Grinder that makes the hummmmmmmmm sound

Bought a 3/4 HP Milwaukee pedestal grinder at a garage sale. Weighs maybe 230 lbs with the pedestal. Supposedly it ran at the seller's place, but when he brought it to Chuck's garage, it would not run and makes the hummmmmm sound instead. I trust Chuck 100% as I have known him for a while.

So I bought it for $20, hoping for a quick fix.

My own hypothesis is that it may not even be the cap, but some contact became loose in the starting circuit due to moving this grinder from one garage to another.

i
Reply to
Ignoramus29131
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Reply to
JR North

mild Hmmmmm - could be contacts not closing, open start cap or windings or disconnected wire loud Hmmm - seized armature, shorted start cap

Reply to
Bill Noble

So, the starting switch is dirty (it's a grinder, that's very possible), or the capacitor needs retirement. Those grinding wheels are flywheels, give 'em a spin then apply power. It's probably good to go!

Capacitor-start motors' little foibles are the BEST thing that ever happened to garage-sale prices...

Reply to
whit3rd

I tried, does not work.

Reply to
Ignoramus29131

It is a quiet hummmmm

i

Reply to
Ignoramus29131

Could be. I've had cap connections slip lose and a hum with no rotation is the symptom.

Give the wheel a good spin by hand before throwing the switch and see if it runs OK. If it does, its the cap, the centrifugal switch or associated wiring.

You could opt to never fix the start circuit if spinning it by hand is easy enough.

Reply to
Paul Hovnanian P.E.

On Sat, 10 Oct 2009 11:11:00 -0500, the infamous Ignoramus29131 scrawled the following:

Could be, so check it all.

Doublecheck your end play, too. If the armature hits on the end, it'll short and you get it humming a few bars. On some motors, end play is adjusted with paper washers. Wild, huh?

-- Seen on a bumper sticker: ARM THE HOMELESS

Reply to
Larry Jaques

Spinning it by hand is problematic, because of 1) the guard and 2) there is considerable friction, I would say more than I would consider normal. I will look into that. It could also be the way it had always been. By now, I have taken it off the pedestal and opened up the bottom box with the cap. I will do some more testing later.

A cap costs abour 4 bucks at McMaster.

i
Reply to
Ignoramus29131

Friction? In a motor?

Thats your problem.

Find oout what is causing the Friction

Gunner

"Lenin called them "useful idiots," those people living in liberal democracies who by giving moral and material support to a totalitarian ideology in effect were braiding the rope that would hang them. Why people who enjoyed freedom and prosperity worked passionately to destroy both is a fascinating question, one still with us today. Now the useful idiots can be found in the chorus of appeasement, reflexive anti-Americanism, and sentimental idealism trying to inhibit the necessary responses to another freedom-hating ideology, radical Islam"

Bruce C. Thornton, a professor of Classics at American University of Cal State Fresno

Reply to
Gunner Asch

On Sat, 10 Oct 2009 23:03:05 -0500, the infamous Ignoramus29131 scrawled the following:

You can get a stick and use it as a safe wheel spinner. The considerable friction is what gets me. That does not sound like any grinder motor I've ever spun. The lack of friction is what is notable and why you normally see grinders spinning for several minutes after they're turned off. Could there be an electric brake (energize to release) in there? I've never heard of one. Most motor braking that I've seen is done by applying DC across the windings to allow magnetism to stop the armature.

Those are easy enough to test. Just charge it up and touch it to your tong...Oops, never mind.

-- Seen on a bumper sticker: ARM THE HOMELESS

Reply to
Larry Jaques

I have taken it apart (which was educational in itself). One of the bearings is bad and causes considerable friction. The starting circuit looks like there is nothing wrong with it.

I am going to buy the following at McMaster-Carr:

1) Suitable double sealed bearings. 2) Starting capacitor.

I will also vacuum it inside and then blow out the dust with compressed air. I will most likely sell this grinder afterwards, as I already have several grinders and one wire wheel on a 1 HP motor.

This is my favorite activity, buying broken stuff and fixing it.

i
Reply to
Ignoramus31605

It was a bad bearing, see my another post.

Well, tongs are made from iron, so it should not be a problem.

Reply to
Ignoramus31605

On Sun, 11 Oct 2009 13:22:05 -0500, the infamous Ignoramus31605 scrawled the following:

If you had been 'Murrican, you'd have translated that as "tongue", where I stopped typing and said "Oops."

-- The best and safest thing is to keep a balance in your life, acknowledge the great powers around us and in us. If you can do that, and live that way, you are really a wise man. -- Euripides

Reply to
Larry Jaques

Use an ESR meter on the capacitor. Even if they have the right capacitance, the ESR can still be too high for the capacitor to do anything in a low impedance circuit.

Reply to
Michael A. Terrell

The ESR test reading is only a small portion of a capacitor's condition. An ESR reading for an AC cap will mean very little to someone with no experience testing AC caps, if they don't have a known-good, or good new cap of approximately the same value and voltage to compare the reading to. Acceptable ESR readings for AC caps are higher than the acceptable readings for most DC caps, of the same value and voltage.

About all that can be determined with a DMM or VOM is that the cap is, or isn't shorted, unless one is inclined to create test circuits.

Without testing for Value, Leakage (uA internal current) at the cap's working voltage, Dielectric Absorption and ESR, one can't really evaluate a cap's condition.

The best approach is likely to be to buy a new cap for the motor and be done with it.

Reply to
Wild_Bill

WB, I agree with you on the cap. I will have the replacement tomorrow.

i
Reply to
Ignoramus30792

Fine. Spend money on parts you may not need. Some of us are good enough techs to find the OEM's datasheet to see what the ESR spec is for a new capacitor. Some techs are offended by the low grade 'Shotgun' method of repair. It wastes parts and raises repair costs.

BTW, I record the ESR of the new caps for future reference.

Reply to
Michael A. Terrell

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