When we tested converters, it seemed they pulled between .2-.3A/HP of the slave motor. This could be 10-15A just sitting there on the single phase side. Rotary converters are pretty efficient "when" loaded between 20-90%. The SS can easily pull 10A of three phase just sitting from the servos being fired up and all the fans and such running. The SS also does not have a counter balance and has to hold more power on the Z servo to keep it in position. I can't remember what they do under power down.

Our other shop has 2 of them and they love power but even with 2 of those and three others, the bill does not exceed 600. I would guess the reason you have a 50HP is because of the amperage?? I know the breaker in the SS is 100A! They really do not need it. You could sneak by with a 25HP converter on that machine for what you are doing.

We almost had to run the SS machines with converters so this is interesting. The only thing I can recommend if you only need control functions, you can add a circuit to only power the 120V control of the machine until you are ready to fire up the rest of it. You may also try testing with the estop on to see if that can drop consumption a bit.

If you wanted to back up to a 25HP converter, just find a slave motor and retune the caps a bit. My self or others here could walk you through it. I am interested to see what happens here because this could be a scenario for me soon. I have a lower HP Haas on a converter right now but need to get an SS.