DC Motors
If a D.C. shunt motor is working at full load and if shunt field circuit suddenly opens

Motor will come to stop
This will make armature to take heavy current, possibly burning it
This will result in excessive speed, possibly destroying armature due to excessive centrifugal stresses
Nothing will happen to motor

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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 lower H.P. should be selected
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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