75 kva 480-240 xmfr backfed

[tadavidson]

A potential customer wants 15 kw electric heaters. The voltage is 480. He has a 240 3 phase supply (Open Delta) supplying a xmfr which is designed as a 480-240 but being back fed to produce the 480. He is putting on 240 getting out only 455. Voltage from the xmfr is x1-204, X2- 255, X3- 320
75

 

[petersonra]

A potential customer wants 15 kw electric heaters. The voltage is 480. He has a 240 3 phase supply (Open Delta) supplying a xmfr which is designed as a 480-240 but being back fed to produce the 480. He is putting on 240 getting out only 455. Voltage from the xmfr is x1-204, X2- 255, X3- 320
75

It does not matter whether the 240 V supply is delta (open or otherwise) or wye as long as it really is 3 phase 240V. have you measured the voltage on the supply side line to line for all three lines?

Presumably the output of the 3 phase xfmr is 3 phase. Are the X1, X2, X3 values you mention with respect to some neutral or ground? What is the line to line voltage there?

I suspect a wiring issue with the xfmr or that you don't really have 3 phase on the 240V side.

 

[tadavidson]

Yes sir 240 volts phase to phase in coming to the Transformer.

 

[tadavidson]

455 phase to phase.

 

[ptonsparky]

Does your transformer have taps on either the H or X side

 

[electrofelon]

A potential customer wants 15 kw electric heaters. The voltage is 480. He has a 240 3 phase supply (Open Delta) supplying a xmfr which is designed as a 480-240 but being back fed to produce the 480. He is putting on 240 getting out only 455. Voltage from the xmfr is x1-204, X2- 255, X3- 320
75

If you havnt grounded the 480 side, and those measurements are phase to ground on the 480 side, then that would not be abnormal. The system would be floating and you are just reading capacitive coupling that isn't perfectly in the center. Then as Tim said, maybe the taps are not in their nominal position giving you the 455 phase to phase?

 

[tadavidson]

I will open the xmfr and check the connection s on what is now the secondary. There is a "jumper connection, tap 6, that says 455.

 

[tadavidson]

Does your transformer have taps on either the H or X side

Yes on what would be the H which is now the X

 

[texie]

A potential customer wants 15 kw electric heaters. The voltage is 480. He has a 240 3 phase supply (Open Delta) supplying a xmfr which is designed as a 480-240 but being back fed to produce the 480. He is putting on 240 getting out only 455. Voltage from the xmfr is x1-204, X2- 255, X3- 320
75

Your line to line low voltage is probably due to tap settings. Problem is though, taps will not behave the same on the now secondary side. And your line to ground readings on the now secondary side are meaningless as the secondary is floating.
In addition,you are going to run into other code issues with this set up. If you leave it ungrounded then you will need ground detectors. It you corner ground it you will then need equipment rated for this including the load. Not all equipment is suitable for use on a 480 corner grounded system. At much better solution is to get a 240 delta primary to 480Y transformer. They are reasonably readily available and solve whole lot of negative issues.

 

[tadavidson]

Your line to line low voltage is probably due to tap settings. Problem is though, taps will not behave the same on the now secondary side. And your line to ground readings on the now secondary side are meaningless as the secondary is floating.
In addition,you are going to run into other code issues with this set up. If you leave it ungrounded then you will need ground detectors. It you corner ground it you will then need equipment rated for this including the load. Not all equipment is suitable for use on a 480 corner grounded system. At much better solution is to get a 240 delta primary to 480Y transformer. They are reasonably readily available and solve whole lot of negative issues.

I will suggest another xfmrs as the best solution. The load is 4, 15kw heaters.

 

[jim dungar]

Yes on what would be the H which is now the X

Per standard convention, like ANSI:
"H" means high voltage, not necessarily the primary.
"X" means low voltage not necessarily the secondary

Occasionally, on step up transformers, some manufacturers use H for the primary and X for the secondary.

 

[Besoeker]

A potential customer wants 15 kw electric heaters. The voltage is 480. He has a 240 3 phase supply (Open Delta) supplying a xmfr which is designed as a 480-240 but being back fed to produce the 480. He is putting on 240 getting out only 455. Voltage from the xmfr is x1-204, X2- 255, X3- 320
75

If the turns ratio is designed for 480V to 240V it is likely higher than the voltage ratio to take into account transformer regulation so reversing it is likely to mean you will get lower than 480V out. Which seems to what's happening in your case.

 

[Jraef]

Your line to line low voltage is probably due to tap settings. Problem is though, taps will not behave the same on the now secondary side. And your line to ground readings on the now secondary side are meaningless as the secondary is floating.
In addition,you are going to run into other code issues with this set up. If you leave it ungrounded then you will need ground detectors. It you corner ground it you will then need equipment rated for this including the load. Not all equipment is suitable for use on a 480 corner grounded system. At much better solution is to get a 240 delta primary to 480Y transformer. They are reasonably readily available and solve whole lot of negative issues.

Bingo.

I will suggest another xfmrs as the best solution. The load is 4, 15kw heaters.

If the heaters are the ONLY load, then they will likely not care if it's a corner grounded Delta (or ungrounded with GFP). But in addition to the load itself, not all CIRCUIT BREAKERS are rated for 480V delta. Many small inexpensive 480V breakers are "slash rated" meaning they can ONLY be used where the source is a grounded Wye 480/277V system. If you can find a transformer with a secondary that is 480Y, that's a better choice all around.

Also, transformers used for step-up must now by Code be identified for that use by the manufacturer. It doesn't need to be on the nameplate itself (but that helps), it can be identified as such in the data sheets but if you are having it inspected, have those data sheets with you at inspection to avoid a call back. One of the reasons behind this by the way is because of the potential effects of compensating techniques used in the transformer design resulting in undesirable effects, which is what Besoeker was referring to in his post. That then likely means the transformer you picked would not have been to Code, because it likely was designed for step-down use only.

 

[ptonsparky]

Bingo.



If the heaters are the ONLY load, then they will likely not care if it's a corner grounded Delta (or ungrounded with GFP). But in addition to the load itself, not all CIRCUIT BREAKERS are rated for 480V delta. Many small inexpensive 480V breakers are "slash rated" meaning they can ONLY be used where the source is a grounded Wye 480/277V system. If you can find a transformer with a secondary that is 480Y, that's a better choice all around.

Also, transformers used for step-up must now by Code be identified for that use by the manufacturer. It doesn't need to be on the nameplate itself (but that helps), it can be identified as such in the data sheets but if you are having it inspected, have those data sheets with you at inspection to avoid a call back. One of the reasons behind this by the way is because of the potential effects of compensating techniques used in the transformer design resulting in undesirable effects, which is what Besoeker was referring to in his post. That then likely means the transformer you picked would not have been to Code, because it likely was designed for step-down use only.

Is this a problem more so with 3 phase vs single phase transformers?