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
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I'm sure you spun the wheels while 'on'? Works fer me. JR Dweller in the cellar
On Sat, 10 Oct 2009 11:11:00 -0500, Ignoramus29131

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I tried, does not work.

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

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It is a quiet hummmmm
i

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try spinning the wheel up to near proper speed with another motor - just hook a rubber hose to the other motor and put hose around the shaft on the grinder - spin it up, pull other motor away and power grinder on - if it doesn't keep running, either the main (run) winding is open or there is a broken wire
wrote:

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On Oct 10, 9:11am, Ignoramus29131 <ignoramus29...@NOSPAM. 29131.invalid> wrote:

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...
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Are you sure the motor is not connected for 240 V and you are trying to run it on 120? Engineman

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Ignoramus29131 wrote:

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.
--
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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
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On Sat, 10 Oct 2009 23:03:05 -0500, Ignoramus29131

Friction? In a motor?
Thats your problem.
Find oout what is causing the Friction
Gunner
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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
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On Sat, 10 Oct 2009 23:03:05 -0500, the infamous Ignoramus29131

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.
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It was a bad bearing, see my another post.

Well, tongs are made from iron, so it should not be a problem.
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On Sun, 11 Oct 2009 13:22:05 -0500, the infamous Ignoramus31605

If you had been 'Murrican, you'd have translated that as "tongue", where I stopped typing and said "Oops." <sigh>
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Larry Jaques wrote:

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.
--
The movie 'Deliverance' isn't a documentary!

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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.
--
WB
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WB, I agree with you on the cap. I will have the replacement tomorrow.
i
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Wild_Bill wrote:

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.

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A new set of bearings, a $4-$7 capacitor, and with a little effort the grinder should be good for maybe 5-10+ years of maintenance-free operation. Whether repairing/servicing equipment for myself or for a friend, I'd put more value in reliability than saving a few bucks and waiting for an already stressed, used capacitor to fail. I wouldn't want to hear that the friend had scrapped the potentially very good grinder, and replaced it with a POS from HF/sears/other.
In the OP's instance, the new cap is good for him as a fixer-type guy and seller, good for the buyer, and good for the economy.. so I can't see the captastrophy here.
FWIW, ESR meters are an indispensable troubleshooting aid for switchmode power supplies and various other types of electronic gear, but they don't provide a complete evaluation of caps. They're especially useful in that they can be used to check caps in-circuit for a basic, quick "likely good" or "questionable-test further to determine" checks. Out-of-circuit readings are much more reliable, and can indicate "definitely bad" on the excessive readings. I'm aware of the stresses that caps are subjected to in SMPSs. The typical low voltage circuit of an ESR meter can't apply the rated working voltage to the cap, as a Leakage tester does.
It' not as though the grinder repair would include $70 worth of caps, or a repair kit, to replace "shotgun" all the caps in a piece of video equipment, furchristsake.
Good luck finding specs for the cap in an aged machine motor, or a cap that's only marked with an in-house stock number.. but the point in looking escapes me. The acceptable ESR for an AC cap that was possibly manufactured with a 20-30% tolerance on the cap's value, isn't likely to be very specific.
Currently produced components products' specs are readily available, and I've referred to Panasonic, Nichicon and others literature from time to time.. not the same as looking for a missing cap's specs when all I've had is a GE motor's stock number for a fairly new motor.
An ESR meter reading of 3.2 or 7 ohms for an AC cap doesn't mean anything if the person interpreting the reading doesn't have experience with AC caps for that particular application. I've worked from the EIA charts regarding capacitor test parameters. They're essentially basic guidelines.
You're not the only one that's kept lists of test results of new components and marked packages of incoming new stock, and rechecked parts before installing them. You're not the unique special snowflake that grandma said you were (altered movie reference).
--
WB
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