October 2015
I’m looking to purchase a belt grinder for my workshop to smooth the edge of circuit boards, aluminum cutouts, and the output of my 3D printer. My question relates to the variable voltage controllers and three-phase motors that are available as optional equipment. Given almost double the price of the grinder, can I simply use a surplus variac on a single-phase motor to get a variable speed grinder?
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The short answer is no, you can’t use a variac on an induction motor.
The speed of an induction motor is controlled primarily by the line frequency, not the voltage. If you try to limit the voltage with a variac the motor will likely overheat as it draws additional current to try to compensate for the lower voltage.
What might work for you is a universal (brushed) motor. They are called universal because they can run on either DC or AC. This is the type of motor that is used in home treadmills, handheld routers, jobsite circular saws, etc. The speed can be controlled with a SCR (variable phase) speed control. These motors are usually rated for 120V single phase, up to about 15 amps or so. You could conceivably use a variac to control one of these but the SCR or Triac variable phase speed control will provide better low speed performance.
The speed of an induction motor is dependent primarily on the frequency of the alternating current driving it. Speed controls for 3-phase induction motors are often called VFDs which stands for Variable Frequency Drive. They work by converting the incoming AC power to DC and then use a 3 phase inverter to convert it back into AC with the ability to vary the frequency from as low as a few Hz to 120Hz or more for motors that can tolerate the increased speed.
If you attempt to reduce the speed of an induction motor by controlling the voltage with a Variac, you will likely overheat the motor without it slowing down much, because the current will increase as you reduce the voltage. Inexpensive variable speed power tools are powered by universal motors that have brushes and a commutator and these can be controlled with a Variac, though the common triac light dimmer type circuit is far more economical.
If you can find a grinder powered by one of these then your idea will work, but most bench mounted tools use induction motors.
Also note that the nominally fixed speed motors of any type usually contain an integral cooling fan under a cover, mounted on the shaft. These are good enough, but if you run the motor slower for a significant time, you could fatally overheat the motor.
If it gets hot, add a separate cooling fan so the motor can take It!
