Question

If a DC motor designed for 45°C ambient temperature is to be used for 55°C ambient temperature, then the motor ___________

(a) Of lower HP should be selected

(b) Of higher HP should be selected

(c) Can be used for 50°C ambient temperature also

(d) Is to be de-rated by a factor recommended by manufacturer and select the next higher H.P. motor

The question was asked in an interview for internship.

My question is from DC Motor Basics -2 in division DC Motors of DC Machines

Answer 1

Right choice is (d) Is to be de-rated by a factor recommended by manufacturer and select the next higher H.P. motor

The explanation: The ambient temperature is one of a factor in derating.  If the ambient temperature given is high, the insulation may reach its maximum temperature limit quickly.  Hence, the motor may have to be de-rated, and for application next motor must be used.