VFD's and AC motors in Tools and Tool Making Posted April 22, 2020 · Edited April 22, 2020 by Dane Lance Ah, timgunn, thank you! That puts it all in perfect perspective. I was ignoring the voltage variance by the VFD vs amps in the sub-60Hz range. I understood it concerning the base+ speeds. The trade off between the two different motors then, is that the 4-pole will have more torque in the sub-60Hz up to base speed, but lose in the torque department to the 2-pole beyond 75Hz. The 4-pole does provide a broader speed range, 900RPM to 4500RPM (30Hz to 135Hz), theoretically that is, if balance is the only limiting factor and what I read about manufacturer balancing holds true. The 2-pole speed range would only manage 1800PM to 4500RPM (30Hz to 75Hz). Additionally, the 4-pole has less torque in the upper range, but more in the lower range. Almost seems opposite of what I really want. If frequency were theoretically unlimited in a motor, then at some point beyond base speed, the motor will not be able to increase RPM once torque falls to that necessary to simply keep the motor (or the entire device it is attached to) turning as long as amperage remains constant and only frequency is increasing. I get torque and RPM, at least as it applies to HP in reciprocating engines, things get a bit funkier with electric motors and even more so with a VFD. Will the AC motor operate the same on the VFD as far as back EMF and sync? What I mean is, with no load, back EMF acts like an amperage throttle, so no load is less amp draw and synchronous, or near synchronous RPM. Add a load, RPM drops, back EMF decreases, which allows more amps, motor makes more torque to try to get back to rated RPM. This is more like a constant speed mode. This is why it's often said you can replace a gas engine of a given HP with an electric motor of lesser rated HP. In practice, however, you can get away with it only for intermittent use. Continuous duty cycle needs a 1 to 1 swap. Or, do 3-phase motors act differently than this? Or better yet, just find out what everyone else uses and steer in that direction. This is making my brain squirm too much this morning.