The efficiency of an electric motor decreases as the load on the motor decreases. Therefore, a motor which is oversized requires more input power to drive a load than a smaller, correctly sized motor.1
The effect is more dramatic with older, standard efficiency motors which have lower part-load efficiency. Most motors operating in Australia are older, standard efficiency motors.
The actual motor power required should be calculated based on the service which is being provided by the equipment driven by the motor.
Understanding the required load will avoid replacing like-with-like, when the original motor may have been oversized. The original motor may be the incorrect size because it:
- may have been selected for a different load
- may have been a forced choice because the correctly sized motor was not available
- was reused from another application
- was selected very conservatively expecting that the load may grow in the future.
Some loads need to run at a constant speed, but some loads may vary, e.g. conveyors in mines, metal stamping presses and escalators. In these instances, one possible approach is to use a load sensing optimiser. This is a motor controller which can sense the load on the system and automatically adjust the power supply (current and voltage) to the motor to maintain speed with the minimum energy required.
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