Grinder Speed Reduction

[Lou Morrissette]

I've been toying with the idea of installing a router speed control on a bench grinder to give me more control when grinding smaller chisels. Has anyone tried this or is it just a bad idea? I'm a little concerned that it might cook the motor. TIA

Lou

[Marcus Carr]

I have heard that this doesn't work, but I have never tried it myself.

Marcus

[James Carmichael]

A router speed control works because routers have universal motors on which the speed can be varied. A bench grinder likely has an induction motor, which has one speed.

I'm no electrical engineer, so don't ask me what all that means, I just know that's how it is.

[Gil Jones]

Router speed controllers are for "universal" motors (Universal motors have brushes). Most, if not all, grinders use an “induction” motor, and the router speed controllers will not work on them.

[Jim Becker]

Nope. You cannot use this kind of thing with an induction motor.

[Lou Morrissette]

Well, so much for that bright idea.:( :D Thanks for setting me straight on that one.

Lou

[Kyle Kraft]

Would a VFD Variable Frequency Drive work? Only thing is a VFD would probably cost more than a Tormek with all the goodies.

[Rick Lizek]

It would have to be a three phase grinder for a VFD to work. Woodcraft and Delta have 2 speed grinders for under $100. Figuring a 3 phase grinder and VFD from www.factorymation.com (http://www.factorymation.com) , it's still pricey.

Getting back to the original question...it would be best to learn to apply a lighter touch. Another option is to use a softer wheel or the other option is to use a coolant. I just put a $20 spray mister nozzle from Enco to use on the grinders out in the shop. Don't know if the guys will accept it yet but it uses compressed air to generate a fine mist that can be directed on the tool edge.

[Marcus Carr]

For what it's worth, treadmill motors are variable speed and there are a lot of abandoned treadmills our there.

Marcus

[Michael Fross]

For what it's worth, treadmill motors are variable speed and there are a lot of abandoned treadmills our there.

Marcus

But then what would I stack my lumber on in the basement?

Michael

[Cliff Rohrabacher]

Finesse not speed.

It's not the grinder speed that's working against you.
The wheel should be dressed with a diamond dresser.
Those old school star wheel dressers are pretty good too.
Once you have trued up the face of your wheel your only remaining issue is manual finesse.

You should be able to resharpen by hand common twist drills as small as you have the ability to see.

[David Epperson]

It would have to be a three phase grinder for a VFD to work.
Nope. Variable Frequency Drives work on single phase as well. I've installed quite a few on industrial equipment - Hoist/crane traverse drive systems use them all the time to ramp speed up/down. In fact some of them can be hooked up to single phase and output either single or 3 phase. They don't develop full rated horsepower/amperage when used as a phase converter.....but for small HP motors it works.

[Noah Katz]

"Variable Frequency Drives work on single phase as well."

They will work with 1-ph input, but still need to be used with 3-ph motors.

They can be gotten for less than $100 on ebay; you could also look for a 3-ph grinder to use with it.

Or get an arbor with stepped pulleys.

[Mike Henderson]

"Variable Frequency Drives work on single phase as well."

They will work with 1-ph input, but still need to be used with 3-ph motors.

They can be gotten for less than $100 on ebay; you could also look for a 3-ph grinder to use with it.

Or get an arbor with stepped pulleys.
Actually, you can use a VFD on a single phase induction motor. The speed of an induction motor (single phase or 3 phase) is related to the line frequency and the number of poles (and the load, but let's ignore that for right now). If you reduce the line frequency, the motor will run slower. For example, 50Hz motors run slower than 60Hz motors, all other things being equal. If you increase the line frequency, the motor will run faster (up to a limit). By line frequency, I mean the line actually feeding the motor, not the line frequency of the power feeding the VFD.

I don't know why we don't see more speed controls on single phase induction motors. Maybe they have reduced torque on startup - I'll have to research that.

Mike

[David Epperson]

By line frequency, I mean the line actually feeding the motor, not the line frequency of the power feeding the VFD.
Whether its supplied by 1ph or 3ph, or supplying 1ph or 3ph, every VFD that I know of converts to DC first and then back to the desired "AC". But Pulse Width Modulated AC is a bit different that regular "house current" AC and it can be a bit hard on some (particularly older) motors that aren't insulated enough for the spikes. I guess you could smooth some of this out a bit by running the VFD output through an isolation transformer..but that's just more cost..and load.


I don't know why we don't see more speed controls on single phase induction motors. Maybe they have reduced torque on startup - I'll have to research that.

Mike
Not so much the reduced torque - though that's some of it - it's more the increased current and voltage behind it....that and they tend to "cog" a bit at really low frequencies (Below 5 or 10 hertz or 10% speed).

[Rod Sheridan]

Hi, a variable frequency Drive cannot be used on most single phase motors due to the starting winding.

Single phase power does not have a phase rotation such as 3 phase (ABC), it simply has a polarity reversal at twice the line frequency.

A single phase motor needs an artifical phase displacement to start, in most single phase motors this is accomplished by using a starting winding, and a centrifugal switch to disconnect the starting winding at aproximately 75% of rated speed. (This is the click you can hear as the motor accelerates, as the centrifugal switch snaps into the open position).

Note that the starting winding, centrifugal switch and starting capacitor if used (which increases startup torque) are only rated for intermittent duty.

Reducing the motor speed to the point where the starting switch would close and allow current to flow through the starting winding, would destroy the starting winding and capacitor in a matter of minutes.

A second problem is that the torque is only applied at twice the line frequency, not 6 times the line frequency as in a three phase system. This will result in shuddering, and the motor may stall as the flywheel effect is proportional to the square of the rotational speed, and may be too low at lower speeds to continue rotating.

A router speed control is suitable for brush type motors only, as it simply controls how much of the sine wave flows to the motor. Using this type of speed control on standard induction motors will result in motor damage.

Regards, Rod.

[Carl Eyman]

Many years ago I had problem in synchonizing the speed of a fractional hp motor driving a canvas belt with a packaging machine driven by am electric/ hydraulic motor with a variable speed control. Since solid state electronics was not as advanced then as now varying the shape or frequency of the AC was not an option. I mounted cams on each of the machines operating micro switches. These switches when open would interrupt the current to the holding coil on the motor starter. Then I set the speed of the packaging machine so it was going a little faster than the than the conveyor. When the two were in sync the two micro switches closed and openned nearly simultaneously. If they got out of sync, one switch stayed open longer than the other. This caused the motor starter to be off for a longer period.

It worked very well. Someone is bound to say but that was hard on the motor starter. It openned and closed about 65 times a minute. How long would it last. The answer: eight years - about 200 days per year.

Granted we were not slowing the motor very much. The minimum time of interruption of current was probably 20* out of 360*, or about 5%. However, I belierve the same methods could be used to slow a motor - even a single phase motor - so long as it did not slow to the point where the centrifical starter switch closed.

Sorry this was so long.

[Carl Eyman]

Many years ago I had problem in synchonizing the speed of a fractional hp motor driving a canvas belt with a packaging machine driven by a hydraulic motor with a variable speed control. Since solid state electronics was not as advanced then as now varying the shape or frequency of the AC was not an option. I mounted cams on each of the machines operating micro switches. These switches when open would interrupt the current to the holding coil on the motor starter. Then I set the speed of the packaging machine so it was going a little faster than the than the conveyor. When the two were in sync the two micro switches closed and openned nearly simultaneously. If they got out of sync, one switch stayed open longer than the other. This caused the motor starter to be off for a longer period.

It worked very well. Someone is bound to say but that was hard on the motor starter. It openned and closed about 65 times a minute. How long would it last. The answer: eight years - about 200 days per year.

Granted we were not slowing the motor very much. The minimum time of interruption of current was probably 20* out of 360*, or about 5%. However, I belierve the same methods could be used to slow a motor - even a single phase motor - so long as it did not slow to the point where the centrifical starter switch closed.

Sorry this was so long.