Using a speed control on a grinder

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Forum topic by Bill White posted 07-20-2017 02:45 PM 342 views 0 times favorited 7 replies Add to Favorites Watch
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Bill White

4874 posts in 3890 days

07-20-2017 02:45 PM

Have a Dayton made Craftsman 7” bench grinder. 3450 rpm., 4.4 amp, 110 volts. Induction motor of course.
Would a fan speed control or light dimmer work to slow the speed without harming the motor?


7 replies so far

View msss's profile


4 posts in 655 days

#1 posted 07-20-2017 05:18 PM

HI- I have a router speed controller and a palmgren 8” grinder. ==the speed controller is used for a single speed router when I need it- and never tried it for another purpose- but will see if I can get you the answer you need.

View bbasiaga's profile


1154 posts in 1925 days

#2 posted 07-20-2017 05:23 PM

I have generally read the advice that you should not use a speed controller on an induction motor. Most of the speed controllers are made for the screamer type motors which work differently. Most grinders have induction motors, so do do that right you would need a VFD, and a way to cool the motor when it was running slower than designed.


-- Part of engineering is to know when to put your calculator down and pick up your tools.

View Gilley23's profile


489 posts in 312 days

#3 posted 07-20-2017 05:25 PM

Lighting dimmers are not the same thing as motor speed controllers, they work differently. You will harm the motor by using a lighting dimmer.

View Loren's profile (online now)


10081 posts in 3577 days

#4 posted 07-20-2017 05:28 PM

There are variable-speed AC induction motors
made but a regular grinder motor is a single
speed motor. The speed is built into the
motor as a function of the hertz rating and
other electrical factors. Technically you
can run a standard US motor a little slower
by running it at 50 hz instead of 60 hz but
it’s not good for the motor. 50 hz motors
run a little faster on US 60 hz electrical
power. It’s not as harmful to the motor to
run it a little faster as it is to run it slower.

I’ve got by for years using a 3400 rpm
grinder. I make sure to use a cup of water
to cool my steel often and use a white
Norton wheel.

View Loren's profile (online now)


10081 posts in 3577 days

#5 posted 07-20-2017 05:30 PM

“Poles and Speed
Every ac induction motor has poles, just like a magnet. However, unlike a simple magnet, these poles are formed by bundles of magnet wire (windings) wound together in slots of the stator core.
In most cases, you can look inside the motor and count the number of poles in the winding; they are distinct bundles of wire evenly spaced around the stator core.

The number of poles, combined with the ac line frequency (Hertz, Hz), are all that determine the no-load revolutions per minute (rpm) of the motor. So, all four-pole motors will run at the same speed under no-load conditions, all six-pole motors will run at the same speed, and so on.

The mathematical formula to remember in helping make this calculation is the number of cycles (Hz) times 60 (for seconds in a minute) times two (for the positive and negative pulses in the cycle) divided by the number of poles.

Therefore, for a 60-Hz system, the formula would be:

60×60 x 2 = 7,200 no-load rpm ÷ number of poles.

For a 50-Hz system, the formula would be:

50×60 x 2 = 6,000 no-load rpm ÷ number of poles.

Using this formula, you can see that a four-pole motor operating on the bench under no-load conditions runs at 1,800 rpm (7,200 ÷ 4 poles). Note that when an ac motor is loaded, the spinning magnetic field in the stator does not change speed. Instead, the rotor or moving part of the motor is restrained by the load from “catching up” to the field speed.

The difference between the field speed of 1,800 rpm in this example and the rotor speed of approximately 1,725 rpm is called the “slip.” Slip varies with the load over a narrow operating range for each motor design.”


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Bill White

4874 posts in 3890 days

#6 posted 07-20-2017 08:07 PM

Thanks everyone. Mine was just a thought, and I will gladly put that brain fart in the trash.


View Mainiac Matt 's profile

Mainiac Matt

7787 posts in 2258 days

#7 posted 07-20-2017 08:18 PM

what Loren said.

Also, single phase induction motors use a starting capacitor to get the motor rolling.

To use a VFD on an induction motor to control speed, you have to have a 3 phase induction motor (you can create the three phase power with a VFD that runs off of single phase as the output is electronically created, but the motor must be wound for 3 phases).

There is not starting capacitor on a 3 phase motor. Instead, the three rotating phases (120 deg apart) create the starting torque to spin the motor up from a dead stop.

-- Pine is fine, but Oak's no joke!

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