Motor Noise

[boozer]

Hi all-

I've been lurking on here for quite some time now, and have been fairly active on the HAMB, but haven't really needed any garage advice until now. So here's the deal, I scored a free quincy 216-L air compressor w/ 60 gal tank, only bummer was that it was set up for 3 phase power. No biggie, I craigslisted a 3 hp single phase Baldor motor (L3609T) for it, and a single phase NEMA 1 motor starter w/ overload relay. I think my motor is having issues, here's what it's doing:

Motor seems to start up and run just fine. After the motor is shut off, it will freewheel for a little while and then make a funny noise/vibration and the motor will slow down suddenly. The issue is almost undetectable with a bare shaft, a little worse with just the sheave (pulley) installed, and sounds terrible when it's belted up to the compressor. While hooked up to the compressor it will nearly seize the shaft in it's tracks after turning the switch off. Has anyone had this happen before?

I had a motor rebuilding shop look at it (changed out the stationary switch contacts, replaced bearings, basic maintenance stuff) with no change whatsoever. I have never heard another motor make a noise like this, but just to make sure, i hopped on you tube (laugh if you will....)and listened to a bunch of big air compressors, and NONE made any noises like this.


HELP!

 

[A_Pmech]

When power is removed and the motor is slowing, does the noise start before or after the start switch resets?

 

[boozer]

I believe it is happening at the same time the centrifugal switch is resetting. There have been a few (1-2) times when the motor hasn't made the terrible noise, and it will just slowly spin to a stop. When this happens all you hear is a gentle "click" when it switches back into the start winding.

 

[Milton Shaw]

I have seen some motor that have a quick stop setup to keep saw etc from turning forever, that might be what is happening. I basically almost puts the motor in reverse. I don't know if the motor shop could change that in the motor or not.

 

[boozer]

I have seen some motor that have a quick stop setup to keep saw etc from turning forever, that might be what is happening. I basically almost puts the motor in reverse. I don't know if the motor shop could change that in the motor or not.

I don't think it has that set-up, because it will do it sometimes, and won't do it others. I would think the motor shop would have mentioned it if it had this type of set-up.....

 

[Greatbear]

What you are seeing is the centrifugal starting switch closing. The single phase motors have a starting winding (and in most cases, capacitors) that are switched on until the motor comes up to speed, then a set of flyweights swing out and opens the switch, letting the motor operate on the run windings. When the motor has no load and is able to coast to a stop, it will slow to a point and the centrifugal switch will close. There is a slight mechanical drag where the weighted mechanism holds the switch closed and this will make a dragging sound.

In some cases a capacitor-start motor will retain a DC charge on the starting cap when the switch opens in mid-AC cycle. When the switch is again closed, this DC charge gets dumped internally into the starting winding, this acts as an electric brake. This is mostly harmless, and intermittent. Larger motors (with larger caps) can have wear on the contacts in some situation, as they will often have a 15kOhm 2 watt resistor across the start cap terminals to act as a bleeder to take off any residual charge before the motor is stopped, saving on contact life.

 

[boozer]

Greatbear, you seem to have a solid understanding of what is going on here. The motor shop told me basically the same thing that you are saying, and even gave me two 10Ohm resistors. Before the resistors, this issue would happen every time like clockwork. Since I installed the resistors (One is connected between the two start capacitors, and one is between the other start cap. and the run cap) now the vibration will happen maybe 50-60% of the time. The capacitors in my motor are quite large, two start capacitors; one at 645-774MFD, the other at 460-552MFD both at 125V. The run cap is 60 MFD at 370V.

Questions:
Why have I never seen any other motors do this?
Would installing larger resistors help?

 

[Greatbear]

You should only have resistors across the terminals of each of the start caps, not the run caps. It's the start caps giving you the issue. You should only need 10-15k Ohm resistors. Don't short-cycle the motor, it takes about 20 seconds or more for the charge to be bled off by the resistors when the motor is running. It's odd that it's happening so strongly, but I've seen this on motors like yours that have more than one very large value start cap.

 

[boozer]

Thanks for the advice Greatbear. I will give it a go. The compressor is in about 3 pieces right now, but should be back together in a week or so, I will have to give it a go then. I might have to come back and ask some questions about sizing my overload relay heater coils.....