Why does a PM AC motor need a VFD?

The brief observations that follow will assume that you have a 3 phase permanent magnet (PM) AC motor. In a PM AC motor, max continuous torque is achieved when the stator MMF (or current) vector is 90deg displaced from the rotor's flux vector. Recall the Torque = Rotor_Flux x Stator_MMF ... i.e. the torque is the vector cross product of the vector Rotor_Flux with the vector Stator_MMF, or Torque = |Rotor_Flux| * |Stator_MMF| * sin(theta), where theta is the angle between the two vectors. Torque is a maximum (and the typical desired operating condition) when the two vectors are displaced by 90deg. And torque is zero when the angle is zero.
When the PM AC motor is powered by a variable frequency drive (VFD), the PM AC motor sends its rotor position feedback to the variable frequency drive so that the variable frequency drive's microprocessor can ensure that the current (MMF) vector injected into the PMAC stator windings is always 90deg displaced from rotor's flux vector.

When the PM AC motor is connected to the grid, the grid has no knowledge of the rotor flux position, and it cannot control the stator current from the grid to always be 90deg displaced from the rotor flux. Over the course of a line cycle, the average displacement angle is zero, so the average vector cross product will be zero. With zero average torque to accelerate the motor, the motor will remain at standstill and will just vibrate.

The variable frequency drive is more like a servo amplifier and absolutely needs speed and position feedback to operate properly with maximum torque available. The PM AC motor has "Rare Earth" permanent magnets of the rotor. There are no windings on the rotor. With the permanent magnets on the rotor the motor is a synchronous. Example 4 pole PM AC motor at 60 Hz the speed fully loaded would be 1800 rpm. The PMAC motor has very low inertia and can change speed nearly instantaneously.

The standard induction motor is asynchronous and has windings on the rotor and does not develop full torque until it reaches the slip rpm. Example 4 pole induction motor at 60 Hz. with no load the rpm will be 1800 rpm, when fully loaded the rpm will be less like 1760 rpm (dependent on motor design). The induction motor has relatively high inertia therefore a change in speed is very slow compared to the PMAC motor.

Bottom line the PMAC motor is designed to operate with a variable frequency drive/Servo amplifier and cannot operate across the line. If line voltage is applied the motor could be damaged.

Buy a VFD for your motor now, 5 hp, 10 hp ...

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