VS grinders

[Andrew Garrett]

I just bought an 8" VS bench grinder from Lowe's. It's a Delta (el cheapo). I paid $140.

I have the same VS grinder in a 6" model that went south a year or so ago. I'm wondering if the VS controls on that grinder can be salvaged and used on, say, a 1HP GE motor.

I'm certain the answer is 'no'. It couldn't be that simple, since the 6" grinder only costs like $80, and 3 phase VS controls are stupid expensive. I just wonder why--after all, it is in fact, running a 110V AC motor with variable speed, is it not?

[Ray Rogers]

I'm pretty sure the answer is 'no' also, Andy. The type of variable speed control used on those sanders is the same as a router uses I think. That type of control only works on electric motors that are built a certain way and all of those motors that I've seen are sub hp motors. So, basically, I think it wouldn't work because the GE motor probably isn't designed the way that type of control would require ...

[Andrew Garrett]

Thanks Ray. I guess that begs the question; Why not make bigger such motors? The technology is obviously cheaper to produce.

[Ray Rogers]

I'm not knowledgeable enough on the detailed workings of electric motors to really answer that question technically but I think I can answer it from a practical point of view. I believe the motors that work with these small speed controls are of a very simplistic design, cheap to make, and electrically unsophisticated. If they were made bigger they would likely draw too much power for the average household 110v line and the controller would have to be built heavier to sink the substantially increased heat. In other words, it would be an inefficient brute force approach that wouldn't be marketable when compared to the already available alternatives....

[Thingmaker]

The controller for the 6" grinder has to handle 2.5 Amps. The controller for the 8" model has to handle 5 Amps. Speed control of an AC motor (without torque loss) is a complicated prospect to begin with, and gets worse as the power increases.

To really have good control over larger motors, both the voltage and the frequency of the power would have to change. Otherwise, torque would be lost as the speed was reduced.

[Andrew Garrett]

I bet the guy who designs a transmission for grinder motors (beyond the three speed pulley deal) will make a fortune!