Dummy Coils

The dummy coils (also called as Idle Coils) are used in some of the electrical machines wounded with wave winding, to have mechanical balance.

When a machine is wound with simplex wave winding the following equation must be satisfied.

Y_c=\[\frac{C±1}{P/2}\]

Where,

Y_c=Commutator Pitch
C=Number of coils or Number of commutator segments.
P=Number of Poles

If the above equation is not satisfied, then two ends of one or more coils are cut-off and insulated, leaving them unconnected to the commutator. These coils are unused and hence called as dummy coils. They are merely to provide symmetrical armature winding. It is suggested to avoid the dummy coils as far as possible.