Contactor Sizing for an phase converter Idler Motor

Your maximum load is about 10 HP, and your maximum starting load is 15 HP. That makes me believe that 15 HP rated contactor would be appropriate. I am not, by any means, an exp[ert on this.

I would be interested in finding out the authoritative answer to this, great question.

Personally, I am considering making a two stage phase converter.

First it would start a 10 HP idler, and then it once the idler is up to speed, it would start another 7.5 HP idler. Much easier on the electrical circuit. The reason for this is that I have a 7.5 HP motor that I do not know what to do with, caps, contactors etc. Any cost to this project, at this point, will be my time.

The thing about induction motors that makes them need the proper contactor is the starting current, not the running current. This current is at the maximum when the contacts first close. It can three times the rated current of the motor. This is why big contactors are used. If you spin up the motor with a smaller pony motor first then the starting current will be lower. ERS

He can buy something like this:

I think the answer depends on how you are starting the 15HP idler. The issue isn't the fact that the motor says "15HP" on it, the issue is how much current does it draw when starting. If you're using capacitor start, you get a pretty large surge current, related to capacitor size, not HP itself. I know the caps are sized based on HP, but I'm trying to explain what the real issue is.

In your situation I'd have the 10HP contactor between the 15HP idler and the compressor. This way you start the idler and let it run. The compressor would start via its contactor exactly as if it were on power company 3 phase (assuming your third leg doesn't sag...).

Steve

gtslabs wrote:

This is how I had my converter set up (sold when I moved). I had a 3HP capacitor start converter. For my one larger machine, I then would switch in a 7.5HP idler with some additional run caps. It started just fine off the 3HP idler, since it was starting no-load. Two stage starting, but way easier than what I had before--pull start a 7.5HP with a rope. I got pretty tired of that.

Steve

This is what I am doing now. However I was wondering if I should use a contactor just to start the 15hp converter. I am using 7 108-130 uF capacitors to start it up in about 2 seconds with the momentary button.

You definitely need a contactor to start the 15 HP idler.

For my 10 HP idler, I use 184 mF of capacitance. It starts just fine. I started off with more capacitance (276 mF), but that made the

That's very encouraging. Thank you Steve. I have 3 more 92 mF run capacitors that I can use as run caps, and plenty of contactors.

You can start a motor, especially with no load, on a much smaller starter than is recommended- they're meant to deal with a unit putting out the full rated power. In truth, I think the hardest part of a starter's job is shutting down under load, you'll get arcing then.

John

Caps aren't too pricy, although you got a good deal on the ones you have. Surplus Center has good prices on them, and if you need a bunch of capacitance for a big unit you can do the trick with DC caps hooked back to back- IIRC, they are wired + to +, but look it up to be sure.

John