DC Motors
The mechanical power developed in a dc motor is maximum when back emf is equal to _____ the applied voltage.

One-third
Half
None of the listed here
Twice

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DC Motors
If a D.C. motor designed for 40°C ambient temperature is to be used for 50°C ambient temperature, then the motor

Is to be de-rated by a factor recommended by manufacturer and select the next higher H.P. motor
Of higher H.P. should be selected
Can be used for 50°C ambient temperature also
Of lower H.P. should be selected

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DC Motors
In variable speed motor

Same commutating field is needed at low speed than at high speed
A stronger commutating field is needed at low speed than at high speed
None of these
A weaker commutating field is needed at low speed than at high speed

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