Convert AC Motor to variable speed?

Thread Starter


Joined Aug 27, 2008
I'm wondering if an AC motor can easily be converted to variable speed. I've got an old vacuum cleaner motor I was thinking of using to drive a home-made lathe. Problem is...I'm not sure if I can put something like a rheostat in the circuit like can be done with direct current. And if so, then is such an animal available from a hardware store, or can it be built? I'm assuming the heat and such would be plenty an issue for the ones for "normal" electrical applications.


Joined Jul 17, 2007
If the motor has brushes, it might be adapable to your application.

However, most lathes I've been acquainted with use stepped pulleys to reduce the spindle speed. This gives a large mechanical advantage along with reducing the effective surface speed. Controlling the motor speed using something like PWM might work, but you would lose the advantage of increased torque provided by the reduction drive.

I'm afraid that attempting to use your vacuum motor without a reduction drive would have unsatisfactory results.

Thread Starter


Joined Aug 27, 2008
The motor does have brushes. I was so focused on the speed, I didn't think of the mechanical advantage, thanks. That in mind, I would actually prefer to use the stepped pulley sort of idea.Just for theoretical purposes though, can this be done with resistance as w/ a DC motor? I'm not sure how to change the frequency. Isn't that limited to 60 Hz by the wall outlet?


Joined May 16, 2005
Changing the speed of a DC motor is better accomplished by "pulse width modulation" (PWM) than with a resistor. Not only is less energy wasted as heat, but the motor has better torque at the lower speeds than with a resistor.

Changing the speed of an AC motor does indeed require changing both frequency and voltage. And yes, the wall outlet is a fixed frequency and voltage. This is why AC speed controllers cost so much.


Joined Jan 18, 2008
An AC/DC (i.e., universal) motor can be controlled with a triac device. Dedicated devices are less than $20 at DrivesWarehouse and many other places. Some people just use a light dimmer, which is even cheaper.

As for lathe speed, some of the finest toolroom lathes ever made (e.g., Monarch 10EE) used DC motor controls, and many other lathes have been retrofitted. There are several advantages to that system. For example, when facing, one can control the speed to keep the relative cutting speed constant. The Monarch does that automatically. Good DC drives also control speed under varying loads quite well.

While a triac control will probably work and is the least expensive option, that method of control results in significant loss of torque. My first choice would be to run the motor on DC and use a DC motor control. The DC drives are available quite cheaply on eBay. One brand to consider is KB Drives.