Help With My Transformer (240v 3 phase)

[fst442]

I have questions about an existing transformer installation. There are no markings on this XFMR at all. The service voltage is 3 Phase 240v(high leg) delta. My voltage at the XFMR primary connections is 230v phase to phase across the board/ 115v to ground on A and C phase/ 206v to ground on B phase. The secondary voltage concerns me. I have 430v phase to phase across the board/221v to (derived) neutral on A and C phase/321v to (derived) neutral on B phase. Please take me back to school.

1:If my primary high leg voltage is proportional by the square root of 3(206/115=1.7) the why is the secondary high leg voltage propertional by the square root of 2(321/221=1.4)?

2:Why do I still have a high leg on the secondary at all?

 

[augie47]

On your 430 volt side... to what point are you measuring to obtain the "phase to neutral" voltage,
(What terminal do you have other than H1-H2-H3)

(It sounds like you have a delta high voltage winding and are getting random voltages to ground)

 

[fst442]

I have 3 sets of H1-H2-H3. One set is -5%, another is 0% and the final is +5%. The primaries are connected to the 0% which I'm guessing is the standard. It doesn't tell me what the 5% deviation is based on. Maybe voltage?

On the 480v side which is my secondary I am measuring from X-1,X-2 and X-3 to X-0 which is my XFMR derived neutral point.

 

[augie47]

If I understand correctly, you are concerned about the reading on the 480 v to neutral on the "H" side.
Since you only have a H1-H2-H3 terminal, you have no neutral to obtain a true reference voltage. Any measurements from H1-H2 and H3 to neutral (XO) or ground will be floating voltages and will like vary.

By Code, you can leave the 480 side ungrounded {but see 250.21(D)}
or you can ground one phase which would give you a reference voltage to ground.

(The 5% taps are a means to regulate the voltage to obtain the desired output)

 

[fst442]

My 480 is on the secondary, the X side. It's a step up XFMR. So my X-0 should be stable in relationship to my X-1,X-2 and X-3. That's my thought process anyway.

 

[augie47]

My 480 is on the secondary, the X side. It's a step up XFMR. So my X-0 should be stable in relationship to my X-1,X-2 and X-3. That's my thought process anyway.

I have not encountered a true "step-up" transformer (only reversed wired step-down) and, from what you state, I would agree with your thinking. I have no answer.

 

[ActionDave]

My 480 is on the secondary, the X side. It's a step up XFMR. So my X-0 should be stable in relationship to my X-1,X-2 and X-3. That's my thought process anyway.

Is the X-O grounded?

 

[kwired]

I have 3 sets of H1-H2-H3. One set is -5%, another is 0% and the final is +5%. The primaries are connected to the 0% which I'm guessing is the standard. It doesn't tell me what the 5% deviation is based on. Maybe voltage?

On the 480v side which is my secondary I am measuring from X-1,X-2 and X-3 to X-0 which is my XFMR derived neutral point.

The fact that your voltage adjustment taps are not all the same probably will mess up your expected voltage ratios. It would be nice to know what the actual rated voltage of both windings is for the 0% taps

 

[ActionDave]

Amend to my question about the X-O.

How many wires are coming into the transformer? What are they connected to?

 

[fst442]

There 3 wires coming in on the H side. No neutral on the line side. Do you think that I have a grounding issue? It hurts not having nameplate info on the XFMR. My comment in my initial post,about the relationship to ground of the 3 phases on my X side and the square root of 2,make me wonder.

 

[jim dungar]

Your primary voltages to Ground or Neutral make absolutely no difference at all, what is important are the voltages H1-H2, H2-H3, and H3-H1.

X1, X2, X3, and X0 are the ANSI designations for a WYE transformer connection, therefore your voltages should be V for X1-x2, x2-X3, and X3-X4 and V/1.732 for X1-X0, X2-X0, and X3-X0. If X0 is not bonded to Ground, then X1-G, X2-G, and X3-G are undefined and can be anywhere from 0 to > V.

You say your transformer is 240 to 480V, but you are feeding it with 230V, so you should be getting 460V out not 430V.

It may be time to start over.
What does your transformer nameplate really say?

 

[Smart $]

There 3 wires coming in on the H side. No neutral on the line side. Do you think that I have a grounding issue? It hurts not having nameplate info on the XFMR. My comment in my initial post,about the relationship to ground of the 3 phases on my X side and the square root of 2,make me wonder.

I'm not certain how you determined which taps are +/- 5%, but I'd have to assume this should be a simple 1:2 voltage ratio, and you are not getting that, i.e. 230:430 is not 1:2. If all primary connections are not made to the same respective tap, you are going to throw off the voltage vectors. 221V A-N + 221V C-N should make 442V 1? A-C... yet you are reading 430.

I have to question why no nameplate, and why are you trying to connect this transformer, before proceeding?

 

[kwired]

Your primary voltages to Ground or Neutral make absolutely no difference at all, what is important are the voltages H1-H2, H2-H3, and H3-H1.

X1, X2, X3, and X0 are the ANSI designations for a WYE transformer connection, therefore your voltages should be V for X1-x2, x2-X3, and X3-X4 and V/1.732 for X1-X0, X2-X0, and X3-X0. If X0 is not bonded to Ground, then X1-G, X2-G, and X3-G are undefined and can be anywhere from 0 to > V.

You say your transformer is 240 to 480V, but you are feeding it with 230V, so you should be getting 460V out not 430V.

It may be time to start over.
What does your transformer nameplate really say?

I'm not certain how you determined which taps are +/- 5%, but I'd have to assume this should be a simple 1:2 voltage ratio, and you are not getting that, i.e. 230:430 is not 1:2. If all primary connections are not made to the same respective tap, you are going to throw off the voltage vectors. 221V A-N + 221V C-N should make 442V 1? A-C... yet you are reading 430.

I have to question why no nameplate, and why are you trying to connect this transformer, before proceeding?

From what I understand from his OP this is existing, has no nameplate, and he is wondering why he is measuring those particular voltages he mentioned.

 

[GearMan]

Primary readings are ok. From the terminal markings, you have a D x Y xfmr. 240D x 480y/277v. A & C phase readings are correct but low due to low primary voltage, primary taps could bring that up 5%. B phase is an issue (induced Y secondary is not a high leg). Possible winding fault?

 

[jim dungar]

From what I understand from his OP this is existing, has no nameplate, and he is wondering why he is measuring those particular voltages he mentioned.

No nameplate, then how do we know it is a 1:2 ratio, 240 to 480V?

But, regardless, if the secondary has X1, X2, X3, and X0 it is wye connected.
The OP mentions taps, so there must be more than just H1, H2, and H3 on the primary. If the tap connections are not identical then the secondary L-N voltages will not be either.

 

[kwired]

No nameplate, then how do we know it is a 1:2 ratio, 240 to 480V?

But, regardless, if the secondary has X1, X2, X3, and X0 it is wye connected.
The OP mentions taps, so there must be more than just H1, H2, and H3 on the primary. If the tap connections are not identical then the secondary L-N voltages will not be either.

We don't know what the ratio is or the actual rated voltage. He was simply asking why his meter readings are what they are.

If secondary has an X0 does that mean it is wye connected. I can't recall seeing a high leg delta unit but know they exist, what do they mark the midpoint of the phase that gets grounded with? Seen suggestions of X4 but do not know if that is a standard practice or not. X4 is used for a lead on dual voltage single phase transformers.

 

[jim dungar]

We don't know what the ratio is or the actual rated voltage. He was simply asking why his meter readings are what they are.

If secondary has an X0 does that mean it is wye connected. I can't recall seeing a high leg delta unit but know they exist, what do they mark the midpoint of the phase that gets grounded with? Seen suggestions of X4 but do not know if that is a standard practice or not. X4 is used for a lead on dual voltage single phase transformers.

A mid-point ground would be labeled X4. Barring extreme rareness it is unlikely to be a 480/240 3PH 4W output.

 

[Smart $]

A mid-point ground would be labeled X4. Barring extreme rareness it is unlikely to be a 480/240 3PH 4W output.

Actually, if this transformer were marked to ANSI standard, would not the primary be X and the secondary be H, being that letter markings of windings are determined by voltage, not primary and secondary.

 

[jim dungar]

Actually, if this transformer were marked to ANSI standard, would not the primary be X and the secondary be H, being that letter markings of windings are determined by voltage, not primary and secondary.

Yes.
ANSI standard is H=high voltage, X=low voltage.

But sometimes I choose to be 'flexible' in the use of terminology when the discussion is general in nature. In this case I felt using H for primary would help make the situation overall less confusing.

 

[GearMan]

Putting ratios, voltage vectors and ANSI standards aside for the moment. Since it is existing, what is the xfmr feeding? Is it a single load? Is it a panel with properly labeled voltage. Is it a 3 or 4 wire load?
Is XO bonded at the xfmr or the panel (if it?s a panel)? Does the load require a NEU? What prompted the trouble shooting, did an OCPD trip or blow? Has the load been in operation prior to the service call? Since this is a code forum, it?s good to see responses based on that in this thread, but may not answer your question. You said you had questions about an existing xfmr installation, the only question I see is ?why do I still have a high leg?? Answer, based on the XO terminal?you shouldn?t. Take a peek at any MFG?s wiring diagram 240D x 480Y, you will find terminal markings identical to your prior posts. H is 240v X is 480Y. Whether that is the xfmr you have, I can only guess. Answers to some of the above questions will change future posts. You may even get the answer to the real question. For now, 430v is too low to feed a 480v load and the NEU is an issue. Personally, I would replace the xfmr.