Generator - PURUSHOTHAM ACADEMICSgpmacademics.weebly.com/uploads/2/9/9/3/29933629/dc_generators.pdf.pdf · greater is the generated e.m.f. It may be noted that separately excited

Principles of Electrical Engineering D.C. Generators

G.Purushotham, Asst. Professor, Dept. of EEE @SVCE,TPT Page 1

D.C GENERATORS

Principle of operation of D.C machines, types of D.C Generators, e.m.f equation of D.C Generator,

O.C.C of a D.C Shunt Generator, Load characteristics of D.C.Generators

GENERATOR PRINCIPLE:

An electric generator is a machine that converts mechanical energy into electrical energy.

An electric generator is based on the principal that whenever flux is cut by a conductor, an e.m.f. is induced

which will cause a current to flow if the conductor circuit is closed. The direction of induced e.m.f. (and

hence current) is given by “FLEMING’S RIGHT HAND RULE”.

The essential components of a generator are:

a) A magnetic field

b) Conductor or a group of conductors

c) Motion of conductor with respect to magnetic field.

SIMPLE LOOP GENERATOR:

Consider a single turn loop ABCD rotating clockwise in a uniform magnetic field with a constant speed as

shown in Fig. 1.1.As the loop rotates, the flux linking the coil sides AB and CD changes continuously. Hence

the e.m.f. induced in these coil sides also changes but the e.m.f. induced in one coil side adds to that induced

in the other. (It is because the coil sides always remain under the influence of opposite poles i.e., if one coil

side is under the influence of the N-pole, then the other coil side will be under the influence of S-pole and

vice-versa.)

i) When the loop is in position no. 1 [see Fig. 1.1], the generated e.m.f. is zero because the coil sides (AB

and CD) are cutting no flux but are moving parallel to it.

ii) When the loop is in position no. 2, the coil sides are moving at an angle to the flux and, therefore, a low

e.m.f. is generated as indicated by point 2 in Fig. 1.2.

iii) When the loop is in position no. 3, the coil sides (AB and CD) are at right angle to the flux and are,

therefore, cutting the flux at a maximum rate. Hence at this instant, the generated e.m.f. is maximum as

indicated by the point 3 in Fig. 1.2.

iv) At position 4, the generated e.m.f. is less because the coil sides are cutting the flux at an angle.

v) At position 5, no magnetic lines are cut and hence induced e.m.f. is zero as indicated by point 5 in Fig.

1.2.

Generator Mechanical

Input

Electrical

Output



vi) At position 6, the coil sides move under a pole of opposite polarity and hence the direction of generated

e.m.f. is reversed. The maximum e.m.f. in this direction (i.e., reverse direction, see fig. 1.2) will be the

loop is at position 7 and zero when at position 1. This cycle repeats with each revolution with each

revolution of the coil.

vii) Note that e.m.f. generated in the loop is alternating one. It is because any coil side, say AB has e.m.f. in

one direction when under the influence of N-pole and in the other direction when under the influence of

S-pole. If a load is connected across the ends of the loop, then alternating current will flow through the

load. The alternating voltage generated in the loop can be converted into direct voltage by a device called

Commutator. We then have the D.C. Generator. In fact, a Commutator is a mechanical rectifier.

Action of Commutator:

If, somehow, connection of the coil side to the external load is reversed at the same instant the current in the

coil side reverses, the current through the load will be direct current. This is what a Commutator does. Fig.

1.3 shows a Commutator having two segments C1 and C2. It consists of a cylindrical metal ring cut into two

halves or segments C1 and C2 respectively separated by a thin sheet of mica. The ends of coil sides AB and

CD are connected to the segments C1 and C2 respectively as shown in Fig. 1.4. Two stationary carbon

brushes rest on the Commutator and leads currents to the external load. With this arrangement, the

Commutator at all times connects the coil side under S-pole to the positive brush and that under N-pole to the

negative brush.

Principles of Electrical Engineering

G.Purushotham, Asst. Professor, Dept. of EEE @SVCE,TPT

i) In Fig. 1.4, the coil sides AB and CD are under N

connects the coil side AB to point P of the load resistance R and the segment C

CD to point Q of the load. Also note the direction of current through load. It is from Q to P.

ii) After half a revolution of the loop (i.e., 180

CD under N-pole as shown in Fig. 1.5. The currents in the coil sides now flow in the reverse direction

but the segments C1 and C2 have also moved through 180

positive brush and segment C2 in contact with

Note that Commutator has reversed the coil connections to the load i.e., coil side AB is now connected to

point Q of the load and coil side CD to the point P of the load. Also note the direction of current through the

load. It is again from Q to P. (The situation is that cu

the connections of the coil sides to the external load are reversed. This means that current will flow in the

same direction through the load.)

Thus the alternating voltage generated in the loop will

appear as direct voltage across the brushes. The reader may note

that e.m.f. generated in the armature winding of a D.C. generator is

alternating one. It is by the use of Commutator

generated alternating e.m.f. into direct voltage. The purpose of

brushes is simply to lead current from the rotating loop or winding

to the external stationary load. The variation of voltage across the

brushes with the angular displacement of the loop wi

in Fig. 1.6. This is not a steady direct voltage but has a pulsating character. It is because the voltage

appearing across the brushes varies from zero to maximum value and back to zero twice for each revolution

of the loop. A pulsating direct voltage such as is produced by a single loop is not suitable for many

commercial uses. What we require is steady state voltage. This can be achieved by using a large number of

coils connected in series. The resulting arrangement is known as


In Fig. 1.4, the coil sides AB and CD are under N-pole and S-pole respectively. Note that segment C

connects the coil side AB to point P of the load resistance R and the segment C


After half a revolution of the loop (i.e., 180o

rotation), the coil side AB is under S

Fig. 1.5. The currents in the coil sides now flow in the reverse direction

have also moved through 180o i.e., segment C1

in contact with negative brush.

has reversed the coil connections to the load i.e., coil side AB is now connected to


load. It is again from Q to P. (The situation is that currents in the coil sides are reversed and at the same time


Thus the alternating voltage generated in the loop will

appear as direct voltage across the brushes. The reader may note

that e.m.f. generated in the armature winding of a D.C. generator is

Commutator that we convert the

generated alternating e.m.f. into direct voltage. The purpose of

brushes is simply to lead current from the rotating loop or winding

variation of voltage across the

brushes with the angular displacement of the loop will be as shown



ct voltage such as is produced by a single loop is not suitable for many


coils connected in series. The resulting arrangement is known as armature winding.

D.C. Generators

Page 3

pole respectively. Note that segment C1

connects the coil side AB to point P of the load resistance R and the segment C2 connects the coil side


rotation), the coil side AB is under S-pole and the coil side

Fig. 1.5. The currents in the coil sides now flow in the reverse direction

is now in contact with

has reversed the coil connections to the load i.e., coil side AB is now connected to


rrents in the coil sides are reversed and at the same time




ct voltage such as is produced by a single loop is not suitable for many




CONSTRUCTION OF D.C. GENERATOR:

The D.C. generators and D.C. motors have the same general

construction. Any D.C. generator can be run as a D.C. motor and vice-

versa. All D.C. machines have five principal components viz.,

i) Field system ii) Armature core

iii) Armature winding iv) Commutator

v) Brushes

i. Field system:

The function of the field system is to

produce uniform magnetic field within which

the armature rotates. It consists of a number of

salient poles (of course, even number) bolted to

the inside of circular frame (generally called

yoke). The yoke is usually made of solid cast

steel whereas the pole pieces are composed of

stacked laminations. Field coils are connected in

such a way that adjacent poles have opposite

polarity.

ii. Armature Core:

The armature core is keyed to the machine shaft and rotates between the field poles. It consists of slotted

soft-iron laminations (about 0.4 to 0.6 mm thick) that are stacked to form a cylindrical core as shown in

Fig. The purpose of laminating the core is to reduce the eddy current losses.

iii. Armature Winding:

The slots of the armature core hold insulated conductors that are connected in a suitable manner. This

is known as armature winding This is the winding in which “working” e.m.f. is induced. The armature

winding of a D.C. machine is a closed-circuit winding; the conductors being connected in a symmetrical

manner forming a closed loop or series of closed loops.

iv. Commutator:

A Commutator is a mechanical rectifier which converts the alternating voltage generated in the

armature in the armature winding into direct voltage across the brushes.

Depending upon the manner in which the armature conductors are connected to the Commutator

segments, there are two types of armature winding in a D.C. machine viz,

a) Lap winding b)Wave winding



v. Brushes: The purpose of brushes is to ensure electrical connections between the rotating Commutator

and stationary external load circuit. The brushes are made of carbon and rest on the Commutator.

E.m.f. Equation of a D.C. Generator:

We shall now derive an expression for the e.m.f. generated in a D.C. generator.

Let, φ = flux/pole in Wb

Z = total number of armature conductors

P = number of poles

A = number of parallel paths

= 2 ….. For wave winding

= P ….. For lap winding

N = speed of armature in r.p.m.

Eg = e.m.f. of the generator

= e.m.f./parallel path

Flux cut by one conductor in one revolution of the armature, dφ = Pφ (webers)

As the conductor cuts the flux of all the P poles.

Time taken to complete one revolution, dt = 60/N (seconds)

E.m.f. generated/conductor,

�φ

�� =

�φ

�� ⁄ =

�φ�

�� (��)

E.m.f. of generator, Eg = e.m.f. per parallel path

= (e.m.f./conductor) × No. of conductors/parallel path

= �φ�

�� ×

�

�

Eg = φ��

�

�

� volts

Where A = 2 ….. For wave winding = P ….. For lap winding

TYPES OF D.C. GENERATORS:

The magnetic field in a D.C. generator is normally produced by electromagnets rather than permanent

magnets. Generators are generally classified according to their methods of field excitation. On this basis , d.c.

generators are divided in to the following two classes :

(i) Separately excited generator

(ii) Self excited generator

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



Separately Excited Generator:

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 (Eg = P φ Z N/ 60 A). 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 in

practice. The D.C. generators are normally of self – excited type. Figure shows the connections of a

separately excited generator.

Armature current, Ia = IL

Terminal voltage, V = Eg- Ia Ra

Electric power developed = Eg Ia

Power delivered to load = Eg Ia -Ia Ra = Ia2 (Eg – Ia Ra) = V Ia

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

(i) 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. Fig. shows the

connections of series generator



Armature current, IA = Ise = IL = I (say)

Terminal voltage, V = Eg – I (Ra +Rse )

Power developed in armature = Eg Ia

Power delivered to load = Eg Ia – Ia 2 Ra (Ra +Rse) = Ia [Eg – Ia ( Ra + Rse ) ] = V Ia (or) V IL

(iii) 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.

Figure shows the connections of a shunt wound generator.

Shunt field current, Ish = V/Ra

Armature current, IA = IL + Ish

Terminal voltage, V = Eg – Ia Ra

Power developed in armature = Eg Ia

Power delivered to load = V IL

(iv) Compound Generator: In a compound - wound generator there are two sets of field winding on each

pole–one is in series and the other in parallel with the armature.

A compound generator may be:

(a) Short Shunt

(b) Long Shunt



Short shunt: this generator in which only shunt field winding is in parallel with the armature winding.

Series field current, Ise = IL

Shunt field current, Ish = (V + Ise Rse)/ Rsh

Terminal voltage, V = Eg – Ia Ra – Ise Rse

Power developed in the armature= Eg Ia


Long Shunt: this generator in which shunt field winding is in parallel with both series field and armature

winding.

Series field current, Ise = Ia = IL + Ish

Shunt field current, Ish = V/Rsh

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

Power delivered in the armature = Eg Ia


CHARACTERISTICS OF A D.C GENERATOR:

Following are the three most important characteristics of curves of a DC generator:

1. No-load saturation characteristic (Eo/If):

It is also known as magnetic or open-circuit characteristic (O.C.C). It shows the relation between

the no-load generated e.m.f in armature Eo, and the field or exciting current (If) at a given fixed speed. It is

just the magnetization curve for the material of the electromagnets. Its shape is practically the same for all

generators whether separately-excited or self-excited.

2. Internal Characteristic (E/Ia):

It gives the relation between the e.m.f, E actually induced in the armature and the armature current

(Ia). This is of interest mainly to the designer.

3. External Characteristic (VL/IL):

It is also referred to as performance (c/s) or sometime voltage-regulation curve. It gives relation

between the terminal voltage (VL) and the load current (IL). This curve lies below the internal (c/s) between

it takes into account the voltage drop over the armature circuit resistance. The values of (VL) are obtain by

subtracting (Ia Ra) from corresponding values of (E). This is of great importance in judging the suitability

of a generator for a particular purpose.



Separately- Excited Generator:

Open-circuit characteristics (Eo/If):

The arrangement for obtaining the necessary data to plot this curve is shown in fig. The exciting or

field current (If) is obtained from an external independent D.C source.

It can be varied (If) from zero upwards by a rheostat and its value read by an ammeter (A) connected

in the field circuit as shown.

Now, the voltage equation of a D.C generator is:

Eg = φ��

��

�

� volts

Hence, if the speed is constant, the above relation becomes:

E = K.Φ

It is obvious that when (If) is increased from its initial small value, the flux (Φ) and hence generated

e.m.f, Eo increase directly as current while the poles are unsaturated. This is represented by straight

portion (o d) in fig. But as the flux density increases, the poles become saturated, so a greater increase in (If)

is required to produce a given increase in voltage than on the lower part of curve. That is why the upper

portion (d b) of curve (o d b) bends over as shown.

Self- Excited Generator:

Open-circuit characteristic or magnetization curve for self-excited generator:

The (O.C.C) or no-load saturated curves for self-excited generators whether shunt or series-

connected, are obtained in a similar way. The field winding of the generator whether (shunt or series wound)

is disconnected from the machine and connected to an external source of direct current as shown in fig. The

field or exciting current (If) is varied rheostatically and its value read on the ammeter (A). The machine is

driven at constant speed by the prime mover and the generator e.m.f on No-load is measured by voltmeter

connected across the armature. (If) is increased by suitable steps (starting from zero) and the corresponding

Values of (Eo) are measured. On plotting the relation between (If) and (Eo) a curve of the form shown in fig.

is obtained. Due to residual magnetism in the poles, some e.m.f is generated even when (If=0).



Hence, the curve starts a little way up. The straight curvature at the lower end is due to magnetic

inertia. It is seen that the first part of the curve is practically straight.

Now, connect the field windings back to the armature and run the machine as shunt generator. A

shunt generator will excite only if the poles have some residual magnetism and the resistance of the shunt

circuit is less than some critical value, the actual value depending upon the machine and upon the speed at

which the armature is driven. Suppose curve in fig. to represent the open-circuit characteristic of a shunt

generator. With increasing excitation. Then, for a shunt current (If) OA, the e.m.f. is AB and.

Corresponding resistance of shunt circuit

The resistance line OB represents smaller resistance to which the machine will build up and represent

the maximum voltage AB. If field resistance is increased, and then slope of the resistance line increase, and

hence the maximum voltage to which the generator will build up at a given speed, decreases. If field

resistance increased so much that the resistance line does not cut the O.C.C at all (like OE) then obviously

the machine will fail to excite, there will be no “build up" of the voltage. The value of the resistance

represented by the tangent to the curve is known as critical resistance RC for a given speed.



Load Characteristics of DC Shunt Generator:

Internal characteristics:

Consider the dc shunt generator shown in fig. the internal characteristics is E vs IL

Intially the induced e.m.f is not dependent on the load current IL or armature current Ia. but as load current

increases, the armature current increases to supply load demand. As Ia increases armature, armature flux

increases.

The effect of armature flux which is produced by the armature current on the main field flux is called

armature reaction. Due to armature reaction main flux gets disturbed. Hence lesser flux gets linked with the

armature. This reduces the induced e.m.f .

external characteristics

for dc shunt generator we know tha E = V+Ia R

a neglecting other drops as load current IL increases, Ia

increases. Hence drop Ia Ra increases. but the value of armature resistance is very small,the drop in terminal voltage

as IL changes from no load to full load is very small.this is shown in below fig. hence dc shunt generator is called as

constant voltage generator.

Internal Characteristics

External Characteristics

Principles of Electrical Engineering


Load Characteristics of DC Series Generator:

Internal characteristics:

In case of series generator Ia=I

As load current increases, Ise increases. The flux

The induced emf E is proportional to flux. So induced emf also increases.

increasing nature.

As Ia increases, armature reaction increases but the effect of armature reaction is negligib

to increase in E. But in case of high load current saturation occurs and flux remains consta

case,due to armature reaction E starts decreasing as shown in fig by dotted line.

External characteristics:

As IL = Ia increases ,thus the drop Ia

nature as E increases but it will be below internal characteristics due to drop I

fig.

Note: in self excited series generator, open circuit characteristics

circuit, Ia= IL = 0 hence induced e.m.f is zero. Thus open circuit characteristics is possible only by

separately excited the field winding.

Load Characteristics of DC Compound

The characteristics depends on

compound generator

In cumulatively compound ɸ = ɸsh

producing more flux. Thus individual emf increases and total terminal vo

increases, the various voltage drops and armature reaction drop also increases.hence there is drop in the

terminal voltage.


Generator:

=Ise=IL

As load current increases, Ise increases. The flux ɸ is directly proportional to Ise. So

The induced emf E is proportional to flux. So induced emf also increases. Thus internal characteristics of E is

reaction increases but the effect of armature reaction is negligib

in case of high load current saturation occurs and flux remains consta

case,due to armature reaction E starts decreasing as shown in fig by dotted line.

a (Ra+Rse) increases. thus the external characteristics is also of rising

nature as E increases but it will be below internal characteristics due to drop Ia (Ra+R

in self excited series generator, open circuit characteristics cannot be obtained. In open

0 hence induced e.m.f is zero. Thus open circuit characteristics is possible only by

DC Compound Generator:

The characteristics depends on whether generator is cumulatively compound or differentially

+ ɸse. as load current increases,Ia increases hence I

producing more flux. Thus individual emf increases and total terminal voltage also increases.


D.C. Generators

Page 12

is directly proportional to Ise. So flux also increases.

Thus internal characteristics of E is

reaction increases but the effect of armature reaction is negligible compared

in case of high load current saturation occurs and flux remains constant.in such

) increases. thus the external characteristics is also of rising

+Rse) as shown in above

cannot be obtained. In open

0 hence induced e.m.f is zero. Thus open circuit characteristics is possible only by

whether generator is cumulatively compound or differentially

increases hence Ise also increases

ltage also increases. But as Ia




If drop in voltage due to increasing IL is more dominating than increase in V due to increase in flux

then the generator is called under compounded generator and its characteristics is dropping in nature. As

shown in fig.

If drop in V due to armature reaction and other drops is much less than increase in V due to increase

in flux then the generator is called over compounded and its characteristics is rising in nature.

If the effects of the two are such that on full load current V is same as no load induced e.m.f. i.e.

these effects are neytralising each other on full load then generator is called flat compounded or level

compounded.

In differentially compound the net flux is difference between the two fluxes.(i.e ɸ = ɸsh - ɸse) as IL

increases, ɸsh is almost constant but ɸse increases rapidly. Hence total flux is reduces. hence the induced emf

and terminal voltage also decreases drastically.

There is a drop due to armature resistance, series field resistance, armature reaction due to which

terminal voltage drops. Thus we get the characteristics of differentially compound generator.

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Generator - PURUSHOTHAM ACADEMICSgpmacademics.weebly.com/uploads/2/9/9/3/29933629/dc_generators.pdf.pdf · greater is the generated e.m.f. It may be noted that separately excited