OK Multi-Idler phase converters.
RPC's or rotary phase converters are a reasonably simple unit for creating 3 phase from single phase within reason. There are a lot of pluses to them and a couple minuses. One of the BIG negatives to them is the required starting current is several times that of what starting the same motor on 3 phase would be. This rears its head around 30 HP and by 50 HP its real dominate. So long story short, while not impossible, starting a single motor 100 HP RPC would draw somewhere in the neighborhood of 400 amps, would require a HUGE bank of start caps and even the wiring would be difficult as you will never get 400 amps across a bunch of 12 or 10 gauge wire running to the caps, and finding 1/4 inch slip on connectors for 0/2 copper wire,,, well I never seen them. So how do you do a 100HP RPC? With 5 20HP idler motors that are in parallel. The wiring is a bit of a challenge but the idler branch circuits are only going to require #8 wire even though the output and input are 0/2 copper. The best way to understand who it works is to understand how it starts, at least how I do it. We are going to assume that all motors are the same model here and the run capacitor values are the same for each of them. So when you push the start button, the input contactor closes and the start capacitor contactor control timer closes connecting the start caps to the first idler. The timer runs for the time needed to start the first 20 HP motor. Once that timer runs out, the start cap contactor opens, which sends power to the first output branch contactor which closes and applies power to the output bus. The rest of the idlers are all attached to the output bus via there own contactors and the run caps for those motors are all connected to the idler motors via another contactor. The reason for the run capacitors to have their own contactors is the output voltage on the first motor if it had 100 HP worth of run capacitors connected to it would be somewhere around 600 volts. This voltage could back feed and cause serious issues. SO back to the start sequence. Once the first branch output contactor closes and applies power to the output bus. There is 3 phase power to start the next motor. So a second set of contacts on that branch contactor sends power to the second idler capacitor control timer that holds open the second idler motor capacitor contactor. At the same time it sends power to the idler motor branch output contactor that connects the idler to the output bus starting the motor. Once the branch run capacitor countdown timer expires the run cap contactor closes and you now effectively have a 40 HP RPC running. That timer also sends power to the next branch run cap timer and branch circuit output contactor starting the next idler motor and then switching the run capacitors into the branch circuit. This continues until the 5 motors are running and then the master RPC output contactor is closed connecting the 3 phase output to the 3 phase circuit breaker panel feeding the shop. Power is NOT applied to the RPC output until all motors are running. Now, there are some serious benefits to this design. First is that it's very expandible. If you have enough input power available, you could build this design to ANY size. Second is that if you wire control bypass switches in, you can control the size of the RPC based on the current need for power. Of course you can run one 5 horse lathe on a 100 HP RPC but it's gonna cost you a mint in electric bill to do it.
I have never really sat down to consider how to do automatic control.. and I never thought much about bringing more capacity online if only one motor is running and the rest are bypassed, but I don't think it would be hard in either case. I ahve only done one 100HP setup. The rest were 40's and a couple 60's to run big CNC machines. But again, if you had 400 AMP service in the shop. There is no reason you couldn't build one that was 200 HP like this. And because the branch circuits are only 20 HP each the internal wiring on them wouldn't need to be huge welding cable. Only the input output bus would require the large wire.
