compressor motor caoacitors

I need to change both of the capacitors on my compressor.

Motor is around half horse power. Can't see any value on the caps. anyone got similar motor who can tell me what the uf number should be on them ?

Mike.

IF you clean the ends of the tin cans you should find the values stamped on them. Plan B, disconnect, discharge and measure the value on a good multi meter, but bear in mind that it may have 10% tolerance and IF faulty then the reading may be incorrect ! Does the spec plate give a Hp value, as about a half horse is not a good guide, clearly 2 pole, 220/240V Why do you want to change them ? If it starts when the tank is empty but then struggles to restart with air in the tank, it is likely the non return and or the unloader have failed. many good commpressors fail or burn out due to this fault and not being wired through a DOL starter.  Good luck Noel.

Edited By noel shelley on 27/10/2021 12:12:16

Hi Noel,

I think one of the caps has shorted out as its throwing my rcd out, which is rather sensitive.

It tried to start then trips very soon after switch on.

Cleaned the end of one cap and nothing. Will try the other.

Which one is start cap, large or small one ?

Mike.

If you check the cap connections you will find 1 of them gets switched out of circuit by the centrifugal switch, that's the start cap.

Emgee

At this time of year it could be dampness, try warming it up in an airing cupboard or play a fan heater on it, then try it ! If grinding has been done in the area there may be a build up of metal particles. RCD or MCB ? Mark all wiring and disconnect the caps and do an insulation test this will show if the windings are sound before you spend out on caps that should be 400v. Will it start with the belt off ? Is the centrifugal switch in good order and working ? Noel.

The start capacitor value will be between 80/110 micro farads and the value is not critical and the run around 10 micro farads. The start capacitor, the big one is a special AC electrolytic with a short time rating, once the motor is up to about 70% of full speed, the centrifugal switch opens and disconnects the capacitor. The run capacitor should have a working voltage of 400 as its in continue use. Both caps in parallel when the motor is stationary and one (10) when running. Most multi meter with a capacitance range, have working voltage of 9 volts and motor capacitor really need to be tested at full working voltage, 339.4, hence 400. John.

A google search will bring up several references for calculating motor capacitor sizes. Including at least one online calculator.

This YouTube is as good as any **LINK**

https://www.youtube.com/watch?v=DJyq6Q58HWA

Regrettably the forum software can't handle mathematical formulae.

Clive

Edited By Clive Foster on 27/10/2021 17:11:24

Changed the smaller one and now working and not tripping the RCD.

Thanks for input.

Mike.