DC Generator Lecture

Direct Current Generators

Introduction

The energy converted from mechanical to

electrical is known as generator action.

The energy converted from electrical to

mechanical is known as motor action.

Electrical Energy Mechanical Energy

Motor action

Generator action

Electromagnetic Induction

Whenever the conductor cuts the

magnetic flux lines, a voltage will be

induced in that conductor.

e = vind = N d /dt

Motional Voltage

When the conductor moves from its initial

position to final position in the magnetic

field, the motional voltage can be

expressed as

e = Blv sin

e = induced voltage in V

B = magnetic flux density in Wb/m2

l = active length of the conductor in m

v = speed of the conductor in m/s

= angle b/w conductor and magnetic field

Examples

A conductor of 0.65m is length is moving in a

magnetic field with a speed of 35m/s. The

magnetic flux density of the field is 0.18

Telsa. Determine the induced voltage in the

conductor.

A 1.5m length of the conductor moves at a

velocity of 20m/s with an angle of 350.

determine the induced voltage if the

magnetic flux density is 0.9Wb/m2

Practice Problem

A 200V is induced by the conductor of

16m in length when moves at a velocity of

25m/s with an angle of 650. Determine the

magnetic flux density.

Lenzs Law

It determines the direction of the induced voltage.

The voltage will be induced if the

conductor moves in the magnetic field

and cuts the flux lines. As a result of

this voltage, current will flow through

the conductor if the conductor circuit is

closed.

The magnetic field produced by this

current will always oppose the motion of

the conductor. This is known as Lenzs law.

DC Voltage Output from DC Generator

To generate the voltage by the generator,

following things are necessary

A magnetic field

Conductor or group of conductors

Motion of conductor w.r.t. magnetic field

Consider a coil rotating that is rotating in a

clockwise direction in uniform magnetic field

with a constant speed as shown in figure.

At position 1, the coil is moving in parallel to the

lines of flux and it is not cutting any lines of

flux. As a result generator emf is zero.

At position 2, the voltage induced is maximum

because coils A and B are moving at right

angles to the lines of flux.

At position 3, the generator voltage is again

zero, because it is in reverse of position 1. The

coil A is at bottom instead of the top.

At position 4,

The induced voltage is again maximum because it

is in reverse of position 2.

Position 5 is same as position 1.

Voltage induced in the coil is an AC form. An

alternating current will flow through the load

when load is connected across the ends of the

generator coil. This induced voltage can be

converted into DC by a device known as

commutator.

Sinusoidal voltage Pulsating voltage

DC Generator Construction

Stator

It is the stationary part of the dc generator, which provides mechanical support for the machine and contain field poles.

Rotor

It is the inner part and normally rotates within the poles and provides the slots in its outer periphery.

Armature winding

These are the insulated copperconductors that are placed inside theslots. Within the slots, these areconducted either in lap or wave winding.

Commutator

It is used as a converter and made up ofhard copper segments. Its function is tocovert the AC voltage generated in thearmature windings into DC voltage acrossload circuit. It is also known asmechanical rectifier.

Brushes

These are made of carbon and kept in

touch with the commutator by suitable

pressure of the control spring. Their main

function is to provide proper electrical

connection between the commutator and

the load circuit.

Induced EMF of a DC Generator According to Faradays law of

electromagnetic induction

Ec = d / dt

Net emf generated is equal to the product of emf per conductor and the number of conductors in series per parallel path.

Eg = PZN

60A

The electromagnetic torque will be

Tem = PZ Ia

2A

Parameters used in DC Generators,

Ec is the induced emf per conductor in V,

Eg is the induced emf in V,

P is the number of poles of the generator,

is the flux per pole in Wb,

N is the speed in rpm,

Z is the total number of the armature

conductors,

A is the number of parallel paths,

A 4-pole wave wound generator having 40

slots and 10 conductors placed per slot.

The flux per pole is 0.02 Wb. Calculate the

generated emf when the generator is

driven at 1200rpm.

A 6-pole, lap wound generator has 600

conductors on its armature. The flux per

pole is 0.05Wb. Calculate generated

voltage when the speed of the generator

is 1000rpm. Also find the electromagnetic

torque if the generator supplies a current

of 120A

Practice Problem

The armature of a 6-pole lap-wound

generator has 90 slots. The armature is

rotating at a speed of 600rpm to generate

a voltage of 200V. If each coil has 5 turns,

calculate the flux per pole of the

generator. Also, find the electromagnetic

torque it the armature current is 115A.

Types of DC Generator

Separately Excited DC Generator

Its field winding is energized from an independent external dc source.

Rf is the field resistance,

Rais the armature

resistance,

RLis the load resistance,

Ia is the armature current,

ILis the load current,

Vt is the terminal voltage,

Eg is the induced voltage,

From circuit,

Ia = IL

Vt = Eg Ia Ra

Power developed by armature

Pd = Eg Ia

Power delivered to load

PL = Eg Ia I2

a Ra = Ia( Eg Ia Ra) = Vt Ia

Self Excited Generators

Its field winding is generated from the

output of the generator itself. Types are,

1. Series generator

2. Shunt generator

3. Compound generator

Series generator

The field winding is connected in series with the

armature so that the same armature current can

flow through the winding as well as the load.

Ia = Isc= IL

Terminal Voltage:

Vt = EgIa (Ra + Rse)Power developed by

armature:

Pd = Eg Ia

Power delivered to load

PL = Ia[EgIa (Ra + Rse)] = Vt Ia

Shunt Generator

The field winding is connected in parallel with the

armature winding.

Shunt field current

Ish = Vt / Rsh

Armature current

Ia = IL + Ish

Power developed by armature

Pd = Eg Ia

Power delivered to load

PL = Vt IL

Compound Generator

The generator with both series and shunt field windings. It is divided into two types,

short shunt and long shunt compound generators.

Series field current Ise = IL

Shunt field current

Ish = Eg/Rsh

Ish = (Vt + Ise Re)/Rsh

Terminal voltage

Vt = Eg Ia Ra Ise Rse

Power developed in

armature Pd = Eg Ia

Power delivered to load

PL = Vt IL

Long Shunt Compound GeneratorField winding is in parallel with both in series and

armature winding.

Series field current

Ise =Ia=IL+ Ish

Shunt field current

Ish = Vt /Rsh

Ish = (Vt + Ise Re)/Rsh

Terminal voltage

Vt = Eg Ia (Ra + Rse)

Power developed in

armature Pd = Eg Ia

Power delivered to load

PL = Vt IL

Practice Problem

A 40kW, 230V shunt generator has a field

resistance of 60 and a armature resistance of 0.04 . Calculate the generated voltage.

Saturation Curve of a DC Generator When the current through the shunt field coil is

zero i.e. If = 0, a small magnitude of voltage is denoted by the point a. As the shunt field increases the induced voltage increases proportionally represented by point b and c.

After the point c, by increasing the

current, the induced voltage increase in a

smaller magnitude represented by point d

which is also called saturation point.

After point d, by increasing current the

magnitude of the induced voltage will be

the same.

By decreasing current, the voltage will

follow the line which meet with point e.

Field Circuit Resistance

The shunt field resistance is normally

considered as the field circuit resistance. In a

self-excited dc generator, the voltage buildup

process depends on the field circuit resistance.

At lower value of Rf generator will build up the maximum voltage Eo.

If the Rf increases gradually the generator build up voltage will be reduced.

After sometime the Rf line become tangent to the open circuit characteristics or magnetization curve and generator will not build any more voltages.

This is called critical condition of the dc generator. The critical speed is the speed at which the field circuit resistance becomes the critical resistance.

Voltage Buildup Process

Consider a shunt generator, which is running

at a constant speed to buildup a voltage. The

buildup process can be described by following

steps:

The armature of the generator is driven at a

certain speed without the shunt field. A small

magnitude of voltage Er will appear across the

generator terminals due to residual flux in the

poles.

Now generator is driven with a shunt field. A small magnitude of current will flow through the field winding which will produce the mmf. This mmf sets up a flux, which will aid the residual flux.

The increased mmf increases the total flux which will increases the generated voltage.

The increased generated voltage will

increase the if. As a result, mmf will be

increased and it will increase the Vg.

This buildup process will continue until

to get the VNL. The VNL can be obtained

from the point from the point of

intersection b/w the lower value of Rf line

and the magnetization curve.

After that by increasing if the voltage will

not increase, it is due to the magnetic

material property.

Reasons for Failure of Buildup

Process

Absence of residual flux

Initially, small magnitude of residual flux is required to buildup the voltage from zero to small magnitudes. Sometimes, a separate dc voltage source is used to get a residual flux for short period of time.

Higher value of shunt field resistance

If the shunt field resistance is higher than the critical resistance the generator will not buildup any voltage.

Reverse field connections

If the field winding is connected in such a

way that the flux produced by its

magnetomotive force opposes the residual

flux. The generated voltage will decrease

rather increasing.

Theory of Commutation

The coils or conductors moves from one

pole to another during the operation of

the generator. The direction of the current

in the coil reverses by the combined

action of the commutator and brushes.

The brushes normally fixed on the

commutator segments. The brushes move

from one segment to other segment of the

commutator when the coil rotates in

clockwise direction.

A short period of time is required for the

reversal of the current. The coil

undergoing commutation will induce the

voltage due to rate of change of current

i.e. L di/dt. This voltage in known as

reactance voltage. As a result of

reactance voltage, spark occurs at the

brushes of the machine.

Armature Reaction It is the opposite action that comes from the

armature conductors of the dc generator. In a dc generator, the current in the armature conductors produces an undesirable effect in its operation.

The flux due to main poles act inside the field system of the generator to generate the voltage.

The current flowing through the armature conductors creates flux.

This armature flux acts in opposition to the main flux. As a result, the resultant flux is distorted. Therefore, the opposite action of the armature flux on the main flux is known as armature reaction.

The terminal voltage is slightly reduced due to

armature reaction. At no load condition, the

current in the armature conductors is zero.

There is only one flux f produced by the field winding that is uniformly distributed over the

pole phases.

Flux due to Field, Armature and

Resultant Flux

Techniques to Cancel the Armature

Reaction

Some techniques are,

Brushes may be shifted from neutral position at no-

load to the new neutral position under loaded

condition.

Interpoles or commutating poles may be inserted in

between the main poles.

Use of compensating windings with the armature

winding.

Cancellation of Armature Reaction

Compensating Windings

These are used to cancel the armature reaction due to armature flux on a large generator. These are placed on the slots of the pole faces of the main field poles. The compensating winding is connected in series in such a way that the current through the compensating winding is opposite to the current in the armature conductors.

The flux produced is equal and opposite to the

armature flux. As a result, the armature flux

cancelled completely.

Interpoles

The interpoles are called the commutating

or narrow poles that are placed between

the main field poles. The interpole

winding is connected in series with the

armature winding. The current through

the interpole setup a flux. The flux is

equal and opposite to the flux due to

armature winding, thus canceling the

armature flux completely.

Characteristics of a Series Generator

Initially, the generator is driven without the load, no current will flow through the series field. As a result, the mmf force in the series field is zero. In this case, the terminal voltage will be equal to Er which is produced by the residual flux.

Then the generator is driven with load. This generator will delivers current to the load through series field. The series field will produce the flux that aids the residual flux. The generated voltage will be increased to Eg.

Uses

The series generator is used for welding and

electro-plating purposes. It is also used for

boosting of a dc transmission lines to

compensate the transmission line drop.

Characteristics of a Shunt Generator

The shunt generator has higher value of field

circuit resistance that the armature circuit

resistance. So, less current will flow through the

shunt field winding and most will pass through

the load circuit. The terminal voltage can be

calculated by,

Vt = Eg Ia Ra

Current through the armature conductors

develop the flux that distorts and weakens

the main field flux. As a result, the

resultant flux is reduced. The terminal

voltage reduces slightly. It also reduces

cut to armature circuit resistance drop.

By increasing load, terminal voltage

reduces and in result it reduces the field

current, as a result, Vt will also be reduced

and gradually it will reach to the saturation

region of the magnetic materials.

The IL is increased to reduce the Vt until to

get the breakdown point. At this saturation

point, the magnetic circuit fall its

saturation property.

The terminal voltage reduces sharply

towards zero with a change in the field

current.

Voltage Regulation of a DC Generator

It is used to identify the change in terminal voltage with increase in the generator load.

It is defined as the change in voltage from no-load to full load and it is expressed as percentage of full load terminal voltage.

VR = (Vnl Vfl) x100

Vfl

Losses of a DC Generator

Copper Losses

When current flows through the copper

wire, I2R losses occur in that wire.

Armature copper loss.

Shunt copper loss.

Series copper loss.

Interpole field copper loss.

Compensating field copper loss.

Mechanical Losses

These occurs due to following reasons

Friction between the shaft and bearings, commutator and brushes and the rotating armature and surrounding air.

Iron or Magnetic Loss

This loss is divided into two types, eddy current loss and hysteresis loss.

Eddy current loss

A voltage will be induced due to rotation of

the armature in the magnetic field. As a

result of this voltage, a small magnitude

of current will flow through the body of

the armature core. This current is known

as eddy current and the power loss due to

this is known as eddy current loss.

Pe = keB2

mf2t2V

Hysteresis Loss

The armature conductors are changing

their position between the poles. Due to

new position of the conductor, the

magnetic field also changes. Therefore,

this loss occurs due to reversal of

magnetic field.

Ph = Bm1.6f V

Efficiency of a DC Generator

It is defined as the ratio of output power

to input power. The efficiency of

generator depends upon load. Therefore,

it varies with load.

Generator output,

Po = Vt IL

Variable loss,

Pv = IaRa

Constant loss = Pc

2

Input power,

Pin = Po + Pv + Pc

Generator efficiency,

= Po / Pin

The efficiency will be minimum when the

denominator of the efficiency is minimum.

The efficiency will be maximum when the

constant loss is equal to variable loss.

Practice Problem

A speed of a 10kW, 220V, 4-pole wave

wound generator is 1000rpm. The

armature has 400 conductors the full

copper loss is 500W. Determine the flux

per pole and the efficiency of the

generator. Consider the shunt field

resistance of 60 and the total brush drop of 2V.

Practice Problems