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What will happen, with the increase in speed of a DC motor?

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What will happen, with the increase in speed of a DC motor?

(a) Back emf increase but line current falls.

(b) Back emf falls and line current increase.

(c) Both back emf as well as line current increase.

(d) Both back emf as well as line current fall.

I had been asked this question in class test.

Asked question is from DC Motor Basics -1 topic in chapter DC Motors of DC Machines

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Right option is (a) Back emf increase but line current falls.

The explanation: In case of DC motor, the speed is proportional to the back emf (Ea ∝ N). So, with the increase in speed, the back emf also increases. Therefore, armature current is also decreased, in case of series motor, armature current is equal to the line or load current.

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