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Why dummy coils are connected in DC machine?

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Why dummy coils are connected in DC machine?

(a) To reduce eddy current losses

(b) To enhance flux density

(c) To amplify voltage

(d) To provide mechanical balance for the rotor

This question was addressed to me at a job interview.

My enquiry is from Ways to Reduce Effects of Armature Reaction in portion Armature Reaction of DC Machines

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Right choice is (d) To provide mechanical balance for the rotor

Explanation: Dummy coils are connected in armature winding of type wave winding. In a lap winding Yc= +/-1 irrespective of the number of armature coils so that coils can always be chosen to completely fill all the slots (C =1/2 US). In a wave winding the number of coils must fulfil the condition C =P/2 Yc +- 1 while at the same time C must also be governed by C =1/2 US.

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