Figure 4.20: The single coil and commutator (θ=90 ◦ )
ThetorquerippleintheDCmotorcanbereducedsigni fi cantly by increas-
ing the number of coils. For the coil shown in Figure 4.19, if three coils AX,
BY and CZ are used and if they are symmetrically distributed around the sta-
tor with intervals of 120 ◦ , then the torque produced by each coil is the same
as that shown in Figure 4.23 but shifted from one another by 120 ◦ .There-
sultant torque is shown in Figure 4.24. Comparing the fi gures of Figure 4.19
and Figure 4.24, it is easily understood that use of multiple coils with proper
arrangement can reduce the torque ripple.
Let us de fi ne a ratio r tam as
Maximum Torque .
r tam =
For a multi-coil system equipped with commutation device, if the number of
coil M is odd, then it can be proved that,
r tam = 2M
π sin( π
And if M is even, then
r tam = M
π sin( π
Therefore, when the number of coil, M, is increased to a certain level, r tam
tends to a value close to 1, i.e., the generated torque can be considered constant
if the input current is constant.