Q:

DC motor rpm slower than the rated rpm

A seperately excited shunt dc motor of 336 KW with rated rpm from 500-700, when it is operated on 420 rpm what will be the effect of current on it?

A:

If the armature voltage is lowered with the field held constant the current will be proportional to load torque. If the load requires the same torque at the lower speed, the current will not change. If the load requires less torque at the lower speed, the torque will be less. If the motor is self cooled, the temperature will increase. That will also have some influence on the speed and current at the lower voltage.
There is no general rule defining the lowest speed that the motor can operate with no danger to the motor. The motor should have a maximum insulation temperature rating marked on the rating plate. Operation above that temperature will shorten the life of the motor. Unless the motor is cooled by a separately powered blower, it depends on the speed of the rotor to cool itself. Heat dissipated in the motor is mostly determined by armature current. Armature current is mostly determined by load torque.
Motor manufacturers can provide speed vs. torque curves, curling curves and other information that can be used to predict the insulation temperature under various operating conditions. Without additional information from the manufacturer, it is very difficult to predict what operation is safe for the motor.

