The size and rating of an electric motor are primarily determined by its maximum temperature rise under specific load conditions and the maximum torque required. These factors help engineers choose the right motor for the application to ensure efficiency and durability.
Types of Motor Ratings
Electrical motors are classified based on their operating conditions and load handling capabilities. Below are the common motor rating types:
Continuous Rating
- A motor with a continuous rating can deliver a specified load indefinitely without the temperature exceeding permissible limits. It can also handle a 25% overload for up to two hours under the same thermal conditions.
- Example: A motor rated at 100 kW continuous can run indefinitely at 100 kW and up to 125 kW for two hours without exceeding its temperature limit.
Continuous Maximum Rating
- This refers to the maximum load a motor can carry continuously without the provision for overload. It is used where overload conditions are not expected.
Intermittent Rating
- In intermittent rating, the motor is designed to deliver a specified output for a short, fixed duration—such as 1 hour, 30 minutes, or 15 minutes—without exceeding the allowable temperature rise.
Estimation of Motor Rating
- From the above discussion, it is evident that the most critical limitation in motor operation is the maximum permissible temperature rise. Therefore, motor ratings are calculated based on average temperature rise.
- This average temperature rise is dependent on the heating effect, which is proportional to the square of the current and the duration of the load.
- For instance, if a motor carries different loads (L1, L2, etc.) for different durations (t1, t2, etc.), the average heating is given by the equation below:
