Page 609 - Basic Electrical Engineering

The direction of the induced EMF in the coil sides is determined by
applying Fleming’s Right-Hand rule. When the coil is connected to the load,
current will flow through the load as has been shown. Current from coil side

a′ flows to the load through commutator segment C′ and brush B′. From the
load, the current returns to the coil through brush B, commutator segment C

and then to the coil side a. No current can flow from commutator segment C
to C′ or from C′ to C as there exits a layer of insulation between them. With

such an arrangement when the coil rotates in the magnetic field, a
unidirectional current will flow through the load as has been shown in Fig.

7.4 (a) and (b). In Fig. 7.4 (a), current in the coil flows from coil side a to coil
side a′. After half revolution the direction of current is changed from a′ to a.
In every half revolution of the coil, this change in direction of current will

take place. The current through the load resistance, however, will be
unidirectional because the connections from the armature coil to the load

have been taken through brush and commutator.
It is observed from the output current wave shape that we are getting a

fluctuating dc and not a constant dc. In actual practice, in a dc generator,
instead of using a single coil a large number of coils are placed on the

armature slots so as to generate a considerable amount of voltage.
Consequently, a large number of thin commutator segments are used to make
the commutator assembly. The sum of the EMFs induced in the armature

coils when connected to the load through the brush and commutator
arrangement will be a steady dc current flowing through the load. The brush

commutator assembly in a dc generator, in fact, works like a full-wave diode
rectifier.