The effects of changing the supply frequency of an induction motor.

 1. Rotational speed is a direct function of frequency.

2. Cooling fan speed is a direct function of rotational speed.

3. Back emf decreases as the motor slows down.

4. Current increases with reduced back emf.

5. The magnetic capacity of the motor's magnetic circuit is designed to the relationship: Voltage/frequency ( V/f ).

            If the frequency drops, then the V/f ratio goes up. This means that the magnetic flux increases and the motor needs a larger magnetic circuit. Without it, the magnetic circuit can become overloaded. This is called saturation and it leads to a rapid increase in current draw and a corresponding large increase in temperature.

            If the frequency increases, the V/f ratio drops with no issues as the magnetic circuit will remain plenty large enough for the magnetic flux. But the motor may have a worse power factor.