| I'm not sure what the "arty-farty" part of that was. A "CNC converter" | is simply a rotary phase converter that supplies well-balanced three | phase power from a single phase source, suitable for running CNC | machinery. For purposes of this discussion, let's just say that CNC | machinery is any machinery containing sensitive electronic control | circuits that might not operate properly if the manufactured three phase | power was not well-ballanced (nearly identical voltage readings between | any two phases), and the phase angles being 120 degrees apart. It is | DES's contention that a normal three phase motor cannot be used to | produce clean three phase power with 120 degree phase angles, and that | he uses some "special motor" to overcome this. My questions are: (1), | is there any reason why a three phase motor wouldn't produce 120 degree | phase angles on the output when used to make a rotary phase converter, | and (2), if there is, what modifications would be necessary to make it | do so?
I think the problem here is that whoever is promoting the "CNC converter" is dwelling too much on the existance of the problem, and not enough on just how his particular product is able to effectively solve the problem better than something else (at least in the same price range).
So the question is "what (the hell is) a CNC converter?" ... and this is asked NOT in terms of what it is FOR, but rather, how it accomplishes its goal. So answering "it is a phase converter for CNC machinery" is not the answer being sought. What is being sought is how this converter is different from other converters that are considered unsuitable for CNC machinery.
Sales people tend to give that former (wrong) answer. If you ask a sales person "how does it work", they tend to think you asked "how WELL does it work" and answer "It works very well".
Engineers don't give tiddly about sales drool. They want to know what makes it tick, and whether it actually solves the problem at hand, and fits into other issues like reliability, maintainability, etc.
Smart managers (yes, there are some around) listen to their engineers.
Dumb managers (all the rest) listen to the sales drool.
Now to your questions:
| is there any reason why a three phase motor wouldn't produce 120 degree | phase angles on the output when used to make a rotary phase converter,
A three phase motor BY ITSELF, which is designed with 120 degree angled armatures, will produce consistent angle differences of 120 degrees between the phases. But the "BY ITSELF" is the issue here. If you are running a syncronous single phase motor driving a three phase generator then you'll get some nice three phase output if you don't load it so hard as to drag the mover motor out of sync. But consider what happens if you run this genset with an inductive squirrel cage motor. You will get 120 degrees between the 3 lines from the generator, but you won't get a consistent frequency. It will be lower that your mains frequency and will change with load.
Now, if you are taking just ONE line of power from that generator, and comparing that to 2 lines from the mains (opposite poles of single phase) you'll see either frequency variation or phase variation depending on the design and load. Frequency variation makes things rather useless. Phase variation within a certain window can be used. Just what that window is depends on the application.
| and (2), if there is, what modifications would be necessary to make it | do so?
It's a matter of degree (I love that pun). How perfect do you want? Even the utility won't be perfect because of factors (ooh, another pun) like the loads from other customers. But it is usually close enough for everything but the most picky equipment.
An idler motor will give you a consistent frequency, but the phase will vary depending on how much the load causes the idler rotor to lag. The specifics I can't tell you, but I've read of cases where it lagged as much as 30 degrees under normal loads. Your mileage may vary.
There are probably a number of things to do to adjust that phase or reduce the variation in it. A larger idler motor would help. Or maybe a large inertia on that motor.
A system that senses the phase error and adjusts things to correct it would be the way to go. There are probably a number of things that can be adjusted, from applied voltage on the idler, to adding varying amount of the single phase power to compensate. Mixing voltages from single phse doesn't get you anywhere because it's all one dimensional. But once you have something different than 180 degrees, you can mix varying quantities of voltage to get any phase angle you want (within practical limits).
What does the "CNC converter" actually do? That I don't know. And that is the whole of the technical questions. No engineer can advise you on whether it is suitable for your needs without first knowing what that means. And unfortunately, when marketing names a problem based on what problem (market) it is intended for, rather than on what kind of design it has, then it leaves other engineers in the dark.