Page 627 - Basic Electrical Engineering

the figure that as the armature rotates in the anticlockwise direction, current
in the conductors of a coil passing under the brushes would change. This will
ensure that at any point of time any conductor on the upper half of the

armature will have cross current and on the lower half will have dot current.
There will be continuous rotation of the armature in one direction. The

reversal of current in the coils passing under the brush is called commutation.

7.7.2 Changing the Direction of Rotation

The direction of rotation of the armature will change if the polarities of

supply to either the armature or to the field windings are changed. If the
polarities of supply to both the field and armature are changed, the direction

of rotation will remain unchanged. This has been shown diagramatically as in
Fig. 7.17.

Thus, to change the direction of rotation, we have either to change the
polarities of supply to the armature or to the field.

7.7.3 Energy Conversion Equation

When the armature of the dc motor starts rotating because of the interaction
between the field flux and the current-carrying armature conductors, EMF
will be induced in the armature as the conductors are cutting the field flux.

This EMF and the EMF induced in the armature when the machine is
working as a generator is the same. However, now this EMF will oppose the

supply voltage, V. Since the EMF induced opposes the supply voltage, it is
also known as back EMF, E such that
b