Originally Posted by dannronn
A single phase motor has a mechanical switch inside that cuts the start winding and capacitor ( if it is a capacitor start motor) out at approx. 80% of full load speed. If you use a VFD on this type of motor at low speeds the start winding and capacitor will be energized and burn up after time. But you can use a VFD on a permanent split capacitor motor as they are designed so both start and run winding are energized all the time and the capacitor gives enough phase shift to start and run. VFD's were designed to vary the speed on 3 phase motors with frequency modulation and constant voltage therefore giving constant torque. You can also buy a single phase VFD that will give you a 3 phase output which comes in handy if you have a 3 phase motor but only single phase power. DC motors can vary speed by a variable resistance in series with the shunt field (called field weakening) if it is a shunt motor. On the other hand a permanent magnet DC motor can vary speed only by varying the voltage with some kind of SCR drive, or a high wattage variable resistor ( not recommended , lots of heat).
I wont disagree about capacitive start motors, but not all single phase motors have capacitor start, which is centrifugal. I've been a licensed electrician for over 35 yrs, and I've installed enough frequency drives on single phase, 3 ph. and DC motors to know what can or can't be done.
AB does make frequency controllers for capacitive start motors also that work, and wont burn up these motors. Not sure how many others also make them. But thesee are not controllers that the average hobbyist would spend the money on, as there are cheaper options, like buying a drill press with pulleys to change speed.
No offense, but I don't need a class on how single ph, 3 ph., and DC motors operate.