electric motor 5

Note: The source of the technical material in this volume is the ProfessionalEngineering Development Program (PEDP) of Engineering Services.

Warning: The material contained in this document was developed for SaudiAramco and is intended for the exclusive use of Saudi Aramco’semployees. Any material contained in this document which is notalready in the public domain may not be copied, reproduced, sold, given,or disclosed to third parties, or otherwise used in whole, or in part,without the written permission of the Vice President, EngineeringServices, Saudi Aramco.

Chapter : Electrical For additional information on this subject, contactFile Reference: EEX21604 W.A. Roussel on 874-1320

Engineering EncyclopediaSaudi Aramco DeskTop Standards

Motor Starter Control Circuits

Engineering Encyclopedia Electrical


Saudi Aramco DeskTop Standards

CONTENTS PAGE

CONTROL CIRCUIT COMPONENTS............................................................. 1

Pushbuttons.......................................................................................... 2

Selector Switches ................................................................................. 8

Position Versions...................................................................... 8

Circuit Breaker Control Switch .............................................. 10

Indicator Lights .................................................................................. 15

Full-Voltage............................................................................ 15

Transformer Type................................................................... 16

Push-to-Test Type .................................................................. 17

Control Relays ................................................................................... 18

Electromechanical .................................................................. 18

Solid-State .............................................................................. 20

Timer ...................................................................................... 20

Control Power Transformers (CPT)................................................... 21

Voltage Ratings ...................................................................... 21

Volt-Ampere Ratings.............................................................. 22

Fusing..................................................................................... 23

Wiring ................................................................................................ 24

Types ...................................................................................... 24

Sizes ....................................................................................... 25

Contactor............................................................................................ 26

AC Coils ................................................................................. 27

DC Coils ................................................................................. 28

Circuit Breaker................................................................................... 29

Overload Relay .................................................................................. 32

MANUAL STARTER CONTROL CIRCUIT LOGIC..................................... 34

Control Logic Description.................................................................. 34




Toggling ................................................................................. 34

Pushbutton.............................................................................. 35

Mechanical Tripping .............................................................. 36

Reset ....................................................................................... 36

FULL VOLTAGE NON-REVERSING CONTROL CIRCUIT LOGIC .......... 38


Three-Point............................................................................. 38

Two-Point (Hand-Off-Auto)................................................... 41

Overload Relay Contact ......................................................... 43

Run/Stop Indicator (Pilot) Lights ........................................... 43

Medium Voltage Control Logic ......................................................... 44

Interposing Relay ................................................................... 44

CT Secondary Circuit ............................................................. 47

NEC Requirements ............................................................................ 48

Accidental Grounds................................................................ 48

Voltage Limitations ................................................................ 50

FULL VOLTAGE REVERSING CONTROL CIRCUIT LOGIC.................... 51


Full-Speed Reversing (Small Motors) .................................... 51

Stop Before Reversing (Medium Motors) .............................. 54

Time-Out Before Reversing (Large Motors) .......................... 55

Mechanical and Electrical Interlocks...................................... 56

REDUCED-VOLTAGE AUTOTRANSFORMER CONTROLCIRCUIT LOGIC............................................................................................. 57



Transition Timer..................................................................... 60

Incomplete Sequence Timer ................................................... 61

Tap Selection.......................................................................... 61

REDUCED-VOLTAGE WYE-DELTA CONTROL CIRCUIT LOGIC.......... 62






Transition Timer..................................................................... 64

MULTI-SPEED CONTROL CIRCUIT LOGIC............................................... 65


Two-Speed Two-Winding Motors.......................................... 65

Two-Speed Single-Winding Motors....................................... 67


Multiple O/L Relays ............................................................... 69

TYPICAL ELECTRONIC (SOLID-STATE) CONTROL CIRCUIT LOGIC.. 70


Solid-State Interface Devices ................................................. 75

Current Feedback ................................................................... 75

Programmable Features .......................................................... 76

CIRCUIT BREAKER CONTROL CIRCUIT LOGIC ..................................... 77


Circuit Breaker Control Switch .............................................. 79

Closing Solenoid .................................................................... 80

Anti-Pumping Relay............................................................... 81

Trip Solenoid.......................................................................... 81

Relay Interface ....................................................................... 81

Indicator Lights ...................................................................... 82

Spring-Operated Mechanism.................................................. 82

GLOSSARY..................................................................................................... 83




LIST OF FIGURES

Figure 1. Typical Motor Starter Schematic Showing Control Circuit as“Ladder Diagram”.......................................................................... 1

Figure 2. Normally Open Pushbutton............................................................ 2

Figure 3. Normally Closed Pushbutton ......................................................... 3

Figure 4. Types of Contact Assembly Blocks ............................................... 4

Figure 5. NEMA Type Designation (Reference NEMA Std. No. 250-1991) .............................................................................................. 5

Figure 6. Ratings and Test Values for AC Control Circuit Contacts at50 or 60 Hertz (Reference NEMA-ICS2-1988) ............................ 6

Figure 7. Ratings and Test Values for DC Control Circuit Contacts(Reference NEMA-ICS2-1988) ..................................................... 7

Figure 8. Position-Type Selector Switch ....................................................... 8

Figure 9. Schematic Diagram of Selector-Type Position Switch .................. 9

Figure 10. Typical Circuit Breaker Control Switch....................................... 10

Figure 11. Band, Row, and Stage Identification for a Typical CircuitBreaker Control Switch................................................................ 11

Figure 12. Example of Contact Operation for a One-Stage, Six-ContactSwitch .......................................................................................... 12

Figure 13. One-Stage, Six-Contact Circuit Breaker Control SwitchUsing Slip/ Pull Contact Position................................................. 13

Figure 14. Continuous and Interrupting Current Ratings for

Typical Circuit Breaker Control Switches.................................... 14

Figure 15. Lamp-Base Styles......................................................................... 15

Figure 16. Typical Voltage Ratings for Transformer-Type IndicatingLights ........................................................................................... 16

Figure 17. Push-To-Test Indicating Light ..................................................... 17

Figure 18. Typical Control Relays................................................................. 19

Figure 19. Typical Voltage Ratings for Control Power Transformer ............ 21

Figure 20. Typical Volt-Ampere Ratings for Control Power Transformer.... 22

Figure 21. Type of Controller Required per SAES-P-114............................. 24

Figure 22. Typical Operating Characteristics for AC Coils........................... 27

Figure 23. Typical Operating Characteristics for DC Coils........................... 29




Figure 24. Branch Circuit Short Circuit Protection (Control CircuitConductors Contained Within Controller Enclosure) .................. 29

Figure 25. Branch Circuit Short Circuit Protection (Control CircuitConductors Extending Beyond Controller Enclosure)................. 30

Figure 26. Overcurrent Protection ................................................................. 31

Figure 27. Example of Overload Relay Ratings ............................................ 33

Figure 28. Fractional Horsepower Toggle Switch Starter ............................. 34

Figure 29. Integral Horsepower Pushbutton Starter (Manual)....................... 35

Figure 30. Full Voltage Non-Reversing Motor Starter (Three-PointControl Circuit) ............................................................................ 39

Figure 31. Full Voltage Non-Reversing Motor Starter (Two-PointControl Circuit) ............................................................................ 42

Figure 32. Medium Voltage Starter With Interposing Relay and CurrentTransformers ................................................................................ 45

Figure 33. Example of Incorrect Control Circuit Wiring............................... 48

Figure 34. Example of Correct Control Circuit Wiring ................................. 49

Figure 35. Full Voltage, Full-Speed Reversing Motor Starter....................... 52

Figure 36. Full Voltage (Stop Before) Reversing Motor Starter ................... 54

Figure 37. Reduced-Voltage Autotransformer Motor Starter ........................ 58

Figure 38. Equivalent Single-Phase Circuit for Autotransformer MotorStarter........................................................................................... 59

Figure 39. Reduced-Voltage Wye- Delta Motor Starter ................................ 62

Figure 40. Multi-Speed Starter for Two-Speed Two-Winding Motor........... 66

Figure 41. Multi-Speed Starter for Two-Speed Single-Winding Motor........ 68

Figure 42. Typical Electronic Solid-State Motor Starter ............................... 71

Figure 43. Typical Manufacturer Ratings for Solid-State Starters ................ 72

Figure 44. Voltage Ramp Function for Solid-State Motor Starter................. 73

Figure 45. Torque Speed Characteristics for Current-Limit Function ........... 74

Figure 46. Standard Breaker Control Scheme (With DC Close, DC Tripand AC Spring Charging Motor) ................................................. 78



Saudi Aramco DeskTop Standards 1

CONTROL CIRCUIT COMPONENTS

Motor starter control circuits are often illustrated by means of a schematic “ladder diagram”similar to the one shown in Figure 1. With reference to this Figure, it is noted thatcomponents used for control circuits include pushbuttons, selector switches, indicating lights,control relays, control power transformers, auxiliary contacts, contactors, overload relays,circuit breakers or MCPs, and electrical wiring. The following sections of this InformationSheet describe the physical construction, optional variations, and mechanical and electricalratings of these components.

Figure 1. Typical Motor Starter Schematic Showing Control Circuit as “Ladder Diagram”




Pushbuttons

One component typically used in the control circuit of motor starters is the pushbutton. Thepushbutton is used to energize the control circuit. A simple pushbutton is composed of a setof stationary contacts, a set of moving contacts, an operating plunger, a return spring, and ahousing to hold the assembly together. Pushbuttons are basically one of two types: one withnormally open contacts, and the other with normally closed contacts.

For the normally open pushbutton (Figure 2), the operating plunger is held up by a returnspring. This spring holds the moving contacts away from the stationary contacts, and it,thereby, keeps the control circuit open and de-energized. Depressing the plunger causes themoving contacts to engage the stationary contacts, thus closing the circuit. Normally openpushbuttons are typically used to start a process.

Figure 2. Normally Open Pushbutton




In the normally closed pushbutton (Figure 3), the design of the contacts is opposite to that ofthe normally open pushbutton. For this type, the contacts open, and the process stops whenthe plunger is depressed. As a result of the internal tension caused by the return spring, theaction of both the normally open and normally closed pushbuttons is momentary. When theoperator’s finger is removed from the pushbutton, it returns to its original state.

Figure 3. Normally Closed Pushbutton




The pushbuttons, which are illustrated in Figures 2 and 3, have only one set of contacts.However, pushbuttons can accommodate multiple sets of contacts, both normally open andnormally closed. Some manufacturers construct multiple-contact pushbuttons as a singleunified assembly with the desired number of contacts included. Other manufacturers offer thepushbutton actuators and the current carrying contacts as separate assemblies that can bemixed and matched to suit the requirements of the application. For pushbuttons where thecontacts are offered as separate block assemblies, as many as eight blocks (circuits) can bemounted side by side or stacked in tandem and operated from one pushbutton. The contactassemblies are generally mounted in a transparent housing that allows visible inspection ofcontact condition and status. Contact assemblies are typically available in a variety offunctional styles. Figure 4 lists some of the common functional styles offered bymanufacturers.

Figure 4. Types of Contact Assembly Blocks

To address specific application needs of selected pushbuttons, accessories are available foruse with the pushbuttons. Some of the accessories available for use with pushbuttons includea padlockable cover to allow locking the pushbutton, protective shrouds to preventinadvertent operation, and rubber boots to provide additional sealing of the pushbutton againstdust and water.




In addition to other considerations, pushbuttons are designed and manufactured to meetvarious environmental requirements. They are considered to be dust tight, water tight, and/oroil tight if, when properly mounted in a suitable enclosure, the assembly meets the applicabledesign requirements given in the standards used to qualify enclosures for electrical equipment(NEMA Standards Publication No. ICS 6, and ICS 250). In accordance with these standards,pushbuttons are identified by a “type” number. Figure 5 lists some of the more common typedesignation numbers applied to pushbuttons and the environmental conditions they protectagainst.

Figure 5. NEMA Type Designation (Reference NEMA Std. No. 250-1991)




With regard to current carrying capability, pushbuttons are identified as being either standard-duty or heavy-duty. In accordance with NEMA standards, standard-duty pushbuttons havecontact rating designations of B600, B300, B150, P600, or P150 as shown in Figure 6 foralternating current and Figure 7 for direct current. Heavy-duty pushbuttons have contactrating designations of A600, A300, A150, N600, N300, or N150 as shown in Figure 6 foralternating current and in Figure 7 for direct current.

Figure 6. Ratings and Test Values for AC Control Circuit Contacts at 50 or 60 Hertz(Reference NEMA-ICS2-1988)




Figure 7. Ratings and Test Values for DC Control Circuit Contacts (Reference NEMA-ICS2-1988)




Selector Switches

Position Versions

A position-type selector switch (Figure 8) is similar to a pushbutton in the sense that it servesthe same function, which is to energize a control circuit. However, unlike the momentaryaction of the pushbutton, the position-type selector switch, once set, maintains its contactengagement without the need of a seal-in interlock.

Figure 8. Position-Type Selector Switch

Position-type selector switches come in two functional styles; two position rotary switchesand three-position rotary switches. The two-position switch has moving contacts andstationary contacts that allow two switch positions, one off position and one selected position.The three-position switch has moving contacts and stationary contacts that allow three switchpositions, one off position and two separately selected positions. These switches areillustrated schematically in Figure 9.




Figure 9. Schematic Diagram of Selector-Type Position Switch

Like the pushbutton switch, the selector-type position switch is offered by somemanufacturers as a two-part assembly. One part is the rotary handle, and the other part is aseparate contact assembly block. Each contact block contains one set of either normally-openor normally-closed contacts. The contact blocks listed in Figure 4 for pushbutton switches arealso available for position selector switches. As many as four contact block assemblies can bemounted side-by-side or in tandem, and then operated from one rotating handle.

Selector-type position switches are also designed to meet environmental requirements. Theyare identified by the same type number designation, and must comply with the same standards(NEMA Standards Publication No. ICS 6, and ICS 250), as pushbutton switches . Theexamples of type designations listed in Figure 5 for pushbutton switches also apply to positionselector switches.




Finally, the current-carrying capability of contacts for selector-type position switches are ratedusing the same method as used for pushbutton switches. The selector switches are identifiedas either standard-duty or heavy-duty. In accordance with NEMA standards, standard-dutyselector switches have contact rating designations of B600, B300, B150, P600, or P150 asshown in Figure 6 for alternating current and in Figure 7 for direct current. Heavy-dutyselector switches have contact rating designations of A600, A300, A150, N600, N300, orN150, as shown in Figure 6, for alternating current and Figure 7 for direct current.

Circuit Breaker Control Switch

A circuit breaker control switch is a rotary power switch designed for heavy duty controlsystems. Built with a spring operating action that returns the switch to its original or neutralposition, it is well suited for circuit breaker control where momentary contact is required.

A typical circuit breaker control switch (refer to Figure 10) consists essentially of an operatinghandle, face plate, control housing, frame contact assembly and rotor contact assembly. Theframe contact assembly and rotor contact assembly form a contact stage. Switches areidentified by the number of stages that they contain, and may be built with from one to eightstages mounted on the steel operating shaft.

Figure 10. Typical Circuit Breaker Control Switch




Each contact stage of a circuit breaker control switch has a minimum of two, and a maximumof up to twelve rotary positions (refer to Figure 11). The frame contacts are positionedaround the frame at 30o intervals, and they are identified in the same manner as the numberson a clock. At every position location on the frame, there are two contacts in line (a set) perstage. The roller contact assembly is made up of from one to six rollers (depending on therequirements for the specific switch). As the switch is operated, the roller contacts internallybridge the adjacent sets of stationary contacts, completing their connected circuits.

Figure 11. Band, Row, and Stage Identification for a Typical Circuit Breaker Control Switch




For purposes of instruction and documentation, the contacts of this type of switch areidentified by combination of bands and rows. Viewing the switch shown in Figure 11 fromthe handle end, a terminal row is identified as the row of contacts at one of the clockpositions. Thus, for a given switch, there are twelve possible rows, which are identified asrows one through twelve and are located in the same position as the hours on a clock.Viewing the switch in Figure 11 from the side, the individual bands are identified as the set ofcontacts located in one clockface (plane). The band nearest the handle end is band “A”, thesecond band is band “B”, etc. Bands “A” and “B” constitute stage one, and bands “C” and“D” constitute stage two. To complete a circuit, the roller contact bridges a set of stationarycontacts in the same row. As an example, completing the circuit between the contacts locatedin row 12 of bands A and B is noted as (A12-B12).

By varying the combination of contact rows, bands, and stages, the number of contactarrangements that are possible from a circuit breaker control switch is almost unlimited. Toassure that users are aware of the contact arrangements for a given switch, manufacturers usea schematic diagram and table to illustrate the operation of each switch. Figure 12 shows oneexample of the type of diagram and table used to convey this information. In this case, theswitch is assembled with six rows of contacts at clock positions 11, 12, 1, 5, 6 & 7, and twobands (one stage). As illustrated by the diagram and noted in the accompanying table, one setof contacts is closed as the switch is moved through each of its six functioning clockpositions.

Figure 12. Example of Contact Operation for a One-Stage,Six-Contact Switch




In addition to rotary motion, circuit breaker control switches are typically provided with alateral movement (push-pull) of the handle and shaft. This position is referred to as the slip orpull contact, and it is used for trip lockout. Figure 13 gives an example of a circuit breakercontrol switch that has this feature, and it shows the relevant wiring diagram for the circuitbreaker.

Figure 13. One-Stage, Six-Contact Circuit Breaker Control SwitchUsing Slip/ Pull Contact Position




The circuit breaker control switch is a very durable switch designed for rugged duty andcapable of carrying and switching higher currents than the position-type selector switch. Asan example, at 240 volts the heavy-duty position selector switch is typically capable ofinterrupting a circuit with 3 amperes, while the circuit breaker switch, at the same voltage,will interrupt 20 amperes. The range of continuous and interrupting current ratings for typicalcircuit breaker control switches is shown in Figure 14.

Figure 14. Continuous and Interrupting Current Ratings forTypical Circuit Breaker Control Switches




Indicator Lights

Indicating lamps are another component used in motor starter control circuits. Their functionis a relatively simple but important one, which is to report the status of the control circuit and,as a result, the status of the equipment controlled. A variety of colors are used for the lens ofthe indicating lights to report or warn of various circuit conditions. Colors typically offeredby manufacturers include red, green, blue, amber, white and clear. The most commonly usedtypes of indicating lights include the full-voltage, transformer and push-to-test types.

Full-Voltage

In a full-voltage indicating light, the lamp operates at the full voltage of the control circuit.This voltage is applied directly to the terminals at the body of the light. For reasons of safety,the maximum voltage rating for this type of indicating light is 120 volts AC/DC. Othervoltage ratings available for this style of light are 6, 12, 24, 28/32, and 48 volts AC/DC.

The full-voltage indicating light is manufactured in two lamp-base styles: the candelabra styleand the bayonet style. Figure 15 shows an illustration of the two lamp-base styles.

Figure 15. Lamp-Base Styles




Transformer Type

The transformer type of indicating light is essentially designed for use with circuit voltageshigher than 120 volts. The unique feature for this type of indicating light is that it comes witha built-in or attached transformer that connects to the higher voltage circuit and steps thevoltage down to a safe level for the lamp. Although intended primarily for use with circuitvoltages above 120 volts, the transformer-type indicating light is also available in lowervoltage ratings. Figure 16 lists the voltage ratings typically available for this style of light.Similar to the full-voltage style, the transformer indicating light is available in both thecandelabra and bayonet lamp-base styles.

Figure 16. Typical Voltage Ratings for Transformer-TypeIndicating Lights




Push-to-Test Type

The push-to-test type indicating light has the unique feature of allowing its lamp to be testedfor satisfactory operation without disturbing the control circuit that it is monitoring. This testis accomplished with the use of a pushbutton that is provided as part of the indicating lightassembly. The schematic diagram shown in Figure 17 illustrates how the test is accomplishedwithout disturbing or altering the control circuit. As seen in Figure 17, operating thepushbutton for the indicating light completes the circuit from one side of the supply voltage,through the light, to the other supply line. In this manner, relay and pilot contacts are leftundisturbed, while the lamp is energized and de-energized for verification of operation.

Figure 17. Push-To-Test Indicating Light

The push-to test light is available as both a transformer style indicating light and also a full-voltage style indicating light. For the transformer style, it is offered in voltage ratings of120/110, 240/220 and 480 volts. In the full voltage style, it is offered as a bayonet base lampor LED (Light Emitting Diode) at voltage ratings of 12, 24 and 120 volts AC/DC.




Control Relays

A control relay is a component that is used in a motor starter’s control circuit to interfacebetween a pilot device and the circuit that the pilot device controls. In effect, the control relayallows the pilot device to control a current that is too large for the contacts of the pilot device.Pilot devices used in motor controls typically monitor parameters such as time, pressure,liquid level, and heat. When the pilot device activates, it allows the control relay to pickup,which in turn allows a higher current control circuit to be energized. For some cases, morethan one pilot device may have to be activated before a control circuit is permitted (throughthe control relay) to be activated.

Several types of control relays are available for use in control circuits. Some common typesinclude electromechanical, solid-state and timer relays. Most of these relays are available ineither 4- or 6-pole configuration. Many types can be easily converted to 8- or 10-poles byadding an additional 4-pole unit. Contacts are convertible between normally open (NO) andnormally closed (NC).

Electromechanical

One type of control relay is the electromechanical type (Figure 18a). This relay uses anelectromagnet to move the output contacts from open to closed and closed to open. Relays ofthis type are referred to as alternating current relays, if designed for actuation from an ACsource, or direct-current relays, if designed for DC operation.

The contact construction for electromechanical relays may be convertible, fixed or universal.Convertible construction allows contacts to be changed in the field from normally open tonormally closed and vice versa. Fixed construction means contacts are either normally openor normally closed and cannot be changed. Universal construction provides both a normallyopen and a normally closed set of contacts on each pole of the relay, but only one or the othermay be used.

The classes of electromagnetic control relays are designated in terms of their contact ratingsby means of letters and numerals. The letter indicates the rating of the contacts in accordancewith Figure 6 (Ratings and Test Values for AC Control Circuit Contacts at 50 or 60 Hertz) orFigure 7 (Ratings and Test Values for DC Control Circuit Contacts), and the number indicatesthe maximum voltage rating of 600, 300 or 150. As an example, a relay designated A600 is arelay which has class A600 contacts (per Figure 6), is suitable for use at alternating-currentvoltages through 600 volts, and has a thermal continuous current rating of 10 amperes.




Figure 18. Typical Control Relays




Solid-State

The solid-state control relay (Figure 18b) performs the same function as theelectromechanical control relay; that is, it controls the flow of current in a control circuit inresponse to a lower level signal from a pilot device. However, the method used by the solid-state device to close and open the current flow is very different. The solid-state relay usessemiconducting devices (i.e. SCR’s, transistors) instead of open-air contacts for the switchingoperation.

The solid-state relay is constructed with a totally encapsulated body that provides protectionof internal components against shock, vibration, dirt, and other environmental hazards.Because the relay is constructed of solid-state devices, there are no moving components towear, thus yielding a longer service life than the electromechanical type, provided the ratingsof the solid-state relay are not exceeded.

The solid-state relay requires two voltages to be applied to it for operation of its solid-statecontacts. One voltage is a nominal fixed line voltage (e.g. 120/110 VAC), and the othervoltage is an input voltage modulated by the pilot device. For typical solid-state relays, theinput voltage may range from 5 volts up to 120 volts AC with respective currents of only afew milliamperes. Application of the two voltages will in turn cause the solid-state contacts(either normally open or normally closed) to operate. A typical alternating current rating for asolid-state contact is 132 VAC, 2 amperes continuous and 5 amperes inrush current.

Timer

The timer control relay is available as either an electromechanical or solid-state type relay.The relay can be purchased with normally open contacts, normally closed contacts, or acombination of the two types. This type of relay functions as described above for either theelectromechanical or solid-state type, except that it has a built-in timing circuit that delays theoperation of the relay contacts.

Timing relays are offered with two types of timing modes. One is for on-delay applicationand the other is for off-delay application. For the on-delay application, once the relay isenergized to operate, it goes through its preset timing cycle before it operates its contacts.The de-energizing operation for the on-delay type has no time delay cycle. For the off-delayapplication, contact operation on energizing the relay is normal (no time delay); however, onde-energizing, the relay goes through its preset timing cycle before the contacts are operated.

Time delay cycles for this type of relay are available in ranges from less than one second tofive minutes. Current and voltage ratings are the same as for electromechanical and solid-state relays without time delay.




Control Power Transformers (CPT)

The voltage needed to operate the control circuit is normally taken from the main powercircuit of the motor starter, as shown in Figure 1. To connect the power circuit to the controlcircuit, a control power transformer (CPT) is used. The transformer is a standard single-phasedesign consisting of two windings connected by a common core. Power is transformed fromthe primary to the secondary winding in accordance with the principles of magnetic induction.The control power transformer is selected and sized according to the voltage rating and volt-ampere capacity of its windings.

Voltage Ratings

Control circuits are designed to operate at relatively low voltage levels (110 to 120 volts AC)in order to provide safety for operating personnel. Motor starter power circuits, however,operate typically at one of several higher voltage levels. For this reason, control powertransformers are available in a variety of voltage ratings and voltage ratio combinations.When sizing a control transformer to the power circuit, the winding voltage ratings must beselected to match the voltages of the control and power circuits. More commonly, thetransformer is sized and provided by the manufacturer as an integral part of the controller.Figure 19 lists the voltage ratings for typical control power transformers available from onemanufacturer.

Figure 19. Typical Voltage Ratings for Control Power Transformer




Volt-Ampere Ratings

In addition to the voltage rating of the windings, the control power transformer is also rated inaccordance with it volt-ampere capacity. With the voltage rating of the windings identified,the volt-ampere rating determines the maximum current that the winding can carry withoutoverheating. As an example, for a transformer rated at 500 VA with a primary winding of480 volts and a secondary winding of 120 volts, it is determined that the maximum safeprimary current is 500 VA/ 480 V = 1.04 amperes. The maximum safe secondary current is500 VA/ 120 V = 4.2 amperes.

Typical volt-ampere ratings available for control power transformers range from 50 VA to2000 VA. Similar to the voltage rating of the windings, the volt-ampere rating for thetransformer is commonly determined by the manufacturer when the transformer is supplied aspart of the controller. Figure 20 lists the volt-ampere ratings of control power transformerwith NEMA sized contactors 00 through 6 as typically provided by one manufacturer.

Figure 20. Typical Volt-Ampere Ratings for Control Power Transformer




Fusing

With reference to Figure 1, control power transformers are typically provided with two fusesin the primary winding circuit and one fuse in the ungrounded leg of the secondary windingcircuit. The fuses are normally contained in a three-pole fuse block mounted on top of thetransformer. The physical dimensions of the primary and secondary fuses are intentionallymade different to prevent installing them in the wrong pole of the fuse block.

The single fuse located in the ungrounded leg of the secondary winding circuit basicallyprovides overcurrent protection for the control circuit conductors and components, but it mustalso provide overcurrent protection for the transformer secondary. In accordance with theNational Electric Code, this fuse must meet the requirements of NEC Article 240 forprotection of the control circuit conductors, and it must also meet the requirements of NECArticle 450 for protection of the transformer secondary.

The two fuses located in the primary winding circuit provide overcurrent protectionspecifically for the transformer. Required sizing for these fuses must be in accordance withNEC Article 450. In general, for transformers rated 600 volts, nominal or less the protectionmust comply with one of the two following requirements:

• Transformer shall be protected by a device on the primary side rated or set atnot more than 125 percent of the rated primary current of the transformer.

• Transformers with an overcurrent device on the secondary side rated or set atnot more than 125% of rated secondary current shall not be required to have anindividual overcurrent device on the primary side if the primary feederovercurrent device is rated or set at a current value not more than 250% of therated primary current of the transformer.




Wiring

Types

The types of control wiring used for combination controllers are determined and provided bythe manufacturer as an integral part of the combination controllers. The wiring types used areselected in accordance with customer, NEMA and NEC standards.

Figure 21 shows the types of controllers that Saudi Aramco requires. Referring to this figurehelps to identify the combination controllers used by Saudi Aramco that have their wiringprovided as an integral part of the controller. With reference to Figure 21, it is noted thatcombination controllers are required for induction motors rated 600 volts and below andgreater than 1 to 100 horsepower. For induction and synchronous motors rated 4000 voltsand above and 1500 horsepower or less, class E2 combination controllers are an approvedselection. The wiring for these controllers is provided by the manufacturer as an integral partof the controllers.

Figure 21. Type of Controller Required per SAES-P-114




With regard to the type of control wiring used for low-voltage controllers, 16-SAMSS-503specifies that all wiring used for motor controllers rated 600 volts and below must be strandedelectrical grade copper with 600-volt insulation.

With regard to the type of control wiring used for medium voltage controllers, 16-SAMSS-506 specifies that conductors must be stranded copper, rated 600-volt NEC Type SIS orTHHN. Each wire must be identified with a thermoplastic, slip-on wire marker withpermanently printed characters (snap-on and adhesive type markers are prohibited).

Sizes

With regard to the size of control wire used in combination starters, the manufacturer selectsthe size (as was done in selecting wire type) in accordance with customer specifications, andNEMA and NEC standards.

Saudi Aramco requirements for control wiring for low-voltage controllers are specified in 16-SAMSS-503. The requirement per this specification is that all conductors must be aminimum size of 2.5 square mm (14 AWG).

Saudi Aramco requirements for control wiring for medium voltage controllers are specified in16-SAMSS-506. The requirement per this specification is the same as for low-voltagecontrollers, which is that all conductors be a minimum size of 2.5 square mm (14 AWG).




Contactor

In a motor starter circuit, the contactor is normally considered to be part of the power circuitbecause it makes and breaks the circuit carrying the motor current. However, the contactor isunique in the sense that a portion of its elements are connected in the power circuit (the mainmotor current contacts), while the remainder of the elements are connected in the controlcircuit (coil and auxiliary contacts).

With regard to the contactor elements contained in the control circuit, Figure 1 (TypicalMotor Starter Schematic) shows the connection of the contactor coil (labeled “M”) and theauxiliary contacts (labeled “Ma” and “Mb”) for a full-voltage non-reversing starter. The mainfunction of the contactor coil is to magnetically open and close the main contacts, also labeled“M”, thus controlling the operation of the motor. The coil is physically mounted on a corethat is constructed of thin, individual metal laminations riveted together. The core and coilform an electromagnet. Energizing the coil causes an armature (mechanically linked to themain contacts) to operate, thus opening and closing the main contacts. Contactors areavailable with either AC or DC voltage coils.

The auxiliary contacts associated with the main contactor are identified with the capital letterM and the subscripts a and b (Ma, Mb). The auxiliaries may be contained within the contactorassembly, or they may be provided as an attachment accessory. They are commonly providedin sets of two with one normally open contact and one normally closed contact. The contactsare designed to interlock the main contactor with the control circuit and are thus designed tocarry no more than 10 amperes continuously.




AC Coils

Contactors with alternating-current coils are available in a wide range of coil voltage ratings(24 volts AC to 600 volts AC). In accordance with NEMA Standard ICS 2-110, alternating-current operated contactors must withstand 110 percent of their rated voltage continuouslywithout injury to the operating coil and must close successfully at 85 percent of their ratedvoltage. Figure 22 lists the typical operating characteristics of AC coils for contactors ofNEMA size 00 through 6.

Figure 22. Typical Operating Characteristics for AC Coils




DC Coils

Contactors are also available with coils that operate on direct current. Typical voltage ratingsfor these coils range from 24 to 250 volts DC. In accordance with NEMA Standard ICS 2-110, direct-current operated contactors must withstand 110 percent of their rated voltagecontinuously without injury to the operating coil and must close successfully at 80 percent oftheir rated voltage. Figure 23 lists the typical operating characteristics of DC coils forcontactors of NEMA size 00 through 6.

Figure 23. Typical Operating Characteristics for DC Coils




Circuit Breaker

As described in the preceding Module EEX216.01 “Motor Starter Components andStandards”, circuit breakers (instantaneous trip and inverse time) are normally used to provideshort circuit and ground fault protection for the power circuit of the motor. Circuit breakersare ordinarily not considered part of the control circuit for the motor starter.

However, under certain conditions, the circuit breaker that provides the motor branch circuitshort circuit protection, may also be used to protect the control circuit conductors. Theconditions under which this is allowed are given in NEMA Standard ICS 1-112.61. Ingeneral, control-circuit conductors that do not extend beyond the motor controller enclosureare considered protected by the circuit breaker providing branch circuit short circuitprotection if the rating or trip setting of the breaker does not exceed the values shown inFigure 24.

Figure 24. Branch Circuit Short Circuit Protection (Control Circuit Conductors ContainedWithin Controller Enclosure)




When control circuit conductors extend beyond the motor controller enclosure, then they areconsidered to be protected by the circuit breaker providing branch circuit short circuitprotection if the rating or trip setting of the breaker does not exceed the values shown inFigure 25.

Figure 25. Branch Circuit Short Circuit Protection (Control Circuit Conductors ExtendingBeyond Controller Enclosure)




For both cases above (conductors contained within the enclosure, and conductors extendingbeyond the enclosure), when the rating or trip setting of the circuit breaker exceeds the valuesgiven in the relevant figure (Figure 24 or Figure 25), the control circuit must be protected by asupplementary overcurrent device (e.g. fuses) with a rating not to exceed the values given inFigure 26.

Figure 26. Overcurrent Protection

In accordance with 16-SAMSS-503, ARAMCO requires each low-voltage combination motorcontroller to have a control power transformer (CPT) to supply the control circuit. The CPTmust be provided with primary current-limiting fuses and a secondary general purpose fuse.




Overload Relay

The overload relay is another component in the motor starter that is connected in both thepower circuit of the motor and the control circuit of the motor starter.

As illustrated in Figure 1, the thermal sensing elements of the overload relay for low-voltagestarters are connected directly in series with the conductors that carry the current to the motor(for medium voltage starters, the overload relays are connected through current transformers).In this manner, the motor’s load current is monitored to determine if the level of currentexceeds the rating of the thermal elements. In accordance with the time-currentcharacteristics for the relay, an overcurrent continuing for a predetermined time will cause therelay contacts to operate.

Overload relays are available from manufacturers in a three-pole configuration, with threethermal elements (one in each pole), or as a single-pole relay with one thermal element. Bothconfigurations are provided with one normally closed contact that opens when an overloadcondition is detected. For three-phase motors, one three-pole relay or three individual single-pole relays can be used to provide the overload protection. The advantage of using threesingle-pole relays is that this arrangement provides good protection against a “single-phase”condition for the three-phase motor. On the other hand, three-pole overload relays provideonly limited protection against a “single-phase” condition.

The normally closed relay contact (identified as “OL” in Figure 1), is connected in the controlcircuit of the motor starter. When mechanically operated by the thermal sensors, in responseto a timed overcurrent, the contact interrupts the current flow to the contactor coil. Thisaction in turn opens the contactor and shuts down the motor.

The contact of the overload relay which operates in the control circuit is designed,manufactured, and rated in accordance with the NEMA ICS-2 ratings shown above in Figure6 (“Ratings and Test Values for AC Control Circuit Contacts at 50 or 60 Hertz”), and inFigure 7 (“Ratings and Test Values for DC Control Circuit Contacts”).

The current rating of an overload relay is expressed in amperes at an ambient temperature of40oC. Ratings for overload relays are given as a range of motor full-load amperes.




As an example of overload relay ratings, Figure 27 lists a few of the current ratings offered byone manufacturer for one type of overload relay for low-voltage starters. Reviewing theindividual ratings shown in this figure gives a good indication of the range of currents thateach relay is capable of handling.

Figure 27. Example of Overload Relay Ratings

The actual level of current that the overload relay will operate at is determined by the rating ofits thermal elements. The thermal elements (sometimes called heaters) have a rating that issimilar to the overload relay in the sense that it is also based on the motor full-load current.But it is a separate rating with a much smaller range in amperes. The rating of the selectedthermal elements, when mounted in a compatible overcurrent relay, determines the time-current operation of the relay.

With respect to time of operation, overload relays with inverse time-current characteristics aredesignated by a NEMA specified class number (10, 15, 20 or 30). The class number indicatesthe maximum time in seconds required for the relay to operate when carrying 600 percent ofits current rating.

The rating and selection of thermal elements, together with a description of relay time classesare described in detail in Module EEX 216.04 “Selecting Low-Voltage Motor Starters”.




MANUAL STARTER CONTROL CIRCUIT LOGIC

Control Logic Description

Manual motor starters are used where only on-off operation is required for small, single-phaseor three-phase motors, and full voltage across-the-line starting is acceptable. Generalapplication includes control of small AC and DC motors where remote control is not required,where the operator is in attendance at the driven load and needs control at that location, andwhere conditions eliminate any hazard due to sudden restarting of motors upon restoration ofpower. Size of motors controlled with manual starters are typically limited to 10 horsepoweror less.

Toggling

The simplest type of manual starter is a 1-, 2- or 3-pole toggle operated switch used forinfrequent starting and stopping of small motors. The toggle operated switch consists of abasic snap-action mechanism that connects the motor to the line in the “on” position anddisconnects the motor when in the “off” position. The toggle switch has a third positioncalled the “trip” position, which is the position the handle is left in when the thermal devicetrips the starter. When in the “trip” position, the handle must be manually moved to the “off”position before it can be switched to the “on” position. When a toggle operated switch ismounted inside a NEMA type enclosure, the toggle handle that operates the contact assemblyprotrudes through the enclosure cover.

To provide running overload protection, the manually operated switch contains a thermaldevice to open the circuit on overloads (Figure 28). The thermal unit is typically a plug-inelement that is keyed for proper positioning in the switch. Elements are typically rated toprovide 115% to 125% protection with the rating marked directly on the element.

Figure 28. Fractional Horsepower Toggle Switch Starter




Pushbutton

Manual starters can also be provided with pushbuttons for operation of the starter. For thiscase, the pushbuttons are mechanically coupled to the contact assembly as illustrated inFigure 29. The pushbutton type starter has a “start” pushbutton to connect the motor to theline, an “off” pushbutton to disconnect the motor, and a pop-out trip indicator that serves as a“reset” pushbutton to reset the starter following a trip operation by the thermal device.

Figure 29. Integral Horsepower Pushbutton Starter (Manual)




Mechanical Tripping

Tripping or opening of the manual type starter is accomplished by one of two methods. Thefirst method is to operate the toggle handle (or pushbutton) manually. The toggle handle (orpushbutton) is mechanically linked to the mechanism that operates the contacts. To assurethat the contacts move with sufficient velocity to reliably make and break their rated current,the mechanism is designed to provide a very quick “snap-action” operation. Manuallyoperating the toggle handle (or pushbutton) is generally intended for opening the motor circuitunder normal conditions with no overcurrent.

The second method of tripping the manual starter is accomplished automatically by thethermal device when an overcurrent exists for a period of time exceeding the time-currentcharacteristics of the device. The thermal device is mechanically linked to the contactoperating mechanism in a manner similar to the toggle handle. When an overcurrent isdetermined to exist for a longer than allowed period, the thermal device mechanicallyoperates the contact opening mechanism in the same manner as the toggle handle.

Reset

The thermal device provides running overload protection for the manually toggle operated (orpushbutton operated) starter by opening the circuit on overloads. When the thermal devicetrips the starter, the starter toggle handle is left in the “trip” position. The toggle handle (orpushbutton) must be then manually reset from the “trip” position to the “Off” position, aftercooling, before it can be physically operated to the “On” or closed position. This resettingprocedure is a safety precaution that is built into the starter switch and is known as a “tripfree” design. The trip free design of the starter is accomplished through the construction ofthe contact mechanism and its method of coupling to the thermal device. In addition torequiring the toggle handle to be reset to “Off” after overload tripping, the trip free designprevents holding the switch closed against a sustained motor overload.

Although manual starters are equipped with overload protection, they do not have the form ofprotection known as “low-voltage” or “under-voltage” protection. As a result, the motor isnot protected against overheating that can be caused by low-voltage operation. In addition tothis lack of protection, the further problem exists that a power failure or other loss of voltageto the motor circuit will cause the motor to stop, but not be disconnected from the supplycircuit. For this condition, the starter contacts remain closed and the motor will restartimmediately on restoration of power. Such an effect can be hazardous when machineoperators or maintenance personnel who are working on the motor are taken by surprise onthe sudden restoration of power.




The above information on manual starters is provided for the general knowledge of theparticipant. Saudi Aramco standards do not permit use of manual starters. In accordancewith 16-SAMSS-503, controller specifications for motors rated 0.75 kW (1.0 hp) or less allowusing only a molded case circuit breaker with three-pole thermal-magnetic trip unit, or acombination controller. For motors greater than 0.75 kW (1.0 hp) to 75 kW (100 hp) 16-SAMSS-503 requires use only of a combination controller with either a three-pole thermal-magnetic molded case circuit breaker, a three-pole overload device and a three- polemagnetic-trip only molded case circuit breaker, or a three-pole overload device and a three-pole motor circuit protector (MCP).




FULL VOLTAGE NON-REVERSING CONTROL CIRCUIT LOGIC

The full voltage non-reversing motor starter is relatively simple in construction, easy tomaintain, and the least expensive of all AC motor starters. This type of starter is used forcontrol of three-phase motors where full-voltage starting is acceptable and where the motor isto start up and run in one direction only.

The control circuit used for control of the full voltage non-reversing starter is normally one oftwo types, either the three point control circuit (Figure 30), or the two point control circuit(Figure 31). This Information Sheet describes the logic and requirements for these circuits asused in both low- and medium-voltage starters.


Three-Point

Figure 30 shows a three-point (or three-wire) control scheme with the control wiring tappedoff the line-side terminals of the starter. Since the control circuit derives its current supplyfrom the same branch circuit that supplies the power to the motor, it is also disconnectedwhen the disconnecting means ahead of the starter is opened. The three-point circuit gets itsname from the fact that this arrangement requires three points for connection of thepushbuttons to the starter.




Figure 30. Full Voltage Non-Reversing Motor Starter(Three-Point Control Circuit)

To trace the operation of the three-point control circuit, it is important to first note two itemsof information regarding the components. First, the pushbuttons are the momentary type.That means that the pushbuttons open or close for as long as they are pushed. Once released,the pushbuttons return to their original state (either normally open or normally closed). Thesecond item that is important to note is that all components are shown in the diagram in theirde-energized state.




With these two items of information noted, the operation of the circuit can be describedbeginning with the closing of the start button. When the start button is pushed, the circuit tothe starter’s contactor coil is completed, and the main contacts of the starter close allowingcurrent to flow to the motor. At the same instant, the red indicating light is turned on, and theauxiliary contacts Ma and Mb operate. Contact Ma, which is normally open, changes to theclosed position completing the circuit around the start button so that the circuit to the coil ismaintained when the button is released. Contact Ma is referred to as the seal-in contact, as thecontrol circuit is now sealed-in until the stop button is operated. Auxiliary contact Mb, whichis normally closed, opens when operated, turning off the green indicating light. Under theseconditions, the motor is connected directly across the line, and starts at full voltage. When itis desired to stop the motor, momentary depression of the stop button breaks the controlcircuit, releasing the starter contacts, which removes the power to the motor. As before, theauxiliary contacts Ma and Mb operate in unison with the main contactor. By the time the stopbutton is released, the Ma contact of the starter has opened, blocking the circuit to the startercoil, and the Mb contact has closed, turning on the green indicating light. The motor can onlybe restarted by once again pushing the start button.

Voltage Self-Protection, also known as undervoltage protection, is an important protectioncharacteristic of the three-point control circuit. The action of this characteristic can bedescribed by again noting that when the start button for the three-point control circuit inFigure 30 is pushed, the contactor coil is energized, and the Ma auxiliary contact seals the coilcircuit closed. Following this, if the line voltage dips too low or fails altogether, the coil willnot be able to hold the contacts closed. Generally, the contactor coil is unable to hold thecontacts closed if the voltage falls below 50% to 60% of normal. Thus a prolonged (morethan a few cycles) drop of voltage at the starter terminals can cause the starter to open.Should the starter open due to voltage failure, the Ma contact also opens releasing the seal-inconnection across the coil. As a result, the starter cannot reclose on return of voltage. Inorder to close the starter after it has opened because of low voltage or voltage failure, the startbutton must be pushed again. This voltage protection characteristic of the three-point controlcircuit eliminates the hazard of uncontrolled restarting of a motor.




Seal-In Contact Ma is an auxiliary contact physically located on the main starter contactorand mechanically linked to operate in unison with the main contacts. It is a normally opencontact, meaning that it is in the open position when the contactor coil is de-energized. TheMa contact is often referred to as the seal-in contact since, as described above, it is used toseal in the circuit that energizes the contactor coil after the start button is pushed. The maincontactor has a normally closed auxiliary contact identified as Mb. The Mb contact iscommonly used for operation of indicating lights or other control functions. When needed,an Mb auxiliary contact can be added to a contactor as an accessory attachment.

Two-Point (Hand-Off-Auto)

The two-point control circuit, as shown in Figure 31, uses a position type selector switch inplace of pushbuttons. The position-type switch is similar to the pushbutton in the sense that itserves the same function, which is to energize the control circuit. However, unlike themomentary action of the pushbutton, the position-type selector switch, once set, maintains itscontact engagement without the need of a seal-in interlock. Position-type switches arenormally provided with either two or three selectable positions. The two-point control circuitgets its name from the fact that this arrangement requires two points of connection betweenthe source of control voltage (terminal 1 of the control-circuit fuse) and the contactor coil.The two-point control circuit in Figure 31 shows the use of a three position switch whichoffers a third or “auto” position for use when it is desired to switch the control of the motor toan external source.

With reference to Figure 31, the operation of the two-point control circuit can be describedbeginning with the manual movement of the selector switch to the “hand” position. Operatingthe selector switch to the “hand” position energizes the starter contactor coil, which in turncloses the main contacts and allows current to flow to the motor terminals. Because theselector switch maintains engagement, it does not require a seal-in circuit and thus remains inthe closed state until manually changed. When it is desired to stop the motor, the selectorswitch is manually operated to the “off” position. This action de-energizes the contactor coiland opens the main contacts to stop the motor.

A unique characteristic of the two-point control circuit is that following a power failure, themotor will restart upon return of power. As a result, the two-point control scheme is typicallyused in cases where the motor is required to be controlled by a remote device, such as athermostat, pressure switch, float switch, or limit switch.




Figure 31. Full Voltage Non-Reversing Motor Starter(Two-Point Control Circuit)




Overload Relay Contact

Figures 30 and 31 show the normally closed contact of the overload relay (marked “OL”) inseries with the starter contactor coil “M”. When an overload condition occurs, the overloadcontact opens, de-energizing the contactor coil and stopping the motor.

Automatic Reset of the overload contacts after operation and cooling is a feature provided onsome overload relays. The automatic reset is a convenient function for use with motorslocated in a remote areas. Relays having the automatic reset feature can also be adjusted andused in the manual reset mode. The manufacturer normally furnishes this type of relay set tothe manual reset mode. The customer then has the option of adjusting it to the automatic resetmode.

Manual Reset of overload relays is the more commonly used type of reset mode. This modeprovides added safety by requiring an intentional reset of the relay before the motor can berestarted. In most cases, this prompts an inspection by the operator to determine the cause oftrip before restarting. To accomplish a manual reset, the operator, following an overload tripand cool down period, must go to the overload relay location and physically push or operatethe reset button (plunger). For two-point control circuits, the manual reset function ispreferred to eliminate the hazard of uncontrolled restarting of motors.

Run/Stop Indicator (Pilot) Lights

Run and stop indicating (pilot) lights are optional pilot devices for motor starters. Thestandard practice is to use a green light to indicate that the motor is switched-off (de-energized and not running), and a red light to indicate that the motor is switched-on(energized and running). As illustrated in Figures 30 and 31, and described above, the redlight is connected directly across the terminals of the starter contactor coil and thus is turnedon and off in unison with the energizing and de-energizing of the coil. Operation of the greenlight is controlled by the normally closed auxiliary contact Mb and thus is switched on and offdirectly opposite that of the red light.




Medium Voltage Control Logic

The control logic for medium voltage motor starters is very similar to the control logic usedfor low-voltage starters. However, there are some differences. Two of these differencesinclude the use of interposing relays and the use of current transformers. The followingparagraphs describe the reasons for using these components.

Interposing Relay

Interposing relays are special relays used in the control circuits of larger size contactors. Therelays interface (or interpose) between the relatively high coil current required by the largesize contactors and the pushbuttons or control contacts used to switch-off or break theircurrent. As an example, Figure 32 shows one type of medium voltage starter that uses aninterposing relay in its control circuit for this purpose.




Figure 32. Medium Voltage Starter With Interposing Relayand Current Transformers




Closing the start button of the control circuit shown in Figure 32 energizes the coil (MR) ofthe interposing relay and causes its three contacts (labeled MR) to close. Closing MRcontacts in turn energizes the contactor coil (M) which closes the main contacts that supplycurrent to the motor terminals. In this manner, the relatively high current required by themain contactor coil (M) is switched on and off by the two higher current rated contacts of theinterposing relay, while the start and stop pushbuttons need only make and break therelatively lower current of the interposing relay coil.

To give an example of the relatively higher currents required by larger size contactor coils,refer to the discussion in the previous Information Sheet “Control Circuit Components”, andthe relevant figures, Figure 22 (Typical Operating Characteristics for AC Coils ) and Figure23 (Typical Operating Characteristics for DC Coils ). The data provided in these figuresshows the volt-ampere burden of the contactor coil increases in accordance with the size ofthe contactor. With reference to Figure 22, it is noted that the coil for a NEMA size 0contactor (in the open position) presents a burden of 160 VA to the control circuit. However,larger size contactors present larger burdens, with a size 6 contactor representing a burden of2900 VA. If a contactor coil with a burden of 2900 VA were applied to a 120-volt controlcircuit, the control contacts (i.e. pushbuttons) would be required to break coil currents ofapproximately 24 amperes.

Noting the relatively high coil current required for larger size contactors, refer to Figure 6(Ratings and Test Values for AC Control Circuit Contacts at 50 or 60 Hertz) of the previousInformation Sheet and consider the amount of current that control contacts are capable ofbreaking. As seen in Figure 6, the breaking current ratings for typical control contacts aremuch lower than the expected coil current of larger size contactors. Even heavy duty controlcontacts (i.e. A150, A300, A600) are limited to breaking current ratings of 6.0 amperes. As aresult, control circuits used to operate larger size contactors (typically size 5 and larger)normally require the use of an interposing relay. For combination starters, provided bymanufacturers, the need for, and sizing of, interposing relays is done by the manufacturerwithout need of specification from the customer.




CT Secondary Circuit

Medium voltage motor starters normally use current transformers (as shown in Figure 32) tosupply current to the thermal elements (heaters) of the overload relays. Current transformerare used for two reasons.

First, the line voltage supplied to the motor terminals by medium voltage starters is nominallyin excess of 1000 volts. Within Saudi Aramco, medium voltage starters are identified asbeing 5 kV class for use on 4160 volt circuits. Since overload thermal elements are rated fordirect in-line use on circuits rated 600 volts AC and below, they can not be directly connectedto higher voltage lines. As a result, insulated current transformers are used as an insulatinginterface between the higher voltage bus and the 600 volt rated thermal elements.

The second reason for use of current transformers in medium voltage starters (and low-voltage starter using large size contactors) is when there is a need to step down the level ofcurrent supplied to the thermal elements. As an example, manufacturers typically offerthermal elements for direct in-line use at current levels up to approximately 135 amperes.Above this current level, it is necessary to use a current transformer to step down the level ofcurrent to match the current rating of the thermal element. For comparison information,consider that NEMA size 00 contactors are rated at 10 amperes while size 5 contactors arerated at 300 amperes and size 9 contactors are rated at 2500 amperes. As a result, contactorsof NEMA size 5 and larger typically require the use of current transformers to supply currentto the overload relay thermal elements.




NEC Requirements

Accidental Grounds

Accidental grounds occurring in a motor control circuit fed from an intentionally groundedsource, can result in unwanted motor starts if the starting devices (pushbutton, limit switch,pressure switch, etc.) are not properly located. As an example, Figure 33 shows a controlcircuit with the pushbuttons located in the ground leg feed to the contactor coil. With thisarrangement, an accidental ground at any of the points (A, B, or C) indicated in the figure willresult in an unwanted start of the motor.

Figure 33. Example of Incorrect Control Circuit Wiring




The National Electric Code recognizes that this condition can occur where damage to thecontrol circuit, especially long wiring runs to remote pushbutton stations, is a potentialproblem. To protect against this problem, NEC Article 430-73 requires that where one side ofthe motor control circuit is grounded, the motor control circuit shall be so arranged that anaccidental ground in the remote-control devices will (1) not start the motor, and (2) not bypassmanually operated shutdown devices or automatic safety shutdown devices.

Figure 34 shows a control circuit arrangement that satisfies the NEC requirement and preventsunwanted starts due to accidental grounds. The important step taken in wiring the circuitshown in Figure 34, is that care has been taken to place the pushbutton station in the hot legfeed to the coil, and not in the grounded leg. By taking this wiring precaution, it can be seenthat the occurrence of a ground fault cannot start the motor.

Figure 34. Example of Correct Control Circuit Wiring




Voltage Limitations

A number of different voltage levels, both AC and DC, are used for motor starter controlcircuits. Among these are included AC voltage levels of 24, 48, 110, 120, 200, 208, 220, 240,380, 440, and 600 volts. DC voltage levels include 24, 48, 125 and 250 volts. Evidence ofthe large number of voltage levels in use can be seen by reviewing a manufacturer’s catalogand noting the number of different voltage ratings available for contactor coils.

The National Electric Code addresses circuit arrangement (such as proper groundingdescribed above), protection requirements, and wiring requirements for control circuits, but itdoes not address the level of voltage to be used. Although a large number of voltage levelsare in fact used, only a few are used extensively. For most common applications, the level ofcontrol voltage is 120 or 240 volts AC with some uses extending to 480 volts.

As described previously, a number of motor starter control circuits use a control powertransformer (CPT) to step down the level of voltage used in the control circuit for isolatingand safety purposes. In all cases, when a control power transformer is used, the voltage isreduced to between 110 and 120 volts AC. As a result, the control voltage level for circuitswith control power transformers is 110 to 120 volts AC.

In accordance with 16-SAMSS-503, Saudi Aramco requires that all low-voltage starters beprovided with control power transformers unless otherwise specified. Medium voltagestarters normally use a control power transformer as part of their standard design.




FULL VOLTAGE REVERSING CONTROL CIRCUIT LOGIC

The full voltage reversing motor starter is used where it is necessary to be able to start and runa motor in either direction. The direction of rotation of three-phase induction motors is easilyreversed by simply interchanging any two of the three line connections to the motor. Reversalof the motor connections is accomplished through control of the contactors mounted in themotor starter. This Information Sheet describes several types of control logic used in full-voltage reversing starters.


Full-Speed Reversing (Small Motors)

One type of control logic used in full-voltage reversing starters is the full-speed reversinglogic. The circuit arrangement for this type logic allows the direction of a motor to bereversed without pushing the stop button. This mode of operation has the advantage ofreversing the direction of the motor in the shortest possible time. However, use of this type oflogic is acceptable only for small motors with relatively low mass and thus low inertia.

The circuit arrangement for full-speed reversing is shown in Figure 35. The arrangement ofthis circuit is basically the same as for all full-voltage reversing starters, except that theforward pushbutton and the reverse pushbutton have an additional normally closed contactblock. The starter has two contactors, one connected to apply the three phases to the motor sothat the motor starts and runs in the forward direction. The other contactor is connected sothat when it closes, two of the lines to the motor are interchanged, thus reversing the directionof rotation of the motor.




Figure 35. Full Voltage, Full-Speed Reversing Motor Starter




For the circuit shown in Figure 35, starting the motor and operating it in the forward directionis initiated by closing the forward pushbutton. This action energizes the forward contactor(F), which in turn closes the main contacts to the motor terminals, thus starting the motor inthe forward direction. The same action that closes the normally open contact of thepushbutton also opens its normally closed contact. This temporary open circuit prevents anunwanted start in the reverse direction. As the forward contactor operates, it causes itsauxiliary contacts to operate also. The normally open auxiliary forward contact (Fa) closes toseal in the forward coil circuit, and the normally closed auxiliary forward contact (Fb) opensto block the reverse contactor coil circuit.

To reverse the direction of the motor (without first pushing the stop pushbutton), simply pushthe reverse pushbutton. The opening of the normally closed contact of the reverse pushbuttoncauses the coil circuit for the forward contactor (F) to momentarily open. The circuit for thereverse contactor coil (R) circuit momentarily closes. When the forward contactor opens, itoperates its auxiliary contacts (Fa , Fb). Fa opens to keep the forward (F) coil circuit open,while Fb closes to allow the reverse coil (R) circuit to energize. When the reverse contactorpicks up, it closes it main contacts to the motor terminals, which in this case causes lines L1and L3 to be interchanged, reversing the direction of the motor. Auxiliary contacts Ra and Rboperate in unison with the reverse contactor. The normally open Ra contact closes to seal inthe reverse coil circuit, while the normally closed Rb contact opens to block the energizing ofthe forward coil circuit.




Stop Before Reversing (Medium Motors)

The stop before reversing logic is generally used for medium size motors. Stopping the motorfirst, before reversing its direction, eliminates the overcurrent surge and potentially damagingmechanical stress that can result with the attempt to rapidly reverse a large rotating inertia(load). Figure 36 shows the circuit arrangement for a stop before reversing logic. The basicdifference between this circuit and the one given in Figure 35 is the arrangement of thepushbutton contacts.

Figure 36. Full Voltage (Stop Before) Reversing Motor Starter




Operation of the circuit in Figure 36 begins by closing the forward pushbutton to energize theforward contactor coil (F). This action closes the main forward contacts to the motorterminals starting the motor in the forward direction. In sequence with this action, auxiliarycontact Fa closes to seal in the forward coil (F) circuit, and auxiliary contact Fb opens toblock the reverse coil (R) circuit. To reverse the motor when it is running in the forwarddirection, it is necessary to first push the stop pushbutton. Pushing the stop pushbutton de-energizes the forward contactor, which in turn opens the main forward contacts and allows Fato open and Fb to close. At this point, the reverse pushbutton can be pushed to energize thereverse contactor coil circuit. In sequence then, the main contacts of the reverse contactorclose (reversing motor lines L1 and L3), Ra closes to seal in the reverse coil circuit, and Rbopens to block the forward coil circuit.

The circuit logic for the stop before reversing starter requires only that the stop button bepushed before an attempt is made to reverse the direction of the motor. Whether the motor isallowed to come to a complete stop before actually reversing is an option of the operatorbased on the motor’s size and inertia.

Time-Out Before Reversing (Large Motors)

Large motors must be brought to a complete stop before attempting to reverse their direction.This precaution is necessary because of the large mass and resulting high inertia of the motor.This inertia could cause intolerably high motor current and possible mechanical damage ifdirection reversal where attempted while the motor was still turning.

To accomplish this mode of operation, a circuit logic referred to as time-out before reversingis used. The circuit arrangement for this logic is identical to the one shown in Figure 36 withone exception. The exception is that either a timing relay or speed sensor is added to thecircuit.

When a timing relay is used, the relay is connected to begin its timing cycle whenever thestop pushbutton is pushed. The timing relay contacts prevent energizing the contactor coil forthe opposite direction until after the relay has completed its predetermined time cycle. Thecycle is preset to allow the motor sufficient time to come to rest.

When speed sensors are used, the control contacts for the sensors are connected in series withthe contactor coil circuits. The sensors continuously monitor the speed of the motor. Onpushing the stop pushbutton, the sensor control contacts prevent energizing the contactor coilfor the opposite direction until after the motor comes to a stop.




Mechanical and Electrical Interlocks

All reversing starters have two magnetic contactors mounted in one enclosure. The contactorsprovide for connection of the power leads to the motor terminals. One contactor (forwardcontactor) is connected to the apply the three phases to the motor so that the motor starts andruns in the forward direction. The other contactor (reverse contactor) is connected so thatwhen it closes two of the lines to the motor are interchanged, thus reversing the direction ofrotation of the motor.

With regard to the operation of the two contactors, it necessary that they be interlocked toprevent both of them from closing at the same time. If both contactors were to close at thesame time, the result would be a dead short circuit across two of the phases. Interlocking isused to prevent this condition from happening. Both mechanical and electrical interlocks areused. Mechanical interlocks typically use an insulated linkage fastened between the movingassemblies of the two contactors to prevent their simultaneous closing. Electrical interlockingcircuits vary in purpose and complexity, but the basic technique consists of using normallyopen and normally closed contacts in both the forward and reverse coil circuits to maintaineach coil circuit open while the other is closed.




REDUCED-VOLTAGE AUTOTRANSFORMER CONTROL CIRCUIT LOGIC

Reduced-voltage starters are used where full-voltage starting can cause serious problems. Forsome applications, the use of a full-voltage starter with its typical 600% starting currentcauses unacceptable voltage disturbances on the electrical power system. In other cases, therelatively high starting torque that accompanies full-voltage starting may mechanicallyoverstress the driven equipment. For these, as well as other reasons, reduced voltage startersare sometimes used instead of full-voltage starters. The reduced-voltage autotransformer isone type of reduced-voltage motor starter used for this purpose. The following paragraphsdescribe the control logic and operation for this type of starter.


The autotransformer motor starter incorporates the use of autotransformer coils to supply areduced voltage to the motor during starting. Figure 37 shows a typical control circuit for anopen-delta type autotransformer starter while Figure 38 presents an equivalent single-phasecircuit for the starter at the instant of starting.




Figure 37. Reduced-Voltage Autotransformer Motor Starter




Figure 38. Equivalent Single-Phase Circuit forAutotransformer Motor Starter

The basic principle used to apply reduced voltage to the motor can be described by referringto the equivalent circuit show in Figure 38. To begin with, assume that the motor isconnected to the 50% tap on the transformer. With the motor connected to this tap, 50% ofthe full voltage is applied to the motor, therefore the current Im drawn by the motor is 50% ofthe full-voltage starting current. However, by transformer action, the line current IL (theprimary current of the transformer) is only 50% of the motor current (the secondary current ofthe transformer). Thus, the line current drawn from the system is only (0.50)2 or 0.25 (25%)of the full-voltage starting current. The reduction of starting torque is proportional to thesame percentage-tap-squared factor as for the reduction in starting line current.

With reference to Figure 37, the operation of the control circuit that allows theautotransformer to apply reduced voltage and start the motor begins with the closing of thestart pushbutton. This action energizes and closes start contactor S, which connects eachautotransformer to the line and energizes the motor through the transformer taps. At the sameinstant, the timing relay TR is energized to begin its preselected timing cycle. As the motoraccelerates toward full speed, the timing relay operates to open contactor S, whichdisconnects the autotransformer and closes run contactor R, which connects the motor directlyto the lines so that it runs normally on full voltage.





Interlock protection for the S and R contactors is necessary to prevent their being closed at thesame time. A review of the control circuit in Figure 37 shows that if both contactors werepermitted to close at the same time, a portion of the autotransformer winding would beshorted out placing an overvoltage on the remaining portion. To prevent this condition fromhappening, the S and R contactors are electrically interlocked by the normally open andnormally closed contacts of the timing relay (TR). As a result of their placement in thecontrol circuit, the timing relay contacts maintain each coil circuit open while the other isclosed. In addition to electrical interlocking, it is common practice for the S and R contactorsto be mechanically interlocked. Mechanical interlocking of the S and R contactors is done byconnecting their moving assemblies with an insulated mechanical link.

Transition Timer

For the autotransformer starter shown in Figure 37, the switching operations are made by themagnetic contactors in combination with the timing relay TR. The timing relay, also knownas the transition timer, initiates the transfer from reduced voltage to full voltage operation.The time period, beginning with the closing of the start pushbutton and continuing until theautotransformer is switched out of the circuit and full voltage applied, is called the transitiontime. The allowed transition time is the time estimated necessary for the motor to accelerateto full speed while at reduced voltage. The timing relay (transition timer) is preset to theallowed transition time before attempting to start the motor.




Incomplete Sequence Timer

For the reduced-voltage autotransformer starter, the autotransformer is expected to carrycurrent only during the short starting cycle. As a result, the physical size of transformers usedin these starters are smaller than the ones that would be expected to carry full currentcontinuously. To protect against a malfunction of the control circuit that would require theautotransformer to carry current for too long a time during the start cycle, an incompletesequence timer (TS) is connected in the control circuit. The incomplete sequence timer has athermal element connected in a manner that causes it to begin carrying current as soon as thestart pushbutton is closed. The thermal element senses the period of time that current isflowing, which in turn is the same amount of time that the autotransformer is carrying startingcurrent. Should the current carrying time period exceed the allowed period, the TS relaycontacts open. When the TS contacts opens, they open the TR coil circuit and thusdisconnects the motor. Additionally, the TS timer has a thermal memory that will prevent anattempt to re-start the motor until sufficient time has elapsed to allow the heater element of theTS, and the autotransformer windings, to cool.

Tap Selection

Autotransformers are normally constructed with taps rated 50%, 65%, and 80%. For aspecific application, the tap is selected on the criteria of starting the motor using the minimummagnitude of line current while realizing motor starting torque sufficient to overcome thecounter-torque of the driven load. The motor must additionally produce a net torquesufficient to accelerate rotational masses to full speed within a period of time that is less thanthe duty rating of the transformer. The timer relay, TR, must be set slightly longer than theanticipated acceleration time to prevent a transition to full-voltage before the magnitude of thestarting current has reduced to less than 150% of motor full-load amperes, but not longer thanthe duty rating of the autotransformer.




REDUCED-VOLTAGE WYE-DELTA CONTROL CIRCUIT LOGIC

Another type of reduced-voltage motor starter is the wye-delta starter (Figure 39). This typeof starter uses the method of connecting the motor windings into a wye configuration to startand then switches the windings into a delta configuration to run. The following paragraphsdescribe the logic for this type of starter.

Figure 39. Reduced-Voltage Wye- Delta Motor Starter





A six-lead motor that is normally connected delta for rated voltage can be started withreduced inrush current and torque by connecting the windings wye during acceleration. Afterthe motor is accelerated to full speed, magnetic contactors quickly reconnect the windings todelta for rated operation. Starting the motor in this manner, with the windings connected in awye connection, is equivalent to starting on a 57% autotransformer tap and providing themotor with 33% of full-voltage starting torque.

Operation of the control circuit shown in Figure 39 begins by closing the start pushbutton.This energizes and closes start contactor S which closes Sa and picks up contactor 1M. At thesame time Sb opens to block 2M from picking up. The main contacts of the S contactorconnect motor terminals T4, T5, and T6 to make a wye connection. The 1M contacts connectthe supply lines L1, L2, and L3 to the motor terminals T1, T2, and T3. With the windingsconnected in this wye arrangement, the motor accelerates to full speed. With the closing ofSa, the time delay relay TR is energized and begins timing the transition (starting) period.After the timing relay TR times out, its contacts open, dropping out contactor S. Althoughauxiliary contact Sa opens, contactor 1M remains closed due to the sealed-in 1Ma auxiliarycontacts. Auxiliary contact Sb closes to pickup 2M. With 1M remaining closed, the 2Mcontacts now connect the motor windings into a delta circuit by connecting supply lines L1,L2, and L3 to the motor terminals T4, T5, and T6. The motor now operates at full (deltaconnected) voltage.

The wye-delta type starter has the advantage of being relatively inexpensive since it does notrequire the use of resistors or autotransformers. However, offsetting this advantage are a fewdisadvantages. First, there is no method of adjusting the starting torque. If the one-thirdnormal torque cannot turn the motor or should the acceleration be too slow, the wye-deltastarter is not a practical choice. Second, use of this type starter requires a special motor withall six leads brought out to the terminal box. Finally, the starter as shown is an open transitiontype, which means the motor is briefly disconnected for the transition from wye to delta.





Interlock protection for the S and 2M contactors is absolutely necessary since the closing ofboth contactors at the same time would result in a three-phase short circuit directly across thepower supply lines. To prevent S and 2M from closing at the same time, they are interlockedboth electrically and mechanically. Electrical interlocking is accomplished using the normallyclosed Sb and 2Mb auxiliary contacts to maintain each coil circuit open while the other isclosed. Mechanical interlocking of the S and 2M contactors is accomplished using thestandard manner of interconnecting their moving assemblies with an insulated mechanicallink.

Transition Timer

The reduced-voltage wye-delta starter, starts the motor with the windings connected in a wyeconfiguration and then, using its contactors, goes through a transition to a delta windingconfiguration. The time required for the motor to accelerate to full speed while connected inwye is referred to as the transition time and is preset by the setting of the timing relay TR.When the TR relay times out, its contacts open, dropping out the starting contactor (S), whichin turn switches in the 2M contactor that completes the transition by reconnecting the windingin delta. Allowing the motor to run for a longer than necessary time period while connectedin the wye configuration does no harm. However, in the wye configuration, the motor hasonly one-third of its full-load torque capability, and it is necessary to transition to the deltaconnection to provide the motor with full torque capability.




MULTI-SPEED CONTROL CIRCUIT LOGIC

Full voltage magnetic starters are available for operating multi-speed motors at differentspeeds. A typical multi-speed starter consists of a group of contactor assemblies in a singleenclosure, each contactor operating the motor at one speed.

The operating logic of multi-speed starters is determined by the principle on which multi-speed motors are based. In brief, this principle states that given a fixed frequency of ACsupply voltage, the speed of rotation of a three-phase motor depends on the number of polesin the motor. An increase in the number of poles will decrease the motor speed. Thus,changing the number of poles in a motor, changes the speed at which it rotates.

Multi-speed starters are basically used with two types of AC squirrel-cage induction motorsthat provide for changing the number of their poles to accomplish multi-speed operation. Onetype is a motor that has one winding, but the winding is designed in a manner that allows it tobe reconnected to change the number of its poles. The winding for this motor is known as a“consequent pole” type and can be reconnected to obtain two different numbers of poles, withspeeds in a ratio of 2-to-1. This motor is commonly referred to as a two-speed, single-winding motor.

The other type is a motor that has separate windings that provides for changing connections togive a different number of poles. In this case, each winding produces a certain number ofpoles for a certain speed, but the two speeds do not have to be in the ratio of 2-to-1. And, ifone or both of the separate windings on the stator are of the consequent-pole type, then themotor may operate at 3 or 4 speeds. This type is typically called a two-speed, two-windingmotor.

The following paragraphs describe the control logic for a two-speed two-winding starter and atwo-speed single-winding starter.


Two-Speed Two-Winding Motors

Figure 40 shows the control logic for a typical multi-speed starter for a two-speed two-winding motor. As described above, this type of logic accomplishes the changing of motorspeed by separately and individually energizing the two windings of the motor. To operatethe motor at low speed, the low speed start pushbutton (L) is closed. This action causes thelow speed contactor coil (L) to pickup, closing the main contacts to energize the low speedwinding. Auxiliary contact La then closes to hold the coil circuit closed and allow the motorto continue running at low speed.




To change to high speed operation, the high-speed pushbutton is closed. The mechanicallinkage between the normally open and normally closed contacts of the high-speedpushbutton cause the circuit to the low-speed coil to be opened (de-energizing the low-speedwinding), before the circuit to the high-speed coil is allowed to close. The high-speedcontacts close, energizing the high-speed winding. Auxiliary contact Ha closes to seal-in thehigh-speed coil circuit and allow the motor to continue operating at high speed. Pushing thestop pushbutton at any time opens both contactor coil circuits and stops the motor.

Figure 40. Multi-Speed Starter for Two-Speed Two-Winding Motor




Two-Speed Single-Winding Motors

Figure 41 shows a typical multi-speed starter used for a two-speed single-winding motor. Inthis example, the two different speeds are accomplished by using the starter contactors toreconnect the single winding from a delta connection (for low speed) to a wye connection (forhigh speed). However, it is important to note that the multi-speed starter shown in Figure 41differs from the wye-delta starter described above in Figure 39. The basic difference is thatthe wye-delta starter described in Figure 39 does not change the effective number of poles inthe motor it controls. It only reconnects the winding from wye to delta and thus operates themotor at the same speed for each connection. On the other hand, the starter shown in Figure41 actually changes the number of poles by reconnecting the individual circuits of thewinding. For the low-speed delta connection, the starter contactors connect the motorwinding in a configuration that places two circuits per phase in series (i.e. winding circuit T4-T1 is in series with T1-T6, T6-T2 is in series with T2-T5, and T5-T3 is in series with T3-T4).For the high-speed wye connection the motor winding is reconnected for two winding circuitsin parallel per phase (i.e. winding circuit T4-T1 is in parallel with T4-T3, T6-T1 is in parallelwith T6-T2, and T5-T2 is in parallel with T5-T3).

Low-speed operation of the starter shown in Figure 41 begins with the closing of the low-speed start-pushbutton (L). This action picks up the low-speed contactor and connects themain supply lines to motor terminals T1, T2, and T3 (the low-speed, two-series, deltaconfiguration). At the same time, auxiliary contact La closes to hold the coil circuit closedand allow the motor to continue running at low speed.

To change to high-speed operation, the high-speed pushbutton is pushed. The mechanicalinterlock between the low- and high-speed pushbuttons causes the circuit to the low-speedcoil to be opened (de-energizing the delta winding configuration), while the circuit to thehigh-speed coil is closed. The high-speed contactor has five mechanically interlocked maincontacts that close at the same time reconnecting the winding to a two-parallel wyeconfiguration. Specifically, motor terminals T1, T2, and T3 are connected together by two ofthe contacts to form the star point of the wye, while the three supply lines are individuallyconnected to motor terminals T6, T5, and T4 to complete the three parallel legs of the wye.At the same time, auxiliary contact Ha closes to seal-in the high-speed coil circuit and allowsthe motor to continue to operate at high speed. Pushing the stop pushbutton at any time opensboth contactor coil circuits and stops the motor.




Figure 41. Multi-Speed Starter for Two-Speed Single-Winding Motor





For the two types of multi-speed starters described above, interlock protection is necessary toprevent short circuits. Both types of starters require that the low-speed and high-speedcontactors be interlocked to prevent them from being closed at the time. For the examplesshown, the interlock between the contactors is accomplished with the standard insulatedmechanical link connected between their moving assemblies.

In addition to mechanical interlocking, auxiliary contacts Hb and Lb electrically interlock theH and L contactors.

Multiple O/L Relays

Multi-speed starters require the use of overload relays to sense overload conditions and stopthe motor in the same manner as other type starters. This includes connecting the overloadrelay thermal sensor (heater) in the motor current circuit to sense a potential overload, whilethe relay contacts are connected in the control circuit to open the contactor coil circuit whennecessary.

However there is one difference. The difference is that for two-speed starters controlling twowinding motors (Figure 40), it is necessary to use two overload relays, instead of just one, toprotect the two windings. This is necessary for two reasons. First, the starter, whencontrolling the two-winding motor, energizes only one winding at a time. As a result, it isnecessary to individually connect an overload relay in each winding circuit to provideoverload protection for each winding. The second reason for requiring two separate overloadrelays for the two-winding motor is that each winding has a different current rating. As aresult, the current rating for each set of thermal elements must be different in order to providethe correct level of overload protection for each of the two windings.

Figure 41 shows a two-speed single-winding motor protected with two overload relays usingsix thermal elements (heaters) connected in the six motor lines (T1 through T6). However,this motor could be protected using only one overload relay with three thermal elements. Toprotect the motor using only one overload relay, the thermal elements must be electricallyconnected in motor line leads T1 T2, and T3. Connecting the six thermal elements T1, T2,and T3 places them in the path of the motor current for both the low-speed and high-speedwinding connection. In comparison, if the thermal elements where connected in the circuit atmotor terminals T6, T5 and T4, the thermal elements would only be effective when the high-speed contactor was closed.




TYPICAL ELECTRONIC (SOLID-STATE) CONTROL CIRCUIT LOGIC

Electronic solid-state motor starters provide reduced voltage starting for AC squirrel-cageinduction type motors. The functional operation of the solid-state starter is similar to that ofreduced voltage magnetic type starters, however, the technology and components used aredifferent. A typical solid-state motor starter (as shown in Figure 42) uses power type SCRs(silicon controlled rectifiers) to control the flow of current to the motor and electronic sensingdevices to detect overload and fault conditions. Solid-state starters are available only as low-voltage starters for nominal voltage ratings of 208V through 575V. Figure 43 gives anexample of the ratings offered by one manufacturer for one style of solid-state starter. Withregard to this figure, it should be noted that the nominal horsepower ratings are listed forreference only, and the starter should be selected based on motor full-load amperes.

Saudi Aramco standards do not, in general, specify the use of solid-state type starters for lowvoltage motors. However, for completeness, and as background information for theParticipant, a description of typical control logic for solid-state starters is included.




Figure 42. Typical Electronic Solid-State Motor Starter




Figure 43. Typical Manufacturer Ratings for Solid-State Starters





The typical solid-state starter uses six high-current SCRs, arranged two per pole (for bothpositive and negative current flow), to switch the motor load current on and off. Turning theSCRs on and off is accomplished by sequentially timed voltage pulses supplied from anelectronic “firing” circuit to the gate terminals of the SCRs. The conduction period of current,and consequentially the starter’s output voltage level, is made to increase whenever thecontrol-logic module delivers gate pulses earlier in the forward-bias period of each SCR.Voltage level decreases when the pulses are delayed. Voltage output is zero whenever gatepulses are interrupted.

The distinction of the SCRs to control the voltage and current level supplied to the motor,coupled together with the logic of the internal electronic circuits, give the solid-state starterthe capability of providing soft starts. The soft starts are governed by using one or both oftwo adjustable functions controlled by the starter logic circuit. One is a voltage/time rampand the other is a current-limit function.

The voltage/time ramp is the more basic of the two functions. For this function, the voltagesupplied by the SCRs to the motor terminals is controlled in a manner that causes the voltageto increase linearly with time until full line voltage is applied (Figure 44). The start of theramp is modified by adding an initial voltage step as shown in Figure 44. For typical solid-state starters, this voltage step is adjustable from 10 to 80% of full motor volts. The reasonfor including this initial voltage is to adjust for the negligible level of motor torque at lowvoltage. If the voltage ramp were allowed to start at zero, the motor may not begin to moveuntil several seconds after initiating the start.

Figure 44. Voltage Ramp Function for Solid-State Motor Starter




The second adjustable function used to control the soft start operation is the current-limitfunction (Figure 45). For this function, load current is sensed via the current transformers,and compared with a preset limit (typically 300 to 550%) of full-load current. The currentallowed to flow through the SCRs to the motor is adjusted to remain at or below the presetvalue during the starting period. A precaution for this adjustable function is that whenadjusted for very low currents, and starting a partially loaded motor, there is a possibility thatthe motor may not rotate, drawing the limited level of starting current, until it overheats ortrips out.

Figure 45. Torque Speed Characteristics for Current-Limit Function




Solid-State Interface Devices

Solid-state starters are supplied together with their electronic overload protection, currentsensors and logic controls packaged in a NEMA type enclosure as an integral package. Start,stop and reset pushbuttons are normally mounted on the enclosure cover together with anarray of indicating lights and LED’s (light emitting diodes) that report the status of suchparameters as motor operation, ramp starting functions, phase rotation, undervoltage trip,overcurrent trip, phase loss trip, and phase current unbalance.

Interface connections to the starter basically include connection of the three power supply lineleads and three motor leads. Pushbuttons for local operation are mounted on the enclosurecover, however, the starter is equipped with a terminal strip that allows connection of remotepushbuttons for start, stop, and reset functions at a 120 volt control level. An external tripsignal is supplied from the starter logic control circuit for connection to the customer’sprotective device. Normally open and normally closed auxiliary contacts contained in thestarter assembly are also brought out to the terminal strip for use by the customer.

Current Feedback

The control for this type of starter is accomplished by the internal electronic logic circuitsmonitoring and comparing the current and voltage signals from the sensors. In this manner,the voltage and current feedback signals are used to control the soft start voltage ramp andcurrent-limit functions on starting, the switching-off of the SCRs for unacceptable overloads,and as necessary the sending of a trip signal to the external protective device to provideprotection for short circuit, undervoltage, and single-phase conditions.

Voltage feedback is accomplished with a direct wiring connection between the starter lineleads and the control logic circuit as shown in Figure 42. This method of feedback is possiblebecause the solid-state starters are available only as low-voltage starters (208V to 575V), thusthe voltage signal can be fed directly to the logic circuit and reduced as necessary withresistance dividers.

Current feedback must be achieved with the use of current transformers. The level of currentin the motor leads is too large to use directly in the logic circuit and must be reduced bycurrent transformers. The current transformers are sized in accordance with the rating of thestarter and the requirements of the control logic circuit. The current transformers are thensupplied as an integral part of the starter.




Programmable Features

Solid-state starters have a number of functions that can be programmed by the customer tosatisfy specific application requirements. Some of the functions typically available forprogramming include:

• Full-Load Running Current Trip - The current trip function providesoverload protection for the motor and is typically programmable over a rangeof 100 to 120% of full-load current. The current trip circuit works on aninverse time principle, similar to a thermal overload relay or circuit breaker.Current trip settings are normally made in accordance with motor servicefactors.

• Start Time Delay - The start time delay function is used to inhibit the inversetime current trip while the motor is being accelerated to speed. Typicalmaximum start delay times up to 20 seconds are available for programming.

• Current Limit - Programming of this function controls the available startingcurrent (which is proportional to torque) during the starting period. The torquein turn determines the time required for a motor to accelerate to full speed.Typical ranges available for programming are 200 to 500% of full-load current.

• Time Ramp - This function allows programming of the rate at which voltage isapplied to the motor terminals during startup. By starting the motor with acontrolled voltage ramp, inrush current is limited and acceleration is smooth. Atime range of from 1 to 40 seconds is typically available for programming thevoltage to increase linearly up to the full-rated voltage of the motor.

• Initial Voltage - This function works together with the voltage/time rampfunction to allow the ramp to start with an initial voltage step instead of startingfrom zero. Programming the voltage/time ramp function to start from zerocould result in the motor standing idle during the first few seconds of thestarting period. The range available for programming the initial voltage step istypically from 10 to 80% of full motor voltage.




CIRCUIT BREAKER CONTROL CIRCUIT LOGIC

Low-voltage and medium-voltage power circuit breakers are generally used for control oflarger size motors. The types of breakers commonly used for this purpose include magneticair, vacuum, and SF6 gas power circuit breakers. The breakers are typically operated by aspring charged mechanism with a control circuit similar to the one shown in Figure 46. Theoperating logic of the breaker’s control circuit is the control logic for this type controller.

In accordance with SAES-P-114, Saudi Aramco specifies that low-voltage power circuitbreakers be used as the controller for induction motors 600 volts or less and greater than 100horsepower. For induction and synchronous motors 4000 volts or greater and less than 1500horsepower, a power circuit breaker may be used as the controller. For induction andsynchronous motors greater than 1500 horsepower, a power circuit breaker must be used asthe controller.




Figure 46. Standard Breaker Control Scheme (With DC Close, DC Trip and AC SpringCharging Motor)





The breaker control circuit shown in Figure 46 is typical of the type used for many powercircuit breakers. An AC supply source drives a universal motor to charge the closing springsof the operating mechanism. DC control voltage is used to actuate the closing or trippingmechanisms of the breaker by briefly energizing the coils of the spring release (closing)solenoid or the trip solenoid. Auxiliary and limit switches are used to assure correctsequencing of the control operations. The following paragraphs describe the control logic forthe main components of this circuit.

Circuit Breaker Control Switch

Closing and opening of the circuit breaker is initiated by operation of the circuit breakercontrol switch (similar to the type described above in Figures 10 through 14). This switch isnormally mounted on the front panel of the switchgear enclosure or on a remote control panel.The contacts of this switch are identified in the circuit diagram of Figure 46 as CS/T (controlswitch trip) and CS/C (control switch close).

Operating the close contacts (CS/C) energizes the spring release coil (SR), which in turnreleases the energy in the mechanism closing springs and closes the main contacts of thebreaker.

Operating the trip contacts (CS/T) energizes the trip coil (TC), which releases the energy inthe mechanism opening springs and opens the main contacts of the breaker.

However, successful closing and/or tripping of the breaker depends on the status of otherauxiliary, limit, and relay contacts. Details of the closing logic are described below under theheading of “Closing Solenoid”, while details of the tripping logic are described under theheading of “Trip Solenoid”.




Closing Solenoid

In normal operation, the mechanism closing spring (the source of energy used to physicallyclose the breaker) is charged by the spring charging motor as soon as control power is appliedto the breaker ( AC power to secondary contact-block points 55 and 56). The motor limitswitch (LS1), which is normally closed when the spring is discharged, opens after the springis charged and disconnects the motor. At the same time, motor limit switch (LS2), which isnormally open when the spring is discharged, closes after the spring is charged to prepare thespring release circuit for operation.

The signal to close the breaker is initiated by closing the contacts CS/C which places the DCcontrol voltage directly across the secondary contact-block points 6 and 7. However,energizing the spring release coil (SR) is also controlled by the status of the relay (Y), theauxiliary contacts (b), the latch check switch (LC), and the motor limit switch (LS2).

For normal conditions, when the closing springs are fully charged (completely extended), themotor limit switch (LS2), will be closed. If the breaker is correctly positioned in its draw-outcell, its levering device is not engaged and the closing spring is charged, the trip latch will befully engaged on the trip shaft and thus the trip latch switch (LC) will be closed. Also, theauxiliary switch (b) is normally closed when the breaker is open and the relay contact (Y) isnormally closed when the “Y” relay is de-energized.

Thus, with the four sets of permissive contacts closed, the SR coil is energized through Y, b,LC and LS2 when the control switch contacts CS/C are closed. Energizing SR mechanicallyreleases the closing springs, closing the breaker, opening the limit switch (LS2), and openingthe auxiliary contact (b) to disconnect the SR coil circuit. At the same time, LS1 closes toenergize motor (M) and recharge the closing springs. With the opening of auxiliary contact(b), relay coil (Y) is energized through resistor (R). This action opens the normally closedcontact (Y) to maintain the SR coil circuit open and prevent the re-energizing of the SR coil inthe event that: a close-open operation occurs, and the closing spring has time to recharge(approximately 5 seconds), and the close signal still persists.




Anti-Pumping Relay

The principle function of the “Y” anti-pumping relay is to prevent close-open cycling of thebreaker through any period that the close signal might remain active. Close-open cycling canoccur if a breaker is closed into a fault, immediately trips in response to the fault, and thenattempts to reclose because the close signal is still present. (A similar pumping can occur ifthe breaker is closed under normal conditions and a short or ground in the control circuitcauses an unintended trip signal to be present).

As described above, once the SR coil is energized and allowed to operate, the “Y” relay picksup through resistor “R”. Once energized, the “Y” relay remains energized until the closesignal is released to allow the “Y” relay coil circuit to open. As long as the “Y” relay ispicked up, its open contact prevents unwanted re-energizing of the SR coil.

Trip Solenoid

With the breaker in the closed position, the signal to trip the breaker is received throughsecondary contact-block points 9 and 10. The initiating contacts can be either the breakercontrol switch “trip” contacts (CS/T) or the protective relay contacts (PR). Closure of eitherset of contacts places the DC control voltage directly across the trip coil. Energizing the tripcoil causes the mechanism opening spring to release, thus opening (tripping) the breaker.Two auxiliary contacts (a), which are closed when the breaker is closed, open when thebreaker opens and interrupt the trip coil circuit.

Relay Interface

The control logic of the circuit breaker provides for tripping of the breaker by one or moreprotective relays. This capability is known as the relay interface for the logic circuit. Therepresentation of a single contact, PR, on Figure 46 represents the control function of severalprotective relays any one of which can act alone to trip the circuit breaker. The wiringconnection for the relay contacts is made directly across the control switch “trip” contact(CS/T).




Indicator Lights

The control circuit of Figure 46 contains two indicating lights, one green and one red. Thegreen light is connected in the normal manner using an auxiliary contact to make and break itscircuit. When energized, the green light indicates that the breaker is open.

The red light serves two functions. When energized, it indicates that the breaker is in theclosed state. But more importantly, when illuminated, it serves as a positive indication thatthe trip circuit is in fact continuous and ready to be tripped. It is capable of providing thispositive indication because of the manner in which it is connected in the circuit. The redindicating light is connected directly in series with the trip coil (TC) and its auxiliary contacts(a).

Spring-Operated Mechanism

The type of mechanism commonly provided on low- and medium-voltage power circuitbreakers is the spring-operated mechanism. This type of mechanism has two major parts: onepart is dedicated to charging or storing energy in the operating springs, the other part is theassembly that opens and closes the breaker.

The mechanism is typically an electrically-operated version equipped with a universal-typemotor for automatic charging of the closing springs. It is equipped with a spring releasedevice for electrically closing through a control switch, pushbutton, or other circuit makingcontacts. A shunt trip device is supplied for remote tripping through a control switch, relay orother device. In the absence of control voltage, or whenever desired, the closing spring canbe charged by using an emergency charging handle. Once the springs are charged, handclosing and tripping of the breaker can then be accomplished by pushing close and trip plateslocated on the mechanism’s front panel.

The breaker control is so arranged that for normal operation the spring charging motor isenergized as soon as control power is applied to the breaker. The motor will typically chargethe closing spring in approximately 5 seconds. When the closing spring is fully charged, themotor is cut off. Releasing the closing springs to close the breaker causes the opening springto be charged, in preparation for a trip operation. At the same time, the control circuitautomatically energizes the motor to recharge the closing springs.




GLOSSARY

air magnetic breaker A type of medium voltage circuit breaker with its contacts inair. An electromagnet built into the arc chutes aids inextinguishing the arc.

circuit breaker control A rotary power switch designed for heavy dutyswitch control systems.

combination starter A complete motor starter consisting of a disconnect device, amagnetic contactor, and protective devices for short circuitand overload. All devices are assembled in a singleenclosure.

contactor A magnetic device that has sufficient capability to connectand disconnect the electric circuit of a motor under normaland overload conditions.

control circuit The circuit that carries the electric signals directing theperformance of the controller but does not carry the mainpower circuit.

control relay A component that is used in a motor starter’s control circuitto interface between a pilot device and the circuit that thepilot device controls.

control power A transformer used to draw control power from thetransformer (CPT) main power circuit of a motor starter.

full-voltage starter A type of motor starter that applies full voltage to the motorterminals during the starting period.

interposing relay Special relays used in the control circuits of larger sizecontactors to interface (or interpose) between the relativelyhigh coil current required by the large size contactors and thepushbuttons or control contacts used to switch-off or breaktheir current.

manual starter A simple type of motor starter that provides full-voltage, on-off type operation for small single-phase and three-phasemotors.




motor circuit protector A magnetic-only molded case circuit breaker used in low-voltage combination starters. This device has onlyinstantaneous functions to protect the motor, starter, andbranch circuit from short circuit and ground fault currents.

overload relay A device that is used to sense an overload on a motor circuit.The most common type uses a heater that heats a bi-metallicstrip that operates a set of contacts.

pilot device Control and indicating devices used in motor control circuits.These include indicating lights, switches, and pushbuttons.

reduced-voltage starter A type of motor starter that applies less than full-voltage tothe motor terminals during the starting period.

reversing starter A type of motor starter that provides for reversing thedirection of rotation of the motor.

relay An electric device that is designed to interpret inputconditions in a prescribed manner and after specifiedconditions are met to respond to cause contact operation orsimilar abrupt change in associated electric control circuits.

solid-state contactor A contactor whose function is performed by semiconductingdevices

three-wire control The most common type of control used to start and stop amotor.

two-wire control This type of control automatically starts and stops a motordepending on the set points of a pilot device.

vacuum circuit breaker A specific type of circuit breaker designed to interrupt the arcinside a container that is under vacuum. The vacuum limitsionization of gases and makes the circuit breaker lighter andmore compact.

electric motor 5

Documents