phase converters for big iron

[Mike Gemeny]

A few things I forgot to mention and then a couple of questions. Even though the caps were balanced for the lathe motor, when I start the grinder, the lathe motor gets even quieter. Does this mean that perhaps I should be dropping out the caps once I have a motor running? I don’t think so, because when I balanced the cap bank I was looking for the sweet spot in motor happiness. Perhaps its an indication that the more motors I have running the happier they all are, which has been my assumption.
 
If the latter were the case then a rotary approach would not have much in the way of limits for motors.
 
Also, the impression I have always had from machine shop circles was that commercial rotary phase converters were the same as motors as far as the stator and rotor were concerned. The only differences being that it has no output shaft and would have some other stuff (like my bank of caps) on the outside. But again, I don’t have experience with commercial rotary phase converters ether. If there were differences between the stator and rotor of a commercial rotary phase converter and a three-phase motor, I would love find out what they are.
 
Yes, Kevin is quite correct that solid-state phase converters are available. In fact some will even allow you to control the speed of the motors, although one that elaborate is most likely not what Brian would want.
 
I also agree with Paul that for a conventional power supply “I would expect power supplies to be every bit as tolerant of distorted waveforms as motors are, even assuming that these converters produce major distortion.” And for switching computer power supplies I would bet that an old 3 phase motor idling in the corner would go a long way toward cleaning up any artifacts in the solid state converter output.
 
I would expect a solid-state approach to handle a wide range of loads, but I don’t have any experience with them. I would also expect a solid-state approach to have a strict upper limit on capacity. Is that the case? Perhaps Kevin could get his brother to share more information with us on his experiences.
 
On phase order I just want to point out what I hope is obvious to everyone but no one has said, if you swap any two phases you have reversed the phase order.
 
Jim brings up 208v, which brings up an issue not yet discussed, that is delta vs. Y.
 
My guess is that motors and generators are generally delta, and that most distribution systems are Y. I can accept being mistaken about this, and a rich discussion is sure to help us all.
 
Most of you must by now be familiar with the distribution systems used in the states as far as one leg being 110v and both legs giving you 220v on single phase service. With that in mind how can we best approximate that in a 3 phase building? We use a Y configuration of second windings on the last set of 3 transformers. If each secondary winding is set for 120v, and we combine 2 windings we don’t get 240v, we get 208v.
 
Given thusly: 
(120+120)*sin(360/3)=207.85 (or 208)
 
If you wanted 220 or 230 or 240 you use a set of 3 transformers and most likely wire the primaries as Y and the secondaries as delta.
 
Dwight mentioned RMS voltage when referring to a square wave. I was a bit confused by that. My understanding of RMS is that it is a way of expressing the energy under the curve of a sign wave such that the total energy was expressed as that of the peek of a square wave with equivalent energy. If that is the case then RMS and peek for a square wave would be the same but to convert between RMS and peek for a sign wave you multiple or divide by the square root of 2 or 1.414. I’m not trying to criticize Dwight at all here, I suspect he makes a good point, I’m just not sure I fully understood his post.
 
Clarification and other opinions always welcome.
 
Mike.