# Thread: formula for capacitor on 3 phase motor

1. I have bought a 3 phase air compressor. Its 4kw (5Hp) 380v (French voltage) 50Hz. It was wired in star, I have changed it to delta. So I can run it on 220v three phase. Now I know I can possibly run it on single phase if I put a capacitor across two of the winding's. I saw someones calculations on the internet. Please tell me if I have worked this out right ? ;

14u/A x 10.5A = 147uf

So I guess I need a 220v / 147uf inline Capacitor ?

Last edited by dudz; 12-11-2015 at 08:43 PM.

2. Ok. Seems its not that easy. (Thanks Dean)

For those who are interested in this concept. Here is a link that might be of value. http://www.electro-tech-online.com/threads/3-phase-converter-schematic-miller-system.100563/

Last edited by dudz; 12-11-2015 at 10:11 PM.

3. I will probably buy a 220v 3 phase inverter or find a used single phase motor.

4. I have never thought of wiring a 3 phase motor as single phase. Don't you have to have a centripetal switch as well as the capacitor? I didn't like to reply in case Jazz came along and told me that was only for cap start and I was being silly

Why would you want a single phase motor if you buy an inverter? Surely it would drive the motor you have, assuming you put it back the coils back the way they were.

5. Originally Posted by Robin Hewitt
Don't you have to have a centripetal switch as well as the capacitor?
No centripetal switch, the additional start capacitor circuitry to provide enough phase shifted power to start the motor (which must be disconnected for run conditions) can be on a timer relay or be switched using voltage sensing, it used to be common for rotary converters to be spun up using a rope with a knot at the end on a pulley with a slot in one side, if you had end access to the motor you could even use a starting handle ;-)

I bought and read books on all this stuff before discovering Inverter VFDs, if you need a machine running next week, or tomorrow VFD is the way to go, if you have no time pressure you can get a machine running with caps, relays and miscellaneous gubbins but the VFD driven machine will be smoother and better.

- Nick

6. Originally Posted by Robin Hewitt
Why would you want a single phase motor if you buy an inverter?
Think you missed the " or " Robin.!!

Why would I say anything you say was Silly unless it was Silly or wrong.?

7. Originally Posted by JAZZCNC
Why would I say anything you say was Silly unless it was Silly or wrong.?
I thought it was a trap, you were all waiting for me to break the group silence and put my foot in it

8. Originally Posted by Robin Hewitt
I thought it was a trap,
Nah.!!. . . It's close to xmas and I'm a bird lover so I'd never Trap a Robin. .!!. . . . Pheasant got much more meat on it and easier to trap..

9. I have read from 40µF/kW, thats near to your amount, in a thumbrule there are 3Amps/kW. But maybe you need a separate start capacitor, which is switched off after a second or so. But, you can't run such a big motor with only one fase.
The Motor need some 24Amps to run and triple of it to start. With a small motor maybe half as strong, you can make this.
Last edited by uli12us; 13-11-2015 at 01:55 PM.

10. Originally Posted by uli12us
I have read from 40µF/kW, thats near to your amount, in a thumbrule there are 3Amps/kW. But maybe you need a separate start capacitor, which is switched off after a second or so. But, you can't run such a big motor with only one fase.
The Motor need some 24Amps to run and triple of it to start. With a small motor maybe half as strong, you can make this.
I agree. I haverun several 3ph motors in Delta using caps with a timer switched extra "starter" cap. the system works well for motors up to about 3HP, above that and you will notice the lights dim on start up, and that will be the same with any neighbours connected to the same phase. G.

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