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Inductors in Circuits

If the load c...

Question

If the load current and flux of a DC motor are held constant and voltage applied across its armature is increased by 5%, the speed of motor will

A

reduce by 5%

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B

increase by 5%

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C

depends on other factor

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D

remain unchanged

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Solution

The correct option is A increase by 5%
If the load current is constant then the armature current is also constant. Therefore back emf of DC motor is proportioned to the terminal voltage. So, 5 % increase of terminal voltage means that increase in 5 % of back emf. The speed of DC motor is directly proportional to back emf and inversely proportional to flux per pole. Since the flux is also constant, the speed is now only depends on the back emf and the speed is directly proportional to the back emf. Hence speed is also increased by 5 % with the increase 5 % of terminal voltage.

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