Capacitor Diagnostics Fun
Capacitor Diagnostics Fun
(OP)
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.





RE: Capacitor Diagnostics Fun
RE: Capacitor Diagnostics Fun
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
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RE: Capacitor Diagnostics Fun
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Capacitor Diagnostics Fun
RE: Capacitor Diagnostics Fun
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
RE: Capacitor Diagnostics Fun
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
RE: Capacitor Diagnostics Fun
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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RE: Capacitor Diagnostics Fun
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.
RE: Capacitor Diagnostics Fun
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Capacitor Diagnostics Fun
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
RE: Capacitor Diagnostics Fun
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Eng-tips forums: The best place on the web for engineering discussions.
RE: Capacitor Diagnostics Fun
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Eng-tips forums: The best place on the web for engineering discussions.
RE: Capacitor Diagnostics Fun
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com