Sanity check for the electric motor guru's?

[Mark Bolton]

We have a sander with a 240V single phase 5Hp TEFC motor on it. I cant say that I ever really notice it til now but I guess in hind sight the sander was perhaps getting louder (somewhat of a growling sound) over time. Never peaked my interest. But then the amp meter on the sander started to climb. Couple times employees would say no matter what they did they couldnt keep it out of the red on the meter. This was again over time. Finally we were running some material on Saturday and I noticed the amp meter riding constantly on the high side of normal (around 28-30 amps). Motor is rated at 25 and I cant say I know for sure but I really think I remember it no-load idling at less than 20.

I thought for sure we had a run capacitor going south. So I pulled the cover off, check the capacitor spec's, and for the heck of it I popped the belts off the motor sheave and started the motor with zero load. Same high amperage condition, same growling, (idling at 22-24 amps with nothing connected to the motor shaft). I did this trying to rule out a bad bearing up top. The motor shaft is rock solid, no radial or end play, and spins as free as a bird.

Swung by Grainger this morning and picked up a new 370V 50Muf run capacitor. Came back to the shop, installed the cap. and while it was apart stuck in a fresh set of belts, hit the button and unblievably it showed the exact symptom of a bad START capacitor? It tried to start, rocked back and forth a few times, and tripped on overload. So I reset the overload and hit the button and gave the drums a roll by hand and it fired right up AND ran quieter than ever but still with moderately high amps (about 20-21).

I started it several times rolling by hand and headed back to grainger for a new start capacitor (should have just bought both the first time). Got a new 125VAC 600Muf start capacitor, back to the shop, stick it in, re-tension the belts, and the thing fires right up. It is still no-load idling at 20-21 but its running fine, and sanding is seemingly fine.

Im wondering if the motor is on its way out? Why was it starting with the old start cap. and when switched the run cap. the start cap then seemed bad?

We are planning to just watch it but we have a slew of work coming up and Im wondering if I shouldnt just order a replacement motor to have on hand but hate to toss 600-700 bucks out the window if its not needed.

Appreciate any of you motor guys input.

[Mike Cutler]

Mark

Keep the money in your pocket for now.
More than likely both caps were bad, or just getting old, and when you put the new one in. The leakage through the other became more apparent.
Both caps are in parallel until the centrifugal switch opens, so they can effect each other.
Electric motors are pretty stout little buggers, but you might want to think about that centrifugal switch as a candidate for replacement. You might not have an open as much as a high resistance contact now, and the new caps are masking it.

[Mark Bolton]

Oh man Mike,

That sounds great to me!!!

So whats the remedy? Ship it off to the motor shop? It most definitely seems to be shutting down differently with the two new caps. (faster/harder).

[Bill George]

If the centrifigal switch was bad, as not opening the start cap would blow. If the run cap was replaced and did not fix, then I think the bearings were going bad. What happens when you spin the motor by hand with no load?

[Matt Day]

If the centrifigal switch was bad, as not opening the start cap would blow. If the run cap was replaced and did not fix, then I think the bearings were going bad. What happens when you spin the motor by hand with no load?

As said above: “The motor shaft is rock solid, no radial or end play, and spins as free as a bird.”

[Dan Friedrichs]

Entirely possible you fixed the problem. The ammeter will not show a "linear" increase with load (ie - it could be 20A at no load and 25A at full load, due to the reactive component of the current), so I wouldn't take the no-load reading as indicating a problem, necessarily. If the shaft spins freely and has no radial wobble, bearings are probably fine. As Mike said, check the centrifugal switch (you may be able to hear it "click" in or out - especially on coast-down. Otherwise, if you have a clamp current meter, measure the current in the start cap and make sure it's zero once the motor is up to speed).

In any case, there's not much that can go wrong:
1. Caps
2. Centrifugal switch
3. Bearings
4. Windings (much less likely than the other 3, especially if it hasn't been badly abused).

If you can confirm the centrifugal switch is operating and the bearing feel good, you probably don't have anything to worry about.

[Bill George]

Growling when running, could be a winding issue or bearings, regardless of spin checking. Take to a motor shop. BTW is this a Chinese motor?

[Mark Bolton]

Thanks guys. Switch is definitely operating. Good solid click on wind down.

[Mike Cutler]

Mark

Sounds like you're all set.
I mentioned the switch as a possibility only. Switches can degrade over time and get carbon deposits on the individual contacts, and the mechanical mechanism can "get weak", and the Open distance isn't what it was new. In either, or both cases combined, a switch "open" which should be infinity, might be something less. Not closed, but somewhere in between. It may seem fine with a DMM on the bench, but once you put potential back to it, it can arc across.
No matter what, it's good that it was just the cap's. They're relatively cheap.

[Bill George]

But its still growling and drawing high amps when running idle?

[Mark Bolton]

Its no longer growling and upon further inspection it seems like the meter may be part of the issue as well. We noticed today when the machine is shut down the meter needle only falls back to the 5 amp mark, not zero, so wondering if the meter is reading high. I havent put the clamp meter on it yet but it seems to be running perfectly now.

I have always read and heard that a failing run capacitor can show up as noise/growling, heat, high amp draw. I will rule out the amps when I put the clamp meter on it and will likely toss in a new amp meter. Today when the meter held at 5 after shutting down I pecked on the meter three or four times and it fell back to zero.

My gut is Mike nailed it with the caps being in parallel and I should have just changed them both out.

Sadly in our area there is only one electric motor shop and they will not even look at small AC motors. Their bench charge alone is probably a third or half the cost of a new motor.

Im feeling good about it and the sander ran all day today so fingers crossed that 30 dollar worth of caps did the trick.

[Bill George]

Wonderful news, no there are not many real motor shops around anymore. Too many throw away motors on the market.