Motor rating is defined, as the output of a motor under prescribed working conditions, with a temperature rise below specified limits.
Motors suffer various losses like core loss, stator loss, rotor loss, winding loss, and friction loss. All these losses result in the production of heat.
As the load on the motor increases, the heat generated also increases. To maintain a healthy state of motor, the heat generated has to equal the heat dissipated.
There are as such two major types of motor ratings:
Continuous duty motors, are motors, which are meant to give the continuous rated load specified on the nameplate. Such motors can be operated at these load continuously, without causing overheating.
Intermittent duty motors are meant for taking loads above the maximum continuous rating, for a short duration, such as for one hour or so. This allows the motor, to get enough time for dissipating the heat generated, in the time intervals when the motor is not running.