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Why brushes shifting method is not employed in practical commutation?

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Why brushes shifting method is not employed in practical commutation?

(a) Expensive

(b) Construction problems

(c) Causes demagnetization

(d) Used practically

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The above asked question is from Commutation Process -1 topic in portion Commutation Process & Excitation Methods of DC Machines

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The correct choice is (c) Causes demagnetization

To explain: Brushes are located at GNA’s, a small voltage is induced in the commutating coil. It opposes current commutation as the commutating coil is cutting the flux which has the same sign as that of the pole being left behind. It could be partially remedied by shifting the brushes towards MNA but that causes direct demagnetization and is therefore not employed in practice.

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