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DC Generators

Engr. Muhammad Waseem Zeeshan Ashrafi

Electrical Machines

Tutorial Lecture No.02

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EMF equation of DC generator

Let,

Z=Total no. of conductors

φ= Flux /pole (Wb)P= No. of Poles

A= No. of parallel paths (A=2 for Wave & A=P for Lap winding)

N= Speed of Armature (in RPM)

Eg= emf of the generator = emf/parallel path

Flux cut by one conductor on one revolution of the armature,

dφ = P φ (Wb)

Time Taken to complete one revolution,

dt = 60/N (Second)

According to Faraday’s Law,

(volts)

Hence,emf of generator, Eg = emf per parallel path

= (emf/conductor) x No. of conductors in series per parallel path

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Types of DC generators:

The magnetic field in a dc generator is normally produced by electromagnets

rather than permanent magnets. Generators are generally classified according to

their methods of field excitation.On this basis, DC generators are divided into the following two classes:

(i) Separately-excited DC generators

(ii) Self-excited DC generators

The behavior of a d.c. generator on load depends upon the method of field

excitation adopted.

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Separately-Excited D.C. Generators

 A D.C. generator whose field-magnet winding is supplied from an independent

external D.C. source (e.g., a battery etc.) is called a separately-excited generator.

The voltage output depends upon the speed of rotation of armature and the field

current (If ).

The greater the speed and field current, greater is the generated e.m.f.

It may be noted that separately excited D.C. generators are rarely used inpractice. The d.c. generators are normally of self-excited type.

Following figure shows the connections of a separately-excited generator.

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Self Excited D.C. Generators

 A D.C. generator whose field magnet winding is supplied current from the

output of the generator itself is called a self-excited generator.

There are three types of self-excited generators depending upon the

manner in which the field winding is connected to the armature, namely;

(i) Series generator 

(ii) Shunt generator 

(iii) Compound generator 

Q: How is self-excitation achieved?

When the armature is rotated, a small voltage is induced in the armature

winding due to residual flux in the poles.

This voltage produces a small field current in the field winding and flux per pole

to increase.

The increased flux increases the induced voltage which further increases the

field voltage which further increases the field current.

These events take place rapidly and the generator builds-up to the ratedgenerated voltage.

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(1) Series Generator:

In a series wound generator, the field winding is connected in series with

armature winding so that whole armature current flows through the field winding

as well as the load.

Following Figure shows the connections of a series wound generator.

 Armature current, Ia = Ise = IL

Terminal voltage, V = Eg - Ia(Ra + Rse)

Power developed in armature = Eg.Ia

Since the field winding carries the whole of load current, it has a few turns of thick

wire having low resistance.

Series generators are rarely used except for special purposes e.g., as boosters.

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(2) Shunt Generator:

In a shunt generator, the field winding is connected in parallel with the armature

winding so that terminal voltage of the generator is applied across it.

Following figure shows the connections of a shunt-wound generator.

The shunt field winding has many turns of fine wire having high resistance.

Therefore, only a part of armature current flows through shunt field winding and the

rest flows through the load.

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(3) Compound Generator:

In a compound-wound generator, there are two sets of field windings on each Pole; one is in

series and the other in parallel with the armature.

 A compound wound generator may be:

(a) Short Shunt generator: in which only shunt field winding is in parallel withthe armature winding [Fig. A]

(b) Long Shunt generator: in which shunt field winding is in parallel with

both series field and armature winding [Fig. B]

Fig. A (Short Shunt)   Fig. B (Long Shunt)

Note:The two windings may be connected

1- To aid each other (cumulative compounding) or

2- To oppose each other (differential compounding).

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Losses in a D.C. Machine:

The losses in a DC machine (generator or motor) may be divided into three Classes:

(1) Copper losses

(2) Iron or core losses

(3) Mechanical losses

 All these losses appear as heat and thus raise the temperature of the machine.

They also lower the efficiency of the machine.

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Power Stages:The various power stages in a d.c. generator are represented diagrammatically

in Fig:

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Characteristics of D.C. Generators

Following are the three most important characteristics or curves of a d.c. generator :

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These types of  DC generators are generally more expensive than self-excited

DC generators because of their requirement of separate excitation source.

I. Because of their ability of giving wide range of   voltage   output, they are

generally used for testing purpose in the laboratories.

II. Separately excited generators operate in a stable condition with any variation

in field excitation. Because of this property they are used as supply source of 

DC motors, whose speeds are to be controlled for various applications.Example- Ward Leonard Systems of speed control.

Applications of Separately Excited DC Generators

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The application of shunt generators are very much restricted for its

dropping voltage characteristic.

They are used to supply power to the apparatus situated very close to its

position. These type of    DC generators   generally give constant

terminal voltage for small distance operation with the help of field regulators from

no load to full load.

I. They are used for general lighting.II. They are used to charge battery because they can be made to give constant

output voltage.

III. They are used for giving the excitation to the alternators.

IV. They are also used for small power supply.

Applications of Shunt Wound DC Generators

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These types of generators are restricted for the use of power supply because of their 

increasing terminal voltage characteristic with the increase in load current from no

load to full load.

They give constant current  in the dropping portion of the characteristic curve. For 

this property they can be used as constant current source and employed for various

applications.

Applications of Series Wound DC Generators

I. They are used for supplying field excitation current in DC locomotives

for regenerative breaking.

II. This types of generators are used as boosters to compensate

the voltage drop in the feeder in various types of distribution systems

such as railway service.

III. In series arc lightening this type of generators are mainly used.

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Applications of Compound Wound DC Generators

 Among various types of DC generators, the compound wound DC generators are

most widely used because of its compensating property.

We can get desired terminal voltage by compensating the drop due to armature

reaction and ohmic drop in the in the line. Such generators have variousapplications.

I. Cumulative compound wound generators are generally used lighting, power 

supply purpose and for heavy power services because of their 

constant voltage property. They are mainly made over compounded.

II. Cumulative compound wound generators are also used for driving a motor.

III. For small distance operation, such as power supply for hotels, offices, homes

and lodges, the flat compounded generators are generally used.

IV. The differential compound wound generators, because of their largedemagnetization armature reaction, are used for arc welding where

huge voltage drop and constant currentis required.