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randyjaco

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#1
I have a Quincy air compressor with a Baldor 5hp single phase 220vac motor. The motor is now crapping out. When the power is turned on, the motor hums loudly and turns slowly until the breaker blows. With no load the motor spins right up and runs smoothly. The first thing I did was check the pump. It turns easily, so the problem is not there. Next I ordered new replacement caps. (Not cheap with the Baldor name on them ). I hooked up the new ones exactly like the old ones. I turned on the power; and no change , humming and slow turning till the breaker shuts it down.
I have had this unit for several years, so there is no warranty. About 6 months ago fiber board that houses the starter contacts over heated/burnt and I replaced it. Everything worked fine until last week. I rechecked the fiber board and it looks fine. So at this point I am out of possible solutions.
Anybody have an Idea on what I should do next?
My shop is at a standstill without my compressor, so I need a solution ASAP. Also I am electronically challenged. I am also a little miffed, as I bought the Quincy/Baldor compressor so I wouldn't have to deal with such problems 8-(
Thanks
Randy
 

markba633csi

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#2
Randy: Sounds like a bad or burnt up connection related to the starting contacts OR you have a bad winding in the motor- hate to say it, but a new motor might be in your future
 

JimDawson

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#3
I agree with @markba633csi.

I think a new motor is in your future. But don't do what I did, make sure you get the same RPM motor. I screwed up and got a 1725 RPM when the original was a 3450 RPM. Turned my 5 HP compressor into a 2.5 HP compressor, on the other hand the pump will last forever at the lower RPM and the motor is not working at all.
 

WCraig

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#4
Any competent motor shop would be able to check the capacitance on your new and old caps. That way, you would know for sure if your old cap was really bad. Perhaps your new cap was bad or wasn't the right millifarad rating? If the caps are OK, then you have a more serious problem.

Craig
 

Alexander McGilton

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#5
The statement that the motor runs smoothly under no load, then pulls the breaker under load suggests that there is a short in the one of the primary winding. Under no load the difference in coil length is negligible so it runs smoth, under load there is reduce inductive reactance and already low resistance so it pulls more power. The same thing happened to my lathe where it blow smoke and growled under load, yet ran smooth under no load. I checked the resistance and acrose the two main windings and there was a fifty percent difference, so there was no choice but to replace it.
 

Ulma Doctor

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#6
Hi Randy,
just for giggles, have a look at the start capacitor, it may be faulty.

i'm assuming you have a run and a start cap for a motor running on single phase doing compressor duty.
in a no load situation , the run cap may be enough to get the motor started and run normally,
but when you would start under load , the absence of a good start capacitor would produce the same symptom you originally described.

secondary to that, there is a check valve in some air compressor tanks, that when faulty, will prevent air to enter the tank and therefore build up excessive pressure in the tank charge hose. the excessive pressure could prevent the motor from starting similar to what you originally described as well.

i have run into both scenarios in the field
 
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benmychree

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#7
I agree with @markba633csi.

I think a new motor is in your future. But don't do what I did, make sure you get the same RPM motor. I screwed up and got a 1725 RPM when the original was a 3450 RPM. Turned my 5 HP compressor into a 2.5 HP compressor, on the other hand the pump will last forever at the lower RPM and the motor is not working at all.
If your compressor is belt drive, simply double the motor pulley size to run the pump at its correct speed.
 

SubtleHustle

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#8
Hi Randy, while the motor is "grunting" have you tried "push starting" it by hand? You need to be very careful that you don't get tangled up in it, but if you can get it started with a good push, I would guess its your start capacitor..
 

Chipper5783

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#9
Hi Randy,
Umma is talking about the unloaded feature. The line to the tank should de pressurize when the pump shuts off (the brief hiss when the compressor shuts off). Is there any chance that is staying pressurized up to the valves in the head?

Does the motor smell burnt at all?

Try doing an uncoupled start with the starting capacitor disconnected. The motor will probably just humm, which is fine. However if it starts the same whether the start windings are enabled or not - then you may be able to conclude that there is an issue in the starting circuit?

Regards, David
 

Z2V

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#10
Randy, you already said that you replaced the caps with no change. I’m with @Ulma Doctor on looking at the check valve.
 

Ulma Doctor

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#11
another possibility comes to mind.
inside the motor, on the side opposing the shaft end, there is the centrifugal start switch.
the centrifugal start switch effectively switches the start capacitor in and out of the starting circuit.
it is a normally closed set of contacts that is controlled by flyweights and spring tension.
when the motor gets to about 80% of it's nameplate RPM during use, the start capacitor circuit drops out because the flyweights open the circuit.
if there is a burned or otherwise faulty connection or contact, the circuit will not function as intended, also giving the aforementioned symptoms

i have seen many failures of this type as well
 

FOMOGO

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#12
Quincy compressors normally don't use a check valve at the tank. That function is controlled at the pump. I would take the motor in and have it checked out, if you have someone locally available. Like Doc said, the centrifugal switch could be the culprit. Mike
1543672276589.png
 

randyjaco

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#13
Wow Guys, thanks for all the responses. It does look like a new motor is in my future:( Taking it to a shop is not much of an option. Most of the shops around here are backed up. The last time I took a motor in, it was going to take a month for them to get to it. The estimated cost was close to the cost of a new motor:confused:
I need a running compressor. I have now been without one for over a week.
Thanks again
Randy
 

JimDawson

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#14
If your compressor is belt drive, simply double the motor pulley size to run the pump at its correct speed.
True, but I can't quite fit double the pulley size in the space available, more like about 150%. But the compressor actually serves my general shop air needs as it is, that and I didn't want to spend $130 for a new pulley. If I do need more air I just switch in the backup 5 HP compressor. :)
 

warrjon

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#15
My first guess would be the start capacitor. A 5hp motor would have a switched start capacitor and a run capacitor. It is very common for the centrifugal switch to jam and not switch out the start cap, 30ms and the start cap is fried.

Pull the capacitor out and use an ohm meter on high range the reading should start low and slowly raise to infinity.
 

Cooter Brown

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#16
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