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recoilless
04-20-2006, 03:04 PM
Bought a Hardinge TM mill w/ 440 only motor. Machine came with 2- 240x480 acme transformers mounted on outside as previous owner had 220 3-phase at his business. Machinist friend of mine is selling me a 3 hp RPC to handle phase conversion. Guy on PR Machinist site told me to use 3- transformers with RPC because of "...relatively poor voltage regulation from the rotary converter" but I wasn't able to get a good explanation from him. Can any of you weigh in on this matter. Thanks in advance

SGW
04-21-2006, 10:14 PM
I'm not sure what the guy is talking about, but my knowledge is limited.

Here's my guess though: two of the 3 legs are going to be good solid 220, as they are the same as what you have coming out of your wall socket. The oddball is the generated 3rd phase. It's whatever your particular motor happens to generate. It will be approximately correct, but it could vary quite a bit. I assume if you put everything through a 3-phase transformer things would get evened out. Whether it's worth the effort, I don't know.

I think another way of balancing the output is with capacitors across various pairs. Size and configuration subject to experiment.

Another thing capacitors can do is fix up the phase shift problem. Two of the 3 wires (the ones equal to the ones coming out of your wall socket) are going to be 180 degrees out of phase, and that has the entire electrical grid behind it. It's not going to change. The 3rd (generated) leg will be somewhere in between, but the phase angles between the 3 phanses will not be the uniform 120 degrees it should be. Installing appropriate capacitors can help fix that up.

J. R. Williams
04-22-2006, 11:19 AM
Recoil
The two transformers will do a good job for converting 240 three phase to 480 three phase when connected in "open Delta" . The total power delivery capability is reduced when compared to using three transformers. Many small electrical customers have the "open delts" service from the power company.

JRW

kf1002002
04-22-2006, 12:44 PM
If you can use the capacitors to get the phase voltages equal the phase angles will look after themselves. I suspect, however that the capacitors will have to be sized for each individual 3 phase motor you connect. The transformer open delta connection is fine and is in widespread use as a previous poster stated. A little problem with the rotary phase converter is that the motor makers never envisioned this use of a motor so as far as I can see there is little theoretical analysis of it.
One thing which few people realize is that running a 3 phase motor on unbalanced phase voltages results in heating of the rotor without undue heating of the stator. This is probably not a big deal with home shop machine tools as they are rarely loaded up to the hilt and then only for a short time.
I doubt that the transformers will do anything to balance the phase voltages.

Ken

recoilless
04-24-2006, 12:20 PM
Thanks for the replies. Since I posted the question, I have
1. Picked up a 3 hp rotary converter (Phase a matic)
2. Had an electrician come out, showed him what I wanted. He does a lot of work at the mills around here and said he would take care of it as he has a lot of experience with this type of work.

Thanks again, Recoilless