Page 606 - Basic Electrical Engineering

two-pole construction having only one coil on the armature. The two
terminals of the armature coil are to be brought out for connection to the load
circuit. For this, two arrangements are possible, i.e., (i) through brush and

slip-ring arrangement, and (ii) through brush and commutator arrangement.
We will consider these two arrangements side by side and see the nature of

output voltage and load current.

7.1.1 Nature of Load Current When Output is Taken Out Through Brush and

Slip-ring Arrangement

In brush and slip-ring arrangement the coil terminals are connected to two
conducting slip rings which are fitted on one side of the rotating shaft. The

carbon brushes are placed on the slip rings and are fixed. Connections from
the brushes are taken to the load. When the shaft is rotated by a prime mover,

the coil rotates. The slip rings are nothing but extensions of the coil-end
connections. The slip rings also rotate as the coil rotates. The brushes sit on

the slip rings and make sliping contact with the coil and are able to connect
the EMF generated to the load for supply of current.
In Fig. 7.2 (a), the coilside a of coil a–a′ is under North pole and the

coilside a′ is under South pole. After half revolution the positions of the
coilsides change as shown in Fig. 7.2 (b). The direction of EMF induced in

the coilsides have been shown. The EMF induced will cause current to flow
through the load resistance as shown. It is noted that the direction of current
through the load resistance has changed after half revolution of the coil.

In Fig. 7.2 (a), current flows from a to a′ and after half revolution of the
coil, current in the coil flows from a′ to a. After every half revolution, current

in the coil will get reversed.
When the armature is rotated continuously by the prime mover, the EMF

induced in the coil is alternating in nature as can be seen from Fig. 7.2 (a) and
(b). The direction of EMF induced in the coil sides a and a′ has been

determined by applying Fleming’s Right-Hand rule. We can conclude that
when a coil is rotating in a magnetic field, an alternating EMF is induced in it