EMT 462 ELECTRICAL SYSTEM TECHNOLOGY Chapter 2 : DC Machines By: En. Muhammad Mahyiddin Ramli

EMT 462EMT 462ELECTRICAL ELECTRICAL

SYSTEM SYSTEM TECHNOLOGTECHNOLOG

YY

EMT 462EMT 462ELECTRICAL ELECTRICAL

SYSTEM SYSTEM TECHNOLOGTECHNOLOG

YYChapter 2 :Chapter 2 :DC DC MachinesMachines

Chapter 2 :Chapter 2 :DC DC MachinesMachines

By:En. Muhammad Mahyiddin Ramli

Chap 2: DC Machines 2

ContentsContentsContentsContents

Introduction DC Machines Construction DC motors : Principles of

Operation, Equivalent circuit & Characteristics

DC generators : Principles of Operation, Equivalent circuit & Characteristics

Review


Introduction: What are DC Machines?

Are DC generators that convert mechanical energy to DC electric energy. Are DC motors that convert DC electric energy to mechanical energy.

Chapman S.J., “Electric Machinery Fundamentals”


IntroductionIntroduction

DC machine can be used as a motor or as a generator.

DC Machine is most often used for a motor.

Cutaway view of a dc motor

DC motors are found in many special industrial environments

Motors drive many types of loads from fans and pumps to presses and conveyors The major advantages of dc machines over generators are

easy to control 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.


Types of DC Motors

DC motors are classified according to electrical connections of armature windings and field windings.

Armature windings: a winding which a voltage is induced Field windings: a winding that produces the main flux in machines

Five major types of DC motors:- Separately excited DC motor Shunt DC motor Permanent Magnet DC motor Series DC motor Compounded DC motor


DC Machines ConstructionDC Machines Construction

DC motor stator with poles visible

Rotor of a dc motor


DC Machines Construction

DC machines, like other electromechanical energy conversion devices have

two sets of electrical windings

field windings - on stator

amarture windings - on the rotor.

.


DC Machines DC Machines ConstructionConstruction The stator of the dc motor 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 DC Machines ConstructionConstruction 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.


DC Machines DC Machines ConstructionConstruction

The coils are connected in series through the commutator segments.

The ends of each coil are connected to a commutator segment.

The commutator consists of insulated copper segments mounted on an insulated tube.

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

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


DC Machines DC Machines ConstructionConstruction

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.


Review of magnetismReview of magnetism

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.

The flow of electrical current in a conductor sets up concentric lines of magnetic flux around the conductor.



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. The thumb will indicate the direction of motion The forefinger will indicate the direction of the magnetic

field The 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 Operation


Current in DC Motor


Magnetic Field in DC Motor


Force in DC Motor


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, which is converted to electrical power. For the motor, e is developed electromagnetic torque, which used to drive the mechanical load.


ARMATURE winding are defined as the winding which a voltage is induced.

FIELD windings are defined as 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.



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 min

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

mAAIE



Generation of Unidirectional Voltage

As the rotor is rotated at an angular velocity (), the armature flux linkage () change and a voltage eaa’ is induced between terminal a and a’. The expression for the voltage induced is given by Faraday’s Law

dt

deaa

'

Two pole DC generator

a) Flux linkage of coil aa’; b) induced voltage; c) rectified voltage


The internal generated voltage in the DC machines defined as:

KEA

Where EA = armature voltage

K = motor constant

= flux

= rotation per min

Generation of Unidirectional Voltage


DC Motor Equivalent CircuitDC Motor Equivalent 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.

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


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 - Combining Radj with field resistance (RF).


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.

The magnetization curve of a ferromagnetic material ( vs F)

The Magnetization Curve of a DC The Magnetization Curve of a DC machinemachine

IF mmf flux


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

AE

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 Magnetization Curve

AE

The magnetization curve of a dc machine expresses as a plot of EA versus IF, for a fixed speed ω0

The induced torque developed by

the motor is given as

Aind IK


The equivalent circuit of The equivalent circuit of Separately ExcitedSeparately Excited DC MotorDC Motor

F

FF R

VI

AAAT RIEV

AL II

Separately excited motor is a motor whose field current is supplied from a separate constant-voltage power supply.


The equivalent circuit of a The equivalent circuit of a ShuntShunt DC DC MotorMotor

F

TF R

VI

AAAT RIEV

FAL III A shunt dc motor is a motor whose field circuit get its power directly across the armature terminals of the motor.


How Shunt response to load? - Speed-Torque Characteristics

KEAAAT RIKV

AAT RIKV

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

Induced Voltage

Substituting the expression for induced voltage between VT and EA.

AAAT RIEV

Since then, current IA can be expressed as

K

I indA

indAT

K

R

K

V 2)(

Finally, solving for the motor's speed yield

Aind

T RK

KV


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

ind then , with constant VT,

otherwise it affect the torque-speed curve

This equation is a straight line with a negative slope. The graph shows the torque-speed characteristics of a shunt dc motor.

indAT

K

R

K

V 2)(

Speed-Torque Characteristics


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.

KEA



In order for the motor speed to vary linearly with torque, the other term in this expression must be constant as the load changes.

The terminal supplied by the dc power source is assumed to be constant – if not, then the voltage variations will effect the shape of the torque-speed curve.

However, in actual machine, as the load increase, the flux is reduced because of the armature reaction. Since the denominator terms decrease, there is less reduction in speed and speed regulation is improved (as shown in previous slide).

If a motor has compensating windings, of course there will be no flux-weakening problem in the machines, and the flux in the machine will be constant



Speed Control of Shunt DC Motor

Two common ways in which the speed () of a shunt dc machine can

be controlled.• Adjusting the field resistance RF (and thus the field flux)• Adjusting the terminal voltage applied to the armature.

The less common method of speed control is by• Inserting a resistor in series with armature circuit.


1 : Changing The Field Resistance

F

T

R

V

K

A

AT

R

EV

loadind

1. Increasing RF causes IF

6. Increasing τind makes

to decrease.

2. Decreasing IF decreases .

3. Decreasing lowers EA

4. Decreasing EA by increasing IA

5. Increase IA by increasing )( Aind IK

with the change in IA dominant over the change in flux ().

and the speed ω increases.


1: Changing The Field Resistance7. Increasing speed to increases EA = K again.

8. Increasing EA decreases IA.

loadind 9. Decreasing IA decreases until ind 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


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

4. Increasing ω increases EA =(Kω )

2: Changing The Armature Voltage

2. Increasing IA increases

)( Aind IK3. Increasing τind makes loadind increasing ω.

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

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

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


2: Changing The Armature Voltage

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

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


3 : Inserting Resistor in Series with Armature CircuitAdd 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


3 : Inserting Resistor in Series with Armature CircuitAdd resistor in

series with RA

Equivalent circuit of DC shunt motor

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


The Series DC Motor

Equivalent circuit of a series DC motor.

The Kirchhoff’s voltage law equation for this motor

)( SAAAT RRIEV


Induced Torque in a Series DC Motor

The induced or developed torque is given byAind 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 that a series motor give more torque per ampere than any other dc motor, therefore it is used in applications requiring very high torque, example starter motors in cars, elevator motors, and tractor motors in locomotives.


To determine the terminal characteristic of a series dc motor, an 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 :

The Terminal Characteristic of a Series DC Motor.

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


The Terminal Characteristic of a Series DC Motor.Also, EA = K, substituting these expression yields:

)( SAind

T RRKc

KV

We know ;

cIA

2c

Kind

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


The Terminal Characteristic of a Series DC Motor.Substituting the previous equation for VT yields:

)( SAind

indT RRKcK

cKV

The resulting torque – speed relationship is

Kc

RR

Kc

V SA

ind

T

1

One 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.


The Terminal Characteristic of a Series DC Motor.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.

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


Speed Control of Series DC MotorMethod of controlling the speed in series motor.

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. This is only one efficient way to change the speed of a series motor.

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.


The Compounded DC Motor.

A 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) and the 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


The equivalent compound DC motor (a) Long-shunt connection (b) Short-shunt connection

shu

nt

series

shu

nt

series

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.



The equivalent compound DC motor (a) Long-shunt connection (b) Short-shunt connection

shu

nt

series

shu

nt

series

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.



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 windingThe positive (+) sign is for cumulatively compound motor

The negative (-) sign is for differentially compound motor



The Torque Speed Characteristic of a Cumulatively Compounded DC Motor

The cumulatively compounded motor 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.


The Torque Speed Characteristic of a Cumulatively Compounded DC Motor

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 Torque Speed Characteristic of a Differently Compounded DC Motor

In a differentially compounded DC motor, 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.


Speed Control in the Cumulatively Compounded DC Motor

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.


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 thisequipment 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 losses

Pout =out m

I2R losses Mechanicallosses

Core loss

Pconv = Pdev = EAIA=indω

Pin =VTIL



Electrical or Copper losses : Copper losses are the 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 = IA

2RA

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 : The brush drop loss is the 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 : These are the hysteresis and eddy current losses

occuring in the metal of the motor.

Mechanical loss : These are 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) : These are other 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 is 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.


DC Generators

DC generators are dc machines used as generator. There are five major types of dc generators, classified according to the manner in which their field flux is produced:• Separately excited generator: In separately excited generator, the

field flux is derived from a separately power source independent of the generator itself.

• Shunt generator: In a shunt generator, the field flux is derived by connecting the field circuit directly across the terminals of the generators.

• Series generator: In a series generator, the field flux is produced by connecting the field circuit in series with the armature of the generator.

• Cumulatively compounded generator: In a cumulatively compounded generator, both a shunt and series field is present, and their effects are additive.

• Differentially compounded generator: In differentially compounded generator: In a differentially compounded generator, both a shunt and a series field are present, but their effects are subtractive.


DC Generators

These various types of dc generator differ in their terminal (voltage-current) characteristic, and the application is depending to which is suited.

DC generators are compared by their voltages, power ratings, efficiencies and voltage regulations:

%100

fl

flnl

V

VVVR

+VR = Dropping characteristics

-VR = Rising characteristic


Equivalent Circuit of DC Generators

The equivalent circuit of a DC generator

A simplified equivalent circuit of a DC generator, with RF

combining the resistances of the field coils and the variable control

resistor


Separately Excited Generator

Fig : Separately excited DC generator

A separately excited DC generator is a generator whose field current is supplied by a separately external DC voltage source

VT = Actual voltage measured at the terminals of the generatorIL = current flowing in the lines connected to the terminals.EA = Internal generated voltage.IA = Armature current.

AL II


The Terminal Characteristic of A Separately Excited DC Generator

The terminal characteristic of a separately excited dc generator (a) with and (b) without compensating windings (EA = K)

For DC generator, the output quantities are its terminal voltage and line current. The terminal voltage is VT = EA – IARA (IA = IL)

Since the internal generated voltage EA is independent of IA, the terminal characteristic of the separately excited generator is a straight line.

Take note about the axes between motors ( and ind) and generators (VT and IL)


The Terminal Characteristic of A Separately Excited DC Generator

When the load is supplied by the generator is increased, IL (and therefore IA) increase. As the armature current increase, the IARA drop increase, so the terminal voltage of the generator falls. (Figure (a) PREVIOUS SLIDE)

This terminal characteristic is not always entirely accurate. In the generators without compensating windings, an increase in IA

causes an increase in the armature reaction, and armature reaction causes flux weakening. This flux weakening causes a decrease in EA = Kω which further decreases the terminal voltage of the generator. The resulting terminal characteristic is shown in Figure b (PREVIOUS SLIDE)


Control of Terminal VoltageWe control torque-speed in DC Motor, while in DC Generator we control VT

The terminal voltage of a separately excited DC generator can be controlled by

changing the internal generated voltage EA of the machine.

VT = EA – IARA

If EA increases, VT will increase, and if EA decreases, VT will decreases. Since the

internal generated voltage, EA = KΦω, there are two possible ways to control the

voltage of this generator:

1. Change the speed of rotation. If ω increases, then EA = KΦω increases, so VT = EA - IARA increases too.

2. Change the field current. If RF is decreased, then the field current increases

(IF =VF/RF ). Therefore, the flux Φ in the machine increases. As the flux rises, EA= K ω must rise too, so VT = EA – IARA increases.


The Shunt DC GeneratorA shunt DC generator is a DC generator that supplies its own field current by having its field connected directly across the terminals of the machine.

Figure : The equivalent circuit of a shunt DC generator.

F

TF

AAAT

LFA

R

VI

RIEV

III

Because of generator supply it own field current, it required voltage buildup


Voltage Buildup in A Shunt Generator

Assume the DC generator has no load connected to it and that the prime mover starts to turn the shaft of the generator. The voltage buildup in a DC generator depends on the presence of a residual flux in the poles of the generator.

This voltage is given by resA KE

This voltage, EA (a volt or two appears at terminal of generators), and it causes a current IF to flow in the field coils. This field current produces a magnetomotive force in the poles, which increases the flux in them.

EA, then VT increase and cause further increase IF, which further increasing the flux and so on.

The final operating voltage is determined by intersection of the field resistance line and saturation curve. This voltage buildup process is depicted in the next slide


EA may be a volt or two appear at the terminal during start-up

Voltage buildup occurred in discrete steps



Voltage Buildup in A Shunt GeneratorSeveral causes for the voltage to fail to build up during starting which

are :• Residual magnetism. If there is no residual flux in the poles,

there is no Internal generated voltage, EA = 0V and the voltage will never build up.

• Critical resistance. Normally, the shunt generator builds up to a voltage determined by the intersection of the field resistance line and the saturation curve. If the field resistance is greater than critical resistance, the generator fails to build up and the voltage remains at the residual level. To solve this problem, the field resistance is reduced to a value less than critical resistance. Refer Figure 9-51 page 605 (Chapman)

Critical resistance


• The direction of rotation of the generator may have been reversed, or the connections of the field may have been reversed. In either case, the residual flux produces an internal generated voltage EA. The voltage EA produce a field current which produces a flux opposing the residual flux, instead of adding to it.

Under these conditions, the flux actually decreases below res and no voltage can ever build up.



The Terminal Characteristic of a Shunt DC Generator

Figure : The terminal characteristic of a shunt dc generator

As the load on the generator is increased, IL increases and so IA = IF + IL also increase. An increase in IA increases the armature resistance voltage drop IARA, causing VT = EA -IARA to decrease.

However, when VT decreases, the field current IF in the machine decreases with it. This causes the flux in the machine to decrease; decreasing EA. Decreasing EA causes a further decrease in the terminal voltage, VT = EA - IARA


Voltage Control for Shunt DC GeneratorThere are two ways to control the voltage of a shunt generator:1. Change the shaft speed, ωm of the generator.2. Change the field resistor of the generator, thus changing the

field current.

Changing the field resistor is the principal method used to control terminal

voltage in real shunt generators. If the field resistor RF is decreased, then the

field current IF = VT/RF increases.

When IF , the machine’s flux , causing the internal generated voltage

EA. EA causes the terminal voltage of the generator to increase as well.


The Series DC Generator

Figure : The equivalent circuit of a series dc generator

A series DC generator is a generator whose field is connected in series with its armature. Because the field winding has to carry the rated load current, it usually have few turns of heavy wire.Clear distinction, shunt generator tends to maintain a constant terminal voltage while the series generator has tendency to supply a constant load current.

The Kirchhoff’s voltage law for this equation : )( SAAAT RRIEV


Terminal Characteristic of a Series Generator

The magnetization curve of a series DC generator looks very much like the magnetization curve of any other generator. At no load, however, there is no field current, so VT is reduced to a very small level given by the residual flux in the machine. As the load increases, the field current rises, so EA rises rapidly. The IA (RA + RS) drop goes up too, but at first the increase in EA goes up more rapidly than the IA(RA + RS) drop rises, so VT increases. After a while, the machine approaches saturation, and EA becomes almost constant. At that point, the resistive drop is the predominant effect, and VT starts to fall.

Figure : A series generator terminal characteristic with large armature reaction effects


The Cumulatively Compounded DC Generator

Figure : The equivalent circuit of a cumulatively compounded DC generator with a long shunt connection

A cumulatively compounded DC generator is a DC generator with both series and shunt fields, connected so that the magnetomotive forces from the two fields are additive.



The total magnetomotive force on this machine is given byFnet = FF + FSE - FAR

where FF = the shunt field magnetomotive forceFSE = the series field magnetomotive forceFAR = the armature reaction magnetomotive force

NFI*F = NFIF + NSEIA - FAR

F

ARA

F

SEFF

N

FI

N

NII *



The other voltage and current relationships for this generator are

F

TF

SAAAT

LFA

R

VI

RRIEV

III

)(


Another way to hook up a cumulatively compounded generator. It is the “short-shunt” connection, where series field is outside the shunt field circuit and has current IL flowing through it instead of IA.

Figure : The equivalent circuit of a cumulatively DC generator with a short shunt connection



The Terminal Characteristic of a Cumulatively Compounded DC Generator

When the load on the generator is increased, the load current IL also increases.

Since IA = IF + IL, the armature current IA increases too. At this point two effects

occur in the generator:

1. As IA increases, the IA (RA + RS) voltage drop increases as well. This tends to cause a decrease in the terminal voltage, VT = EA –IA (RA + RS).

2. As IA increases, the series field magnetomotive force FSE = NSEIA increases too. This increases the total magnetomotive force Ftot = NFIF + NSEIA which increases the flux in the generator. The increased flux in the generator increases EA, which in turn tends to make VT = EA – IA (RA + RS) rise.


Voltage Control of Cumulatively Compounded DC Generator

The techniques available for controlling the terminal voltage of a cumulatively

compounded DC generator are exactly the same as the technique for controlling the

voltage of a shunt DC generator:

1. Change the speed of rotation. An increase in causes EA = K to increase, increasing the terminal voltage VT = EA – IA (RA + RS).

2. Change the field current. A decrease in RF causes IF = VT/RF to increase, which increase the total magnetomotive force in the generator. As Ftot increases, the flux in the machine increases, and EA = K increases. Finally, an increase in EA raises VT.


Analysis of Cumulatively Compounded DC Generators

The equivalent shunt field current Ieq due to the effects of the series field and armature reaction is given by

F

ARA

F

SEeq N

IN

NI

F

The total effective shunt field current is eqFF III *

NSE = series field turns

NF = shunt field turns

FAR = armature force

IA = armature current

where,


Field Resistance

IA (RA + RS)

VT at no load condition will be the point at which the resistor line and magnetization curve intersect.

As load is added, mmf increased thus increasing the field current Ieq and the resistive voltage drop [IA(RA + RF)].

The upper tip triangle represents the internal generated voltage EA.

The lower line represents the terminal voltage VT


The Differentially Compounded DC Generator

)( FAAAT

F

TF

FLA

RRIEV

R

VI

III

A differentially compounded DC generator is a generator with both shunt and series fields, but this time their magnetomotive forces subtract from each other.

The equivalent circuit of a differentially compounded DC generator


The Differentially Compounded DC Generator

The net magnetomotive force is

Fnet = FF – FSE – FAR Fnet = NFIF – NSEIA - FAR

And the equivalent shunt field current due to the series field and armature reaction is given by :

F

ARA

F

SEeq N

IN

NI

F

The total effective shunt field current in this machine is

eqFF III *

or

F

ARA

F

SEFF N

IN

NII

F*


Voltage Control of Differentially Compounded DC Generator

Two effects occur in the terminal characteristic of a differentially compounded

DC generator are

1. As IA increases, the IA (RA + RS) voltage drop increases as well. This increase tends to cause the terminal voltage to decrease VT.

2. As IA increases, the series field magnetomotive FSE = NSEIA increases too. This increases in series field magnetomotive force reduces the net magnetomotive force on the generator, (Ftot = NFIF – NSEIA), which in turn reduces the net flux in the generator. A decrease in flux decreases EA, which in turn decreases VT.

Since both effects tend to decrease VT, the voltage drop drastically as the load is increased on the generator as shown in next slide





The techniques available for adjusting terminal voltage are exactly the same as

those for shunt and cumulatively compounded DC generator:

1. Change the speed of rotation, m.2. Change the field current, IF.


A long journey start with a single step.

- Confucious

Documents

EMT 462 ELECTRICAL SYSTEM TECHNOLOGY Chapter 2 : DC Machines By: En. Muhammad Mahyiddin Ramli