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I know that the maximum rated torque of a motor is limited to the rated speed of the motor. Above nominal speed, particularly when a VSD is used, the iron core saturates to a point that the maximum torque starts decreasing. I would like to determine the point of saturation. Any document or link would be appreciated.



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Hello Bob


The torque limitation in an induction motor is due to the flux limit.

There is a maximum amount of flux that can be concentrated in the iron. If you increase beyond this, the inductance of the winding reduces and the current increases etc and that is the point of saturation.


When using a motor with a VFD, the objective is to keep the flux at the normal design flux of the motor.

If we consider the equivilent circuit of the motor, we find that the magnetising current is essentially determined by the line voltage across a shunt inductive element, so the magnetizing current is determined by the voltage across the reactance of this element. i = v/z, and the reactance of this element is i/2 x pi x f x L

Hence the magnetizing current is inversly proprtional to f the frequency.

With a VFD, as we reduce the frequency below line frequency, we must also reduce the voltage to keep the V/Hz constant.


As we increase the frequency above line frequency, we must keep the V/Hz constant in order to retain the same flux, however our maximum voltage is limited to the line voltage and if the motor has the same voltage rating as the line voltage, we are not able to raise the motor terminal voltage above line frequency and so as the frequency is increased, the flux reduces, so we get a reducing torque.


Best regards,


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Thank you very much for your professional reply. I understand the theory behind. It`s true that the shaft power is directly proportional to the product of speed and torque. It`s also true that because of saturation of the core, this does not hold true.



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