Rheostat for Bench Grinder

in article 65d94$3fe4a15e$4503c914$ snipped-for-privacy@msgid.meganewsservers.com, buckaroo at snipped-for-privacy@covad.net wrote on 12/20/03 11:27 AM:

It may be practical for universal motors that really are series wound motors.

Bill

feasible or even

convert it to variable

$3 one designed for

motor or worse yet burn

gizmo for this

series wound

But bench grinders are not generally series wound.

Or universal or PM motors..

Best regards, Spehro Pefhany

Out of curiousity, where does one find a bench grinder for $40?

In response to your question, if the grinder has a conventional 1750 or 3600 rpm synchronous induction motor, it really doesn't lend itself to variable speed use. Motors like this are common in 1/4 through 1/2 HP grinders.

On the other hand, a $40 grinder may have some sort of brush type or ac/dc motor which lends itself to variable speed operator that may or may not be controllable through use of something akin to an ordinary lamp dimmer. The problem here is that the dimmer may not respond well to an inductive motor load and the voltage spikes it produces.

It all depends on the HP of the motor and its type of design, plus what sort of a speed reduction you are searching for.

Harry C.

Not possible. Only way to properly speed it down is to change the frequency fed to it.

buckaroo

Pretty much every bench grinder I've seen uses an induction motor. At least all the ones designed to run off the mains.

Harry- Ryobi 6" wheels......Model BGH615............ $39/Home Depot......... 2.1 Amp.....Induction Motor......3600 RPM.......Looking for RPM around 2100-2400 Based on what you already said probably won't work

Thanks for helping

Just about anywhere these days. I bought a scary-large** 8" Ryobi for about US$70 including a wheel dressing tool, the 6" ones are cheaper, the no-name China-made ones cheaper again. It won't get you a Baldor, of course. Harbor Fright has 6" no-name China ones (with lamp) for US $35.99 at the moment.

Best regards, Spehro Pefhany

Thanks to All..... I am convinced that this is not possible.... I do know that they make some grinders with variable speeds but I assume that they are wired differently. I knew it sounded too easy...

Actually it isn't a matter of how they are wired, it's a matter of the motor used. Those might very well be universal motors. Easy way to tell, many universal motors have ports to replace their shoes. TTYL

Why would you want to slow down a grinder? (Asking as someone with no expertise at all with them. You can have my Black and Decker grinder which slows down if you look at it cross-eyed - or if you use it to grind something - in return for one that doesn't slow down!)

Why slow down a grinder?... When sharpening chisels and lathe tools on a bench grinder a slower speed is much better since it will not burn (blue) the steel. They make bench grinders that run at a slower speed as well as variable speed ones. I just happen to have bought a "normal speed" one and was just thinking that maybe a rheostat may work also but I have found out that it is not true..... at least for this induction motor. I use to know some about electricity, etc but as I get older my mind destroys all of my old data....

They are generally asynchronous motors, so forget the rheostat or dimmer, you will burn the motor down.Speed adjustment to an asynchronous motor (i.e.brushless motor)is possible only by changing the frequency of the AC.In the lab, when I was still studying, we burnt a motor down (it was smoking!)by applying on it 150 volts instead of 220. Visit our website

-- Dimitris Tzortzakakis,Greece

No cheap way here....but, you could use a dc motor/tach feedback and apply power to it with a pwm amplifier that supports tach feedback. Your drive signal could be a very small 0 to 10 volts DC signal...in the milliamp range. This would give you precision throughout the range. Of course you would probably have the most expensive bench grinder in the state!.........Ross

Thanks! That makes sense. I'm not brave enough to sharpen my chisels on a grinder - I'm afraid of ruining them. I do it by hand.

I can relate to that!

Ross- You sound like McGiver....LOL I knew it was a long shot, but I wasn't sure.... Now I am! Thanks to everyone

Enjoyed your reply....have a great holiday season...Ross