FUNDAMENTAL OF FUNDAMENTAL OF ELECTRICAL ENGINEERINGELECTRICAL ENGINEERING

EMT 113/4EMT 113/4

CHAPTER 2:CHAPTER 2:

DC MACHINESDC MACHINES

SUBTOPICSSUBTOPICS

Introduction to DC Machines

DC motors : Principles of Operation, equivalent circuit & Characteristics

DC generators : Principles of operation, equivalent circuit & Characteristics

INTRODUCTION TO INTRODUCTION TO DC MACHINESDC MACHINES

WHAT ARE DC MACHINES?WHAT ARE DC MACHINES? DC generators that convert mechanical energy to DC

electric energy.

DC motors that convert DC electric energy to mechanical energy.

• often used as a motor.

• found in many special industrial environments. Motors drive many types of loads from fans and pumps to presses and conveyors

• Advantages: easy speed and torque regulation. • However, their application is limited to mills, mines

and trains. As examples, trolleys and underground subway cars may use dc motors.

• In the past, automobiles were equipped with dc

dynamos to charge their batteries.

Important parts:

- STATOR : provides mechanical support for the machine, consists poles and yoke - ROTOR / ARMATURE : the rotating part, shrouded by fixed poles on the stator-COMMUTATOR : mechanical rectifier, which changes the AC voltage of the rotating conductors to DC voltage - BRUSHES : conduct the current from the commutator to the external circuit - WINDINGS

•uniform magnetic flux is established by fixed poles mounted on the inside of the stationary number called STATOR•May use permanent magnet as poles or wind the field windings (excitation coils) around the poles• Advantage of wound machine: easy to control the flux in the machine by regulating the direct current in the field winding

DC motor stator with poles visible

Rotor of a dc motor

DC machines, like other

electromechanical energy

conversion devices have

two sets of electrical

windings:

1) field windings - on stator

2) amarture windings - on the rotor.

• The stator of the dc machine has poles, which are excited by dc current to produce magnetic fields.

• In the neutral zone, in the middle between the poles, commutating poles are placed to reduce sparking of the commutator. The commutating poles are supplied by dc current.

• Compensating windings are mounted on the main poles. These short-circuited windings damp rotor oscillations.

DC Machines Construction

• The poles are mounted on an iron core that provides a closed magnetic circuit.

• The motor housing supports the iron core, the brushes and the bearings.

• The rotor has a ring-shaped laminated iron core with slots.

• Coils with several turns are placed in the slots. The distance between the two legs of the coil is about 180 electric degrees.

• The coils are connected in series through the commutator segments.

• Ends of each coil are connected to a commutator segment.

• Commutator consists : insulated copper segments mounted on an insulated tube.

• Two brushes are pressed to the commutator to permit current flow.

• Brushes are placed in the neutral zone, where the magnetic field is close to zero, to reduce arcing.

• The commutator switches the current from one rotor coil to the adjacent coil

• The switching requires the interruption of the coil current.

• The sudden interruption of an inductive current generates high voltages .

• The high voltage produces flashover and arcing between the commutator segment and the brush.

DC MOTORS : DC MOTORS : Principles of Operation, Principles of Operation,

equivalent circuit & equivalent circuit & CharacteristicsCharacteristics

Introduction

TYPES OF DC MOTOR

Five major types of DC motors:

• Separately excited DC motor• Shunt DC motor• Permanent Magnet DC motor• Series DC motor• Compounded DC motor

classified according to electrical connections of armature windings and field windings.

• ARMATURE winding : the winding which a voltage is induced.

• FIELD windings : the windings that produce the main flux in the machines.

• The magnetic field of the field winding is approximately sinusoidal, thus AC voltage is induced in the armature winding as the rotor turns under the magnetic field of stator.

• The COMMUTATOR and BRUSH combination converts the AC generated voltages to DC.

DC Motor Operation

To understand the operation of a DC motor, we need to know the basic mechanism of the DC Motor – The Electromagnetism.

LETS REVIEW..!!LETS REVIEW..!!

The magnetic lines around a current carrying conductor leave from the N-pole and re-enter at the S-pole.

"Left Hand Rule" states that if you point the thumb of your left hand in the direction of the current, your fingers will point in the direction of the magnetic field.

Lines of flux define the magnetic field and are in the form of concentric circles around the wire.

Review of Magnetism

The poles of an electro-magnetic coil change when the direction of current flow changes.

• The motor has a definite relationship between the direction of the magnetic flux, the direction of motion of the conductor or force, and the direction of the applied voltage or current.

•Fleming's left hand rule can be used: - thumb will indicate the direction of motion- forefinger will indicate the direction of the magnetic field- middle finger will indicate the direction of current.

• In either the motor or generator, if the directions of any two factors are known, the third can be easily determined.

DC Motor OperationDC MOTOR OPERATION FLOWS:

1. Uniform magnetic field is created by poles

2. The armature conductors are forces to carry current by connecting them to DC power source

3. The current direction in the conductors under each pole is kept the same by commutator

4. According to Lorentz force equation, a current carrying conductor when placed in a magnetic field experiences a force that tends to move it (F=ilB)

5. All conductors placed on the periphery of a DC motor are subjected to the forces

6. These forces cause armature to rotate in the direction of the torque developed by the motor.

DC Motor Operation

DC Motor Operation : Current

DC Motor Operation : Force

DC Motor Operation : Magnetic Field

Basic principle of operation• The generated voltage of a DC machines having (p) poles and (Z) conductors on the armature with (a) parallel path between brushes as below :

K

a

pZEA 2

where K = pZ /(2πa) = machine constant

• The mechanical torque which also equal to electromagnetic torque, is found as follows:

AAA

me IKIE

In the case of a generator:

m is the input mechanical torque, (converted to electrical power)

For the motor:

e is developed electromagnetic torque, (drive the mechanical load)

• The induced or generated DC voltage (EA) appearing between the brushes is a function of the field current (IF) and the speed of rotation () of the machine. This generated voltage is :

FA IKE '

Where K’ = voltage constant = rotation per mina

• If the losses of the DC machine are neglected, the electrical power is equal to the mechanical power

mAAIE

Important NoticeImportant Notice

KEA nKEA `

apzK 2/

Wherep : no of polesz : no of conductorsa : no of current path

Equation of Induced voltage when speed, w (in radian per second / Angular speed)

Equation of Induced Voltage when speed, n (revolution per minute/ Run per minute/ rotation per minute (rpm)

apzK 60/`Wherep : no of polesz : no of conductorsa : no of current path

• The internal generated voltage in the motor KEA

• From the equation,

EA is directly proportional to the flux () in the motor and speed of the motor ().

• The field current (IF) in DC machines produces a field magnetomotive force (mmf)

• This magnetomotive force (mmf) produces a flux () in the motor in accordance with its magnetization curve.

IF mmf flux

The Magnetization Curve of a DC machineThe Magnetization Curve of a DC machine

• Since the field current (IF) is directly proportional to magnetomotive force (mmf) and

• EA is directly proportional to the flux, the magnetization curve is presented as a plot EA versus field current for a given speed.

The magnetization curve of a dc machine expresses as a plot of EA

versus IF, for a fixed speed ω0

NOTE : To get the maximum possible power, the motors and generators are designed to operate near the saturation point on the magnetization curve (at the knee of the curve).

• The induced torque developed by the motor is given as

Aind IK

Equivalent CircuitEquivalent Circuit

RA

Armature circuit (entire rotor structure)

The brush voltage drop

Field Coils

External variable resistor used to control the amount of current in the field circuit

NOTE: Because a dc motor is the same physical machine as a dc generator, its equivalent circuit is exactly the same as generator except for the direction of

current flow.

Equivalent circuit of dc motor

Simplified Equivalent CircuitSimplified Equivalent Circuit

• The brush drop voltage (Vbrush ) is often only a very tiny fraction of the generated voltage in the machine – Neglected or included in RA.

• Internal resistance of the field coils is sometimes lumped together with the variable resistor and called RF

Simplified quivalent circuit of dc motor

Separately excited DC motorSeparately excited DC motor

Separately excited motor is a motor whose field current is supplied

from a separate constant-voltage power supply.

F

FF R

VI

AAAT RIEV

AL II

Shunt DC motorShunt DC motor

A shunt dc motor is a motor whose field circuit get its power

directly across the armature terminals of the motor.

F

TF R

VI

AAAT RIEV

FAL III

Shunt DC Motor : Terminal Characteristics

• Consider the DC shunt motor. From the Kirchoff’s Law

AAAT RIEV

• Induced Voltage

KEA

• Substituting the expression for induced voltage between VT and EA.

AAT RIKV

• Since, then current IA can be expressed as

K

I indA

Aind

T RK

KV

• Finally, solving for the motor's speed yield

indAT

K

R

K

V 2)(

Torque-speed characteristic of a shunt or separately excited dc motor

This equation is a straight line with a negative slope.

Shunt DC Motor : Terminal Characteristic

indAT

K

R

K

V 2)(

Torque-speed characteristic of a motor with armature reaction present.

• Affect of Armature Reaction (AR) will reduce flux as the load increase (ind also increase), so it will increase motor speed ().

If the motor has compensating winding, the flux () will be constant.

Shunt DC Motor : Speed Control

1 : Changing The Field Resistance (flux affected)

F

T

R

V

K

A

AT

R

EV

loadind

1. Increasing RF causes IF

6. Increasing τind makes

to decrease.

2. Deceasing IF decreases . (graph flux vs current)

3. Decreasing lowers EA

4. Decreasing EA increases IA

5. Increasing IA increases )( Aind IKwith the change in IA dominant over the change in flux ().

and the speed ω increases.

Shunt DC Motor : Speed Control

loadind

7. Increasing speed to increases EA = K again.

8. Increasing EA decreases IA.

9. Decreasing IA decreases until at a higher speed ω

Decreasing RF would reverse the whole process, and the speed of the motor would drop.

The effect of field resistance speed control on a shunt motor’s torque speed characteristic: over the motor’s normal operating range

Shunt DC Motor : Speed Control 2: Changing The Armature Voltage

Armature voltage control of a shunt (or separately excited) dc motor.

1. An increase in VA increases IA [= (VA – EA)/RA]

4. Increasing ω increases EA (=Kω )

2. Increasing IA increases )( Aind IK3. Increasing τind makes loadind increasing ω.

Shunt DC Motor : Speed Control

5. Increasing EA decreases IA [ = (VA – EA)/RA]

6. Decreasing IA decreases τind until loadind at a higher ω.

The effect of armature voltage speed control on a shunt motor’s torque

speed characteristic

The speed control is shiftted by this method, but the slope of the curve remains constant

Shunt DC Motor : Speed Control 3 : Inserting Resistor in Series with Armature Circuit

Add resistor in series with RA

The effect of armature resistance speed control on a shunt motor’s torque – speed characteristic

Equivalent circuit of DC shunt motor

Additional resistor in series will drastically increase the slope of the motor’s characteristic, making it operate more slowly if loaded

Shunt DC Motor : Speed Control

This method is very wasteful method of speed control, since the losses in the inserted resistor is very large. For this it is

rarely used.

indAT

K

R

K

V 2)(

The above equation shows if RA increase, speed will decrease

Series DC Motor

Equivalent circuit of a series DC motor.

The Kirchhoff’s voltage law equation for this motor

)( SAAAT RRIEV

Series DC Motor: DC motor whose field windings consists of relatively few turns connected in series with armature circuit

Series DC Motor : Induced Torque • The induced or developed torque is given by

Aind IK

• The flux in this motor is directly proportional to its armature current. Therefore, the flux in the motor can be given by

AcIwhere c is a constant of proportionality. The induced torque in this machine is thus given by

2AAind KcIIK

This equation shows, torque in the motor is proportional to the square of armature current. So, series motor give more torque per ampere than any other dc motor, therefore it is used in applications requiring very high torque, e.g. starter motors in cars, elevator motors, and tractor motors in locomotives.

Series DC Motor : Terminal Characteristic

• To determine the terminal characteristic of a series dc motor, analysis will be based on the assumption of a linear magnetization curve, and the effects of saturation will be considered in a graphical analysis

• The assumption of a linear magnetization curve implies that the flux in the motor given by :

AcI

)( SAAAT RRIEV

• The derivation of a series motor’s torque-speed characteristic starts with Kirchhoff’s voltage law:

KcI indA

From the equation; the armature current can be expressed as:

2AAind KcIIK

• Also, EA = K, substituting these expression yields:

)( SAind

T RRKc

KV

22 c

KKcIAind

• Substituting the equations so the induced torque equation can written as

indK

c

Therefore, the flux in the series motor can be written as :

We know ; c

I A

• Substituting the previous equation for VT yields:

)( SAind

indT RRKcK

cKV

Kc

RR

Kc

V SA

ind

T

1

• Disadvantage of series motor can be seen immediately from this equation. When the torque on this motor goes to zero, its speed goes to infinity.

In practice, the torque can never go entirely to zero, because of the mechanical, core and stray losses that must be overcome.

• However, if no other load is connected to the motor, it can turn fast enough to seriously damage itself.

NEVER completely unload a series motor, and NEVER connect one to a load by a belt or other mechanism that could break.

Figure : The ideal torque- speed characteristic of a series dc motor

Series DC Motor : Speed Control

1. Change the terminal voltage of the motor. If the terminal voltage is increased, the speed also increased, resulting in a higher speed for any given torque.

Kc

RR

Kc

V SA

ind

T

1

2. By the insertion of a series resistor into the motor circuit, but this technique is very wasteful of power and is used only for intermittent period during the start-up of some motor.

Method of controlling the speed in series motor :

Compounded DC MotorA compound DC motor is a motor with both a shunt and a series field

Two field windings : - One is connected in series with armature

(series field)

- Other is connected in parallel with the armature

(shunt field).

The equivalent compound DC motor

a) Long-shunt connection (cumulative compounding), (b) Short-shunt connection (differential compounding)

shu

nt

series

shu

nt

series

Compounded DC Motor

• If the magnetic fluxes produced by both series field and shunt field windings are in same direction, that is, additive, the dc motor is cumulative compound. If the magnetic fluxes are in opposite, the dc motor is differential compound.

• In long shunt compound dc motor, the series field is connected in series with armature and the combination is in parallel with the shunt field.

•In the short shunt field compound dc motor, the shunt field is in parallel with armature and the combination is connected in series with the series field.

Compounded DC Motor• The Kirchhoff’s voltage law equation for a compound dc motor is:

)( SAAAT RRIEV

• The currents in the compounded motor are related by :

FLA III F

TF R

VI

• The net magnetomotive force given by

F net = F F ± FSE - FAR

FF = magnetmotive force (shunt field)

FSE = magnetomotive force (series field)

FAR = magnetomotive force (armature reaction)

The effective shunt field current in the compounded DC motor given by:

F

ARA

F

SEFF N

FI

N

NII *

NSE = winding turn per pole on series winding

NF = winding turn per pole on shunt winding

The positive (+) sign is for cumulatively compound motorThe negative (-) sign is for differentially compound motor

Cumulatively Compounded DC Motor: Torque Speed Characteristic

• Has a higher starting torque than a shunt motor (whose flux is constant) but a lower starting torque than a series motor (whose entire flux is proportional to armature current).

• It combines the best features of both the shunt and the series motors. Like a series motor, it has extra torque for starting; like a shunt motor, it does not over speed at no load.

• At light loads, the series field has a very small effect, so the motor behaves approximately as a shunt dc motor.

• As the load gets very large, the series flux becomes quite important and the torque speed curve begins to look like a series motor’s characteristic.

• A comparison of these torque speed characteristics of each types is shown in next slide.

Fig (a) The torque-speed characteristic of a cumulatively compounded dc motor compared to series and shunt motors with the same full-load rating.

Fig. (b) The torque-speed characteristic of a cumulatively compounded dc motor compared to a shunt motor with the same no-load speed.

The techniques available for control of speed in a cumulatively compounded

dc motor are the same as those available for a shunt motor:

1. Change the field resistance, RF

2. Change the armature voltage, VA

3. Change the armature resistance, RA

The arguments describing the effects of changing RF or VA are very similar to

the arguments given earlier for the shunt motor.

Cumulatively Compounded DC Motor : Speed Control

Differentially Compounded DC Motor: Torque Speed Characteristic

• The shunt magnetomotive force and series magnetomotive force subtract from each other.

• This means that as the load on the motor increase,IA increase and the flux in the motor decreased, (IA)As the flux decrease, the speed of the motor increase, ()This speed increase causes an-other increase in load, which further increase IA,Further decreasing the flux, and increasing the speed again.

• All the phenomena resulting the differentially compounded motor is unstable and tends to run away.

• This instability is much worse than that of a shunt motor with armature reaction, and make it unsuitable for any application.

DC Motor Starter

In order for a dc motor to function properly on the job, it must have some special control and protection equipment associated with it. The purposes of this equipment are:

1. To protect the motor against damage due to short circuits in the equipment

2. To protect the motor against damage from long term overloads

3. To protect the motor against damage from excessive starting currents

4. To provide a convenient manner in which to control the operating speed of the motor

DC Motor Problem on Starting• DC motor must be protected from physical damage during the starting period.

• At starting conditions, the motor is not turning, and so EA = 0 V.

• Since the internal resistance of a normal dc motor is very low, a very high current flows, hence the starting current will be dangerously high, could severely damage the motor, even if they last for only a moment.

• Consider the dc shunt motor:

A

T

A

ATA R

V

R

EVI

When EA = 0 and RA is very small, then the current IA will be very high.

Two methods of limiting the starting current :• Insert a starting resistor in series with armature to limit the current flow (until

EA can build up to do the limiting). The resistor must be not permanently to avoid excessive losses and cause torque speed to drop excessively with increase of load.

• Manual DC motor starter, totally human dependant

Inserting a Starting Resistor in Series & Manual DC Motor

Fig : A shunt motor with a starting resistor in series with an armature. Contacts 1A, 2A and 3A short circuit portions of the starting resistor when they close

Fig : A Manual DC Motor

Human dependant: • Too quickly, the resulting current flow

would be too large.• Too slowly, the starting resistor could

burn-up

DC Motor Efficiency Calculations

To calculate the efficiency of a dc motor, the following losses must bedetermined :

• Copper losses (I2R losses)• Brush drop losses• Mechanical losses• Core losses• Stray losses

Stray lossesI2R losses Mechanicallosses

Core loss

Pout =out m

Pconv = Pdev = EAIA=indω

Pin =VTIL

Electrical or Copper losses : losses that occur in the Armature and field windings of the machine. The copper losses for the armature and field winding are given by :

Armature Loss PA = IA2RA

Field Loss PF = IF2RF

PA = Armature LossesPF = Field Circuit Losses

• The resistance used in these calculations is usually the winding resistance

at normal operating temperature

Brush Losses : power loss across the contact potential at the brushes of the machines. It is given by the equation:

PBD = VBDIA

Must consider RS for series and compound DC Motors

Magnetic or core loss : Hysteresis and eddy current losses occuring in the metal of the motor.

Mechanical loss : Friction and windage losses. • Friction losses include the losses caused by bearing friction and the

frictionbetween the brushes andcommutator.

• Windage losses are caused by the friction between rotating parts and air inside the DC machine’s casing.

Stray losses (or Miscellaneous losses) : losses that cannot be placed in one of the previous categories. (Is about 1% of full load-RULE OF THUMB) [[pg 318,Electric Machinery and Transformers, BHAG S. GURU] and [pg 525, Electric Machinery Fundamentals, STEPHEN J. CHAPMAN]

Rotational losses : when the mechanical losses, Core losses and Stray losses

are lumped together. [pg. 193 Electromechanical Energy Devices and Power

System, ZIA A. ZAMAYEE & JUAN L. BALA JR.]

It also consider as combination between mechanical and core losses at no load

and rated speed.[pg 317, Electric Machinery and Transformers, BHAG S. GURU] and [pg 593, Electric Machinery Fundamentals, STEPHEN J. CHAPMAN]

Motor efficiency :

%100

%100

XP

PP

XP

P

input

lossesinput

input

output

Speed Regulation

The speed regulation is a measure of the change speed from no-load to full load. The percent speed regulation is defined

Speed Regulation (SR):

%100

%100

X

or

X

fl

flnl

fl

flnl

+Ve SR means that the motor speed will decrease when the load on its shaft is increased.

-Ve SR means that the motor speed increases with increasing load.