The size and rating of a motor for any task are determined by its maximum temperature rise under given load conditions and the maximum torque required.
Different ratings for electrical motors are as under:
Continuous Rating-
It is based on the maximum load in which a motor can deliver for an indefinite period without its temperature exceeding the specified limits and also possessing the ability to take 25% overload for a period of time not exceeding two hours under the same conditions.
For example, if a motor is rated continuous 100 KW, it means the motor has capable for giving an output of 100 kW continuously for an indefinite period of time and 125 KW for a period of two hours without its temperature exceeding the specified limits.
Continuous Maximum Rating-
It is the load capacity as given above but without overload capacity.
Intermittent Rating-
It is based on the output which a motor can deliver for a specified period; say one hour or the ½ hour or the ¼ hour without exceeding the temperature rise.
Estimation of Motor Rating-
From above discussion, it is clear that the primary limitation for the operation of the electrical motor is maximum permissible temperature rise. Hence motor rating is calculated on the basis of its average temperature rise.
The average temperature rise depends on the average heating which is proportional to the square of the current and the time of duty under load condition.
For example, if a motor carries a load L1 for time t1 and load L2 for time t2 and so on, the average heating is given by
But we know that the heat is proportional to the square of current so the size of the motor is given as-
