480/400VAC Stepdown

Hi all. I've got a robot coming in from Europe in a few weeks, and we need to go from 480VAC to 400VAC for it. The robot is 3PH, no neutral and 12kVA.

As far as I can tell, a couple of 1PH transformers in open delta should be fine for this. The vendor will graciously supply us with a European 3PH/3PH transformer for $2100; it's open frame and we'd have to construct an enclosure for it as well (or have a custom enclosure fabbed).

So, question is... for a load consisting primarily of servo drives and a control system, is there any concern in using a pair of 1PH in open delta? Or, if anyone has a source for an OTS 480/400 transformer, that would be fine too. And, yeah, I know that several of the majors can sell me a multitap that will go 480/380, but the vendor is specifying 400VAC, so in this case "that's what it takes."

Thanks!

Search for a supplier of "autotransformers" in your area.
This kind of transformer is exactly suited to stepping up or down where the ratio is less than 3/2. They are less expensive than transformers that are isolated between primary and secondary.

Equipment from the EU normally works OK with 380V, even if the nominal voltage is 400V. But since you have to spend a transformer anyway, you should get the proper one. And I wouldnt experiment with using single-phase transformers.

Here is one supplier:
http://www.marcustransformer.com/uploads/marcus_product.pdf1/Type MAT0001.pdf
There isnt a 400/480 version in their selection chart. But since they custom make them you could have one made for you.

We have had good luck with this guy: http://www.controlledmagnetics.com/ An 11 kVA 480 Delta/ 400 Wye transformer costed us about $700 in 2007, when copper was way more expensive, and took 3 weeks to build. Of course, you would still need an enclosure for it.

Call them and talk - they do almost any kind of custom build to your specs. I think a single 3-phase transformer will be smaller in size that any combination of 3 single-phase ones. Autotransformers better be avoided for servo systems - you are not only converting the voltage, you are also trying to isolate higher harmonics.

I converted two machines to 400 volt using a transformer I ordered from Dongan. 600 volt primary to 480 volt secondary. Connecting 480 to the primary side will give you about
380 on the secondary and you can tap it up if need be. We have recently sent one of the machines to the bone yard but still have the transformer if you are interested in it let me know. It is overkill for what you need at 35 kva.

Thanks, guys. Jesper, Marcus is just building two single phase in open delta in a single box. At least, the diagrams you linked show that.

LL, I'll try calling Controlled Magnetics and see what they say. They are at least supplying an isolation transformer. I still don't know if I really need that, or if I can just go with a pair of autotransformers.

Jimee, thanks dude but I think that 35kVA is major overkill for this one! Hey, I can make you a great deal on a 1990 Motoman robot though... maybe we should swap.

Anybody else have any thoughts??

Paul T said:

They are at least supplying an isolation transformer. I still don't know if I really need that, or if I can just go with a pair of autotransformers.

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I should state it one more time: if you need to supply power to servos or VFDs, autotransformers are a big 'no-no'. Talk to the Controlled Magnetics guy about this - he knows this stuff really well.

I've lost an Allen-Bradley servo drive once due to an internal short in the main machine autotransformer (that was the way the machine builder supplied and I did not realize that until the damage happened).

Paul T said:

Jesper, Marcus is just building two single phase in open delta in a single box. At least, the diagrams you linked show that.

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No, they only show one of three phases. What you see is the possible combinations (what tabs there are) of one of the phases.

LL's comment about higher harmonics is quite valid. Cant comment about if the control system for the robot is particularly sensitive. I would guess it is designed to be installed at less-than-optimal power supply conditions. But ask the supplier.

JesperMP said:

No, they only show one of three phases. What you see is the possible combinations (what tabs there are) of one of the phases.

LL's comment about higher harmonics is quite valid. Cant comment about if the control system for the robot is particularly sensitive. I would guess it is designed to be installed at less-than-optimal power supply conditions. But ask the supplier.

Click to expand...

Ha. That was my first stop... all I got was:

"3 transformer should be OK"
"pay attention on wiring to avoid phases inversions from one to the other"
"I am surprised that it is more efficient compared to a 3-phase transformer..."

That's verbatim. Not especially helpful! Especially the "3 transformer" comment... I'm not sure that "Open Delta" even translated.

OK... I still wish I understood this better than I do. LL, thanks for restating it more clearly. I'll have to poke around and see what I can find regarding the issues using autotransformers... do the harmonics lead to higher temps than the windings are rated for? Less efficient? Both? I had thought that the only issues using open delta for stepdown were less capacity and possible overcurrents if you tried to add a neutral on the output side.

Meanwhile, I'll call the guys at CM; they should be there by now. And I really hope they can do better than $2100!

Hi again.

An autotransformer is more efficient than a 'regular' isolated transformer, and cheaper too. However it does not suppress harmonics as already stated, and a catastrophic failure can lead the full incoming voltage to pass through.
This wiki page explains it much better:
http://en.wikipedia.org/wiki/Autotransformer

Is there an isolation transformer for the control voltage in the robots control panel ?

Can I assume youhave verified that the 60Hz US instead of the 50Hz European will not be a problem with any of the robot's devices?

Tom Jenkins said:

Can I assume youhave verified that the 60Hz US instead of the 50Hz European will not be a problem with any of the robot's devices?

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Well, it's not according to the vendor <g>. And we saw one run at their NC facility a few weeks ago. Seriously, 60Hz is ok. I just don't want their $2100 open frame transformer, which I would then have to build an enclosure for.

I have to admit that finding a 480/400 stepdown, or at least an isolating stepdown, turned out to be harder than I thought it would. Controlled Magnetics gave me a verbal of about $950; I'm having them turn it into a written quote. Controlled Power gave me a quote of $4500, for a high K-rating isolation transformer. I'm planning to place an order with CM this afternoon, as soon as they get me a written quote.

Jesper, thanks for the link. That's some help to get me started.

The application, BTW, is a large materials handling robot by Staubli. This is an RX260, which has a 2.6m reach and 100kg payload. We have a process cell that runs product through three ovens in sequence. A shuttle carries the product into the cell; the robot removes it from the shuttle, moves it to each oven in turn, then places it back on the shuttle after the last oven. This system has been in place for ten years; we just need to replace the 1990-vintage Motoman robot now in use, which was old when we got it ten years ago. Should be a fun project!

Square D brouchure
http://ecatalog.squared.com/pubs/Electrical%20Distribution/Low%20Voltage%20Transformers/Non-Linear,%20Energy%20Efficient,%20K-Rated/7400HO9501.pdf

Page 10 has diagram for using two standard 600-120/240V transformers connected in an open delta for 480-400.

Are you sure your machine does not need a 4-wire source?

We use Dongan all the time for 480 to 400. They have numerous combination of Auto or Iso, open frame or 3R, copper or aluminum.

I use Marcus all the time. excellent products.
If you are fine with autotransformer, check MAT15B22 (480/380V 15kVA - note that 380/400 in Europe is the same kind of deal you get here with 460/480V or 115/120V, it's only 5% difference).