1. Home
  2. Electronics Repair
  3. Capacitor Start, Capacitor Run Motor

Capacitor Start, Capacitor Run Motor

I havd an air compressor that has the Subject line type of motor,

115vac. It only has about 200 hours on it. I had left it on for a couple of days recently and found that my circuit breaker in my garage was tripped. I restet it and the compressor started(surprised me because I didn't remember leaving it on.). It ran about 2 seconds and the circuit breaker kicked, again. Before I tore into the compressor, I made sure it wasn't the circuit breaker by plugging into the utility room circuit, 23 seconds later, it kicked that breaker. The strange thing is that it sounds different than before. It did have a deeper, stronger sound when the motor was running, now it sounds kinda puny. When the motor starts and runs, it turns the compressor pump an actually builds a small amount of pressure before it kicks off. I have pulled the starting cap and the run cap to get the numbers off, and have found them online. However, before I buy them, I was wondering if this sounds more like the main winding (not the startup winding) may be burned out. I haven't opened the case on the motor to look, yet, but I wouldn't think that the winding would NORMALLY short (thus more current), but sometime during the few days that I had left the compressor on, we had a lightning hit that destoyed a dvd player and an audio amp. It could be coincidence. Anyway, what would cause the motor to pull excess current. Would the run cap being bad cause this? I have checked both caps with a DMM, and all I can say is that they are not shorted.
Reply to
Robowang
Loading thread data ...

Short answer: replace the capacitors.

Reply to
Charles Schuler

I agree. The capacitors are cheap to replace. In general the run capacitor value is much less than the start capacitor value.

Reply to
Ken Layton

replace. In general the run

So, a bad capacitor could cause a hi current condition?

Reply to
Robowang

Not exactly ... see my short answer. Circuit breakers have two trip times. A stalled motor can evoke the longer trip time because a capacitor failure does not get the motor turning.

Reply to
Charles Schuler

I ordered the caps. However, The motor does start. It actually gets to where I would guess that the start winding is cut out, maybe 50-60% of it's speed. then it kicks the circuit breaker. When the compressor is first started, it is very easy to rotate the pump. Of course, as the pressure is built up in the tank, the compressor is going to work the motor harder. But it kicks out almost immediately. To pull that much current, the main winding would have to be near shorted. I have seen older motors lose the coating on the windings, partially shorting the windings and lose resistance due to losing wire length in the winding. This will cause the motor to not be as efficient at the same current draw. Or I have seen an open in the winding, which kills the motor, totally. Also, if the start capacitor is bad, wouldn't that just cause the motor not to turn at start up and just hum and jiggle? I think the start cap on a single phase motor is just to cause enough of an out of phase condition to get the rotor to start rotaing, then once it starts turning, the start winding (and start cap) is cut out and the main winding will continue the rotation in the direction the start winding phase pulled the rotor orignally. It's all smoke and mirrors. I'll get back when I get the caps. I hope it is just the caps, as a new 6hp peak motor is close to $200.00. thanx guys.

Reply to
Robowang

Of course the motor could be bad ... but the point is that the capacitors are more often bad. It is simply a matter of statistics, which is what you'll mostly get when posting here.

My experience with motors and capacitors is 10:1 capacitor failures versus winding failures. Just me ... no science ... just statistics.

Good luck.

Reply to
Charles Schuler

Hi!

As has been mentioned, your compressor's problem is likely the run capacitor. Even if the start cap were bad as well, the motor might still be able to start and get up to running speed just fine.

I don't think you can stress the capacitor enough with a simple meter test to make it fail. It sounds like the run cap breaks down with stress and time before turning into a short or more load than the circuit can handle. With time it might get worse, such that your meter test would reveal a short circuit.

I think you will find the replacement capacitors solve the problem, but I would still like to hear how it goes.

William

Reply to
William R. Walsh

I had almost the exact same problem about a month ago and it was just a bad run cap.

- Mike

Reply to
Michael Kennedy

That makes me feel better. Yeah, I think if it is a cap that it's likely that it's the run cap. I just don't have any experience with electric motors. The only thing I have ever experienced was replacing a start cap in a refrigerator. It wouldn't start up, just hum. The motor was a low rpm version and you couls actually start it by spinning the shaft until it got up to speed. A start cap fixed it. I just wasn't for sure about the run cap. It's been along time since I had any training in that area, but it seems like I remember that the run cap was to help the motor make torque efficiently once it reaches peak speed. I have used a heavy duty cap checker and even it wasn't effective. The best way to check big cap is substitution. But you gotta have an extra. I know a DMM is pretty useless for a cap out of the circuit, but it's all I have. I should have the new caps by the end of the week, So I'll let everyone know what happens. Thanx for the input.

Reply to
Robowang

Can you confirm that the start winding is actually cutting out. It may be that the centrifugal switch is the problem.

Not really. All you need is a few shorted turns which then act as the shorted secondary of a transformer.

One simple (in principle) test would be to remove the compressor load entirely and power only the run winding. Then, spin it by hand and apply power while it's still turning. It should come up to speed and run without tripping the breaker if the windings are good.

--- sam | Sci.Electronics.Repair FAQ:

formatting link
Repair | Main Table of Contents:
formatting link

+Lasers | Sam's Laser FAQ:
formatting link
| Mirror Sites:
formatting link

Important: Anything sent to the email address in the message header above is ignored unless my full name AND either lasers or electronics is included in the subject line. Or, you can contact me via the Feedback Form in the FAQs.

Reply to
Sam Goldwasser

Reply to
steve.dumouchel

That would be voltage drop over the distance, and with an induction motor the amperage will increase to keep the power constant.

Reply to
James Sweet

The breaker size is 20 amps. I am not using an extension cord. I did connect to seperate circuit with a 20 amp breaker and it tripped. also. I hope to get the caps this week, but I still haven't been notified that they were shipped. I have a decent Electronics store about 30 miles away, so if I don't hear from the net store today, I will cancel my order and go to the local store this weekend. Thanx guys.

Reply to
Robowang

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.