View Full Version : Motor start capacitor rating

27-11-2012, 09:29 AM
Hi guys

I have a blueway reverse cycle aircon and the motor start capacitor needs replacing. Its a 50uf 450v but the biggest I can find is 30uf 400v, would this be big enough.


monkey spanners
27-11-2012, 10:00 AM
No, it's too small. You can join them in parallel to get to the value you need so 20 +30, 25 + 25 etc would make a 50uf.
Its most likely a run capacitor so make sure this is what you replace it with, also the higher the voltage rating of the replacement the longer it will last.

27-11-2012, 10:13 AM
There are 2 capacitors in there, one is a 93uf which works and the other is the 50uf. Would the 93uf be the start capacitor and the 50uf be the run capacitor?
I am able to get a 20uf and a 30uf, would it work fine with the two wired in parallel?

Rob White
27-11-2012, 12:25 PM

The start cap will be plastic and the run cap will be made from aluminium.

The start cap is only meant to work for a few seconds each time the thing starts
so heat build up is not an issue, but run capacitors are on for the whole run cycle
and therefore they need to be made of a good heat dissipating material such as aluminium.

And as stated above use the same size or connect two together and the higher the
voltage rating the better.



monkey spanners
27-11-2012, 12:26 PM
As a general rule, start caps have a wide tolerance, so yours could be 93uf +- 30% also the voltage range tends to be lower than run caps so maybe 250 or 280V for example.

Run caps tend to have lower tolerance so 50uf +- 5% and a higher voltage range, e.g. 380 or 440v etc.

Start caps are physically smaller for a given rating, which is why they will overheat and fail if used as a run cap.

A look at the systems wiring and start relay will tell which is which. Its imposible to say from here.

A 30 and 20uf in parrallel will work fine, many comercial system come like this from the factory. Join them together and test with your meter to confirm before fitting to the system.

29-11-2012, 10:54 AM
Thanks guys, got it working all good now