OT: Compressor won't start. Cap or ?

[Bryan]

My compressor is sick. It's one of those infernal direct drive things. Italian. I've had it for many years but it doesn't do much work. It gives a couple of feeble grunts and trips the overload. Is there something other than a fritzed capacitor that could do this? The motor turns ok by hand. There's no swelling, burning or other badness evident about the cap. Is there a way I can test the cap? I shorted it and got no sparks. Something else I can test? I have a multimeter but very limited knowledge of electrics.

[RayG]

Hi Bryan,

Sounds like a shorted start cap. Does you multimeter have a capacitance range? If it does you can check if it's reading the right capacitance, but sometimes even if it reads ok, it might still be breaking down at higher voltages.

The other things to check would be to check the windings for continuity, and if you have a megger, check the insulation,

Regards
Ray

[Stustoys]

Hi Bryan,
Can't be much help on the electrical side of things. One thing that comes to mind though. Is the air receiver empty? on larger compressors(and I assume the direct drive ones would be the same) there is a one way bleed valve that releases the air pressure from the compressor when it stops. If that's not working and you still have 80odd psi in the tank the motor might not have enough torque to start.

Stuart

[malb]

Stu,

They actually have a one way (check) valve at the point where the feed pipe enters the tank, and the bleed valve connects into the feed side of the valve, so it dumps the air between the comp outlet valve and the check valve, not the air in the tank. Otherwise there would only be usable air while the motor was running.

Bryan,

Is the overload that it trips built into the pressure switch or are you talking about a breaker or RCD at the switchboard?

For a single phase comp the motor would be a cap start unit to develop the start torque required. This style motor has two groups of windings in the motor. There is a run winding that is energised whenever the motor is powered, and a start winding, connected via a start capacitor and a centrifugal switch, which is energised until the motor reaches about 70% of it's rated speed, when the centrifugal switch opens and disconnects the start circuit.

Failure to start, and tripping out, would be caused by a failure in the start circuit, either of the winding itself, the capacitor, the switch contacts, or the switch mechanism which controls the contact position, or the interconnections between these parts.

[Skew ChiDAMN!!]

I don't s'pose there's anywhere you can safely "give it a spin" when turned on, to see if it'll run with a hand-start?

'Cos if it does, it's the start circuit. Probably the cap, but as Malb said...

And if it doesn't, it's usually expen$ive news. :sigh:

[Bryan]

Ok thanks for the input guys. I now have a few more things to look at and think about.



Is the overload that it trips built into the pressure switch or are you talking about a breaker or RCD at the switchboard?


Neither. There's a separate mini breaker mounted on the motor.

[malb]

The presence of a dedicated breaker at the motor adds another possibility, that the breaker itself is faulty, particularly if it is a minature unit.

To eliminate that as a possibility, I would be trying to substitute with a DIN rail sized unit with similar trip current. If that works, then try to locate a replacement mini unit to complete the repair.

Beyond that, things tend to move rapidly into the hazardous zone if you are not trained and confident in electrical mechanics.

[Bryan]

Thanks Mal. I will add that to my list. Don't worry I will be very careful. I have a lot of respect for lethal things.