Air compressor keeps turning off

[commerou]

Hi, it may not be the best forum to poast these kind of questions, but I might be lucky one of you guys can help me.
I have an air compressor, which is as new(2 years old, but allmost never been used). When i turn it on it runs for about 15 seconds, then the thermal overload protection switch for the motor shuts it off. I can then reset it and it will do the same. Do any of you guys no what could be the problem?

It is a 2.2 kW, 380V, 3 HP motor. I have tried to change the oil in the compressor, but that did do the job.

Hope you can help.

-Commerou

[madworm]

Frozen bearings?

[monochrome]

Someway or the other it it pulling too much amps. The motor could be faulty, the thermal switch could be faulty. Are you using the right oil viscosity? Another option could be the check valve in the tank. If it is stuck open it put a heavy load on the motor when it tries to start, that could cause the motor to overload.

[NiHaoMike]

Check the start and run capacitors.

[tautech]

monochrome has put up the most likely faults.
Is it belt driven or direct coupled?
Single or 3 phase?

Try running the motor by itself.
Check the current draw with a clamp meter is not exceeding rated amps.

[Dave]

Since 380V is a standard line-to-line voltage for 3-phase systems, I'm going to assume it's a 3-phase machine.
Would it be possible for you to attach a thermocouple to the thermal protection switch and measure the temperature? You could quickly see if that thing operates properly.

[Precipice]

3 phase, huh? Is it running backwards? Compressors don't tend to enjoy that...
Does it sound OK? Does the motor actually start? Does it pump air before it cuts out? Any way to see how much current it's actually taking? (or does it feel warm when it's stopped?)

[Stonent]

Are you absolutely sure it's a thermal overload related shutdown and not a faulty pressure sensor?

[SeanB]

2.2kW, 3 phase motor is likely a DOL starting motor. Check cold resistance of the windings ( with power off, and then check it actually is getting all 3 phases, as it is likely that you have dropped a phase in the DOL contactor, or have blown a phase fuse. If you can disconnect the compressor and check it turns freely, both motor and compressor, and that the motor will run without a load. If you have a clamp meter ( even the horrid cheap HF one will do for current measurement, but do not trust it to check mains voltage or resistance, it is way to poor to do that) check each phase is drawing the correct current, as indicated on the motor nameplate.

[dannyf]

Hope you can help.

You probably want to run out if the thermal switch is malfunctioning. Does the motor run hot?

[G7PSK]

The unload or the check valve is stuck and it's trying to start under load, had the same problem myself, you can take them apart and clean the rubber seal but in the end you need to replace it. It most likely looks something like this and will be in the pipe that runs from the compressor to the tank and outlet and a small pipe to the pressure switch.

https://www.google.co.uk/search?q=compressor+unloading+valves&es_sm=122&tbm=isch&imgil=AlW5Xhv_bZ_MaM%253A%253Bhttps%253A%252F%252Fencrypted-tbn1.gstatic.com%252Fimages%253Fq%253Dtbn%253AANd9GcQXujgDLfKsfzb93yFYXM8yqx-e5Xz3FUCuFIgzaJ-zHcLFS1bc%253B250%253B250%253BZZBJdjLLicaW6M%253Bhttp%25253A%25252F%25252Fwww.air-compressor-guide.com%25252Fair-compressor-unloader.html&source=iu&usg=__jFzpsYBUT1hXhMV8K88ICcl84g8%3D&sa=X&ei=TamgU6vgOMXJPO_kgeAJ&ved=0CEgQ9QEwAA&biw=1264&bih=916#facrc=_&imgdii=_&imgrc=jMF5N53axvav2M%253A%3BBVh7vcyrA7Ww-M%3Bhttp%253A%252F%252Fwww.about-air-compressors.com%252Fimages%252F1056.gif%3Bhttp%253A%252F%252Fwww.about-air-compressors.com%252Ftank-check-valve.html%3B286%3B210

[commerou]

So I found the problem, after much trouble checking valves etc. and changing the oil.
Never trust the work of the last man making the plug, it was only getting 2 phases and 0 and ground, a three phase motor should ofcause use 3 phases 
So i rewired it and it is running like a dream.
Thanks for the support and answers on the forum

[tautech]

I wondered about that, but thought no surely that could not be. 

Lesson: never assume. 

Current clamp on each phase would have picked that up.

Good result.