208 to 480 transformer 3phase for Servos

timryder

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Join Date
Feb 2007
Location
Macomb MI
Posts
176
Hey guys... I'm in the middle of a battle with the boss because when I try to tell him what I need he claims I don't know what I'm talking about. So I'm looking for some advice or help to make sure first that we're doing the correct thing and if I so happen to validate my thoughts along the way... great.

Our building has a 208 3 phase ungrounded delta transformer powering it up. We are building custom manufacturing machines that requires it to be 480v 3~. So we clearly need a transformer to step up that voltage. I did the math on all of the servo's which are going to be on this machine alone and they sum up to around 116amps of 3phase 480 FLA. The boss buys a used transformer which is a Delta Wye but it's a step down transformer so he has it wired in reverse so the 208 3~ goes to the WYE secondary which gives me 480 on the primary and then corner grounds the Delta on H3.

When I measure my incoming L1 from the transformer with respect to ground I get nothing (0.05 vac.) Then L2 is 480vac. Finally L3 is 480vac. Across any two I get 480. Well I was getting errors on my Kinetix servo controllers (5000 series) that I've never gotten before and my local distributor helped me deduce it's because I'm missing a phase since he corner grounded the delta. There is a pre-charge amplifier which isn't working correctly in the kinetix or something like this. They recommend that I change the transformer to a Delta/Wye step up or boost transformer as it's better anyways.

So I tell the boss that it's single phasing and i need to change out the transformer because it's the wrong type. Not what I asked for to begin with. He screams and yells and says I don't know what the hell I'm talking about.

Can anyone help me with this?

1.) Is the corner grounded delta the right configuration for an automation machinery which has sensitive servos.
2.) By grounding H3 the corner terminal of the delta, is it actually single phasing h1 and h2?
3.) Is a 208 3~ primary delta and a 480 3~ secondary WYE the right transformer to put in?

Thanks guys sorry for the long winded post
 
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Much much more knowledgeable people than I will probably be around shortly, but I thought most all drive manufactures state NOT using corner grounded delta. You may be able to find in your drive manual about this, if it explicitly states NOT to use corner grounded delta, then you should show your boss.
 
The Kinetix has an optional grounding screw you can remove for the AC and DC Buss which will let you use a Corner Grounded Delta. But it is not recommended. I am more interested in the Phasing question and if the Delta is OK to use. But thank you for your validation.
 
1.) Is the corner grounded delta the right configuration for an automation machinery which has sensitive servos.
2.) By grounding H3 the corner terminal of the delta, is it actually single phasing h1 and h2?
3.) Is a 208 3~ primary delta and a 480 3~ secondary WYE the right transformer to put in?

Thanks guys sorry for the long winded post

I'm no expert either, but I would say:
1) No, not recommended...most VFD makers either prohibit it or have a RFI jumper that needs to be removed to avoid burning up the RFI filter section.
2) No. But, the way that the drive detects the loss of a phase might be "tricked" by having one phase grounded.
3) Yes, at least for the secondary, but I am not certain about what your primary should be.
 
Purposefull Step Up Trtansformer

Tim,

Do you really need to run all of the servos at the same time, under full load?
Can you stagger, or offset the servo moves for testing purposes?

You stated 115 amps at 480 vac 3-phase. Math says 115a multiplied by 1.73 multiplied by 480 = 95 kva.

That load is somewhere between a 75kva, and a 112kva transformer.
If this is truly what you need to do, in your 208v facility, then a financial investment is required. I detect that you are a OEM provider to industry.

Most or all or your customers have a 480/277 center bonded voltage system, and you should be testing your equipment at that incoming voltage.
You are a professional, and should not compromise with the corner-ground, 3-wire delta 480v.

If your company intends on continued large systems to industrial users, you need to have a legitimate 480/277 wye center bonded system in place. If your company only predicts rare, or occasional need for that system, perhaps a short lease of a vacant temporary industrial facility is in order.

If you want to gear-up your current digs for this type of work, contact Jae at Power-Tran in Ferndale. His team can make a purposeful step-up transformer to go from your 3-wire 208 to a 4-wire 480/277 wye system.

Your installing licensed electrician will ensure that the secondary of the new transformer is bonded per NEC (NFPA-70) requirements.
Now the question is, does your facility have a sufficient 208 v system?

If you opt for 75kva step up, your electrician will need to bring 300 amps to the 208 v primary.

If you opt for 112kva step up, your electrician will need to bring 400 amps to the 208 v primary.

(Side note: we often build machines for Europe, and often have to buy transformers, and VFD’s to provide 380v, or 415v 50Hz power just for run-off in our facility that is 480/277 wye.)

Best Regards Tim,

Plastic
 
You are not single phasing. On a corner grounded delta, one phase will read zero referenced to ground, because it already IS referenced to ground. That's exactly what "corner grounded" means. It's totally legit, in fact it's one of two NEC accepted ways of using a delta secondary on a transformer, the other being a very expensive ground fault monitoring system. Back in WWII era and a little beyond, people used to use UNgrounded delta systems for reliability, in a "Damn the torpedoes, full speed ahead!" sense, but it's never been a good idea for a number of reasons. So some time in the late 70s / early 80s the NEC was amended to require EITHER corner grounding of a delta secondary, or ground fault monitoring.

Then along came "power electronics" in the 80s and 90s and it all changed again. Electronic components are made, for the most part, to EXPECT the Line to Ground reference voltage to be no more than about 300V max. that's why 600V rated VFDs are so much more expensive than 480V versions, they have to be made with components that are not as commonly available. So basically, power electronics (VFDs, Servos, UPS, ballasts etc.) are designed for Wye power systems where the L-G potential is 58% of the L-L potential. Inside of them, they have components and systems that utilize that L-G reference for protection and providing return paths for the Common Mode Noise they create. Delta systems provide no path for that noise, and the protection devices become problematic, because if referenced to ground in a Delta system, they are essentially going to be a ground point for the ENTIRE system, until there is a ground fault event somewhere, at which time they vaporize!

Yes, A-B lets you remove their ground reference screw / jumper to prevent the MOVs and Common Mode Noise capacitors from becoming little bombs, but if you read the manual, it tells you that this should be avoided. The Common Mode Noise, with no ground reference now, will go through the roof and start affecting EVERY piece of electronics in the neighborhood. Will it work? Yes, but will it give you constant headaches and all manner of phantom glitchy events? Almost guaranteed. So when the boss gets sick and tired of the problems and repairs, THEN you might be able to point out that his decision to get cheap on the transformer now has likely cost him more in the long run when considering down time on your expensive machine.

Either get a transformer with a Wye secondary, or add another "Drive Isolation Transformer" ahead of the servos (and any other 480V powered electronics), which is a 1:1 ratio shielded isolation transformer that will be 480V Delta primary, 480/277V Wye secondary. That's EXACTLY why those products exist.
 

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