Varying grinder speed with voltage regulator

[Ryan Stokes]

Hi everybody. I currently have an induction motor bench grinder/buffer that is a single speed (3450 RPM). I also have a voltage regulator laying around.

Does anyone know if using the voltage regulator to lower the speed would harm the motor?

I am aware of the possibility of harming the motor if the RPM goes to low because this would lower the speed of the cooling fan to an unacceptable speed. However, I don't plan to go really slow (maybe 1500 RPM).

It's an 8" Grinder/Buffer 120 Volt, 4.8Amps, 60Hz, 3/4 HP

Thanks in advance.

[Jim Becker]

No, you cannot do that...those "voltage regulators" only work with universal motors and even then you have to be careful as for some tools, cooling issues come into play at artificially lower RPM from the design.

[Mike Henderson]

Nope, won't really work. The only safe way to change the speed of an induction motor is with a VFD (variable frequency drive).

There are motors, such as small fans, where voltage is changed to change the speed but unless the motor is designed for that you'll probably cause it to overheat - and not from the slower speed of the cooling fan.

On an induction motor, the slip affects the efficiency of the motor. So a motor with 5% slip will never be better than 95% efficient. A standard 60Hz 3450RPM induction motor under load has about 4% slip (3600-3450)/3600. At 1500RPM you would have about 58% slip. At best, your motor would be 42% efficient (and it would actually be less). So about 60% of the energy input to the motor would be going as heat.

At least that's the best I can remember from my induction motors books. Maybe someone else can give a more detailed answer.

Mike

[Rod Sheridan]

Single phase induction motors can only be run at reduced speed with a VFD if they do not have a centrifugal switch in the starting winding.

Three phase motors (which do not have starting windings) are ideal for speed control using a VFD.

In both cases a voltage regulator is not capable of controlling motor speed without motor damage.

You need to control both frequency and voltage, so a VFD will change voltage in proportion to the frequency (speed) change.

Regards, Rod.

[John McClanahan]

There is another type of speed control. The kind that is used for ceiling fans. It works by clipping the peaks of the AC cycles. Basically turning the power on and off 60 times a second. The speed is changed by the off time vs the on time.

That said, finding a control with the amp rating for your grinder may be difficult. Also, the grinder's power will drop faster than the speed, due to the inductive slip that Mike referred to. Thats due to the motor poles becoming out of sync with the sine waves of the AC current. Thats why a ceiling fan will hum louder when one of these speed controllers is in use.

John

[Carroll Courtney]

Your best bet is go to Woodcraft,they did have a VS 8" grinder for 100.00 due to the fact that they were being phase out----Carroll