Posted by Mike Palmer 1 on 01/09/2019 12:15:15:
I would like advice on controlling the spindle which is powered by a 20V DC brushless motor. Is it possible to use a VFD and change the output via a rectifier to DC?
The short answer is no.
A DC motor with brushes runs on (you've guessed it) DC. Motor speed is proportional to the applied DC voltage.
A brushless DC motor requires switched (AC) voltages across the colis. Essentially the switched waveforms replace the mechanical commutator with electronic switches. Motor speed is directly related to the switching rate, unlike an induction motor where the speed is load dependent to a small degree. Brushless DC motors can be run without position sensors, but the simpler, and hence cheaper, method is to use Hall effect devices on the rotor to determine position.
A VFD outputs a PWM signal where the width of each pulse changes from pulse to pulse. The output emulates a sine wave. The induction motor speed is set by the frequency of the sine wave less a few percent depending upon load. Passing the VFD output through a diode would result in a waveform similar to the input – definitely not DC.
The longer answer is that you need to use a controller designed for the type of motor you want to drive. To do otherwise will result in failure and possibly the release of the magic smoke.
Andrew
Edited By Andrew Johnston on 01/09/2019 21:34:11