I´m conducting some tests with a VSD in a induction motor. I read somewhere that by maintaining the Voltage/Frequecy constant, we could obtain the rated torque no mather the speed of the motor. Being sure that V/F remais constant, I notice that this is not true, because at low speed (under 15 Hz) I could stop the shaft with my hands . Is this a special behavior for the motor at very low speed, a misinterpretation of the V/F "law" or am I missing something important here?

Thank you

Hello pedromega

In order to achieve rated torque from the motor at all frequencies, you must ensure that there is the correct flux in the iron. If you have too much flux, you will have an increased iron loss, but too little and you will not get full torque output.

If we assume that there is no resistive component in the motor circuit, we would have a pure inductor and the flux would remain constant at all frequencies provided that we keep the v/f ratio constant. (the reactance of the inductor is directly proportional to the inductance and to the frequency.)

In practice, there is a resistive component that is constant. At line frequency, the reactance is much larger than the resistive component and so it cna be ignored, however as we reduct the frequency, the reactance reduces and the resistive component begins to become significant. At very low frequencies, the magnetising current is limited by the resistance rather than the inductive reactance. This reduces the current and therefore the flux. At low frequencies, you need to boost the voltage to overcome the resitive component.
Best regards,