Chapter (1)
D.C. Generators
Introduction
Although a far greater percentage of the electrical machines in service are a.c.
machines, the d.c. machines are of considerable industrial importance. The
principal advantage of the d.c. machine, particularly the d.c. motor, is that it
provides a fine control of speed. Such an advantage is not claimed by any a.c.
motor. However, d.c. generators are not as common as they used to be, because
direct current, when required, is mainly obtained from an a.c. supply by the use
of rectifiers. Nevertheless, an understanding of d.c. generator is important
because it represents a logical introduction to the behaviour of d.c. motors.
Indeed many d.c. motors in industry actually operate as d.c. generators for a
brief period. In this chapter, we shall deal with various aspects of d.c.
generators.
1.1 Generator Principle
An electric generator is a machine that converts mechanical energy into
electrical energy. An electric generator is based on the principle 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. Therefore, the essential
components of a generator are:
(a) a magnetic field
(b) conductor or a group of conductors
(c) motion of conductor w.r.t. magnetic field.
1.2 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.
(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 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).
(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 when the
loop is at position 7 and zero when at position 1. This cycle repeats with
each revolution of the coil.
Fig. (1.1) Fig. (1.2)
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.
1.3 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 commutator is mounted on but insulated from the rotor shaft.
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 lead current to the external load. With this arrangement, the
commutator at all times connects the coil side under S-pole to the +ve brush and
that under N-pole to the −ve brush.
(i) In Fig. (1.4), the coil sides AB and CD are under N-pole and S-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 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° rotation), the coil side AB is
under S-pole and the coil side 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° i.e., segment C1 is
now in contact with +ve brush and segment C2 in contact with −ve brush.
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.
Fig.(1.3) Fig.(1.4) Fig.(1.5)
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 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 to the
external stationary load.
, The variation of voltage across the brushes
with the angular displacement of the loop
will be as shown 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 Fig. (1.6)
by a single loop is not suitable for many
commercial uses. What we require is the steady direct voltage. This can be
achieved by using a large number of coils connected in series. The resulting
arrangement is known as armature winding.
1.4 Construction of d.c. Generator
The d.c. generators and d.c. motors have the same general construction. In fact,
when the machine is being assembled, the workmen usually do not know
whether it is a d.c. generator or motor. 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 [See Fig. 1.7].
Fig. (1.7) Fig. (1.8)
(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
D.C. Generators
Introduction
Although a far greater percentage of the electrical machines in service are a.c.
machines, the d.c. machines are of considerable industrial importance. The
principal advantage of the d.c. machine, particularly the d.c. motor, is that it
provides a fine control of speed. Such an advantage is not claimed by any a.c.
motor. However, d.c. generators are not as common as they used to be, because
direct current, when required, is mainly obtained from an a.c. supply by the use
of rectifiers. Nevertheless, an understanding of d.c. generator is important
because it represents a logical introduction to the behaviour of d.c. motors.
Indeed many d.c. motors in industry actually operate as d.c. generators for a
brief period. In this chapter, we shall deal with various aspects of d.c.
generators.
1.1 Generator Principle
An electric generator is a machine that converts mechanical energy into
electrical energy. An electric generator is based on the principle 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. Therefore, the essential
components of a generator are:
(a) a magnetic field
(b) conductor or a group of conductors
(c) motion of conductor w.r.t. magnetic field.
1.2 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.
(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 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).
(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 when the
loop is at position 7 and zero when at position 1. This cycle repeats with
each revolution of the coil.
Fig. (1.1) Fig. (1.2)
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.
1.3 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 commutator is mounted on but insulated from the rotor shaft.
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 lead current to the external load. With this arrangement, the
commutator at all times connects the coil side under S-pole to the +ve brush and
that under N-pole to the −ve brush.
(i) In Fig. (1.4), the coil sides AB and CD are under N-pole and S-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 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° rotation), the coil side AB is
under S-pole and the coil side 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° i.e., segment C1 is
now in contact with +ve brush and segment C2 in contact with −ve brush.
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.
Fig.(1.3) Fig.(1.4) Fig.(1.5)
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 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 to the
external stationary load.
, The variation of voltage across the brushes
with the angular displacement of the loop
will be as shown 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 Fig. (1.6)
by a single loop is not suitable for many
commercial uses. What we require is the steady direct voltage. This can be
achieved by using a large number of coils connected in series. The resulting
arrangement is known as armature winding.
1.4 Construction of d.c. Generator
The d.c. generators and d.c. motors have the same general construction. In fact,
when the machine is being assembled, the workmen usually do not know
whether it is a d.c. generator or motor. 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 [See Fig. 1.7].
Fig. (1.7) Fig. (1.8)
(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
