Ignoramus11916 wrote:
I have a rotary phase converter with a 10 HP idler.

If I was to run a serious 3 phase welder, I would need to increase the
capacity of my RPC. My own thought is to add two more 10 HP idlers (at
very little cost). They would be switched on (if necessary), one after
another, after the first idler properly spins up. Once they are
running, I could turn on the load. That's better than one 30 HP idler
because

1) They do not strain the breakers at startup
2) It would require a lot less starting capacitance
3) I could regulate how much idler power I am using

Does it make sense?

As far as I can see it makes sense. If the idlers are always started at
separate times, you should be able to figure out a way of using the same
start capacitor to start all three idlers. Or you could just use the
three phase output from the first idler to start the others, which would
avoid some complex wiring. If your convertor uses run capacitors as
well, you'll need one for each of the idlers.

It would be really neat if you could figure out a system for starting
the extra idlers when they were needed. Perhaps you could use a system
of current or voltage relays to sense the load on the convertor and
switch in extra idlers as needed? I haven't thought out the details of
it yet, though.

One final point: try to get three identical motors if you can. It is
bound to make your convertor neater.

Best wishes,

Chris