03-06-2013, 04:54 PM
D.C. Generators
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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.
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
© motion of conductor w.r.t. 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.
(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.
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.
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].
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 (1.9). The laminations
(See Fig. 1.10) are individually coated with a thin insulating film so that they do
not come in electrical contact with each other. The purpose of laminating the
core is to reduce the eddy current loss. The laminations are slotted to
accommodate and provide mechanical security to the armature winding and to
give shorter air gap for the flux to cross between the pole face and the armature
“teeth”.
Commutator
A commutator is a mechanical rectifier which converts the alternating voltage
generated in the armature winding into direct voltage across the brushes. The
commutator is made of copper segments insulated from each other by mica
sheets and mounted on the shaft of the machine (See Fig 1.11). The armature
conductors are soldered to the commutator segments in a suitable manner to give
rise to the armature winding. 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.