As you know, when the armature of an electric motor rotates, an e.m.f. of induction opposed to the e.m.f. of the source is generated in the wires of the armature. This back e.m.f. increases with the number of revolutions the armature makes per second. The greater the back e.m.f., the weaker is the strength of the current in the armature circuit. If the motor is idling, the armature rotates faster, and when the motor is doing work, the armature rotates more slowly. Therefore, if the motor is loaded a stronger current passes through the armature circuit and therefore a loaded motor heats up more quickly than one that is idling.