi).

• Seat the brushes by using sandpaper placed between the commutator surface and the brushes, with the rough side against the brush contact surface.

• Do not use emory cloth, because conductive grit from the emory cloth may lodge between the commutator bars and cause two adjacent bars to become shorted.

• Commutator repair is expensive, brushes are cheap—replace them regularly!

CommutatorThe condition of the commutator is fundamental for the performance of DC motors. This means that the commutator must be checked regularly. Look for the following:

• The commutator must be kept free of oil and grease and the slots between the bars must be kept clean.

• Under normal operation condition, the film on the surface of the commutator will be a glossy, dark brown or slightly black colour.

• A polished copper surface or surface wear indicates that the brushes must be replaced.

• A black and thick colouring, which usually occurs due to long overloads with humidity, indicates that there is excessive patina on the commutator. Do not use emory cloth to remove the patina. Instead, use sandpaper or an appropriate milling wheel.



Proper maintenance is the best way to ensure the correct operation and reliability of a motor. Table 1 below gives a typical maintenance schedule for a DC motor.

Table 1: Maintenance schedule

COMPONENT WEEKLY MONTHLY 6 MONTHS YEARLY

Brushes and brush rigging

• Good visual inspection.

• Check for brush wear.

• Check for proper seating of brushes.

• Check the condition of brush holders (look for loose bolts and wear).

• Check condition of pigtail shunts.

• Check length of brushes.

• Replace brushes when manufacturer’s wear mark disappears. Replace with same type and grade of brushes.

• Check for normal wear and free movement in the brush holder. Replace cracked and broken brushes.

• Remove some brushes to check contact with commutator surface.

• Clean brushes and brush holders with vacuum cleaner or jet of dry air.

Commutator • Good visual inspection.

• Check commutator condition and wear.

• Check formation of film. Commutator must be glossy with a slightly dark colour like well-done toast.

• Loose brushes cause sparking, overheating and excessive wear of the commutator and brushes.

• Check height of mica between commutator bars.

• Check commutator surface and the condition of the film. A black coloration or streaks in the film indicate commutation problems.

Bearings • Listen for bearing noise. If bearing sounds noisy, check with screwdriver or stethoscope.

• Check for leakage of grease in the bearing housing. Clean it before operating the motor.

• Check for bearing noise. Replace noisy bearings.

• If required, re-grease according to manufacturer’s recommendations.

• Check for noise on both bearings.

• Replace noisy bearings.

• Remove external bearing caps to check grease.

• Follow manufacturer’s recommended lubrication intervals.

• Detailed analysis of bearings. Follow manufacturer’s recommended lubrication intervals.



COMPONENT WEEKLY MONTHLY 6 MONTHS YEARLY

Frame and armature

• Good visual inspection. • Measure insulation resistance. Follow values recommended by manufacturer. If required, clean motor completely.

Complete motor

• Check vibration. Check for abnormal noise. Check air flow around motor.

• Check all electrical connections and retighten them, if required. Check for signs of bad contacts, arcs, discoloration, overheating. Check motor mounting bolts for tightness as well as all coupling elements.

Table 2 lists the common characteristics of a malfunctioning DC motor, the probable cause and corrective measures for each situation.



Table 2: Troubleshooting

MALFUNCTION PROBABLE CAUSES CORRECTIVE MEASURES

Motor does not start, unloaded

• Armature circuit open

• Short in the interpole coil or in the armature

• Drive system defective

• Brush holder not at neutral plane

• Field circuit open

• Test armature for opens and shorts.

• Check for break or defect in the drive system.

• Set brushes to neutral plane.

• Find and repair open.

Motor starts bumping • Drive system defective

• Short in armature coils

• Short between commutator bars

• Check for break or defect in the drive system.

• Repair armature.

• Check commutator and clean between bars.

Motor starts but stalls when load is applied

• Short in armature coils

• Excess line drop

• Brushes not at neutral plane

• Drive system incorrectly adjusted

• Repair armature.

• Check supply voltage and line drop.

• Reset brushes position to the neutral plane.

Motor overspeeds and speed fluctuates when load is applied


• Field circuit open or no-load field rheostat set too high

• Auxiliary series winding incorrectly connected, differentially compounding


• Repair the open. Lower field rheostat below critical resistance value.

• Check connection and correct it.

Motor overheats during operation

• Overload

• Insufficient air flow

• Short circuit in the armature or field winding

• Check fan rotation and direction, clean air pipes and/or filters. Replace filters, if required.

• Check windings and commutator solder points.

• Repair the coils.

Overheating bearing • Excess grease

• Deteriorated or incorrect grease

• Bad bearing

• Excessive speed or load

• Remove excess grease.

• Re-lubricate with correct grease.

• Replace the bearings.

• Reduce speed or decrease load.



MALFUNCTION PROBABLE CAUSES CORRECTIVE MEASURES

Excessive sparking at brushes when load is applied

• Dirty commutator surface

• Grooves or striations on the commutator surface

• High insulation between bars (mica)

• Insufficient brush pressure

• Loose brushes

• Brushes are below wear marks

• Brush grade or type wrong for load

• Brush rigging broken

• Brushes incorrectly set

• Brushes frozen in the holder


• Short circuit between commutator bars

• Machine the commutator, lower the mica and polish copper bar wedges.

• Clean the commutator surface with a polishing stone (“stone commutator”).

• Adjust brush position in relation to load.

• Check brush pressure.

• Replace brush holder.

• Use brushes specified for type of load.

• Replace brushes.

• Put sandpaper on the commutator surface and seat the brushes.

• Check dimensional tolerance of brushes.

• Adjust brushes, following their markings.

• Find and eliminate short circuit.

Sparking on one brush holder arm or all on brushes

• Error in the distribution of brushes

• Uneven distribution of current

• Poor contact

• Check location of brush holders.

• Check air gap uniformity of the interpoles.

• Retighten bolts.

Excessive sparking when load increases


• Overload

• Adjust overload settings to lower value.





Self-Test 12

1. When operated correctly, a DC motor should run without ,

and .

2. List three typical electrical faults associated with DC motors.

3. Continuity tests of the armature and field circuits can be used to locate .

4. Which of the following test equipment is most effective in conducting insulation tests?

a. ohmmeter

b. wheatstone bridge

c. megger

d. continuity tester

5. A is often used to test for shorted coils on an armature.

6. List three common mechanical faults of a DC motor.

7. Regular mechanical maintenance should include ,

and .

8. A is often used to detect improper alignment of a motor (and the mechanical load).



9. List five causes of excessive brush sparking.


ANSwER kEY H-4


Answer key

Self-Test 11. the type of enclosure

2. armature

3. frog leg, lap and wave

4. lap winding

5. to maintain the current in one direction to the external circuit

6. carbon, electro-graphite, graphite, copper graphite

7. 1, 2

8. Field poles provide the necessary magnetic field.

9. sleeve, ball

10. open

Self-Test 21. magnetos

2. The current for separately excited fields is obtained from a source external to the machine.

3. series and parallel (shunt)

4. series windings

5. rheostat

6. residual magnetism

7. number of poles, flux per pole, rotational speed, number of armature paths, number of active armature conductors

8. flux per pole (excitation) and rotational speed

9. Generated voltage is reduced by one third.

10. residual magnetism

11. no-load terminal voltage

12. flashing the field

13. critical field resistance

14. Field rheostats should be set to their minimum ohmic value.

15. Armature reaction is a distortion of the main field flux caused by current flowing in armature conductors.

16. • A reduction in terminal voltage with increasing load

• Impaired commutation

ANSwER kEY H-4


17. d. small number of turns of large-gauge wire

18. b. in series with the armature

19. compensating windings

20. mechanical, electrical, magnetic

21. bearing friction, windage

22. by laminating the core materials

23. copper losses

24. Efficiency is the ratio of power out to power in.

25. 200 kW

Self-Test 31. connected load, terminal voltage

2. The field coils must be properly connected with respect to the armature and to each other.

3. armature = A; shunt field = F; series field = S

4. a rheostat in the field circuit

5. the impedance of the connected load

6. IR drop of the armature, armature reaction and loss of field current due to a reduction in generated voltage

7. no

8. The rheostat would have to carry the high armature current, resulting in high wattage losses.

9. In the absence of a connected load (open circuit), no current flows through the series field coils.

10. As armature current increases, the strength of the series field increases.

11. The rising voltage characteristic is used to offset the decrease in terminal voltage associated with shunt field excitation in a compound generator.

12. series, shunt

13. cumulative compound

14. differential compound

15. In long shunt, the shunt field is connected across the line. In short shunt, the shunt field is connected across the armature.

16. flat-compound

17. over-compound

18. d. differential compound

19. d. differential compound

20. Connect the shunt field across the source of emf.

ANSwER kEY H-4


21.

22.

A2 A1

F1

L1 + – L2

S2

F2

S1

Terminal box

Self-Test 41. Torque is a twisting or turning force that is capable of producing rotation about an axis.

2. Whenever a conductor carries current in the presence of a magnetic field, a force will act on the conductor at right angles to the field.

3. the direction of force acting on a conductor

4. In a generator, the armature is a source of emf. In a motor, the armature is a load.

5. • Reverse the connection of the armature leads.

• Reverse the connection of both the series and shunt field leads.

6. flux, armature current

7. opposes

8. counter emf (or back emf )

ANSwER kEY H-4


9. flux, speed

10. armature current, armature resistance

11. effective voltage

12. zero

13. armature resistance

14. Resistance is placed in series with the armature.

15. fail to start when voltage is applied

16. The persistence of high starting current may damage the motor.

17. decrease

18. decreases

19. increase

20. The motor would stall.

21. torque, speed

22. speed regulation

23. 5%

24. base speed

25. The increase in armature current (due to a decrease in counter emf ) is greater than the decrease in excitation.

26. armature current

27. the voltage applied to the armature

28. • Weakening of the main field

• Excessive brush sparking

29. interpoles

30. so that the strength of the interpole matches the strength of the armature field

31. 4 amps

32. • Shunt field current = 3 amps

• Armature current = 48 amps

• Voltage across armature = 422.4 volts

• Counter emf = 336 volts

33. 43.8 pounds-feet

34. 2.2%

35. 2472 rpm

ANSwER kEY H-4


Self-Test 51. c. large number of turns of small-gauge wire

2. b. False

3. counter-clockwise (viewed from opposite drive end)

4. a. True

5. • Reverse the direction of current in the armature.

• Reverse the direction of current in the shunt field windings.

6. a. Reverse the armature leads.

7. The rotation remains the same.

8. constant speed

9. The motor can accelerate to unsafe speeds.

10. over-speed sensor (or loss of field relay)

11. drives for paper machines, printing presses, drill presses, lathes, blowers, motor-generator sets

12.

ANSwER kEY H-4


Self-Test 61. b. small number of turns of large-gauge wire

2. The magnetic strength of series field coils varies with load current.


• Reverse the connection of the series field leads.

4. No. Reversing line polarity reverses both armature current and field polarity, maintaining torque in the same direction as before.

5. Since the torque varies with the square of the armature current, changes in mechanical load are accompanied by large changes in operating speed.

6. solid-state drives

7. a. True

8. The sudden removal of load may cause the motor to accelerate to an unsafe speed.

9. high starting torque, poor speed regulation

10. cranes, hoists, locomotives, mine haulage trucks, automobile starters

Self-Test 71. shunt, series

2. shunt

3. cumulative

4. differential

5. In the long shunt connection, the shunt field is connected across the line (armature and series field). In the short shunt connection, the shunt field is connected directly across the armature.


• Reverse the connection of both the series and shunt field leads.

7. reversing the connection of the armature leads

8. the degree of compounding

9. a. True

10. Yes. The degree of compounding may not be sufficient to prevent dangerous over-speed in the event of an open in the shunt field circuit.

11. Yes. The no-load speed hazard of the series motor is not present.

12. torque, speed

13. sudden increases in mechanical load

14. presses, shears, compressors, elevators

ANSwER kEY H-4


15.

Self-Test 81. to start and accelerate a motor

2. because of the arcing that occurs when the current is interrupted

3. The inertia of the armature causes a slower rate of acceleration.

4. A series of copper contacts mounted on a cylinder that is insulated from a central shaft. Stationary contacts surround the cylinder. Moving a handle makes (or breaks) the contacts.

5. a. three-terminal starters

b. four-terminal starters

6. three-terminal starter

7. four-terminal starter

8. rlectromagnetic coil, armature, movable contacts, fixed contacts, spring assembly, blowout coil

9. The mmf (amp-turns) that is required when the armature has pulled in is lower, due to the decrease in reluctance of the magnetic circuit.

10. a. inserting a resistor in series with the coil

b. equipping the contactor with dual coils

11. a principle of magnetism called the motor effect

12. silicon-controlled rectifier (SCR)

13.

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14. L1

L1

L2

L2

F1

F1

A1

A1

A2

F2

Self-Test 91. to maintain a current path to the coil when the start pushbutton is released

2. • Loss of supply voltage

• Opening of the overload contacts

• Operation of the stop pushbutton

3. When the motor is at standstill, the armature does not develop counter emf.

4. • Overheating of the armature conductors

• Excessive starting torque

• An IR (voltage) drop in the supply conductors

5. Current-limit acceleration is the insertion and removal of external resistance to the armature circuit.

6. decreases

7. across the armature leads

8. timing relay

9. dashpot

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10. air

11. Starting resistors are designed for intermittent duty and may overheat.

12. the RC time constant of a resistor and capacitor

13. Pin #2 and Pin #7

14. If the shunt field opens during operation, the motor can accelerate to unsafe speed.

15. field loss relays

16. in series with the shunt field

17. above

18.

F1 F2

S1A1 A2

L1 L2

S2

Self-Test 101. solenoid

2. fail safe

3. The normal or de-energized state is such that the mechanical braking system is applied when the control solenoid is de-energized.

4. Dynamic braking is a method of deceleration that involves reconnecting a DC motor to function as a DC generator.

5. opposite or counter to the direction of rotation

6. Shunt field excitation is necessary for the motor to function as a generator during the braking period.

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7. Since counter-torque is proportional to speed, the braking effect decreases as the motor slows down.

8. an off-delay timing relay

9. the heat loss dissipated by the braking resistor

10. regenerative braking

Self-Test 111. a. visual inspection

b. circuit testing and measurement

2. loose connections, failed components

3. • Tripped circuit breaker, switch or overload relay

• Burned or heat-discoloured components

• Loose connections

• Mechanical problems such as binding or excessive friction

4. • The actuating device travels freely without binding or excessive friction.

• The travel distance provides proper operation of the contacts.

5. The control and power circuits must be tested to ensure that they are both de-energized.

6. • A shading coil is not required in a DC contactor.

• Arcing is more serious in breaking DC circuits and requires the use of blowout coils and greater air gaps.

7. abnormal current inrush, rapid jogging, low voltage, foreign matter between contacts, short circuit, failure of blowout coil

8. filing, interrupting too-high current, excessive jogging, foreign matter on contact surfaces, short circuits, loose connections, sustained overload

9. yes (See explanation, Table 1.)

10. a. sustained overload

b. loose connection of load wires

c. incorrect heater

11. low voltage, coil open or shorted, wrong coil, mechanical obstruction, sealing contact

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Self-Test 121. overheating, sparking at the brushes, excessive vibration

2. open circuit, short circuit, ground, incorrect connection, improper voltage

3. open circuits

4. c. megger

5. growler

6. worn bearings, unbalanced armature, improper alignment, loose coupling, loose mounting bolts

7. cleaning, lubricating, inspection

8. dial indicator

9. dirty commutator surface, grooves or striations on the commutator surface, high insulation between bars (mica), insufficient brush pressure, loose brushes, brushes are below wear marks, wrong brush grade or type for load, brush rigging broken, brushes incorrectly set, brushes frozen in the holder, brushes not at neutral plane, short circuit between commutator bars

7960003541

ISBN 978-0-7726-6822-6

9 780772 668226

learning guide h-4 install dc motors and generators · pdf fileinstall dc motors and...

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