# 3 phase transformer from 3 single phase transformers



## don_resqcapt19 (Jul 18, 2010)

Your line voltage will go to zero assuming that the primary OCPD trips before the transformers are destroyed. You can only ground a single point on the secondary side.


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## qsiguy (Aug 12, 2015)

So if I just ground one of these points it will handle my secondary ground requirement. Correct?

Thank you.


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## Bad Electrician (May 20, 2014)

Your transformers are 1.66 to 1 (400/240), where are you getting 400 VAC (what source)?

You would ground on CB 3 1, 3, or 5 but this leg would not have overcurrent protection.


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## qsiguy (Aug 12, 2015)

That would be a delta corner ground correct? Since the transformer(s) has center taps can't I use one for the secondary ground? This transformer will do 240/120 using these taps.


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## qsiguy (Aug 12, 2015)

...so this new grounded tap would become my grounded neutral effectively making this a wye secondary. I'd rather not do the corner ground method.


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## don_resqcapt19 (Jul 18, 2010)

qsiguy said:


> ...so this new grounded tap would become my grounded neutral effectively making this a wye secondary. I'd rather not do the corner ground method.


No, it is still a delta secondary. If you ground one of the center taps on one of the transformers, you will have a 3phase, 4wire, high leg system. You will have two legs that are 120 volts phase to neutral and the 3rd will be 208 volts phase to neutral. Often called a wild leg system.


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## telsa (May 22, 2015)

Why is this toy necessary?

And where are you ever getting your 400VAC from?


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## tates1882 (Sep 3, 2010)

Sounds like to me you maybe should use a ground fault detector. Yes creating a high leg would probably be a cheaper option, from what I could see of the small drawing and dons reply, you maybe don't have the best grasp on a delta system.


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## qsiguy (Aug 12, 2015)

This is a center tap grounded delta circuit, not some McGyver wiring I came up with. Not sure where the "toy" comment came from. We have a dozen or so systems we've shipped internationally with this non grounded delta-delta circuit and they have been working fine for years. The 400V/3ph is the standard voltage these international customers provide. The only reason I'm inquiring about this now is that one of our new customers in London would like a neutral bonded to earth. I believe the method I am proposing will not effect the operation of my system but was looking for confirmation.

Below is another example of the same thing I came across just drawn differently since it shows a single 3 phase transformer rather than my example using 3 different single phase transformers. I do not need the neutral for any of the devices in the circuit. The customer prefers having a grounded neutral on the secondary. The devices on the secondary are single and 3 phase and do not use a neutral. The leg to leg voltage is all that matters so the ground to leg difference between legs is irrelevant.


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## JW Splicer (Mar 15, 2014)

If the customer wants a wye connection why don't you just give them a 400 delta- 230/133Y?


Will your equipment tolerate a 208V connection? Hook them up Wye.

If you have to choose high leg vs. ground fault detection, I'd go with the latter.


What is it for anyway? Just curious...


Also I'd only center tap the middle bug, if you do the others too you might have to change your pants.


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## qsiguy (Aug 12, 2015)

Due to space constraints we elected to go with the 3 transformer assembly you see at the top (without the red ground strap). These transformers are integrated into the machine and it would be a big and costly job to swap them out assuming we could find a transformer, or transformers, that would do the job and fit. Further I am in Arizona and this system is sitting in London England...

Yes, everything I have on the secondary side will handle 208-240, some single and some 3 phase loads including a water chiller, a couple ozone gas generators, a few water & gas monitoring instruments, and the DC power supplies which feed the controls (PLC, HMI, relays, etc.)

What is the drawback to the "high leg" if not using the grounded neutral for any devices and the leg to leg voltage remains what is required?

What is required to use the ground fault detection option for the entire secondary circuit? Other than 120V GFI outlets occasionally we've never needed to do this on a circuit of this size/load.

This is a water treatment system for a manufacturing plant that requires ultra pure water.

Yes, I was in a hurry to post something on my way out of the office and mistakenly linked up all those taps to ground in my MS Paint quick and dirty drawing. I knew better... Now I am enjoying the wise cracks :jester:


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## telsa (May 22, 2015)

qsiguy said:


> Due to space constraints we elected to go with the 3 transformer assembly you see at the top (without the red ground strap). These transformers are integrated into the machine and it would be a big and costly job to swap them out assuming we could find a transformer, or transformers, that would do the job and fit. Further I am in Arizona and this system is sitting in London England...
> 
> Yes, everything I have on the secondary side will handle 208-240, some single and some 3 phase loads including a water chiller, a couple ozone gas generators, a few water & gas monitoring instruments, and the DC power supplies which feed the controls (PLC, HMI, relays, etc.)
> 
> ...


If your loads are L-L loads the traditional concern about high leg/ wild leg/ stinger loads is entirely gone... You don't have any L-N loads at all.

Am I to presume that this is 18 Meg-Ohms water? 

The Europeans [ IEC ] are big into RCD [ Residual Current Devices ] that detect ground faults. These do not operate quite to American GFCI norms. (4-6 mA)

This may be the basis for some equipment design stipulations. 

They may object to the very principle of an ungrounded SDS (Secondarily Derived System) even if it's integral to a complex machine. 

With your center-tapped delta connection, I should think that their requirement has been met.

%%%%

FYI:

1) An off the shelf NEMA transformer -- 3-phase -- usually has high-side taps down to 432 VAC @ 60 Hz. -- in sizes down to 15 kVA.

It's no biggie for these to be tapped down to 400 VAC. It's a trivial price adder, I should think. 

A three-phase transformer will use less materials, weigh less, cost less than three single phase transformers. Such triplets are actually quite uncommon, as...

2) You could likely get away with using just two (2) single phase transformers to generate your power. ... As an open delta connection. The full 3 phases are still available. 

This economy would depend upon how heavily these units are loaded, as the third unit will be needed once you're past a certain power level. 

The open-delta scheme is as common as dust. 

( Ultra common as a utility service connection, BTW. One big pot -- center-tapped would be mated with a puppy side-kick that generates the third-phase.)


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## Southeast Power (Jan 18, 2009)

don_resqcapt19 said:


> No, it is still a delta secondary. If you ground one of the center taps on one of the transformers, you will have a 3phase, 4wire, high leg system. You will have two legs that are 120 volts phase to neutral and the 3rd will be 208 volts phase to neutral. Often called a wild leg system.


Just do this and be done with it.


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## tates1882 (Sep 3, 2010)

qsiguy said:


> This is a center tap grounded delta circuit, not some McGyver wiring I came up with. Not sure where the "toy" comment came from. We have a dozen or so systems we've shipped internationally with this non grounded delta-delta circuit and they have been working fine for years. The 400V/3ph is the standard voltage these international customers provide. The only reason I'm inquiring about this now is that one of our new customers in London would like a neutral bonded to earth. I believe the method I am proposing will not effect the operation of my system but was looking for confirmation. Below is another example of the same thing I came across just drawn differently since it shows a single 3 phase transformer rather than my example using 3 different single phase transformers. I do not need the neutral for any of the devices in the circuit. The customer prefers having a grounded neutral on the secondary. The devices on the secondary are single and 3 phase and do not use a neutral. The leg to leg voltage is all that matters so the ground to leg difference between legs is irrelevant.


http://www.bender.org/mobile/product-display.aspx?p=IR470LY, try one of these. 

Also when you ask a bunch of ppl online for help don't be offended when they bust your balls a little, it's part of the game. Just because you've built and shipped several of these machines with ungrounded secondary and no ground fault protection doesn't mean it's correct. I'm sure someone will pop up claiming it's safer in theory to not have a grounded system but the folks that write the codes seem to know what's what.


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## don_resqcapt19 (Jul 18, 2010)

qsiguy said:


> ...
> What is required to use the ground fault detection option for the entire secondary circuit? Other than 120V GFI outlets occasionally we've never needed to do this on a circuit of this size/load.
> ...


Ground fault detection would only be required if you left the secondary as an ungrounded system. 250.21(B)


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## JRaef (Mar 23, 2009)

Umm, did anyone tell you that the EU voltage is ALWAYS a Y system, 400Y230V? You don't need a transformer to get 230V irf what you are using it for is single phase 230V loads. But you will not get 120V, which they will HATE you for anyway because over there, they cannot buy anything that is 120V rated unless they wait for it to come from here. So most likely when your stuff gets there, they are gutting your 120V equipment and replacing it the first time anything dies. What is the 120V for? Control voltage? If so, just change to 24VDC, then use auto-ranging 24VDC power supplies so that if connected in North America, it accepts 120V and if connected in the EU, it accepts 230V.


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## qsiguy (Aug 12, 2015)

JRaef, Not using any 120V devices on this system, just 208-240 single and 3 phase. The addition of the center tapped ground/neutral is just to satisfy the customers desire and would require the minimum possible modification to the system. The tricky part of this customers requirement is that they have a global spec so this approved design is a bit of a mix of US and EU specifications. Just this one location has made a fuss over this part of the circuit even though their corporate engineering team has previously approved it.

Tesla, thank you for all the info. I really appreciate it. This location is 50Hz so many of the transformer options we normally offer here in the US we cannot use at most overseas locations. I would much prefer to provide a single 3 phase transformer as the mounting and wiring time would be cut significantly. We did get one of their locations to accept one a few years ago and once they received it they did not like how it had to be shoehorned onto our system and they ended up removing it. The 3 separate transformers allow us to tuck them inside the frame and are quite compact. We've also done the open delta option a few times but normally with 480 mains. One of our main pieces of equipment cannot handle having one leg as high as ~280 so we cannot use it in these applications. I will, however, revisit this option for the 400/3 systems because this will drop the high leg to ~230 which we can handle. Thanks for the reminder.

Thanks everyone for the input. I am going to proceed with the center tapped ground option which I am confident they will accept and will do the job.

Oh, and can someone confirm if this assembly of three 7.5kva transformers is the equivalent to a single 22.5kva three phase transformer? I believe the total capacity of this arrangement is calculated by adding the 3 up but haven't been able to definitively confirm this.


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## Meadow (Jan 14, 2011)

Question, what would happen if all center points were grounded like previous?


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## Southeast Power (Jan 18, 2009)

qsiguy said:


> JRaef, Not using any 120V devices on this system, just 208-240 single and 3 phase. The addition of the center tapped ground/neutral is just to satisfy the customers desire and would require the minimum possible modification to the system. The tricky part of this customers requirement is that they have a global spec so this approved design is a bit of a mix of US and EU specifications. Just this one location has made a fuss over this part of the circuit even though their corporate engineering team has previously approved it.
> 
> Tesla, thank you for all the info. I really appreciate it. This location is 50Hz so many of the transformer options we normally offer here in the US we cannot use at most overseas locations. I would much prefer to provide a single 3 phase transformer as the mounting and wiring time would be cut significantly. We did get one of their locations to accept one a few years ago and once they received it they did not like how it had to be shoehorned onto our system and they ended up removing it. The 3 separate transformers allow us to tuck them inside the frame and are quite compact. We've also done the open delta option a few times but normally with 480 mains. One of our main pieces of equipment cannot handle having one leg as high as ~280 so we cannot use it in these applications. I will, however, revisit this option for the 400/3 systems because this will drop the high leg to ~230 which we can handle. Thanks for the reminder.
> 
> ...


I'm not convinced that the capacity of a transformer would increase if connected to two others. Some others here are experts and would know how to solve that for you. I don't think the answer is x 1.73.
Question.
Why do you use 50amp OCP on the night side and 40 amp OCP on the load side?


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## don_resqcapt19 (Jul 18, 2010)

When you use individual transforms to create a 3 phase transformer, the kVA of the 3 phase transformer is the sum of the kVAs of the individual transformers.


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## qsiguy (Aug 12, 2015)

don_resqcapt19 said:


> When you use individual transforms to create a 3 phase transformer, the kVA of the 3 phase transformer is the sum of the kVAs of the individual transformers.


Excellent, thank you for the confirmation.


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## telsa (May 22, 2015)

meadow said:


> Question, what would happen if all center points were grounded like previous?


Each point of connection is 120 degrees away from the others... in time.

Consequently, they will never be at the same voltage at the same time. :whistling2:


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## don_resqcapt19 (Jul 18, 2010)

meadow said:


> Question, what would happen if all center points were grounded like previous?


It would trip the primary OCPD and/or let the magic smoke out of the transformer. 

Connecting all 3 of the center tap points is really no different than connecting the 3 phases together with a jumper.


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## j johnson (Jul 20, 2009)

There is a transformer called a zig-zag that is yoused to give a y point on delta systems!
if not all ready grounded.


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## Big John (May 23, 2010)

meadow said:


> Question, what would happen if all center points were grounded like previous?


 Good question. I'm not convinced those center taps would ever be at anything except zero volts. But if this were actually a functional way to derive a neutral on a delta system, we'd see this in the field, so I assume it doesn't work.


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## micromind (Aug 11, 2007)

meadow said:


> Question, what would happen if all center points were grounded like previous?


It would blow up. 

Seriously, if one of the center taps is grounded, then the voltage of the other two would be higher than ground. 

There can be only one ground reference.


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## don_resqcapt19 (Jul 18, 2010)

Big John said:


> Good question. I'm not convinced those center taps would ever be at anything except zero volts. But if this were actually a functional way to derive a neutral on a delta system, we'd see this in the field, so I assume it doesn't work.


Once you ground the first center point, the other two will have 120 volts to ground. As I said before, if you connect more than one of the center points, you will either let the magic smoke out or trip the primary OCDP....maybe both.


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## JW Splicer (Mar 15, 2014)

It's exactly the same thing that would happen if you grounded another phase in a wye, or corner grounded delta...


Poof! It's dead...


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## Big John (May 23, 2010)

I'm a yokel: I was imagining the center taps as neutral points instead of just a winding tap. 

Yeah, tying together 120V points would not be a good thing.


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