Question 419Electricity & Electronics - Assistant Engineer

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From the instant of start-up, through the acceleration period, and until the motor reaches rated speed, when is the counter EMF produced in the windings of a DC motor 'zero'?

Amotor is at rated speed
Barmature is not yet turning
Carmature has just begun to turn
Dmotor is almost up to rated speed
AI Explanation

The correct answer is B) armature is not yet turning. When a DC motor is first started up, the armature is not yet turning. Without any motion of the armature, there is no counter EMF being produced in the windings. It is only once the armature begins to turn that a counter EMF is generated, which opposes the applied voltage and limits the current flowing through the motor. The other options are incorrect because: A) The counter EMF is not zero when the motor is at rated speed. C) The counter EMF is not zero as soon as the armature begins to turn. D) The counter EMF is not zero when the motor is almost up to rated speed.

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Official Resources
46 CFR Electrical Engineering 46 CFR Machinery & Electrical USCG National Maritime Center
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