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Open Delta transformer wiring

beckerkumm

Hot Rolled
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Aug 5, 2014
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Wisconsin Rapids WI
I'm wiring two single phase 240-600v transformers for a Smart and Brown lathe. I've attached a couple of pictures of the wiring diagrams as I think they should go. Since I've never wired 600v and would like to survive, I'm hoping someone will look at the diagrams and verify. Input is 240 delta three phase generated from a Phase Perfect. Is there any preference as to which input the 208 to ground leg of the PP should go or doesn't it matter. Basically, one leg to X1 on each, X2-X3 connected on each, X4 connected on both, X1 to H1 on one and X1-H2 on the other.

Thanks, DaveDSCN3428.jpgDSCN3432.jpg
 
I'm wiring two single phase 240-600v transformers for a Smart and Brown lathe. I've attached a couple of pictures of the wiring diagrams as I think they should go. Since I've never wired 600v and would like to survive, I'm hoping someone will look at the diagrams and verify. Input is 240 delta three phase generated from a Phase Perfect. Is there any preference as to which input the 208 to ground leg of the PP should go or doesn't it matter. Basically, one leg to X1 on each, X2-X3 connected on each, X4 connected on both, X1 to H1 on one and X1-H2 on the other.

Thanks, DaveView attachment 197404View attachment 197405

Dave,

I just went back and re-read the entirety of the thread that led to this.

My 2CW is that rklopp is the guy you need to walk it through with because he posted as to having used the same or similar transformers. Not sure if his were Phase-Perfect fed or RPC, though, and I consider that important.

Personally, I'd not touch it. Simple reason is my Phase-Perfect is too 'precious' to me.

P-P manual is adamant about not having a Neutral or ground reference on its output. They recommend a transformer - clearly Delta-Wye if the aim is to re-establish a Neutral.

Not news to any of us, that part.

I HAVE been looking at Delta-Wye 3-Phase transformers that I WOULD be comfortable with. Around US$ 400 + freight so far.

That said, my goal is but 240 Wye, 133 any leg to ground to reduce lethality of shock hazard vs either open Delta or corner-grounded Delta. I don't want 600-volts in the zone, anywhere, nor even 480.

Your shop, your necessity.

So my only real point is... have a care not to fry yourself.

Have a care not to damage the costly to replace Phase-Perfect.

If I just HAD to do this?

I'd at least start by running off my RPC for a while and taking lots of measurements under starting, running, speed changing, and stopping loads and their backtalk, first.

Even then, I might not ever put the P-P at risk. RPC are cheap as dirt, by comparison. Also rugged. I'm happier reserving the P-P for less unconventional loads.

Bill
 
Are you saying the issue is that a PP should only feed an isolation transformer 240d-600y ? Autotransformers which are usually what I see used with machines are not a good idea due to the possibility of the high voltage feeding back to the PP ? Sorry for the case of the dumbs. A 30 hp PP is not a good thing to risk. Dave
 
240V Delta to 600V Delta Xformer Connections

Attached is a connection diagram for the open delta connection. It is shown for step down, but can be used in reverse with no loss of capacity. DO NOT connect the neutral to the center of either of the 1Φ 240V windings.

The wild leg can be connected to any of the 240V leads. If you were running a RPC, I would suggest that the wild leg be connected to one of the outside legs, so that when the voltage fluctuated it would only affect one coil and not both.

On the 600V side, one of the legs needs to be corner grounded. I recommend the center leg, to reduce impedance when a short circuit occurs. Be aware that VFD drives and electronics do not play well with corner grounded systems. Electromechanical controls usually have no problems with it.

Be sure to connect the transformer core and enclosure to the supply circuit ground. Extend the ground to the 600V center tap, and the frame of the machine being powered.

SAF Ω

600D-240D-120 ISO ODHighLeg Small a-125865.jpg
 
Are you saying the issue is that a PP should only feed an isolation transformer 240d-600y ? Autotransformers which are usually what I see used with machines are not a good idea due to the possibility of the high voltage feeding back to the PP ? Sorry for the case of the dumbs. A 30 hp PP is not a good thing to risk. Dave

I'm basically taking Phase Technologies LLC word as to understanding potential risk and failure modes better than I do. They hold the patent. They built the physical hardware.

If you have no manual, those can be had.

The 10 HP, 20 HP, and 30 HP are just columns on the same page in my one (older blue case DPC-A10). Text is the same, page 5:

The three-phase output is delta configured. While the phase-to-phase voltages are equal, the phase-to-ground voltages are not equal. When T1 and T2 to ground voltage is 120 V, T3 to ground voltage will be approximately 208 V. For three-phase loads that are designed for delta connection, the load derives its voltage phase-to-phase, so the phase-to-ground voltage will not affect the operation of the equipment. If the connected load has a neutral connection and requires wye configured power, the output of the phase converter must be passed though a delta-to-wye isolation transformer before connection to the load.

They did the boldface, not I, and I take them at their word.

For my part, I do not see TWO ONLY single-phase transformers in the same light as THREE balanced windings of a classical full-isolation 3-Phase transformer.

SAF's circuit may work with an RPC.

Phase Technologies, LLC is rather firm that it does NOT belong on the output of THEIR device.

It's that damned upside-down pine-tree symbol, y'see.

Rather an innocent looking little f****r for all the mayhem it can deliver when in the wrong place.

:)

So if I am loathe to risk a used 10 HP P-P?

Your 30 HP is arguably worth more consideration than a pair of $15 each transformers are likely to save, pocketbook, OR ass.
 
In addition to my PP issue ( I have emailed them about the transformer usage ) your diagram is much different than mine. Tells me I need more homework. Thanks. Dave
 
Your diagram appears to be a autotransformer connection, which is not suitable for the transformer you show.

The connection diagram provided is for a isolation transformer setup. 240V open delta to 600V open delta. I can't see any reason your PP would have a problem with it. It is a isolated delta load, with no reference to ground or neutral on the PP side.

SAF Ω
 
I get it. My PP manual is in the other shop so you answered the question I posted to them. Sounds like the simplest, cheapest, easiest for me to understand solution is adding a third 2 kva ACME to match the other two. Hit me long enough and I MAY catch on. Dave

Saw the JST post. Am I correct that your response is that the two transformers wired as your diagram are still isolation, just 56% of what three would provide ? Dave
 
You DO NOT WANT TO HAVE THAT GROUND shown on SAF's actual diagram. PP said it, SAF said it, Monarchist said it, and it is worth saying again.

Do what the diagram says for the wiring EXCEPT FOR the ground (and the 600V will be used as the secondary, nor primary, as SAF said)..

Your own diagram seems to show connections between X and H terminals. You don't do that for an isolating setup. The transformer data plate you show is for a perfectly good 240 to 600 unit, so it does NOT get wired as a boost or buck,

Boost or buck is the only reason for making the sort of connection between windings you show. You are not doing that, so keep the H's and X's separate.

Corner grounding is your only option when doing what is basically a delta-delta connection.

SAF's comment about VFDs is true of many, not necessarily all. Treat it as true of all unless you know otherwise, or if there are instructions you have followed to convert the VFD to a corner ground compatible unit.

The problem is voltage to ground. With a wye secondary, and a neutrl point, the voltage to ground is only 57% of the 600V. With corner grounding, it is the full 600V line-to-ground on two lines. Often the input spike protection components are not rated for the higher line-to-line voltage, because the designers expected only the wye voltage.

The conversion often involves disconnecting those components. That works, but also reduces the protection afforded to the VFD.

Yes the general rule is 56% of the full delta rating.

You'd think it was 2/3, but the problem is the current rating of the transformers. The current for the open side has to come from the two transformers, in addition to the current for their own part of the delta, but they are only rated for a given current, regardless of voltage. So that current is not available for their own "phases", and overall rating decreases.
 
What is the size of the 2 transformers you have? 3 transformers are not required.
What is the capacity you need on the 600V side?

Here's a diagram without the ground connection shown.
600D-240D ISO OD Small a-33227-a.jpg
SAF Ω
 
Take a look to the item on the left of the first pic, which I just noticed.

I hope that is a calculator with jumbo buttons, or else the transformer is pretty small physically.

Looks like it could be anything from 1.5 kVA to maybe 5 kVA, depending on how big those buttons are........ and what part of the transformer that covers. Does not look very big.
 
The two transformers are 2 kva each and sized to handle the 2.5 hp S and B motor. I don't need a third for size. As to the grounding, please clarify. I have a ground on the input side as I wire with three hots + ground back to the box ( PP is connected to three phase panel ). I had intended to ground it to the enclosure, machine, and any grounds I found on the existing wiring.
The transformers are grounded by contact to the enclosure but I hadn't intended any other connections. Pardon my slow learning. My first diagram was my attempt to copy what I did when boosting voltage from 240-275 with two BB transformers for my other 550 machine. The motor could be rewired internally to 275 simplifying my life. I don't expect that to be possible with the three speed S and B motor. Dave

The first picture is just a cover or the bottom of the transformer.
 
I get it. My PP manual is in the other shop so you answered the question I posted to them. Sounds like the simplest, cheapest, easiest for me to understand solution is adding a third 2 kva ACME to match the other two. Hit me long enough and I MAY catch on. Dave

Saw the JST post. Am I correct that your response is that the two transformers wired as your diagram are still isolation, just 56% of what three would provide ? Dave

They 'can be' all you need. Per several reminders, above, they have to be wired just right. That is not built-in obvious as it is on a packaged 3-Phase transformer.

Two transformers 'push off' the reference point against the third leg, much as the note above showed the Phase-Perfect had done. Leg to Leg is in balance.

This part is between the P-P and the transformer primary, so you are still at only 240 V delta. You can easily wire it safely, you aren't too concerned about which leg is close or not so close to Earth ground, a bit of proper conduit prevents worry.

That 57% is for Wye, generally safer, but which you will NOT have, anywhere, so it does not enter THIS picture at all.

On the secondary side, you now WILL have 600 V above EARTH, "PE", machinery frame, the floor, some other machine's just-severed power-cord, the puddle of water that leaked in during the storm...whatever.

I live where air-mass thunderstorms are 'standard issue', so Jerry's point about spikes when already at 600 V is 'standard issue' as well, not theory. Ate another Dell laptop PSU just last week ,so is bad enough on 240 V, and why I won't even go near 480 V, let alone 600 V.

Other way around here. I tend to use 600 V-class switchgear on 240 V for the extra safety margin.

Waste of money and a needless risk to run higher voltages when motors are so easy to change.

For a person with a machine-shop it isn't hard to fab adapters, mounts, shaft couplers, and such. Not as if all you had for tools was a hairpin and a butterknife.
 
Take a look in the "New Machine Day Aciera F5" thread over in the Deckel euro-mill forum. You can see how I hooked up my open-delta system. It takes 240 VAC from an RPC up to 575 VAC. A Phase a perfect would be a drop-in replacement as far as the open delta setup is concerned. My setup works nicely. It is corner-grounded on the high side. Since the grounded leg is a neutral, it is not fused.
 
OK. NO ground or neutral connections to the 240V wires.

Ground wire to ground connections on enclosures and chassis and machine motor frame. SAF likes to have a purpose-made ground to the enclosure, regardless of bolt connections in the structure, and I agree. No reason not to, structures get loose over time.

If you bolt the transformers to the lathe frame, I suppose technically you might argue that you not have to do the corner grounding, as they are now part of the machine, and somewhat outside the NEC. But you probably do not satisfy article 670 even so.... May as well do the corner ground, which means connecting the common point between the two transformer 600V windings, to your ground wire.

RKlopp was posting while I was typing.... yes, no fuse in the grounded leg. You can stick a machined dummy brass fuse in the holder in that spot if need be, just do not forget it is there if you change wiring later. I like to paint them bright red, or put red heatshrink on them as an "alarm color" to make sure they look different. You could tag it, and put a label in the wiring compartment.
 
2) 2KVA units wired as shown are rated for 3.46KVA. That will equate to 3.3A @ 600V, 8.3A @ 240V

Check your motor tag for FLA.
NEC value for a 3HP is 3.9A @ 575V
IEC Table for a 3HP is 2.8A @ 600V

Pretty close on sizing, normally you want a factor of 1.5 on transformer ratings powering motors.

For the grounded leg, I skip the fuseholder altogether, only 2 poles are required. Ground to the frame, and pull your grounded 3rd leg from that ground connection. Then there can be no confusion as to where the dummy fuse goes, you won't need one. Technically the NEC requires WHT conductor for the current carrying grounded conductor, essentially treated the same as a neutral conductor.

SAF Ω
 
Isn't .75kva= 1 hp ? Figured the 2.5 hp motor at approx 2kva.

To corner ground i just need to run a ground wire to the H1 connections running into the 600v disconnect which should NOT be fused? I am planning a 240v fused disconnect in the enclosure of the type that disconnects when I open the box. It will not be difficult to add a second one inside to handle the 600v side.

If I weren't trying to keep the S and B original, I would swap the motor and run a vfd with the back gear and Matrix gearbox. Want to take bets that after all this effort, I'll find out the motor is bad and be back to that option? Dave
 
Not with losses. Figure about 1 kVA per HP

You are correct in terms of shaft power, rating power in kW, 1 HP shaft power is 746 W power.

But the motor has losses, and draws at a power factor under 1.0, so 1 kVA per HP of shaft power is about right.

Yes, you can pull the fuse and wire around it. Depending on just how that is done, it can produce as much confusion as the other way.... BUT if you wire straight to the far side of that fuse from the transformer, it is clearer.

If you use a white wire all the way to the transformer, as NEC wants, then confusion is further reduced, since it is harder to mistake it for a regular set of 3 phase motor wires. Someone will have to look in the wiring box to see why the white wire is there. They will see the fuse wired around, so they will know something needs done to wire it back to normal.

Basically, wherever you look at it, it should be clear that the thing has a corner ground, so nobody wires it up later as if it were standard, just using the wires coming out of the machine without looking in the box

You can check the motor unloaded on 240..... it should spin. Might spin slow. If that works, the motor is probably good. You could check it with a megger if you have access to one, to be sure of high voltage insulation. Or, if it is not a lot of trouble, pull it and have it checked at a motor shop. Time is money, and that may take less time overall, while making the result much more certain.
 
Not with losses. Figure about 1 kVA per HP

I don't know that there is a heavy starting load nor peak loading, in this case, but TWO kVA per nominal motor-HP is where the transformer is more assuredly neither a drag on performance, nor likely to run hot if/as/when enclosed in the machine's housing rather than hung on a wall with plenty of air circulation.

Most of these economy DIY projects are being done with fully-potted dry-type, but also 'outdoor' type transformers, after all.
 
I was intending to use my normal SOOW 12-3+g to the machine. Code the white for hot and green of course for ground. No neutral . Three hot into a fused disconnect. From that disconnect to the transformers. From the transformers to a second disconnect with either the fuse replaced or bypassed. That can be white and coded . Since this is all new to me I will sharpie a note in the box that one high leg is grounded. Trying to wrap my head around grounding a machine to a current carrying wire is not something I'm used to. Keep in mind I'm an accountant in real life so this 600v stuff is all new.

There is also a part of me that says put a third transformer in the box and avoid the confusion. Dave
 








 
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