Delta & Shop Fox grinders say "speed controlled induction motors", how?

February 19, 2021 5:59 AM

The product webpages for both of these grinders say they have speed controlled 120v single phase induction motors. (links inside) But the Shop Fox's parts diagram says it uses a potentiometer and the Delta's appears to be a potentiometer labelled "speed controller". (links inside) I have read that induction motors cannot be speed controlled by changing the voltage, how can these grinders be doing it?

1) Shop Fox grinder

The parts diagram for this one says it uses a potentiometer for speed control.

Home Depot sale sale page
https://www.homedepot.com/p/Shop-Fox-6-in-Variable-Speed-Grinder-with-Worklight-W1839/309503853

pdf manual with parts diagram on page 30
https://images.homedepot-static.com/catalog/pdfImages/75/751bcb85-184b-474b-b1dc-16625dc2bace.pdf


2) Delta grinder

This one just says "speed controller" in the parts diagram

Delta product page
https://deltamachinery.com/products/grinding-sharpening/23-197/

parts diagram
https://www.ereplacementparts.com/delta-23197-variable-speed-grinder-parts-c-3275_3621_418334.html

I guess my question is, what am I not understanding? Also, what is probable the reason both have a lower speed limit of 2000 rpms?

Thanks!

posted by atm to Technology (9 answers total)

Best answer: This is a pretty good guide to methods for speed control of induction motors. The parts list for the Delta grinder includes a 'PC board', which I'd assume is implementing the speed control.
posted by pipeski at 6:05 AM on February 19, 2021 [1 favorite]

Best answer: The controller could be doing pulse width modulation to "chop" into a higher input frequency. The upper limit on those sorts of power supplies is usually the quality and quantity of switching diodes and capacitors to discharge fast enough.
posted by nickggully at 6:44 AM on February 19, 2021

Best answer: Above comments are correct you can’t just throw a potentiometer (variable resistor) in line with an ac induction motor.

The potentiometer does get you a low-power control signal that triggers a higher power silicon based switching device. That device turns on and off rapidly (chops) the power going to the motor.

Through pulse width modulation that lets you control power to the motor, longer on pulse and shorter off time is more power. Shorter on pulse and longer off pulse means less power. less power equals slower speed.
posted by sol at 8:21 AM on February 19, 2021 [1 favorite]

Best answer: Yes, pulse-width modulation is the answer. This is also how many LEDs are 'dimmed,' too.

They're either on or they're off, but if you "blink" them just the right way you can dim them to the human eye.
posted by jquinby at 8:44 AM on February 19, 2021

You can't PWM the AC line to induction motors to change their speed, only their torque. (Well, this can indirectly change their speed a little since the slip will increase.) Induction motors can only readily be slowed by either 1. turning off / paralleling poles (cheap, distinct speed steps), 2. rectifying the incoming AC into DC and implementing a VFD to drive the thing (expensive, continuous speeds), 3. using a wound rotor connected through brushes, slip rings, etc., to A. a resistive load to effectively change the rotor back-EMF (mid cost, reduced speeds will drop noticeably under load) or B. fancy power electronics that can effectively reduce the rotor back EMF (expensive, though sometimes quite a lot less than a VFD).

The minimum speed could be an artifact of (1) because you only have so many stator poles or (3) minimum practical speed of the motor before it turns into an expensive heater.
posted by introp at 7:16 PM on February 19, 2021

Response by poster: Introp, so your assessment is that the speeds are being controlled by simply changing the voltage?

Which would mean they heat up a lot at low speeds, correct?

Also, would they still have good torque at low speeds?
posted by atm at 5:32 AM on February 20, 2021

Response by poster: Would PWM also cause heating at low speeds?

I remember reading a few reviews on Amazon where people said they got hot.
posted by atm at 5:34 AM on February 20, 2021

Best answer: Mayyyybe, but as a general rule induction motor speed is controlled solely by frequency and the number of poles in the stator. Having said that, looking at pictures of the circuit board and wiring, I think it's done by simple closed-loop chopping.

Since a grinder is _extremely_ low load (starting it is N times harder than grinding work!) and there's a speed sensor on the shaft, I could certainly imagine doing something like SCRing the input voltage so the rotor slips *A LOT*. Since the lowest speed is > 50% of synchronous speed, I think it'd be possible to pick/design a rotor with a fairly forgiving speed/torque curve. When the sensor indicates below the target speed (a simple comparison), turn the SCRs on; when above, don't. Etc.
posted by introp at 10:47 AM on February 20, 2021

Response by poster: Ok, so what I think I'm hearing is that these grinders are using either PWM or SCR, and there's really no way to know which one, and it doesn't make much difference anyway.

Is that right?

Thanks!
posted by atm at 1:55 PM on February 20, 2021