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What will happen if the series field of a cumulative compound motor is short circuited?

(a) Net flux will increase

(b) Speed will increase

(c) Speed will remain constant

(d) Speed will decrease

The question was asked during an online exam.

Question is from Operating Characteristics of DC Compound Motor in portion Characteristics of DC Motors of DC Machines

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The correct option is (b) Speed will increase

The explanation is: As the series field is short circuited now, current through series field becomes zero and MMF due to series field becomes zero. Hence, total MMF in motor becomes less than previous case, which results in total decrease in the flux created and increase in speed.

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