Running Induction Motors on frequencies other than their Design Frequency.

Induction motors use an iron core and require flux in the iron to operate. In order to achieve the commercial goals of smallest size and lowest price at best efficiency, induction motors are designed to operate at a high level of flux in the iron. The flux is determined by the turns, voltage and frequency. In a modern motor, if the flux is increased by a small amount, the iron losses increase and the iron tends towards saturation. At saturation, the inductance begins to fall and the current increases further. To reduce the flux at a given voltage and frequency, the turns on the stator are increased. This reduces the Iron loss, but a longer length of thinner wire is used and the copper loss increases. Design becomes a balancing act between copper loss and iron loss and so the design is optimised for a given voltage and frequency.

If the voltage applied to the motor is held constant and the frequency is increased, the inductive reactance increases and so the flux reduces. This effectively reduces the maximum torque capacity of the motor and so the motor power rating at the higher frequency remains the same.

If the voltage applied to the motor is held constant and the frequency is reduced, the current will increase and in theory, the torque will also increase. The motor should be able to deliver the same power also, BUT the flux in the iron is now too high resulting in excessive iron loss, and the motor will fail prematurely. Above a very low frequency, (5 - 10Hz) the impedance of the magentising circuit of the motor is primarily inductive and so in order to keep the flux within limits, it is important to keep a linear V/F ratio (Voltage to Frequency ratio). If the frequency is reduced by 10%, the voltage must also be reduced by 10%. Because the flux in the iron remains the same, the torque capacity remains the same and so the power rating of the motor also drops by 10%.

60Hz rated motor on 50Hz

Provided the voltage is dropped by the same proportion as the frequency, it is OK to run a 60Hz motor on 50Hz. The speed will be reduced by the reduction in frequency and the power capacity will also reduce by the ratio of the reduction in frequency.

60 Hz	50 Hz
Line Voltage	Line Voltage
480	400
460	383
440	367
230	191

50Hz rated motor on 60Hz

Provided the voltage is increased by the same proportion as the frequency, it is OK to run a 50Hz motor on 60Hz. The speed will be increased by the increase in frequency and the power capacity will also increase by the ratio of the increase in frequency.

50 Hz	60 Hz
Line Voltage	Line Voltage
415	498
400	480
380	456
230	276

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