Air compressor motor repair - Page 1
Forum section: electronics / repair

author: electrolux
author_rank: Regular Contributor
timestamp: March 22, 2018, 12:08:25 pm
content: I have an air compressor which won't start. When plugged in it attempts to spin while humming before blowing the fuse.I have checked the start capacitor and that is fine. There are no worn through wires anywhere.So the only option now is to test the motor windings, I believe it is a reluctance motor and my question is how can I test the stator?Here are some pics if they are useful:This is definitely not the rotor. /sThis is not a stator. /sThis is the switch unit.

author: orbanp
author_rank: Regular Contributor
timestamp: March 22, 2018, 02:38:55 pm
content: Chances are that the windings are just fine!I have a cheap Chinese air compressor that stopped working after one year (it was only turned on once during that time).In my case it was the centrifugal switch at the end of the motor that switched on the starting capacitor.Cleaning the contacts solved the problem.Does your motor have a centrifugal switch?They also could use a thermistor in series with the capacitor that effectively "disconnects" the cap once the thermistor has warmed up.Next year the overtemp switch quit working, again I could disassemble it and clean it.It has been working fine since then.I guess you get what you pay for it...Good luck, Peter

author: wictor
author_rank: Regular Contributor
timestamp: March 22, 2018, 04:58:16 pm
content: There is also unload or start valve that decreases pressure during start. That can be faulty.

author: 6PTsocket
author_rank: Regular Contributor
timestamp: March 22, 2018, 07:06:14 pm
content: Orbanp. In many years of motor repair, I have never seen a thermistor used to disconnect a a start capacitor. It would be far too slow, they do not go to zero so it would make a lousy switch. Most are positive temp coefficient, the resistance goes up as temp goes up so when the motor starts the resistance is high, preventing the normal high start current. The actual alternative to a centrifugal switch is a current relay, mechanical or solid state, wired in series with the motor.

author: orbanp
author_rank: Regular Contributor
timestamp: March 23, 2018, 12:25:43 pm
content: Hi 6PTsocket,Thanks for the comment!I have not seen the use of PTC resistors as starting switching means either, just read about it, though it sounds feasible solution.The document called the component a thermistor, but they obviously meant the PTC variety and not the NTC one.I also have to mention that my experience is limited to repairing my own faulty equipment, so no "serious motor repair practice".Regards, Peter

author: coromonadalix
author_rank: Super Contributor
timestamp: March 24, 2018, 01:38:54 pm
content: on the photohttps://postimg.org/image/bbfn8kjyf/the switch unitThe top contacts relay / switch arent even with the botton ones, and you have one of them top right darker in color its a bad contact sign / over current ...

author: SeanB
author_rank: Super Contributor
timestamp: March 24, 2018, 02:53:27 pm
content: Is there air pressure in the tank when trying this? If no pressure then your motor run capacitor might be bad ( they generally lose capacitance, and when it drops below 80% of the rating the motor generally will not start), or the pressure switch might be defective, though what often happens is the unloader valve, which is the thin plastic pipe that goes to the side brass connector of the switch direct from the compressor piston outlet, is faulty. Another common issue is the non return valve on the connection to the tank starts to leak, and then the compressor has back pressure and will not be able to start as the motor does not have enough torque.These motors rarely have centrifugal start switches, they are almost all permanent split capacitor type as the motor generally is starting into an unloaded compressor and can get enough momentum into the compressor so that when it does build up enough pressure to pump it will be at full torque and will keep on running. Then when set pressure is reached the switch both disconnects power and also dumps the air in the compressor cylinders via the unloader valve, with the non return valve on the air reservoir keeping the tank from losing pressure via this valve. That is why you must use the compressor switch to power it on and off, as otherwise it might try to start on a full pressure line and will stall.

author: electrolux
author_rank: Regular Contributor
timestamp: March 25, 2018, 01:14:50 pm
content: Quote from: SeanB on March 24, 2018, 02:53:27 pmI've already measured and replaced the start cap so it's for sure not that.There is power getting to the motor since it does try to turn over about an 8th of a turn before stalling, surely that indicates the pressure switch is fine? There is no pressure in the tank.The motor shaft is easy to turn by hand so nothing is seized.All this is what makes me think it is the stator which has something wrong with it which is why I want to test it but I'm not sure how.

author: SeanB
author_rank: Super Contributor
timestamp: March 25, 2018, 04:53:09 pm
content: Can you disconnect the outlet piping, and turn the compressor over a few cycles by hand, feeling that every cycle it does blow air out of the disconnected pipe end of the compressor. Then try running it with power like that, and see if the motor runs no load with the pistons running. Is there any contamination in the oil in the compressor, or has the oil all been turned to black sludge or is full of glitter.

author: G7PSK
author_rank: Super Contributor
timestamp: March 25, 2018, 05:23:56 pm
content: Use a meter to check for continuity on the winding's they will usually be fairly low, so you are looking for open circuit or dead short unless you have the specific resistance for that motor available. Try connecting the motor up without the compressor attached, if the motor still does not start try spinning it with power on, if it continues to run you know there is a problem with the start circuit.

author: james_s
author_rank: Super Contributor
timestamp: March 27, 2018, 12:15:59 am
content: Quote from: 6PTsocket on March 22, 2018, 07:06:14 pmA thermistor controlling the start winding is very common on refrigerator compressors, these normally don't have a capacitor though. I've had to replace quite a few of these PTC thermistors that had burned out.

author: tpowell1830
author_rank: Frequent Contributor
timestamp: March 27, 2018, 02:46:22 am
content: Quote from: electrolux on March 25, 2018, 01:14:50 pmThe pressure switch is connected to the tank, so if there is no pressure in the tank, yes, it will try to start. However, the head pressure is your problem. If the motor starts turning and then stalls, the head pressure is stalling it, and the torque of the motor can't overcome this. There should be a little tube with a Schroeder valve connected to your switch assembly (mechanically) opening this valve to relieve head pressure. Check this to see that it is operating correctly, otherwise, you will burn the motor trying to start.Unfortunately, the system in AvEs vijeo doesn't have the head relief tube/valve, however, a lot can be learned.

author: electrolux
author_rank: Regular Contributor
timestamp: March 27, 2018, 03:06:40 pm
content: Quote from: SeanB on March 25, 2018, 04:53:09 pmI have taken off the compressor housing and the oil is clean as can be. It is pumping air and turning freely when spun by hand.

author: electrolux
author_rank: Regular Contributor
timestamp: March 27, 2018, 03:08:10 pm
content: Quote from: G7PSK on March 25, 2018, 05:23:56 pmWill try this. Thanks!

author: drussell
author_rank: Super Contributor
timestamp: March 27, 2018, 03:32:20 pm
content: You say this motor has a start capacitor, so there must be some kind of centrifugal start switch. Are yousureit is working correctly? Your issue certainly sounds a lot like a start switch problem, which is a very common failure.

author: 6PTsocket
author_rank: Regular Contributor
timestamp: April 04, 2018, 09:52:43 pm
content: Quote from: james_s on March 27, 2018, 12:15:59 amLive and learn.Sent from my SM-G900V using Tapatalk