Electric Motor help

Thread Starter

repairman

Joined Oct 31, 2009
4
I have a 20 year old Sears compresor that sometimes would not start, just ssort of hum and pop the motor breaker. It was complicated by a loose pump pully that would jam. That's fixed now. Now, when power is
applied, the motor runs slow for several seconds, then the motor breaker pops, even with the belt removed. Applying power turns the motor slowly but the shaft is not binding. I disconneced what I thought was the start capicator, on top of the motor, 704 uf, 110v and tried running the motor. It still started to turn slowly with the cap removed, which makes me wonder if this is a run capicitor. Also, I removed the cap to test it with an ohmmeter, and it acts normally, no shorts or opens, resistance builds.
The motor is 110v, 17amp, 3450rpm, a case like a 1/3 or 1/2 horse size.
Do you think replacing the cap will solve the problem or is it something more serious?
 

ELECTRONERD

Joined May 26, 2009
1,147
It certainly could be the cap, depending on how it's connected in your circuit. If one end goes to a positive source and the other a negative, it could no doubt be shorting your circuit. Check for signs that the cap is bad, this may include a bulging top (not on the sides) and sometimes leakage.

Take your multimeter and put it on the resistance setting on both sides of the cap, when the breaker pops you might see a short, which will verify whether the cap is bad or not.

Austin
 

rjenkins

Joined Nov 6, 2005
1,013
If it is a Start cap (rather than Run), there may also be a starter switch or relay unit which switches the cap into circuit. It could be a centrifugal switch on the motor.
 

GetDeviceInfo

Joined Jun 7, 2009
2,192
at that value, it will be a start cap. do a continuity check to make sure your centrifugal switch is engaged and that your start winding isn't open.
 

BillB3857

Joined Feb 28, 2009
2,570
Take the belt off and when you try to start the motor, VERY CAREFULLY help the pulley spin in whichever direction it is trying to go If it is just humming, direction won't matter. If it is a bad start switch, start capacitor or burned start winding, the motor should take off and come up to full speed. Otherwise, I would say it's toast.
 

Thread Starter

repairman

Joined Oct 31, 2009
4
Just to clear some things up:
When the cap is tested with an ohmmeter, the resistance slowly rises to a high nimber. Reversing the leeds does the same. Connecting the ohmmeter across the cap leads with it disconnected is an open circuit,

With the belt off the motor, it turns freely.
When power is applied the motor turns slowly by itself until the breaker pops. This is the case WITH or WITHOUT the cap connected. Spinning the already spinning shaft does nothing to change the speed.
 

SgtWookie

Joined Jul 17, 2007
22,230
My Dad had a Craftsman air compressor that he bought in the 70's. It had a capacitor start motor.

I'll bet that the switch contacts on the centrifugal switch are burned after making/breaking contact many times.

You'll need to open up the motor, polish the switch contacts using a very fine jeweler's file or 600 grit wet/dry sandpaper (removing as little material as possible), make certain that the mechanical portions of the switch are moving freely, and reassemble it.

Don't use emery cloth or regular sandpaper on the contacts; they'll get embedded in the contact material and they will very quickly get burned again. Don't try to make the contacts perfect, just draw sandpaper or a file through them several times to remove the worst of it. You can take the material off, but you can't easily put it back on.
 
The first thing that I would do is replace that capacitor. Shining up the centrifigal swich contacts will help (if there is one). Check the windings with an ohmmeter and check for shorts to ground too. The reason your CB trips is that the motor is trying to get up to rated speed and can't. The time delay built into the CB will hold (for a while) to let the motor get up to speed.

Cheers, DPW
 
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Thread Starter

repairman

Joined Oct 31, 2009
4
The best idea seemed to be to check the switch. The motor is almost impossible to remove without dis-assembling the entire unit, so I decided to check it installed. As I began to release tension on the 4 thru bolts that hold the motor end pieces in place, I heard a "click" just like a centrfugal switch. I tightened the bolts and the motor runs fine so far.
Only time will tell how long
Thanks to all for the suggestions
 
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