Induction motor on compressor died
Short story: My HF compressor http://www.harborfreight.com/cpi/ctaf/Displayitem.taf?itemnumber=47065 died the other day.
Long story: After closer inspection, the overload switch was tripped. I pushed it back in, but the motor would only start for about 1 second not at full speed, then it would trip the 15 A overload switch. Here’s what I know about this compressor / motor: 110V power, Oil-lubed, single-stage, direct drive, dual-capacitor induction motor (not TEFC, open to sawdust). I was able to determine that one of the capacitors was blown. The two capacitors are rated at 70 uF and 80 uF for (up to?) 250V. The 80 uF capacitor was the one blown. $18 later, I had a replacement 80-120 uF capacitor installed. Got everything put back together and flipped the switch. Yee-haw it was running again at full speed! But wait, about 10-15 seconds go by and the overload switch was popped again! Crap. The capacitors were hot too.
So that leads me to really investigate this motor. I unbolted the stator from the frame and got a good look at the armature. See attached pics. The mechanicals seem to run smoothly, so I don’t think it’s getting caught up with a stuck piston or something. After doing a little research, I was surprised to find that the motor did not have any type of centrifugal switch that turns off the start capacitor and winding upon reaching full speed. Apparently, there are 2 windings that are directly wired from the stator to 2 separate capacitors (70 and 80 uF) with the aforementioned 15 Amp overload switch wired in series. This is probably what I get for buying a cheap chinese compressor. The website below was very helpful in describing what kind of induction motor this compressor has:
http://www.iprocessmart.com/leeson/leeson_singlephase_article.htm
Of the types of motors listed on this webpage, the PSC (Permanent Split Capacitor) type fits my compressor motor the best with the exception that it has 2 caps instead of 1. I do not know which capacitor functions as the start capacitor and which one is the run cap.
I have pretty much conceded that I won’t be able to save this compressor, and that a new one is in my near future. However, I just want to know what is going on with this thing. So what is happening if the motor runs ~15 seconds at full speed then trips the overload switch, making the capacitors hot in the process?
Any input is appreciated! -Steve in NM-
Replies
What is up with the rust? Used outdise or near the ocean? Like you said I think you should replace the whole thing. Hey, you get what you pay for. If all is runnning well I really can't guess, but I don't know all that much about motors.
Mike
Just me..
Before doing ANYTHING take it to a Motor rebuilder.. They check everything and will tell you if worth fixing!
Really...
I'm cheep and pretty handy BUT.. Sometimes it makes sense to ask before you buy anything electrical!
EDIT.. I have a degree in Electronics BUT motors.. Well.. I know they rotate!
Edited 5/13/2005 12:50 pm ET by Will George
Edited 5/13/2005 12:52 pm ET by Will George
Don't give up yet!
If you have two caps, each on a different winding, one is probably for start and the other to assist a bit under high torque conditions. This is a bit unconventional, but have seen this before.
From the picture, it seems as if the rotor has rubbed?
Have you megged both stator windings, to see if you have lost insulation?
If you don't have windings specification, it's difficult, but measuring the difference in resistance, between the starting winding and the running winding may give you an idea.
Do you have an amp probe, to check if the overload is actually tripping at 15Amps, as it could also be a faulty overload?
If everything else fails, I would again check the 80uF capacitor you have replaced, the original may be different and has to be exactly replicated. This will be a continuous use heavy duty cap, unlike one used for starting only.
one is probably for start and the other to assist a bit under high torque conditions
Yep that centrifugal switch!
Hey Steve,
I looks like you're armature is rubbing. Given the rust in the photo, I suspect that the stator might have some rust too, and if the plates have expanded due to rust, that might be your problem.
If your armature is rubbing, you're sure to have over heating.
Tom
Hello Wood/compbutcher,
Going on what you describe and those pics I can see that the rotor has rub marks, may be rubbing against the plates or there is some gunk or high spot there.At no time should there be contact between these two- so that is my prime suspect.Wouldn't think that the bearings are to blame-although might be assembly or manufacturing fault.Could also be some insulation breakdown too.It is drawing too many amps.
So if you have no test instruments or sufficient knowledge you can't do more than simple checks. I think you should take it to a motor rewinding shop where it can be checked quickly.
Edited 5/13/2005 10:23 pm ET by mookaroid
Woodbutcher, sometimes you can identify the run capacitor as the one which is usually in a metal can type container that is usually oval vs round. This is because run caps are usually made to be in some sort of oil bath for loger life and durability.
Start caps are almost always the round cylinder, plastic case and are only used by the motor during the first 4-10 seconds of startup. The Start caps are disconnected electrically from the motor when it achieves a certain rpm. A centrifical switch inside the motor performs this task. You should be able to hear the switch when the compressor spins down. Just before it stops you will hear a click as the switch kicks back in. If you don't hear a very obvious click I would suspect the switch, otherwise suspect the start cap or something dragging on the motor armature/stator (like in your pic #2, although it didn't look like it had rubbed too much.). Make absolutely sure that if you replace either of the caps that you replace them with the same value of capacitance (usually expressed in micro-farads, or uf) and that the caps are rated for motorstart or run applications depending on which you are replacing, last but not least the voltage that either cap is rated for should be a minimum of 250vac, but 350-450vac is even better. Never use a cap rated for less voltage than the motor runs on, you could get severely injured by a fragment of the cap as it blow up!
The symptoms you describe sound like the start cap is not doing it's job helping get the motor up to speed during startup, since your breaker blows at just about thew time the start cap should be getting switched out of the motor circuit. You could try to start the motor with the start cap unplugged, if the symptoms do not change it is most likely the start cap is bad and needs replaced. I have used many of the HF motors and have never had one with no centrifical switch for the start winding, but I could easily be wrong, but it would seem to me that leaving a start cap on all the time would result in it blowing up in it's first few time in service, and yes it would be very hot. how many hours approximately does the motor have on it? HF sells the motor on sale every few weeks for 69.95, sometimes with free shipping. The motors have a 1.0 service factor which implies that the motor is always rtunning as hard as it can having nothing in reserve for overload. This reduces the service life obviously. HF also sells a 5hp motor with a 1.1 service factor which means it only runs flat out during a (hopefully infrequent) overload situation. The difference between the two motors is very obvious, yours is about 34 pounds in weight, draws 15 amps, and drive a shaft of 5/8" diameter, whereas the 1.1 service factor motor HF sells weighs 80 pounds, draws 23 amps, and has a 1 1/8" shaft. (BUT unfortunatly costs over 200$ on sale). They both speak fluent chinese which is about the only thing they have in common, I have never seen the large motor on a compressor packaged system from HF, but most other compressor offering in the same price range likely are using the same or very similar components no matter who you get it from. I you want one to last longer, it is gonna cost ya more, no magic here at all. Good luck in your quest for a new system, or repair of this one if that is where you decide to go as an alternative. Remember , that in the world of electric motors, the 98 pound weakling can NEVER become the muscle bound buff- type motor no matter how hard he trys....regards, Joe.
Joe and others,Thanks for the very detailed replies concerning my compressor motor woes. All your comments make sense, even to a guy with no formal electronics training. You mention that the motor should have a centrifugal switch- it does not. The start winding and run winding are wired directly to the 2 capacitors. There is no centrifugal switch in the circuit (much to my dismay). This motor is not a self-contained unit. If you re-check the pictures, you'll see the motor is bolted directly to the side of the cylinder head. The crankshaft of the motor goes through a sealed bearing and attaches directly to the cam. I know exactly what the "click" sounds like on an induction motor with a centrifugal switch. I hear it on my DP, jointer and TS when they spin down (belt driven).
I'm going to see if I can test the windings for a short. I definitely need to clean off the armature, my guess is it's not rust but rather sawdust. In particular, mahogany that I've been working with since January. The motor has a sort of impact shield, but all the internals are open to fine sawdust floating in there.
I hope I can heal this one, but if not I already have my eye on a good belt-driven compressor.
Can you post more pictures of the field windings and anything else that wires go to? My first concern would be fried insulation/shorted windings. If the windings are OK, then it's worth trying to save the motor. Even without a centrifugal switch, there might be separate start and run windings. Some inexpensive motors use a timer of some kind instead, just betting that the machine started OK and got up to speed.
If the capacitor is getting hot, My guess is that would be the culprit. Take it to a good motor shop, and have them test it. It is only about a $10 part.
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