Air compressor capacitor died?

Lost in LA · Aug 7, 2009

Air compressor quit running this afternoon. No burnt smell. Motor was not hot. Just would not start. Power is 230 single phase to a 1-1/2 hp motor and is getting as far as the capacitors.

My best guess is that the cap died--Do not know which one--Probably start.

There are two capacitors. I will replaced both.

The run one is 30 microfarad at 240 vac.
the start cap is 750-800 microfarad at 125 vac. (this one I can purchase)

My catalogs only list the run cap as 30 microfarad at 370 vac. Would this work?

Thanks,
Lost

Butcher · Aug 7, 2009

Yes,as long as it is at least 230 v

9100 · Aug 8, 2009

Not necessarily. Start capacitors are not made to run continuously. You need to be sure that the replacement is run rated. Check your starting switch operation and contacts before spending any money. In my experience, the centrifugal switches fail more often than capacitors. Also, when capacitors fail, they often let you know it by smoking and oozing.

Bill

precisionworks · Aug 8, 2009

In my experience, the centrifugal switches fail more often than capacitors.

+1

If the caps don't look or smell bad, it probably is the switch. Contacts sometimes get burned from the repeated opening & closing, and you may be able to file them smooth or use emery cloth strip to polish them. The springs sometimes break or get weak.

------------
Barry Milton

peterh5322 · Aug 8, 2009

On my 5 HP compressor which has a capacitor start/capacitor run motor, it is the run cap which fails first as it is always in the circuit whereas the start cap is in the circuit intermittently.

For a 240 volt model, the start cap should be rated at least 250 VAC and the run cap should be rated at least 370 VAC.

I have yet to have a centrifugal switch fail, but I have had several run caps fail.

Torque1st · Aug 8, 2009

Many meters have a capacitor test function. Just make sure the cap is discharged and disconnected before testing.

JunkyardJ · Aug 9, 2009

My experience with compressors is mechanical.

Make sure the unloader and check valve are functioning properly if the compressor has those features. I've seen lots of compressors where the check valve failed, so they plugged the unloader. The motor will actually start on some compressors with this done, but one day it fails to start. So, check that stuff to make sure it's working. Then move on to motor problems. :cheers:

Lost in LA · Aug 9, 2009

Thanks everyone for your help.
I went in to work Sunday. Thanks Torque for reminding me of my digital meter. I am so use to using my analog I forgot about it. I tested both caps and got readings in the nano farad range. ( I left meter hooked up until the reading stabilized). I should be getting micro farad readings. I do not think both would be bad?

What readings should I get with a bad cap? Did I do something wrong?

The starting switch and contacts are OK.

Lost

JL Sargent · Aug 9, 2009

Check the capacitance range of your meter. If you are using a Fluke 87 III or older then the range of the meter is not high enough for those caps. A Fluke 87 V will get you there or a meter for HVAC work will do it also. Sounds like your meter is not setup for uF readings.

Lost in LA · Aug 9, 2009

JL- My meter only goes up to 200 micro farads--you learn something new every day--thanks.
It should have shown something on the 30 mf cap.

What kind of readings do you get on a dead cap?

thanks
Lost

macplus · Aug 9, 2009

You can use a higher volt rated capacitor, but with the same MFD rating. I would be real surprized if you had a bad run capacitor. The easiest way to find out what is bad, I would first disconnect the motor from the compressor. Then I would bypass the start capacitor. Since there is now no mechanical loaded applied, the motor should start. If the motor does not start, then the problem could be a bad start switch contact or bad governor switch or bad start winding or a open run capacitor ( since the run capacitor is in series with the start winding, and the start switch and start capacitor is in parallel with the run capacitor )

JL Sargent · Aug 9, 2009

A bad cap can read as an open or a short or out of tolerance. Any meter in ohms mode can test your caps to some degree. When you apply the leads the cap will take a charge and you will see resistance change. Reverse leads and you will see a changing reverse in the resistance.

I have a 5hp single phase Baldor motor on a Quincy 325 that has a weird coast down vibration. Its like when the motor moves through its first critical it has a very high vibration for a second or two. All caps check good, bearings seem good, no cracks in frame. Maybe its the centrifugal switch? Sometimes these motors can drive you coo coo.

Torque1st · Aug 9, 2009

Lost in LA said:
JL- My meter only goes up to 200 micro farads--you learn something new every day--thanks.
It should have shown something on the 30 mf cap.

What kind of readings do you get on a dead cap?

thanks
Lost

Usually you should get a reading of 27-33MF, basically +/- 10% of the marked value.

Search

Largest Manufacturing Technology Community on the Web

Search

Air compressor capacitor died?

Lost in LA

Cast Iron

Butcher

Aluminum

9100

Diamond

precisionworks

Diamond

peterh5322

Diamond

Torque1st

Aluminum

JunkyardJ

Titanium

Lost in LA

Cast Iron

JL Sargent

Diamond

Lost in LA

Cast Iron

macplus

Cast Iron

JL Sargent

Diamond

Torque1st

Aluminum

Similar threads