Help with getting the right voltage output from a transformer.

[hillbilly1]

Or maybe is either field connected wrong or the leads are marked wrong?

I’ve had that happen before, wired a small transformer for a sign many years ago, had it exactly as the diagram shown. It let the magic smoke out. Sent it back to the supply house, who claimed I must of wired it wrong. They came back later and apologized, the manufacturer called back, and said they had a batch that the leads were marked wrong.

 

[tortuga]

This is not a transformer problem and the more time wasted on this is keeping someone from finding the real issue.

The OP @mike1061 has a standard open delta 240V service, where b-c is the open set, he ordered two delta:wye transformers as he needed a 240 wye system for reasons you @Jraef eloquently explain.
Since one transformer was wye-wye mislabeled delta/wye he connected this wye-wye transformer with a floating primary neutral to the open delta system. The way I understand it is the voltage across each primary winding is determined by the distance from each primary phase (a,b,c) to the floating primary neutral point.

Mathematically I think the equation is
Vn0 = (Va+Vb+Vc)/3

As we know Open Delta, the voltage relationship b-n is from the midpoint of the other two phases a & c to b. When you apply a load across b-n , the voltage Vb-n drops significantly more than Va-n or Vc-n.
Much like you were running a large 208V load b-n or a large 1-phase 240V load b-c
The wye-wye floating neutral n0 "slides" toward the stronger phases (a and c).
I suspect the wye-wye secondary voltage (where phases are A,B,C) on Phase B-N can collapse while Phases A-N and C-N spike. Since the secondary was feeding a VFD it likely tripped on under-voltage or phase imbalance immediately.

 

[tortuga]

Brings up a question if one did connect this Wye-Wye to a Open Delta service and tied the Wye-Wye primary neutral to the service neutral would it start to act like a grounding transformer for the service ?
If the utility's primary phases (L1,L2) are slightly unbalanced, the Wye-Wye bank might even try to "fix" the utility's unbalance by circulating current through its primary neutral connection (and get billed for it). I dont know for sure its curious, I could see a situation where transformer windings might be carrying a large load even if the secondary has zero load attached. But the math for that is above my pay grade.

 

[jim dungar]

Brings up a question if one did connect this Wye-Wye to a Open Delta service and tied the Wye-Wye primary neutral to the service neutral would it start to act like a grounding transformer for the service ?

No, it would not act as a winding transformer. It would act as a fault, if the H0 was connected to the source.
The L-N voltages at the source are not equal to those at the wye. The windings of the grounding transformer are connected in a wye-deltacor zig-zag.

 

[winnie]

Brings up a question if one did connect this Wye-Wye to a Open Delta service and tied the Wye-Wye primary neutral to the service neutral would it start to act like a grounding transformer for the service ?
If the utility's primary phases (L1,L2) are slightly unbalanced, the Wye-Wye bank might even try to "fix" the utility's unbalance by circulating current through its primary neutral connection (and get billed for it). I dont know for sure its curious, I could see a situation where transformer windings might be carrying a large load even if the secondary has zero load attached. But the math for that is above my pay grade.

I think that you are asking about the single phase 'base' of the high leg delta, not about the entire 3 phase. If you had a single 240V coil with a center tap, connected L1-L2, and connected that center tap to the supply neutral, then it would act like a single phase version of a grounding transformer. It would derive a neutral point that tries to 'fix' the source neutral. A single phase coil with a center tap derives a low impedance neutral.

But the wye:wye transformer doesn't have a single coil with a center tap; you have three coils on three cores,

If you model the wye:wye transformer as three separate single phase transformers connected wye:wye, then it is pretty easy to show that the transformer will simply 'reflect' the high leg. H1:H0 input sets the X1:X0 output, H2:H0 -> X2:X0, H3:H0 -> X3:X0. There is no path for circulating currents that tries to hold the H0 at the neutral voltage.

What I don't know how to model is what happens if the three coil sets are on the three legs of a common core transformer; I don't know if the shared magnetic flux does anything to try to force the voltages to even out.

-Jonathan

 

[synchro]

Brings up a question if one did connect this Wye-Wye to a Open Delta service and tied the Wye-Wye primary neutral to the service neutral would it start to act like a grounding transformer for the service ?

The leg of the transformer core whose winding is connected between the service high leg and neutral will have 208V across it instead of 139V. That will be well over the volts-per-turn it is capable of handling, and will make that leg go into heavy magnetic saturation. It will very likely drawn enough current to trip the overcurrent protection.

Even if you fed a 3-leg wye-wye transformer capable of a 400/231V with a 240/120V high leg source and the neutral connected, there will be a net zero sequence magnetic flux in the core that's not provided with a suitable return path (like a 5-leg core could provide). The magnetic flux in the 120V legs will flow between each other, but the leg with 208V will not have a low reluctance return path, causing multiple problems.

 

[mike1061]

I’m following up. Thank you all for your help. I am currently emailing with Larson. I want to see what they say. So far, they said nothing important. Only that they want a video of me taking the voltages. I’m going to ask them again about the two different labels. I haven’t had time to go video tape the volt meter.
I’m none too happy with them. When I ordered the transformer from them in the first place, the metal enclosure was different. They denied it until I gave up and installed it anyway. In spite of the pictures I sent them. This could be the same thing all over.
Thanks again
Mike

 

[going_commando]

I remember during COVID we had to special order an Eaton step up 208 pri 480 sec transformer for a propane vaporizer. I think it was a 60kva but I don't quite recall. After 10 weeks it finally showed up from Mexico, and we cracked it open, and they had labelled the windings wrong. We had to send it back and ordered an ACME that we happened to find in stock somewhere.

 

[mr_horsepower]

I wouldn’t say they recommend this set up. This was my way of solving the problem. It is the evaporator motor. Or should I say, it’s the motor that blows thru the coils. The reason I did this was because the transformer for the whole unit is a lot more money. I did and experiment with the unit next to this one and it seems to have worked. This 2nd one won’t. They have replaced the VFD, the board and everything. So they say. Now they are kicking it back to me.
It is an open delta on the line. But the output is supposed to be balanced Y 240.

is it possible, for testing purposes, since the units are right next to each other, to at least jumper your power out of the operable unit into the non-operable unit, so we know for a fact unit B is capable of operating like unit A?

if I was on that roof, it seems like knowing that/ ruling out any problems internal to that unit would have sure comforted me at one or two points along this journey

 

[WD40]

On a 240 Delta an HVAC guy I worked around always unhooked one leg of the fan VFD, I am not expert on why that works but he said the VFD was so over sized because they sold the same packaged unit for single and three phase. I think it was a carrier unit.

 

[kwired]

On a 240 Delta an HVAC guy I worked around always unhooked one leg of the fan VFD, I am not expert on why that works but he said the VFD was so over sized because they sold the same packaged unit for single and three phase. I think it was a carrier unit.

If it is oversized you can do that. The output doesn't care if the supply is single or three phase as all it is doing is taking voltage from the DC bus and creating the output. As long as it is oversized enough that you don't deplete the DC bus faster than it is replenished so to speak it works fine. This is exactly what is done when you use a drive to convert single phase input to three phase output, they typically recommend a drive with twice the motor rating when using it that way, 1.73 times the motor rating should be the minimum needed.

 

[mike1061]

is it possible, for testing purposes, since the units are right next to each other, to at least jumper your power out of the operable unit into the non-operable unit, so we know for a fact unit B is capable of operating like unit A?

if I was on that roof, it seems like knowing that/ ruling out any problems internal to that unit would have sure comforted me at one or two points along this journey

I thought about that. In fact I was planning on doing that until everyone convinced me the transformer was wrong. The bad part is, they are about 30-40’ apart. I would have to leave them jumpered for a while. So that we could make sure it operates properly. It should be could out too, to make sure

 

[mike1061]

On a 240 Delta an HVAC guy I worked around always unhooked one leg of the fan VFD, I am not expert on why that works but he said the VFD was so over sized because they sold the same packaged unit for single and three phase. I think it was a carrier unit.

I will ask the heating guys about this. If it works, this could save the day. I appreciate it.
Thanks
Mike

 

[synchro]

On a 240 Delta an HVAC guy I worked around always unhooked one leg of the fan VFD, I am not expert on why that works but he said the VFD was so over sized because they sold the same packaged unit for single and three phase. I think it was a carrier unit.

I will ask the heating guys about this. If it works, this could save the day. I appreciate it.
Thanks
Mike

I recommend that you disconnect the high leg phase of the open delta POCO supply, which is easy to identify from its higher L-G voltage. And connect the VFD directly without going through a transformer.

The worst choice would be to use the two wires that you measured with the lowest L-L voltage on an averaging meter in your post quoted below. I highly suspect that this voltage is across the "open jaw" of the two open delta transformers. I believe that a third harmonic is reducing the average but increasing the peak of the voltage above that of a sinusoid, while still maintaining an RMS value very close to the other two L-L pairs.

The slightly higher average measurement that's above the RMS on the other two L-L pairs is likely from a third harmonic that flattens the peak voltage but boosts up the "sides" around this peak. A flattened peak voltage could possibly reduce the peak current through the rectifier diodes, which would not be a bad thing.

" This is the problem unit:
Averaging meter
Input, 125,125,217, 250,250,226
...
True rms
Input 123, 123, 215
246, 247, 248"