Posted by Sherman in Idaho [72.47.153.61] on Friday, February 13, 2015 at 15:16:27 :
In Reply to: Re: 3 phase posted by Greg Coffin [67.176.56.160] on Friday, February 13, 2015 at 01:31:58 :
Rotary converters are fine if you have a whole shop to do, and you happen to have a spare 3-phase motor around you can build it with. In theory, though, real three phase is better. With a phase converter, you're providing enough of a third phase to get your motors running, but it's not really balanced three phase power and so your shaft torque still has pulsations in it.
One thing a lot of people don't realize about 3-phase power is that it provides constant shaft torque. This is good on the generator end because there's no pulsations or vibrations in the turbine shaft, and it's equally good on the motor end. Some claim that when doing fine machine work, a mill or lathe powered by a 3-phase motor will make a smoother finish than one powered by a single-phase motor. I'm not a good enough machinist to know if that's true or not, but in theory it ought to be. When you get up to larger motors (I'm going to check out a 500 HP compressor in one of the local mines next week), the smoothness of 3-phase power is absolutely essential. Shaft torque pulsations are severe and would cause fatigue cracking at those kinds of power levels.
Of course good phase balance also assumes you're getting your utility power from a 3-pot bank, hopefully in a wye. Around here, our downtown system is old and uses 2-pot open deltas, since the loads are mostly single-phase, and the little "stinger" transformer is enough to make a few motors run. It's fine for my purposes, but not quite as good as a truly balanced system.