3Ph 5KVA Transformer Troubleshooting with Anomolies

[Open Neutral]

Posting diagram

Posting diagram

I have a pdf of the drawing and could not figure out how to share it.

When all else fails, you can upload a print to sendspace.com or similar, and post the URL here. Such is only valid for a few weeks but it's better than guessing.

 

[kingpb]

I think you need to ask the Owner if the intent is to use the equipment long-term, if so, offer the correct solution and that is to use a VFD on the input side, it will get you where you need to go a lot quicker.

Is anybody else still confused as to the exact arrangement? :?

 

[kingpb]

Even the 15A Fuse, depending on which one it is, may not work. it will depend on the transformer inrush multiplier and duration.

For a standard fuse, if the multiplier is 6 times, and the duration of inrush is 6 cycles, than it may hold in.
If the inrush is greater than 6 or the duration is greater than 6 cycles, than depending on those numbers it may not hold in. A time delay fuse will allow a higher inrush, and/or longer duration to occur, but depending on the combination, even that may not work.

Modeling the transformer and curve together, it looks like a Busmann FNQ-R 20A might be a good fit for coordination purposes. But, if the inrush happens to be above 10 times, then depending on the duration it may still not be good enough.

 

[topgone]

15 KVA TRANSFORMER BANK FUSING

15 KVA TRANSFORMER BANK FUSING

You have 18 amps primary (assuming what you have are three-5 kVA single-phase transformers banked three-phase). As mentioned by other posters, you will expect inrush currents when switching on. Choose a fuse not over 125% of your primary amps = 18 x 1.25 = 22.55, say 20A. If you have the secondaries fused, you are allowed to fuse your primary up to 170% of primary amps (30A). Hope this solves your problem.

 

[jkcowboy]

Reporting success! Thank for all the comments. Here is what I did to fix the transformers:

1. I wanted a go-no go for each transformer. I jumpered a new line from the generator to a fused switch, 15A fuses were fine. Then I jumpered from the switch directly to the transformer. The existing Xfmr wiring had two of the six primary windings running in parallel. I rewired the Xfmr for standard 506 to 240 which uses the entire primary (H1 to H8) and entire secondary (X1 to X4). The secondary X1,X4 leads were lifted at the xfmr and left exposed for reading. It worked just fine. I scratched my head trying to figure what could be happening. What I think happened is someone rewired the primaries with parallel opposing fields which created an inductor vs a transformer with a primary current much higher than fuse settings.

2. I adjusted the primary L2 to connect to H7 vs H8 and the output was exactly 220V at 460 input. Good enough.

3. I then removed my jumpers one at a time converting back to using the installed wiring. All jumpers removed and the xfmr was still running good. The ONLY difference now was the way the xfmr was wired.

3. I repeated the procedure with the other 2 xfmr, rewiring each and lit off the bank. Good start, no noise, no blown fuses.

4. Final voltage at the output fused switch was 220V to ground, 380 V phase to phase. We hooked up the equipment and it ran like a champ.

5. Responding to one of the post. Since the voltage was immediately converted by a DC pwr supply... no need for a VFD.


In the end, someone must have messed with the wiring trying to fix or change something, of course no one fessing up. My test readings turned out to be correct, the xfmrs were good. Restoring the wiring to design returned the system to operation.

An open question to all, what exactly was happening to the two primary phases, obviously wired wrong, that would pop the fuses. Was I correct in my guess that they had created a big inductor?

John King

 

[jkcowboy]

I didn't like the way I phrased my last question.

Question to all: What exactly was happening with the 2 of 6 primary windings, whose mis-wiring was causing the fuses to pop?

My answer: The infinite resistance of the secondary was not being reflected to the primary so the transformer must have become a large 480V inductor with an impedance low enough to pop fuses. Do you concur with my opinion or have a different explanation?

 

[kwired]

I didn't like the way I phrased my last question.

Question to all: What exactly was happening with the 2 of 6 primary windings, whose mis-wiring was causing the fuses to pop?

My answer: The infinite resistance of the secondary was not being reflected to the primary so the transformer must have become a large 480V inductor with an impedance low enough to pop fuses. Do you concur with my opinion or have a different explanation?

My guess is you were inputing 480 volts to a configuration that was set up for a lower input voltage.

 

[jkcowboy]

I don't think the applied voltage is the culprit. When you have a coupled primary and secondary with an iron core the laws of transformers should apply. Infinite secondary impedance = infinite primary impedance. Why exactly was there not reflected infinite impedance? Again, my thought is that the two windings in parallel somehow cancelled each other and decoupled the primary and secondary, thus giving birth to an inductor of sorts.

 

[Smart $]

I didn't like the way I phrased my last question.

Question to all: What exactly was happening with the 2 of 6 primary windings, whose mis-wiring was causing the fuses to pop?

My answer: The infinite resistance of the secondary was not being reflected to the primary so the transformer must have become a large 480V inductor with an impedance low enough to pop fuses. Do you concur with my opinion or have a different explanation?

Back in OP you said Ph1 (L1) connected to H1, H5, and Ph2 (L2) connected to H2, H6.

Without knowing the exact winding configuration, it is still a guess. My guess is that the primary is technically only two windings, each with two voltage taps. H1 and H4 the ends of one winding; H5 and H8 the ends of the other. In betweens H's are voltage taps of their respective winding.

The fact that there was parallel wiring of H terminals says automatically the primary is wired for the lower input voltage, i.e. input voltage to each winding, rather than the higher voltage across both windings in series. However... :blink: ...the stated connections are even wrong for a lower voltage input scheme :ashamed:

 

[jim dungar]

Again, my thought is that the two windings in parallel somehow cancelled each other and decoupled the primary and secondary, thus giving birth to an inductor of sorts.

The primary and the secondary were 'decoupled' everytime you open circuited the secondary.
A transformer always looks like an iron core inductor.

You mention 6 primary windings, this is probably wrong. Most likely it is one primary winding with taps that change the number of turns.

The problem was how the primary windings were connected. It is possible that the incorrect wiring involved only portions of the primary winding. Everything depends on where those 'H" taps are on the windings and how they are interconnected.

In your solution you said L1->H1 and L2-> H8, what did you do with the other 'H' terminals?

 

[jkcowboy]

The transformer has two primary windings each with 4 taps. H1-H4 on the first. H5-H8 on the second. At the most this is like 6 separate windings that you could power individually, series or parallel. The existing wiring when the fault was present was between H1 and H2 and the windings H5 and H6 powered in parallel with 460VAC. I have no idea how the transformer got this way or what the guys were trying to fix or obtain. I only know that it popped 15A time delayed fuses for them and 10A time delay fuses for me.

During a go-no go test, I powered the primary with connections at H1 and H8. H4 and H5 were connected. In other words, the entire primary was energized from H1-H8. It ran fine.

Now back to the question. In a coupled transformer, how can current flow in the primary and pop fuses with an open secondary and no shorts? This should not be possible. The infinite impedance of the secondary should reflect to the primary. In other words a Counter EMF should develop that directly opposes the input voltage and results in no current (of course there are some small losses). Would someone explain why transformer laws no longer seem to apply?

My theory remains that the two energized primary coils (H1->H2 and H5->H6) had opposing magnetic fields, cancelled each other and somehow broke down the transformer action until the device behaved more like an inductor with a reactance = 2(PI)fL if I remember correctly but a defined impedance none the less which would result in a defined current flow.

John

 

[jkcowboy]

Ok, I think I taught myself a lesson and my theory has changed. The solution (as I believe it) is a simple one. Running the two small winding at 460 was too much voltage and most likely saturated the xfmr. When primary voltage is changing and flux is not... that would be bad juju with corresponding primary current and blown fuses.

Forget all that other stuff I was theorizing on.

John

 

[gar]

110817-2133 EDT

Back in post #20 I provided a link for you to Acme and a wiring diagram.

The diagram does not show phasing with dots as is conventional, however the different winding connections were provided.

The way the transformer was incorrectly wired was for a primary voltage of 228 V with the primaries in parallel, and the secondaries in series for 240 V.

What you are not understanding about transformers is that when input voltage in combination with the reciprocal of frequency is too great the magnetic core goes into saturation and the magnetizing current gets very large. i believe that Edison in the development of a successful high efficiency, for day 1879, dynamo discovered that ferromagnetic cores developed saturation as the magnetizing force (ampere-turns) was increased.

If you double the rated voltage at the same frequency to a transformer a very large current is produced from repetitive core saturation.

This saturation effect also occurs at application of a correct voltage to a transformer that is dependent upon the last state of the flux in the transformer core and the point in the AC input when voltage is applied. This inrush is only momentary.

To see this kind of peak inrush current see my photos P6 and P7 at http://beta-a2.com/EE-photos.html .

.

 

[mull982]

Ok, I think I taught myself a lesson and my theory has changed. The solution (as I believe it) is a simple one. Running the two small winding at 460 was too much voltage and most likely saturated the xfmr. When primary voltage is changing and flux is not... that would be bad juju with corresponding primary current and blown fuses.

Forget all that other stuff I was theorizing on.

John

I'll have to admit i didn't read the history on this post however based on what you described in your previous post I'd agree that saturation was your problem with the way you had the windings connected.

 

[jim dungar]

Would someone explain why transformer laws no longer seem to apply?

My theory remains that the two energized primary coils (H1->H2 and H5->H6) had opposing magnetic fields, cancelled each other and somehow broke down the transformer action until the device behaved more like an inductor with a reactance = 2(PI)fL if I remember correctly but a defined impedance none the less which would result in a defined current flow.

John

Transformer laws still apply.

Any opposing flux in the magnetic core does not appreciably affect the inrush current in the primary windings, it would affect the magnetic coupling to the secondary.

You do not have six individual windings. You have two windings with taps on them.

In your OP you said that L1->H1 and L2->H2. This means someone connected your source to two taps on a single winding, effectively creating a short circuit hence the blown fuse.

 

[gar]

110818-0742 EDT

Jim:

Terminals H1 to H2 are one primary coil designed for a nominal 228 V input, and H5 to H6 is a second similar primary coil. H1 and H5 are of the same phasing, both would get a dot to identify this relationship. These two coils can be paralleled with H1 connected to H5 and H2 to H6 and work with a nominal 228 V source.

Applying 480 to this connection forces the core severely into saturation causing a very high current at voltage zero crossings.

I would only describe it as a short when it gets into sever saturation in a portion of the cycle. At this point the coil impedance gets close to the DC resistance in combination with the air core inductance of the coil. If the correct primary voltage was applied it would not be a short.

.

 

[jim dungar]

110818-0742 EDT
Terminals H1 to H2 are one primary coil designed for a nominal 228 V input, and H5 to H6 is a second similar primary coil. H1 and H5 are of the same phasing, both would get a dot to identify this relationship. These two coils can be paralleled with H1 connected to H5 and H2 to H6 and work with a nominal 228 V source.

Correct, if the transformer uses the connection #3 in the link you posted to Acme transformers.

 

[jkcowboy]

Transformer laws still apply.

Any opposing flux in the magnetic core does not appreciably affect the inrush current in the primary windings, it would affect the magnetic coupling to the secondary.

You do not have six individual windings. You have two windings with taps on them.

In your OP you said that L1->H1 and L2->H2. This means someone connected your source to two taps on a single winding, effectively creating a short circuit hence the blown fuse.

Jim,
I read your reply closely and think I may have another misunderstanding. When I look at the connection to a winding with multiple taps, I assumed that the only operable part of the winding was that between the inputs (part that carries current and the that the rest of the coil does not matter (unless being used as an autotransformer)... but maybe I'm wrong here as well. The whole coil has applied changing voltage and perhaps that is the quantity that I should be watching. Let me know if that statement is correct. I need to go dig out one of my old electrical device books that cover transformers again.

Thanks again to all.

John

 

[jim dungar]

Jim,
I read your reply closely and think I may have another misunderstanding. When I look at the connection to a winding with multiple taps, I assumed that the only operable part of the winding was that between the inputs (part that carries current and the that the rest of the coil does not matter (unless being used as an autotransformer)... but maybe I'm wrong here as well. The whole coil has applied changing voltage and perhaps that is the quantity that I should be watching. Let me know if that statement is correct. I need to go dig out one of my old electrical device books that cover transformers again.

Thanks again to all.

John

Sorry, could not follow your reasoning.

Consider, an auto-transformer electrically performs no different than does a isolation transformer. While not usually advised, you could create an auto-transformer simply by using just taps on a primary winding. For example, in the case of the ACME transformer diagram #3, we have been referencing, you could connect a supply voltage to taps H1 and H4, and take off a load voltage from taps H2 and H3 while totally ignoring the winding H5-H8. But your inrush will depend on the design of the H1-H4 winding and the applied voltage.

 

[mbeatty]

You state that the transformers are 240/120, yet you (they) were applying 480 volts. Is that true?

Depending on the way the coils are wound, the inrush can be great, but I'd think that your step-down application would be more likely to hold than reversed, if they are general-purpose. But you could try to apply a low voltage to that winding and show that they do hold and produce the proper high voltage.

You state that the transformers are 240/120, yet you (they) were applying 480 volts. Is that true?

Depending on the way the coils are wound, the inrush can be great, but I'd think that your step-down application would be more likely to hold than reversed, if they are general-purpose. But you could try to apply a low voltage to that winding and show that they do hold and produce the proper high voltage.

This was exactly my question. Are these 480V transformers, or not? :?