Compressor advice needed.

My mates got a clarke warrior 30 compressor 110 v which has decided to pack up. Nothing has seized or over heated and oil level is ok so we don't think it's a mechanical problem. The motor hums when the unit is switched on so power is getting to the coils. We are thinking along the lines of a faulty capacitor or may be the pressure switch. Any ideas on what tests we can do to try and establish what failed.

Mine did that after being sat in a shed for years, I stripped it down (all the plastic gubbins) and turned by hand and it freed up.

Hi Drift it was working ok then it just decided to stop but we can turn it over by hand with no problem

Remember you have 2 sets of windings with a induction motor, so it could be one of them is o/c...If you look at the wiring at the capacitor, you should see three wires...live supply, direct feed from live supply to one winding, and then the live to the other winding from the other side of the cap...
If you disconnect the cap, and measure the resistance through each winding with a meter between neutral and its live feed wire...They may be different readings, that doesnt matter, you are just looking to check that either isnt open circuit...Remember which way the cap is connected as wiring the live feeds the other way round can make the motor preform differently as it splits the phase differently.
If you have a circuit through both windings then reconnect it all back up. Remove the belt and see if the motor starts turning...if not CAREFULLY try and help it start by spinning the pulley...if a shove to get it going then makes it run then your cap is definately shot.

Cheers for that Gregster he did try spinning the pulley to give it a helping hand to start but no joy. I will check the windings to see if there is a o/c on any of the windings. He has just said when working correctly it used to get up to 90 psi before the pump cut out, but just before it failed completely it was only reaching about 45 psi before the pump cut out. Could this point towards the pressure switch failing and would by passing the pressure switch prove anything

. Cheers alan

So long as you have a supply to the motor it should run, might be worth trying to wire it directly bypassing the pressure switch to prove the motor is good...
Got to say, if it was struggling to make pressure this would tend to point to the pressure switch or compressor pump itself, rather than the motor.
If the motor was at fault it would keep running till the pressure switch told it otherwise, and would just take longer to build pressure...
Saying that, if the cap is faulty the motor may not be producing as much grunt, which would mean it would work with a lesser load but then could stall out and stop before the pressure switch turned off. The motor would just then get hot until the termal cutout switched it off.
I am guessing that you are trying to start the compressor on an uncharged tank, and starting with a tank already pressurized will cause a weak/faulty motor to struggle.
You say the motor is buzzing...so that would tend to suggest that the pressure switch is telling it to start...which again points back to the motor.
Check the windings, try the motor with the belt off...if still no good change the cap

Cheers Greg. Will try those things you have suggested and yes he has tried it with a uncharged tank and still no go. This compressor is not belt driven so i think it will be a case of stripping it down. Thanks for your help so far

.

I shouldnt worry about trying to disconnect the compressor...if you can turn the motor by hand (ie nothings seized) and the motor is buzzing but not starting, then you are looking at either one of the windings is open circuit in the motor, or the cap is knackered..If the cap is knackered it can in effect make one of the windings open circuit, as it is wired in series with it.

Hi Greg.Never done this before but am i right in thinking if i set my meter on ohms and place the probes across the terminal and leave for a while it will give the cap a small charge and then if i reverse the probes i should either see no reading which means a o/c or it should slowly go to infinity which would indicate its ok