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Air compressor motor


 
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Keith True
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PostPosted: Sat Aug 18, 2018 7:39 am    Post subject: Air compressor motor Reply to specific post Reply with quote

I have a Baldor 5 HP motor on my compressor that just gave up the ghost.I put in on in 1993,when lightning got my last one.It just gives out a loud hum until the breaker pops.One of the capacitors on the last one was split open and burned,so I figured I would take a look at these before I replaced the whole thing.I took the cover off and found three capacitors,two are plastic bodied and one is metal.I also found that one of the wires was pinched to the metal capacitor,and was shorted.It looked like it had finally wore the insulation away enough to short to the housing.All looks to be fresh arcing,and this motor has never been opened up until today.This was a factory assembly defect.Would that short fry one or all of those capacitors? I don't believe I've seen three on a motor like that before,with one of them being different.When I bought it I got the motor that would start under a load.It has been perfect for 25 years.
 
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Janicholson
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Joined: 30 Jan 2004
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Location: St. Cloud, MN

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PostPosted: Sat Aug 18, 2018 8:06 am    Post subject: Re: Air compressor motor Reply to specific post Reply with quote

Fix the short and try it. If it works cool beans, if not, a motor repair shop can chaec the caps to see if they are toast. Jim
 
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Bob
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PostPosted: Sat Aug 18, 2018 9:13 am    Post subject: Re: Air compressor motor Reply to specific post Reply with quote

CVPost-keith true wrote:
I don't believe I've seen three on a motor like that before,with one of them being different..


Plastic capacitors are high capacitance value, but intermittent-duty START capacitors, aluminum-cased capacitor is a lower capacitance value but continuous-duty "run" capacitor that remains in the circuit after startup .

Unless the short burned a hole in the case of the metal cap, it's unlikely it damaged the caps, but POSSIBLE.

It's also possible for a section of winding to have been damaged, depending upon exactly WHERE in the circuit the short occurred vs. how quick the breaker tripped, or the start switch contacts could POSSIBLY even have been damaged.

As Jim sez, patch up the short and give it a try.
 
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Keith True
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PostPosted: Sat Aug 18, 2018 10:06 am    Post subject: Re: Air compressor motor Reply to specific post Reply with quote

Thanks.I did try the motor after the shorted wire was pulled away from it's contact point.The copper wire is still intact and shiny,so it has not been shorting out for a long time.The wire isn't cooked yet,and the spot on the case is just a pencil point of arc residue.All this is with the belts off the motor.I have had the unloaders hang up before and try to start the motor with 150 pounds of air in the head.That will at least try to start,but this is with the belts off.I won't fool with it too much,but I do remember this motor was $325.in 1993.
 
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Janicholson
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PostPosted: Sat Aug 18, 2018 6:21 pm    Post subject: Re: Air compressor motor Reply to specific post Reply with quote

If it has an internal governor for the start caps/starting system, it may have failed contacts or springs. Jim
 
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Destroked 450
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PostPosted: Sun Aug 19, 2018 7:02 am    Post subject: Re: Air compressor motor Reply to specific post Reply with quote


With it making a humming noise ether the capacitor or capacitors are bad or the motor has a internal short.
You can try swapping the capacitors or take it to a motor shop and have them check it.
We have several 1/2 and 1 hp electric fan motors in our poultry barns, 95% of the time replacing a capacitor fixes one that's stopped running.

I have one of those old 3 capacitor 5 hp Baldor motors on my air compressor, replaced the capacitors in it last year, thinking all 3 where less than $50.
 
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buickanddeere
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PostPosted: Sun Aug 19, 2018 9:06 am    Post subject: Re: Air compressor motor Reply to specific post Reply with quote

CVPost-keith true wrote:
(quoted from post at 14:06:40 08/18/1Cool Thanks.I did try the motor after the shorted wire was pulled away from it's contact point.The copper wire is still intact and shiny,so it has not been shorting out for a long time.The wire isn't cooked yet,and the spot on the case is just a pencil point of arc residue.All this is with the belts off the motor.I have had the unloaders hang up before and try to start the motor with 150 pounds of air in the head.That will at least try to start,but this is with the belts off.I won't fool with it too much,but I do remember this motor was $325.in 1993.


Forget about starting it even under no load unless there are two functional and properly connected start capacitors .
There are premium quality capacitors rated with higher voltage and more mfd . The higher the voltage the less likely to blowup. The higher the mods the more starting torque the motor will have .
Even the metal run capacitor should be upsized in voltage and mfd.
 
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Destroked 450
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PostPosted: Sun Aug 19, 2018 1:32 pm    Post subject: Re: Air compressor motor Reply to specific post Reply with quote


In my experience there's a limit has to how much bigger you can go.
Some of my fan motors call for a 440 volt 15 mfd capacitor, I was out of that size and installed one that was 440 volt 30 mfd.
The motor started and ran but buzzed like it had a short in the winding or something.
I shut it down and locked it out till I could replace it, when I got some more of the correct size capacitors I put one in just to confirm the motor was bad, it ran just fine and is still running.

If it's a on going issue I may try a slightly bigger capacitor but if it's been doing fine and hardly ever gives a issue I put the same size back on.

Just my experience, I'm not a electric motor expert.
 
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rlp in Co.
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PostPosted: Mon Aug 20, 2018 1:35 pm    Post subject: Re: Air compressor motor Reply to specific post Reply with quote

Surplus Center sells capacitors. They also sell used capacitors. I usually buy the new capacitors that I need and buy some used ones to keep as spares.
 
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rustybronco
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PostPosted: Tue Aug 21, 2018 8:11 am    Post subject: Re: Air compressor motor Reply to specific post Reply with quote

If you have an analog VOM you can use it to give a somewhat indication on whether or not the cap is good. On the 1X scale, a fully discharged cap will sweep the needle to the low resistance end of the scale as its being charged up then it will taper back to the higher end.

If it sweeps to the low end and stays there the cap is shorted. if the needle shows no movement at all the cap is open. The bigger the caps value the slower the needle will swing back to infinity.

Those old Simson meters still have a use today. Wink
 
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Keith True
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PostPosted: Tue Aug 21, 2018 8:27 am    Post subject: Re: Air compressor motor Reply to specific post Reply with quote

Update on my compressor motor.I sat down to start serious testing,made sure it had power to both legs,(I did have a magnetic starter go bad before and only send 110 out)I unwrapped the foam from around the capacitors and found the terminal board pushed right out of one can.Big crack down the side,and a blob of white powder inside.I have no idea how long it could have run like that.It was running one minute,and five minutes later all it would do is hum.I really don't know if it could have caused anything else to fry or not.There is absolutely no smell at the open end of the motor,and anything I've ever seen that had something burn out inside has that unmistakable smell after.Now I'm off to find a part,it is my shop compressor and I am lost without it.If I don't have my compressor,torch,and forks on the Bobcat I might as well sit in the house and watch TV.

 
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