Help with finding capacitor

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  #1  
Old 04-06-14, 11:17 AM
Tolyn Ironhand's Avatar
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Help with finding capacitor

Father in law called me having trouble with his air compressor. Said it is hums on start up and then trips the breaker. I tell him it is likely a bad capacitor and you can test them to see if any of them (there are two) are bad. He ended up just swapping out the motor to get things back up and running.

Visited them Saturday and I see the old motor sitting on the bench and he said the new motor runs the compressor just fine. However, he cannot find a replacement start capacitor. He is kind of old school and has only looked in his Grainger, and McMaster Carr catalogs, and couldn't find one. I have looked online and I am having trouble locating one as well.

I tested the cap and it seams that it is bad except one thing is bothering me. If you look at the pictures you can see a resister between the two terminals. When I test the cap the needle swings toward 0 like it should and then starts to back off and stops at about 5 on the 1K setting.

This is not my area of expertise so I need some help to determine if this thing is good or not, and where I can get a new one.



 
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Old 04-06-14, 12:15 PM
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Ok, found out the resister is a bleed down resister. Cut out of the circuit and now when I test the cap needle swings to 5 (On the 1k setting) and then sloooowly drops down towards infinity.

Now I'm thinking this may be good and the centrifugal switch is stuck open.
 
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Old 04-06-14, 12:52 PM
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For your application you have quite a bit of leeway on mfd choices. Here is one at Grainger that will work fine.
DAYTON Motor Start Capacitor,340-408,165v,Round - Capacitors - 6FLV1|6FLV1 - Grainger Industrial Supply

The test you used is not always dependable. The capacitor can check good, but not work in the circuit.

A formula for future reference:

Start capacitor sizing formula
2650 X Full Load Amps Divided by Supply Voltage = MFD
If the full load is not the locked rotor amps then increase MFD by 1/3
for hard start loads such as compressors or augers.


RR
 
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Old 04-06-14, 12:54 PM
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Now I'm thinking this may be good and the centrifugal switch is stuck open.
That's what I was thinking from the start.

WP53781 Motor Start Capacitor - Grainger Industrial Supply
 
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Old 04-06-14, 02:34 PM
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I found the centrifugal switch and it appears to be good. The link you posted PJ is for 120V

@RR So that cap would work even if the MFD is not the same range?
FLA = 15 x 2650 = 39750 So, would that mean I would need about a 397 MFD +1/3?
 
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Old 04-06-14, 03:36 PM
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So that cap would work even if the MFD is not the same range?
FLA = 15 x 2650 = 39750 So, would that mean I would need about a 397 MFD +1/3?
Use 120 for your voltage and you get 331.25 which is in the range of capacitor you have removed. Add 1/3 to get 442 which is still in the operating range of motor. That size will kick it harder at start, but since it will reach speed and centrifugal switch disengages in about a second it will do fine. You probably don't have 'locked rotor' amp draw which the formula refers to so you may use the 1/3 more MFD add for a hard start compressor.
GOOD LUCK!
Have rescued several electric motors that owners thought were trash. Have a good 1/2 hp one I repaired last year. Gave $0.25 for it and by time I had it running like new I had spent another $4.00. Needs a power cord which I will take from a dead window AC. (no cost) Might spring for an on/off switch.

RR
 

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Old 04-06-14, 04:55 PM
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Locked rotor amps is 91.

Centrifugal switch really seams to be to be working after that I have messed around with it more. I am going to see if I have a meter that can measure capacitance and see what that reads.

Thanks for the help so far!
 
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Old 04-06-14, 05:01 PM
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You can also measure the switch for continuity.
 
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Old 04-06-14, 05:07 PM
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You can also measure the switch for continuity.
I did, that is why I'm pretty sure it is working. I could measure between it and the windings of the motor and then see it drop out when I manually moved it.
 
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