Thread: Selecting capacitors for motors

Hi Garry,

Start caps sizing is not especially critical, and there are many different ways to wind a motor, so there is some variability, but the following table should help..



CV is horsepower, so a 1 hp 50hz 220v single phase start capacitor, should be about 25uF, and as it happens, pretty much the same whether it's 2, 4 pole or 6 pole.

Regards
Ray

That table is from Kemet ( a capacitor manufacturer)

Thanks for that Ray, it should save this motor going back into the scrap bin where I found it.

Is there a way to work out the HP of a motor without putting it on a brake? Would loading it up and measuring amps give a reasonably accurate indication?

Garry

I think the capacitor chart displayed is for the permanently connected run capacitors used in that particular type of motor, and would be rated at 450 volts. These are quite dirfferent from starting capacitors, which are electrolytic with values between 100uF and 500uF for the common range of motors 1/2 to 3 HP, with voltage rating the same as the motor, ie 250v. They are switched out of circuit when the motor is partly up to speed, so are intermittantly rated. A common rating for use is max 20 starts per hour. I hope this helps.

Chas

I would suggest that the voltage rating should be approx 30% higher than the supply voltage to allow for spikes etc.

Roger

Hi Chas,
Yes, you are correct, that table is for PSC (Permanent Split Capacitor, or is it Phase Single Capacitor? ) motors. It's PSC anyway

Like this..



If, on the other hand you have a motor with two capacitors and a centrifugal switch then it's most likely a CSCR (capacitor start, capacitor run)

Like so..


There are other variations, but those are fairly common configurations...

I don't have a good chart for CSCR capacitor selection, (maybe Eskimo can help!)

This one might get you in the ballpark, (and they aren't all that critical anyway)



As far as guessing HP goes, measuring the current with a clamp meter is the easiest and quickest..
Actually, the table above will give you a pretty good guide to that as well.


Regards
Ray

PS I agree with what Rogerbaker said about voltage ratings, higher (bigger) is always better.