Variac to control tool motor speed?

Capt Jim · May 4, 2008

I have a very large Variac, and I was wondering if I can use it to control the speed of a 120 volt tool motor...lathe, mill, drill press...etc etc. I understand that as the voltage to the tool motor is decreased...the motor will have to draw more current in order to perform. Perhaps this could amount to excess heat...over stressing the wiring...etc??? Is this a practical way to go...or is it a hazard?
Thanks.
jim

JunkyardJ · May 4, 2008

Won't work with most motors.

It will only work with brush type or fan motors that don't have capacitors to start or run. Most drill presses, mills, and other tools don't use that kind of motor. There are exceptions, but it all depends on the type of motor. most of the standard induction motors don't work with them. Odds are, you can't use it.

JST · May 4, 2008

It will NOT work on any ordinary induction motors. That is surely what you have.

it will NOT work on ANYTHING with a start switch, split phase, capacitor start, 2-value capacitor motors, etc.

It WILL work on the following:

Universal motors..... the ones with brushes, as in small corded drills, etc.

Shaded-pole motors, used for small fans and a few pumps.

"PSC" or permanent capacitor motors, used on fans, blowers, and some gearmotors as well. These have NO start switch, but keep a certain value capacitor connected at all times.

2Slow · May 4, 2008

I have played with the Harbor Freight "router speed control" on a small PSC motor. (little craftsman grinder) and it does not really seem to vary speed on it. It seems to vary torque. The PSC motor always seems to want to get to normal speed and will creep there with no load even on the very "slow" settings.

I due have an old variac and have not tried that, but probably will later today, do you think I wil have different results? I thought a PSC motor would always try to get to synchronous speed unless you varied the frequency.

-Joe

minder · May 4, 2008

2Slow said:
I have played with the Harbor Freight "router speed control" on a small PSC motor. (little craftsman grinder) and it does not really seem to vary speed on it. It seems to vary torque. The PSC motor always seems to want to get to normal speed and will creep there with no load even on the very "slow" settings.

I due have an old variac and have not tried that, but probably will later today, do you think I wil have different results? I thought a PSC motor would always try to get to synchronous speed unless you varied the frequency.

-Joe

The router speed control is most likely a Triac controller for Universal router motors.
You are correct, an induction motor will not vary in speed by Triac or Variac control, as it attempts to run at near synchronism.
M.

JST · May 4, 2008

minder said:
The router speed control is most likely a Triac controller for Universal router motors.
You are correct, an induction motor will not vary in speed by Triac or Variac control, as it attempts to run at near synchronism.
M.

The PSC DOES vary torque, and will only vary speed under load. That is true. It really ends up varying slip.

But with loads that DO have variable torque, such as fans, the PSC will vary speed in a stable manner.

We have a product for the HVAC market that varies PSC blower motor speed and power drawn via a triac control. It does work.

I agree, it isn't very useful for spindle drives on most machine tools, but it MAY be useful for things like coolant pumps etc, which have somewhat predictable and flow-varied loading.

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Variac to control tool motor speed?

Capt Jim

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minder

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