variable voltage effect on motors

It is a single speed, gear-driven unit, my calculations (based on motor speed - 1725 rpm - and ratio of teeth on the drive gears - 1:4) is that it turns at about 430 rpm. This is faster than my modern, moderately-priced drill press - which I can get down to about 310 rpm.

Someone suggested varing the voltage to get the speeds I need using something like the item shown in the link below.

My understanding is that varing the voltage to change motor speeds is tough on the motor unless it was made for that application. Is this correct?

Also, I would imagine decreasing the voltage to get a slower speed would also decrease the power (torque) output of the motor. Correct?

Thanks.

• variac

The rotational speed of an induction motor is determined by the varying magnetic field produced the the AC current input. In theory you could slow it down by changine the frequency of the input power, but that's a lot of trouble & expense.

If it was belt driven, all you'd need is a smaller pulley at the motor. Maybe you can rig some sort of a belt reduction drive ahead of the first gear? Otherwise, you're either stuck with existing speed or will have to change motors to something that can handle variable speeds.

Good luck.

Keith

The speed of true ac motors is pretty much tied to the frequency of the ac power applied to them. You can tell you have an ac motor because the rated rpm is (approximately) a fraction of 3600 rpm. (60 cycles per second x 60 seconds per minute). Induction motors (the most common type) turn slightly slower under load than their theoretical no-load speed. Synchronous motors (such as clock motors) are locked precisely to the power frequency.

You could slow the drill press down using different pulleys. However, don't forget that the torque will go up and might be enough to break something in the press.