Figure 4.22: Current in a single coil when commutation is used.
Figure 4.23: The torque waveform of a single coil with mechanical commutation
If the exciting fi eld is constant, the torque of each coil is proportional to
the input current at a given position implying that the global torque generated
by the multi coils is also proportional to the input current as it is the sum of
torque produced by all the coils. Therefore, if the number of coils is big, then
the relationship between the global torque and input current can be described
by the following equation,
T em = K t I.
where K t is a constant, and generally called as the Torque Constant.This
constant is determined by the motor structure, number of coils, coil turns and
the magnetic fi eld produced by the fi eld exciting system. In a DC motor, these
coils can be connected together to form electrical paths between the brushes
connected to positive and negative pole of power supply . Figure 4.25 shows
the connection of three coils which forms one loop of windings.