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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