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Compressor starting problems.
John Bonnett - 6/1/21 at 06:51 PM

My belt driven compressor has a single phase 3hp motor. A year or so ago the capacitors blew and I had the motor repaired by Rotamec. Since then it has struggled to start in the cold weather normally tripping out the thermal switch or blowing the 13Amp fuse. I'll add a link to a short video which illustrates the problem. The compressor isn't tight and can be rotated easily by hand.

I have spoken to Rotamec but they don't have any suggestions. As far as I remember cold weather didn't bother it before the motor gave trouble.

Any ideas please?

https://www.youtube.com/watch?v=uQpJ81ZWMB0

[Edited on 6/1/21 by John Bonnett]

theconrodkid - 6/1/21 at 07:44 PM

is the unloader working ?

John Bonnett - 6/1/21 at 08:01 PM

quote:
Originally posted by theconrodkid
is the unloader working ?



I've never heard of that let alone know if it's working. I'm sorry I'm a complete dunce with most things electrical so can you explain please?

r1_pete - 6/1/21 at 08:20 PM

My old machine started doing the same in cold weather, I put it down to cold thick oil and the motor aging.

Fresh oil helped, but I ended up replacing it...

rusty nuts - 6/1/21 at 08:26 PM

Unloaded valve could be sticking, normally fitted in the main pump to cylinder pipe and has a small pipe attached . Had similar problems years ago .

theconrodkid - 6/1/21 at 08:35 PM

quote:
Originally posted by John Bonnett
quote:
Originally posted by theconrodkid
is the unloader working ?



I've never heard of that let alone know if it's working. I'm sorry I'm a complete dunce with most things electrical so can you explain please?

as above, when it starts spinning, air is dumped through the valve till the motor gets up to speed then closes, you should be able to hear / feel it.

rusty nuts - 6/1/21 at 09:02 PM

The Mig Welding Forum has a section covering air compressors , you may find better advice there?

daviep - 7/1/21 at 08:54 AM

quote:
Originally posted by John Bonnett
quote:
Originally posted by theconrodkid
is the unloader working ?



I've never heard of that let alone know if it's working. I'm sorry I'm a complete dunce with most things electrical so can you explain please?


It's a small valve often located inside the start stop switch. You should hear a hiss of air when the motor stops when full pressure is reached? If you don't hear the hiss then unloader maybe faulty or blocked up with gunge. There will be a small air pipe going up into the switch, this connects to the unloader.

Regards
Davie

John Bonnett - 7/1/21 at 09:08 AM

Thank you all for your very helpful replies. I'll investigate.

SteveWalker - 7/1/21 at 12:33 PM

I don't know if they do it at all in small compressors, but instead of an unloader valve, large industrial compressors unload by mechanically holding the inlet valve(s) open for starting and stopping.

obfripper - 7/1/21 at 01:21 PM

The contactor in the stop/start switch can also arc when worn which can cause poor starting torque and tripping fuses, most industrial rated 1ph compressors use a 3ph contactor but only use 2 of 3 contactor terminals, so a temp fix is to swap the pair of wires from the worn contactor to the unused pair.

Dave

John Bonnett - 7/1/21 at 04:33 PM

Well I've checked that the unloader valve is working so I don't think that can be the problem.

God point about the contacts on the on/off switch. thank you for that.