fi eld (see Figure 4.18 and Figure 4.19). For a rotating coil, maximum torque
occurs at the positions where the coil axis is vertical to the axis of the exciting
magnetic fi eld; the fl ux encircled by the coil is zero at these positions. On the
other hand, the torque is zero at the position where the coil axis is aligned
with or opposite to the axis of the exciting magnetic fi eld, and the fl ux going
through the coil (linked with coil) is maximum at these positions.
Figure 4.19: Torque acting on single coil as a function of the coil position, θ.
coil, the average torque of the coil over 360 ◦ is zero. That means the coil cannot
sustain continuous rotation if the current is not changed in the coil. To make
the coil rotate continuously, the direction of the current must be changed, or
commutated, according to the position of the coil such that the average torque
becomes positive in the direction of coil rotation. The commutation can be
realized by the mechanical means, that is, using a commutator and brushes to
change the current at the right positions as shown in Figure 4.20. The brush
is made of conductive materials such as graphite. The commutation process
for different rotor positions is illustrated in Figure 4.21.
Using such a commutation system, even if the input current is kept uni-
directional, the current fl owing in the coil can be made to alter at the right
position, as illustrated in Figure 4.22. Correspondingly, the torque generated
is always positive as shown in Figure 4.23.
It can be deduced from Figure 4.23 that, with the commutation system in
use, the average torque is not zero and, therefore, the coil can rotate around
its shaft continuously. However, the torque produced contains rich torque
ripples. This mechanical commutation method, one with commutator and
brush, is used in DC motors where the input current from the power supply is
unidirectional but the current in the armature coils is alternating. The nature
of the input current de fi nes the nomenclature of DC motor.