How does a variable frequency drive (VFD) control a permanent magnet synchronous motor (PMSM) differently than an asynchronous induction motor?

Resolution:
A VFD can use a type of flux vector control where the motor torque producing components can be separated from the flux (or magnetizing) components. This allows the torque and the flux currents to be controlled separately.

For an asynchronous motor the flux is created by induction. The VFD will continuously control flux current at all speeds allowing for good torque control over the full speed range.

A PMSM has magnets to create flux so the flux current is almost zero up to the rated frequency. Therefore the VFD must manage the flux current differently for a PMSM as compared to an induction motor.

The permanent magnets generate a voltage proportional to the speed of the motor so a motor back EMF constant needs to be entered into the drives Synchronous Motor Menu. This is so the drive knows how much voltage to put out for a given speed. The EMF constant will typically be given on the motor nameplate. Units for the EMF constant must be observed, some conversion may be necessary.

Another difference is that an asynchronous motor needs slip so induction can occur to create the magnetic flux in the rotor, PMSM motors do not slip but follow the frequency exactly.

If this article does not answer your question, check out our FAQ database that has answers to more commonly asked questions:

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