Re: [CAD_CAM_EDM_DRO] Phase converter circuit

MARKEHLE wrote:

> While we are talking about it, I would suggest building a rotary converter -
> if you find the parts used, it can be really cheap. I made one out of a 15hp
> motor from an old elevator. The whole thing probably cost me $150, including
> the 3-phase 220-to-440 transformer that is mounted on the side of it to run
> VFD's. With the rotary converter, I get full output from the drives.
>
> It works great... except...
>
> Every time I turn it on, the high-pressure sodium and mercury vapor lights
> in my shop go out, then come on a few minutes later. Does anybody know how
> to reduce the starting current on one of these things? I built it so that it
> starts itself, using a small timer to cut the start capacitors out when the
> rotor come up to speed. Would it help to put a pony motor on it to bring it
> up to speed first?

The pony motor is a crude fix. What you need is low voltage starting.
There are a couple of ways to accomplish it. You can dig up a modest
autotransformer, or wire a step-down transformer to make a 2:1 reduction
in voltage. Or, you can rig up some relays to start the motor on 120 V,
and then switch over to 220. the one tricky bit is to make sure the 120
V relay has dropped out before the 220 v relay engages. If you use a
NC contact on the 120 V relay, then the 220 can't engage until the 120V
relay has dropped out.

The motor will start a lot slower on 120 V, but the current draw should be
a lot less. If the draw is still too much, you may be able to rig a resistor
in series with the 120 V supply. A very small value resistor, probably no
more than 10 Ohms would be about right. (You could use a series
resistor on 240 V, without the transformer, etc. but the resistor wattage
would have to be huge.) There are some schemes that use Wye start,
Delta run, but the relays required get complicated.

There are also schemes using a series inductor to limit starting current.
these work well, but you might have a LOT of trouble finding a suitable
inductor.

Jon