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

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

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DC Motors
In variable speed motor

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

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