... Their reasoning was that running at that slow of a speed would cause the motor to overheat, and it would fry the motor and the electronics. When I told them that I had sanded out thousands of bowls at those speeds with no problems, they told me that it wasn't possible. They said their tests showed the motor ran hotter as it was still drawing 240 volts. Hmm, it runs on volts, but draws amps..... CYA = cover your a--.
Well, Reed you are right and Powermatic Tech support is either not telling the truth or they aren't too bright (I suspect the latter).
The voltage to the motor actually is NOT 240 VAC at slow speed. Below base speed (1750 RPM), the voltage decreases with speed -- not to control the motor speed, mind you, but to prevent the iron core of the field magnets from reaching magnetic saturation. Saturating the core does nothing, but waste energy in the form of heat. As mentioned in another post, if the motor were stalled or bogged down for a long period, it would definitely overheat because it could still draw a pretty hefty current and at very slow speeds the cowled fan on the back of the motor does not provide any meaningful cooling.
In the real world, turners generally do not apply much, if any, load when running that slow.
There was also some mention of the speed not being smooth at very slow speeds. That is a normal condition for that type of VFD system because there is not any true speed feedback to the control electronics. The VFD is known as a "sensorless vector" type which just means that it tries to make a logical guess at the motor speed based on the applied frequency, the motor current, and motor parameters that were entered during the initial set up procedure. This scheme works reasonably well once the speed is above about 200 RPM, but starts deteriorating below that speed. One of the primary reasons why is that motor current is not a nice clean signal -- it is actually as noisy as heck which means that the controller has to do some pretty heavy filtering to get a reasonable guess about the current. Filtering adds a time lag to the measurement and so it is not too hard to imagine trying to control the motor torque for the present need based on what the conditions were at some time in the past. At high speeds, the time lag is just a tiny fraction of a second, but at extremely slow speeds, the time lag can stretch into several seconds.
Once the motor gets down to about 50 RPM, that is about the minimum for smooth, but not especially accurate speed control. At that speed the driving frequency to the motor is dropping to less than 1 Hz. This is where time lags can become several seconds long and the motor is responding with an applied torque now based on a poor estimate of what the load was several seconds ago. The end result is surging which can quickly cause the motor speed to go "divergent" (as in, "going wonky") and have you diving for cover.
Some controllers are much better than others. The Toshiba controller on my Robust is amazingly good. The Delta controller used on the Powermatic and other lathes is sort of OK and some others aren't worth a hoot at slow speeds. Generally speaking, price is a pretty good indicator of quality.
Some cheaper lathes use V/Hz controllers which means that they do not attempt to sense the motor speed, but just simply output a fixed frequency and if the torque load changes then so does the speed (as in, it slows down).
There is also a real honest-to-goodness "vector" controller that requires the use of a speed or position feedback device mounted on the motor that can measure the speed with extremely high precision (like sensing motor shaft position to less than one-tenth of a degree). The feedback device is typically a 4096 line optical encoder. I have several of these type controllers and motors designed for this application. They can actually run all the way down to zero speed and still maintain full torque and smooth output from the motor. At zero speed the motor shafts acts like a stiff spring.
