Rheostat for Bench Grinder

[buckaroo]

Forgot all the electrical schooling I ever had. Is it feasible or even
possible to take a standard $40 bench grinder and convert it to variable
speed by using some sort of rheostat... Possibly a cheap $3 one designed for
dimming lights..... Or will this burn up the grinder motor or worse yet burn
my garage down. If this won't work, do they make any gizmo for this
purpose.

Thanks for Help

 

[John G]

buckaroo
at [email protected] wrote on 12/20/03 11:27 AM:

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

Bill

But bench grinders are not generally series wound.

 

[Spehro Pefhany]

But bench grinders are not generally series wound.

Or universal or PM motors..

Best regards,
Spehro Pefhany

 

[repatch]

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

 

[repatch]

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

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

 

[buckaroo]

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

 

[Spehro Pefhany]

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

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.

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.

** If you don't think an 8" 3500 RPM bench grinder is scary, you need
to think about what happens when one of the wheels shatters (and it
does happen). There is a LOT of energy in those stones.

Best regards,
Spehro Pefhany

 

[buckaroo]

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...

 

[repatch]

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

 

[[email protected]]

Forgot all the electrical schooling I ever had. Is it feasible or even
possible to take a standard $40 bench grinder and convert it to variable
speed by using some sort of rheostat... Possibly a cheap $3 one designed for
dimming lights..... Or will this burn up the grinder motor or worse yet burn
my garage down. If this won't work, do they make any gizmo for this
purpose.

Thanks for Help

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!)

 

[buckaroo]

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....

 

[Tzortzakakis Dimitrios]

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 www.patriko-kreta.com

 

[Ross Mac]

Forgot all the electrical schooling I ever had. Is it feasible or even
possible to take a standard $40 bench grinder and convert it to variable
speed by using some sort of rheostat... Possibly a cheap $3 one designed for
dimming lights..... Or will this burn up the grinder motor or worse yet burn
my garage down. If this won't work, do they make any gizmo for this
purpose.

Thanks for Help

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

 

[[email protected]]

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.

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.

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....

I can relate to that!

 

[buckaroo]

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

 

[Ross Mac]

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