DC Motors
The speed of a dc motor can be controlled by changing

All of the listed here
Aplied voltage
Is flux
Armature circuit resistance

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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

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
Of lower H.P. should be selected

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