Rotary Phase Converter Bucking

[itsmoked]

I've got a 75HP rotary phase converter at a site.

It's to run a shop full of machine tools. One's got a 25HP spindle and a CNC lathe has a 30HP chuck.

The RPC is fed with 400A 240V single phase service.

The problem is that ALL the machine tools require 220V not 240V and unfortunately what's typically delivered is 245V. This puts all the machines out of their warranty operating constraints and causes them to spew insane process stopping alarms in occasional fits. Before things went crazy at this shop a smaller RPC had the same problem and I put a 2kVA buck transformer in front of the RPC which dropped the voltage down to something like 224V which was fine.

Now with this much, MUCH larger unit I don't think I'm even going to find a buck transformer large enough to put in front of the RPC. I want to instead follow the RPC with an open delta pair of two 5kVA bucks. They only weigh 110 lbs each. HA!

Besides the little problem of having to have absolutely no load on the RPC on startup and that means not even a 40W work light (or smoke pours forth from either the idler motor or the wall mounted Box), does anyone see any problem with this after-the-RFC-method?




Keith Cress
kcress -

http://www.flaminsystems.com

 

[BlackJackJacques]

Is there the option of swapping out, or bucking the smaller control transformers for the processes spewing the alarms?

 

[itsmoked]

BlkJk; Thanks for the suggestion.

That is exactly what I'd do if I were stuck in this place. Unfortunately the idiots at Doosan are so cheap the control transformer they install doesn't have a single tap on it, unlike virtually all other machining centers. This would mean I'd have to remove the one provided and replace it with a useful one. That would void the warranty on a brand new $110k machine that seems rather tender to me. Hence, I have to jerk the whole 100A around to the lower voltage instead of just the control voltage (700W) which is all that's having an issue with the voltage.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[aussiejohn2]

Keith,
If you have any single-phase loads then keep an eye on their voltages; the open delta leaves equal-magnitude line-line voltages but displaces the neutral so the line-neutral voltages are not all the same.

JD.

 

[BlackJackJacques]

You would think the manufacturer would approve the ctrl xfrmr mod and still honor the warranty given that voltage variation is normal to the environment and not an issue to the motors, etc. Maybe if you explain and ask. You could make an argument that this is a bit of a design defect on the part of the manufacturer. Another option is to retain the ctrl xfrmr but wire a variac ahead. It would still mod the manufacturer's equipment a wee bit, but perhaps the manufacturer would see this as less of an intrusion. Good Luck

 

[edison123]

Can't you just the feed the existing control transformer via another small step down or a buck transformer? I don't see how any OEM can object to that.

Muthu

http://www.edison.co.in

 

[edison123]

Or what BlackJack said. A variac is still better.

Muthu

http://www.edison.co.in

 

[itsmoked]

The manufacturer/rep has already screwed things up and proven to be a warranty threatener so that's not really a useful solution and while we're tempted to swap out the useless transformer it would mean if he has a warranty issue and the reps come out we can't swap the original transformer back in because then he'd be left with unwarrantable power being supplied again.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[BlackJackJacques]

Sorry to hear you are dealing with a problematic rep. Odds are they will also be problematic when the time comes to honor the warranty under "ideal" conditions. If it is just a rep and not the manufacturer, I'd try to bypass the rep and get someone sensible, or alternatively, ask for a complete refund because their product failed to perform. The latter threat may waken them a bit and break the logjam - especially if you mention you are conferring with legal counsel, etc. Otherwise, the route you mentioned is looking better and should not displace any other ethics-related dilemmas on a working engineer's plate - where you swap out the xfrmr anyway but do it in such a way that it is restorable to original should there be a need for a later inspection. The system is not functioning now and odds are that there will be multiple field mods applied as time goes on, so the rep/manufacturer is being unrealistic as well as unhelpful.

 

[edison123]

Not swap out the control trafo, just the supply to it. You can even hide the buck trafo/variac in plain sight since the rep seems clueless.

Muthu

http://www.edison.co.in