Motor De-rating with Rotary Phase Converter?

[electrofelon]

What size of service do you have? Those take a heck of a big input breaker.

I did a 30hp three phase motor (wide belt sander like the op) on a RPC once. He had a 200 amp service, but it was in a residential area, I don't recall what size transformer it was on. I was a bit worried. We ended up getting one of those large 200 amp breakers that takes up four spaces to feed the RPC. I don't think he's had any problems.

 

[fastline]

Never dealt with a RPC before. Got a customer that picked up a 3-ph wide belt 15 HP sander and a RPC rated for starting a 15 HP motor. From what I understand, the voltage of the two utility legs will continue to be 180 deg out of phase from each other, and the manufactured leg voltage will be somewhere around 90 deg from each utility leg.

In my way of thinking, the motor would have to be derated because the voltage phases will not be symmetrical. But I concede that perhaps the current phases will be symmetrical, and de-rating is not required.

Any gurus out there know the real answer? I did some searching, but am not sure I understand this.

That is simply not true. You are not feeding an RPC 2 phases, you are feeding 1. They will create 120* phasing, but the voltage balance will be all over the place as their balance changes with load, which can lead to high currents and heat.

Because you are single phasing a 3P motor, it has to be derated by SR^3. However, asking one to take across the line hits, yes 50% is about right.

I've designed and built many. They are not perfect but do work. I would personally reach for a VFD.