a home made rotary converter- This is what I was considering using the 2 extra 7.5 hp 3 phase motors for (as rotary converters). So if I was to spin up the slave motors (one at a time), then I could gain back the 30% reduction and could run the 7.5 hp bandmill motor at its full rated capacity?
Do you have a guess as to how many amps of single phase your rotary converter uses by itself?
I was just considering how much I'd have to upsize the breaker and wire for this circuit if I was also running the two other 7.5hp motors as slave motors for the one on the bandmill and knowing the slaves' consumption would help me to calculate the increase in electrical load.
I use a Gerhardt-Werner Rotary converter (that I bought some years back from of all places Grizzly, the home of chi-com machines). Its a made in USA converter and it's never given me any issues. I have it mounted out of the way, overhead, in the trusses..... It's an ODP motor with integreated switch gear.
It's always idling, just produces the T3 leg and it's a 5hp so starting amps should be 23 and running should be about 15 on 230/1. Remember it's never loaded, just idling away.....
I stack my motors, that is, I start the converter and then a surface grinder which is 3 running HP. Then, unless I need the LeBlond Servo Shift (frequency controlled, 7.5 horsepower), I turn off the rotary and let the delta wound motor on the surface grinder provide the T3 excitation and frequency stabilization for the vertical mills. Each mill has a 1.5 HP Wye wound 3 phase motor so the grinder provides enough T3 current to run them.
Keep in mind that a heavily loaded 3 phase motor like a main drive on a lathe or a bandsaw, where the motor isn't starting in a freewheeling condition, requires even more starting amperage on all legs, T3 included.
Probably the only time I extended run the rotary is when the LeBlond is running. I need the frequency stabilization to run the 7.5 horsepower main drive and I have 5 at the rotary plus 3 on each surface grinder.
You can easily tell when there isn't adequate T3 current because when starting a 3 phase motor, it will come to speed slowly, a sure sign of a low T3 current.
A static converter provides T3 current with banked electrolytic capacitors providing the 'oooomph' to get the motor started, once started, the motor produces its own T3 but at a reduced current, thus the reduction (30% or so) in useable power.
You can actually start a 3 phase motor on single phase using what I call the Briggs and Stratton method. A rope wound around the shaft with a pull handle. Turn on the juice and pull start the motor. It's hard on the windings but it works.
I had the rotary before the LeBlond because I wasn't happy with a 30% reduction in useable power (that static conversion creates) plus the extra heat in the motor.
