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Powering 380v lathe via RPC

jpevner

Stainless
Joined
Apr 5, 2005
Location
Central MA
I wish to run a lathe with a three speed 7.5hp 380v 50hz motor from my 240 single phase house supply. Currently I have a 10 hp RPC which has been working well for years for my other three phase stuff. I also have a decent sized 208-380 transformer from when I had a Deckel mill. Originally, I tried using this, but the lathe didn't want to start on the higher motor speeds. Since then, I put in a 208 volt motor controlled by a VFD. That worked for a while, but recently I started getting motor faults appearing on the vfd. So... I would like to go back to the original motor.

I have been searching threads about this, and it seems that it is unwise to try to run 50hz motors on 380v because of the v/hz ratio?? Ok, so maybe I can use a stepup transformer or stepdown wired backwards. In another thread, it seems that the transformer must be rated specifically for this usage by the manufacturer. Again, ok, I can check for that. There is another thread about wye vs delta that is concerning me. Most of the transformers I see seem to convert wye to delta??

Can I use such a transformer, and how should I connect/ground it? Or do I need a special transformer?

regards,

Jon P.

p.s. Please consider in your answers, that I have a science background, but am not a EE or Elec Power engineer.
 
Hi Jon, I'm not an expert either, my 415-ish lathe's powered by a hacked VFD running on 240v rather than a rotary convertor BUT most of Europe runs 380v 50Hz, the UK uses exactly the same motors on 415v 50Hz so there's a bit of tolerance. My understanding is that there's a V/Hz relationship, and I'm aware that a lot of motors will have higher voltage ratings at higher line frequencies, e.g. a 380-415 x 50Hz motor will be rated for 480v x 60Hz (and deliver a little more power, not a lot).

Transformers similarly behave differently, a 50Hz transformer will be safe on 60Hz but draw less magnetising current, t'other way round I'm told is a no-no due to higher magnetising current and overheating?

I'm also looking into this, as I've bought a Mystery Mill with (European) 3-phase motors for spindle, feeds and suds pump (expensive to add three separate VFDs...) I saw an interesting RPC design / implementation at http://www.mig-welding.co.uk/forum/threads/rotary-phase-converter-plans.32135/#post-350783 that may eliminate the need for transformers IF I can a) make it work and b) find a large enough 240/415 motor or run two or more in parallel - European motors seem to stop at 3kw/4HP in 240v and from there up it's 415/600v.

If you do use a Delta-secondary transformer to get your 400v it shouldn't be necessary to ground the delta side other than the equipment safety ground - is your question about the Neutral connection for e.g. 220v primary control transformers etc.? Bit of a bodge, but you could run those from "domestic" power...

Dave H. (the other one)
 
The 50Hz 380 is the same V/Hz (7.6) as 460V/60Hz.

So essentially, it is a 60Hz 480V motor. By running it at actual 380V/60 Hz, you are "under-voltaging" it by 20 % or so.

As for grounding, a delta secondary is best "corner grounded", by grounding one of the 3 phase wires. If the secondary is wye, you can ground the neutral point. The point of grounding is in case of a primary-to-secondary fault (short).

It's not as much of an issue coming from 240V, as it is if coming from 7200V on a feeder, but US code calls for it, and there is no real reason not to.
 
Guys;

Thanks for the thoughts. A further concern has cropped up in my mind. Most xformers seem to be 208/480, so if I put in 240, then wouldn't I be too high
in voltage (over 500)? Even with plus or minus 5% taps. How far can you go? Would it have any merit to use the 220/380v 50 hz xformer I have, and maybe use buck-boost transformers to step it up(used as an autotransformer, of course.

regards,
Jon P.
 
Guys;

Thanks for the thoughts. A further concern has cropped up in my mind. Most xformers seem to be 208/480,
Not "most". That's a Wye => Delta set of numbers, or Delta fed FROM a Wye.
208 => 416 is Wye-Wye, but neither actually 'dominates' the recycled market atall.

Really have to make sure you can see the dataplate, as listers do get it wrong now and then.

Nominal 230 in Delta- 416 Wye secondary might do just fine. JST can check the math for your 380 @ 50 Hz - > 60 Hz comfort-level.

I'm off to fix a meal..

:)

Otherwise, something 230/240 Delta => 460/480 Delta is your more common target, used, and preferably with two or even more adjusting taps. 'Usually' Delta-Delta as mine is.

Another option.. Delta-Wye at 480 secondary gives what? 277 any leg to Neutral rather than 240?

Lethal voltages, either way if you get hung-up on 'em, but Wye can be a tad less so than a corner-grounded, AKA "high leg" Delta can be.

Motor, of course, only cares about leg-to-leg.
 








 
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