Capacitor Diagnostics Fun

[PowerOut]

My air compressor starts and charges the tank, but when pressure drops below the lower limit set point the motor will not start a second time until tank pressure decays sufficiently. The air compressor manufacturer suggests either the motor starting or running capacitor may be faulty. Both capacitors are made by Aerovox, the larger one is rated 25 microFarad and 370 VAC (I think this is the starting capacitor), the smaller one is rated 189-227 microFarad and 165 VAC (I think this is the running capacitor). How can I tell which is the starting and which is the running capacitor - is the larger capacitor rated at 370 VAC the starting capacitor because a larger voltage rating is required during motor starting? Also, how can I diagnose if either capacitor is faulty? Do I just need a continuity check? If so, what range of resistance should I expect to measure across the capacitor leads? Should I reject the faulty capacitor theory since the compressor will both start and run initially? On a parallel path I am also cleaning and verifying proper operation of the pump discharge check valve located on the tank inlet. Thanks in advance for replies.

 

[stevenal]

Sounds more like a faulty dump valve. Compressor shouldn't need to start against tank pressure if dump valve operates at the end of the cycle.

 

[jraef]

Capacitors will either work or not work. If your starting capacitor were bad, it would never start in the first place. No problem in the motor components would have anything to do with the pressure in the tank. Period. So first you need to better identify where the problem lies; in the motor (see below) or in the pressure switch.

The only possible connection I can think of with the motor is if it is overloading when it first charges the tank, then you are having to wait for the motor to cool down. Most small 1 phase tool motors have a device called a "Klixon" thermal protector in it that opens up the circuit when the motor overheats. Some have a manual reset button, usually red, on the motor typically on the non-drive end. Many others will automatically reset when the motor cools down again.

One way to test this (if you are electrically competent) is to remove the pressure switch and put in another manually operated switch or contactor (make sure it is capable of the current rating of the motor). Start the motor and charge the tank to the normal cut-off pressure. Shut it off and when the pressure drops to where the pressure switch should have closed again, try to restart it manually. If it starts, it was a problem with your pressure switch. If it doesn't, hold the manual switch closed for a few minutes and see if it comes on again all of a sudden. If so, it was overloaded and had tripped off, then reset itself. That means the motor is undersized or you have some other load related problem.

JRaef.com
"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." Scott Adams
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[itsmoked]

So what exactly happens when it "doesn't start"?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[PowerOut]

The motor turns over, then actually trips the breaker on the main house panel! This happens whenever the motor tries to start with pressure in the tank. I'm going with the faulty unloading (dump) valve suggested by stevenal (thanks stevenal - I think you nailed the solution). The motor will overload when it tries to start the pump against too high a discharge pressure that has not been relieved by the faulty unloading valve (located in the header between the pump discharge and the discharge check valve). I ordered a new unloading valve and will post again if I have to eat these words.

 

[itsmoked]

Ah ha!

It is simply this. The compressor is taking too high a current at start for the breaker and wiring situation. You can mess with all these other things until you're blue in the face. You will still have this problem. Put the compressor closer to the breaker (electrically) or wire a larger source.

I recently bought a new Sears compressor. It trips the breaker one out of 3 turn ons in an industrial setting. Very annoying.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jraef]

Yes, sorry. I interpreted your "doesn't start" comment to mean that it did even TRY to start.

JRaef.com
"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." Scott Adams
For the best use of Eng-Tips, please click here -> faq731-376

 

[electricpete]

I'm not sure, but I'm leaning toward agreeing with stevenal more than itsmoked.

A motor will draw the same LRC loaded or unloaded. The difference is in the length of time that the LRC is drawn (which the molded case panel breaker TOC will be sensitive to).

A loaded compressor will take more time to accelerate than an unloaded compressor.

If unloading the cylinders by means of a cylinder "dump valve" (I think I've heard it called an unloader valve) removes the mechanical load from the compressor during start, then the start against full pressure will act the same as the start without pressure and will be successful.

Moving the compressor closer to the power to help reduce the source impedance can certainly help speed up the start time but is likely not an easy task. If you look at the cabling and (in consulation with a competent electrician) conclude that you can safely install a higher-rated breaker while still protecting the cabling, that would be an option that might solve the tripping as well.

I also vaguely remember seeing on my home ac compressor some caps called "boost" caps or something like that I think that help boost the voltage during starting.

Is this a system that used to work and just stopped working?

To check a cap: inspect for bulge. Check capacitance if you have a c meter. On your multimeter resistance scale, it should start out looking like a short and gradually increase resistance reading as the cap charges.

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[ccjersey]

easier to see the resistance increase (like electric pete says) as the capacitor charges with an analog meter.

The smaller capacitance 25 microfarad 370 volt rated (usually metal case)capacitor is the run capacitor. The other one with a plastic case usually and a range of capacitance is the start capacitor.

If the tripping behavior is always the same, every time, then it has to be load related, not motor related. You should check the check valve(s) that keeps the tank pressure in the tank and out of the cylinders once the compressor stops. It is sometimes impossible to completely unload the cylinders if the check is faulty.

You would hear air leaking out of the (functioning) unloader all the time if the check were leaking back though. When the compressor stops, there should be a loud hiss of air as the unloader dumps the pressure.

 

[itsmoked]

We just had a guy in here with the same problem. His compressor would trip the breaker 95% of the time when plugged into an outlet out in his backyard but when plugged into the exact same circuit in his garage always started.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Marke]

If there is a problem with the start capacitor or the start switch, there will still be torque generated by the run capacitor, but this torque will be lower than it should be.

Starting against a loaded compressor will require a higher torque to accelerate to full speed. This will be affected by the offloader valve provided that there is one, but if the torque is to low, it will spin at a lower speed until something trips.

I would suggest that you check the switch and capacitor by measuring the current through the capacitor.
You can check the capacitor by connecting across a 230 VAC supply and measure the current through it. Capacitors do loose their capacitance over a period of time.

If the voltage drop is excessive, the start torque will also be reduced and could be below the torque required to accelerate the machine to full speed.

I would sugest the potential problems are:
start capacitor
start switch
low voltage during start
off load valve.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[electricpete]

keith - I'm all for keeping an open mind and trying the easy stuff first. If it's a portable compressor and as easy as pluggin it in somewhere else to see if it works, I would agree that would be the first thing to try.

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[electricpete]

I guess closer to the breaker panel has the best chance of success.

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[itsmoked]

Yeah that's all I'm really saying.. Closer, bigger extension cord, 20A outlet instead of a 15A, maybe even a different 15A breaker. Then go for 'broken stuff' since the op has never said the compressor ever worked, (and now doesn't).

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com