Phase Converter Capacitor Problem

Hi,

I just build a 20 hp phase converter. It worked twice during testing, but now it won't spin up. I have a bank of start capacitors that I think are the problem. They all tested ok with the ohm meter, but when I put power to them, I don't get any voltage across the terminals. They are all rated 220-250v, and I have about 2000 mirco farads total, all wired in parallel. Did I kill them somehow? Is there some other way to test them?

Thanks,

Michael

Wire them in series....

I imagine this is running on 240 volts? If so you likely need caps rated at a minimum of 240 x sqrt (2) or 240 x 1.414 or about 340 volts. If memory serves the next higher standard is 370 volts. Throw a cap meter on em and see if you fried them.

What does wiring them in series do?

I don't have a cap meter. But it sure seems like they're fried. I've never heard of needing the start capacitors at 370 volts. All my run caps are 370. Is there any other way to test them?

Keep the capacitors wired in parallel. Wiring in series would just create a super small capacitor. 250V starting caps are fine since they are only used for a very short time. What problem are you having specifically? You say it won't spin up. Do you mean that the idler motor doesn't turn at all or that it starts but doesn't come up to full rpm? You also said it worked during testing? Did you take things apart and reassemble after this? Have you triple checked the wiring? I can't imagine you killed all the caps at once which makes me think it may be something else. Do you have pics or a diagram you could post?

I agree with most of what David said. Are you using a potential relay and a contactor? Something may have gone bad and the caps are not getting power. Try to find a capacitor tester. Maybe one or more has shorted out. Bob

As others have asked, How are you taking the start caps out once the motor spools up?

You can test start capacitors or run capacitors with just a regular test meter. It would be better to test them with a capacitor test metre. But, If you don't have one, you can charge it up using 240 V for one second and a proper rated switch. Change your test metre to DC and see if it is holding a charge. Deplete the charge using a resistor and then tested again. Depending on where in the sine wave you turned it off, it could be positive 130 V or negative or anywhere in between.
1/ You always want to run start capacitors in parallel for starting power for the third leg.
2/ your parallel lines should be able to handle the capacitance amperage. For example 1000 µF can generate up to 90 A. I would suggest for each 500 µF you go with A minimum 10 gauge wire(which is still pushing the wire capacity but it shouldn't be in the circuit for more than two seconds) back to a distribution block. My last RPC build(which is 20 hp) I ran 900 µF run capacitor's is the starting capacitors on a 6 gauge wire. Every 100 µF can use up to 9 A. You could've simply fried the wire if it was not big enough. Bleed resistors are a must or you will punch holes through the foil in the capacitor with arcing and deplete their capacity in no time.

Can you post some pictures?

Sorry for the tardy reply. The idler motor won't spin at all. When I originally wired it up, I was able to fire it up fine 2 times, then it stopped working. I didn't change anything in between. I'm using a contactor rated for 35 amps, multiplexed with 4 terminals for powering the start capacitors. So I've got up to 140 amps of draw I can put through it. I was able to take 2 of the larger caps over to an electrician who had a tester. They were fine. I have 4 other smaller ones, which I think are the problem.

Can I use run capacitors for starting the motor? (I know I'd need a lot of them, but I can get some used ones really cheap)

Can you post a diagram or picture of your setup? I don't understand what you're talking about multiplexing your contactor. If your two larger caps are testing ok then I would expect to get some movement out of the motor. I'm still curious how you're putting the starting caps into and out of the circuit? Potential relay? manual switch? It sounds like that's where your problem is.