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Old 26-09-2022, 16:59   #1
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Noisy isolation transformer with bizarre outputs

Hi.

I've got a 50-year old Acme brand 15kVA isolation transformer that I'm trying to recommission. On the shore side, I installed a new Hubbell 125/250 50a inlet and #6 cable (240v 50a service) to the transformer and attached the black and red ones to the places in the isolation transformer that the original black and red cables attached to (actually two black cables, one with a turn of red tape on it). I did the same for L1, L2, and neutral going to the distribution panel from the isolation transformer.

When I turn on the disconnect, the transformer hums fairly loudly, and I'm wondering if the humming has anything to do with voltage readings I'm seeing.

I verified 240vac between L1 and L2 at the shore power source, the shore power cable, and the 50a disconnect breaker just inside of the boat, which feeds the transformer. Shore neutral does not come aboard.

After the transformer, I also confirmed 240vac at the distribution breaker panel L1/L2 lugs.

But when I check voltage from L1 to the neutral bus on the distribution panel, I see 175vac; L2 to the neutral bus shows 65vac. That adds up to 240, but not the way I'm used to.

Thoughts on what might be going on here? Are Acme transformers always noisy? Or could crossed wires cause the HUMMMMMMM?
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Old 26-09-2022, 18:34   #2
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Re: Noisy isolation transformer with bizarre outputs

All line frequency transformers "hum" as power goes through them. As the magnetic field switches back and forth 50 or 60 times a second, parts move a tiny bit. The moving parts push on the air, and make noise you can hear.

Why does the shore power neutral not come aboard? This is a transformer with TWO wires in and three out? That doesn't seem right.
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Old 27-09-2022, 03:45   #3
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Re: Noisy isolation transformer with bizarre outputs

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Originally Posted by ItDepends View Post
All line frequency transformers "hum" as power goes through them. As the magnetic field switches back and forth 50 or 60 times a second, parts move a tiny bit. The moving parts push on the air, and make noise you can hear.

Why does the shore power neutral not come aboard? This is a transformer with TWO wires in and three out? That doesn't seem right.
That's interesting about why they hum. I've heard that toroidal transformers are pretty much silent.

As for only L1 and L2 coming aboard, I believe that's fairly standard for 240 systems. Here's a manufacturer's schematic on p.2 of the product brochure: https://www.bridgeportmagnetics.com/...ales-sheet.pdf
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Old 27-09-2022, 07:20   #4
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Re: Noisy isolation transformer with bizarre outputs

It is normal in (many) modern systems to have the neutral and ground come aboard so you can install a working ELCI at the power inlet, even if they are not used elsewhere in the system.

It seems very strange that the center tap off the secondary could be off in voltage...

There is of course no difference between the two incoming hot wires, they would be swappable without the system knowing the difference, so that can't be an issue...

The manual you posted is not completely indicative of the way most marine isolation transformers are wired. Unless you know for sure that the one you are working with is exactly that, I'd be wary of using it as a reference.
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Old 27-09-2022, 07:29   #5
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Noisy isolation transformer with bizarre outputs

Quote:
Originally Posted by ItDepends View Post
It is normal in (many) modern systems to have the neutral and ground come aboard so you can install a working ELCI at the power inlet, even if they are not used elsewhere in the system.

It seems very strange that the center tap off the secondary could be off in voltage...

There is of course no difference between the two incoming hot wires, they would be swappable without the system knowing the difference, so that can't be an issue...

The manual you posted is not completely indicative of the way most marine isolation transformers are wired. Unless you know for sure that the one you are working with is exactly that, I'd be wary of using it as a reference.


Nothing wrong with that wiring. There’s no real point in shore earth going anywhere as the enclosure is non metallic so there’s nothing to connect shore side ground to any way

Only ELCI don’t need any ground connecting to work. So that comment above is misplaced

Nice transformer by the way. The wiring diagrams are correct.
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Old 27-09-2022, 07:33   #6
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Re: Noisy isolation transformer with bizarre outputs

Quote:
Originally Posted by ItDepends View Post
All line frequency transformers "hum" as power goes through them. As the magnetic field switches back and forth 50 or 60 times a second, parts move a tiny bit. The moving parts push on the air, and make noise you can hear.

Why does the shore power neutral not come aboard? This is a transformer with TWO wires in and three out? That doesn't seem right.


It’s split phase. No need for neutral
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Old 27-09-2022, 07:35   #7
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Re: Noisy isolation transformer with bizarre outputs

Quote:
Originally Posted by q240z View Post
Hi.



I've got a 50-year old Acme brand 15kVA isolation transformer that I'm trying to recommission. On the shore side, I installed a new Hubbell 125/250 50a inlet and #6 cable (240v 50a service) to the transformer and attached the black and red ones to the places in the isolation transformer that the original black and red cables attached to (actually two black cables, one with a turn of red tape on it). I did the same for L1, L2, and neutral going to the distribution panel from the isolation transformer.



When I turn on the disconnect, the transformer hums fairly loudly, and I'm wondering if the humming has anything to do with voltage readings I'm seeing.



I verified 240vac between L1 and L2 at the shore power source, the shore power cable, and the 50a disconnect breaker just inside of the boat, which feeds the transformer. Shore neutral does not come aboard.



After the transformer, I also confirmed 240vac at the distribution breaker panel L1/L2 lugs.



But when I check voltage from L1 to the neutral bus on the distribution panel, I see 175vac; L2 to the neutral bus shows 65vac. That adds up to 240, but not the way I'm used to.



Thoughts on what might be going on here? Are Acme transformers always noisy? Or could crossed wires cause the HUMMMMMMM?


How have wired the output of its split phase 120 output if the phases are differently loaded you’ll get uneven voltages.

Do you need split phase on board.
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Old 27-09-2022, 07:54   #8
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Re: Noisy isolation transformer with bizarre outputs

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Originally Posted by q240z View Post
Thoughts on what might be going on here? Are Acme transformers always noisy? Or could crossed wires cause the HUMMMMMMM?

1) you have an open neutral, either in your wiring or in the transformer itself, or you perhaps have somehow picked up the line-side neutral connection in the transformer rather than the load-side neutral.


2) USA design power transformers always hum, larger ones hum more loudly. Victron transformers use a toroidal design that is quieter and lighter but that does not have a barrier layer between line and load. The lack of a barrier layer makes them non-ABYC compliant. Whether the barrier layer provides a meaningful improvement in safety is debatable. The idea behind it is that it will cause a breaker trip in the event of an insulation breakdown while the toroidal designs theoretically could have an insulation breakdown that would go undetected.
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Old 27-09-2022, 08:32   #9
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Re: Noisy isolation transformer with bizarre outputs

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How have wired the output of its split phase 120 output if the phases are differently loaded you’ll get uneven voltages.

Do you need split phase on board.
I don't need split phase, just normal 120 and 240 vac.

The labels were long gone from the transformer cables, so I just swapped the old red, black, and white cables (only red and black on the input) for a new ones. If it was wired wrong from the factory or a previous owner, I wonder how the PO ran any AC electric gear?
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Old 27-09-2022, 08:41   #10
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Re: Noisy isolation transformer with bizarre outputs

Here’s whatI recommend you do:

- shut down and unplug shore power; shutdown all inverters etc.

- disconnect secondary wiring: L1, L2 and N. There should be ground wires too, right?

- with a multimeter in resistance mode, measure the secondary windings: first between L1 and L2 and write down the value. Measure three times, eliminating obvious wrong readouts and averaging the others.

- now measure the same way between L1 and N, as well as between L2 and N.

- now measure between primary side (shore feed) ground and secondary (ship side) ground.

Post the values, we’ll go from there
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Old 27-09-2022, 08:54   #11
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Re: Noisy isolation transformer with bizarre outputs

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Originally Posted by q240z View Post
I don't need split phase, just normal 120 and 240 vac.



The labels were long gone from the transformer cables, so I just swapped the old red, black, and white cables (only red and black on the input) for a new ones. If it was wired wrong from the factory or a previous owner, I wonder how the PO ran any AC electric gear?


In this case the the only way to get 240 vac is via a split phase output you also need 120.

Hence you can just have L1 L2 connected to the primary shore and what you have is a floating neutral output.

The trouble is uneven loading of the two 120 phases will cause the neutral to float resulting in differing live neutral potentials. You could ground the neutral to shore , but this removes one of the more useful aspects of the isolation transformer.
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Old 27-09-2022, 09:02   #12
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Re: Noisy isolation transformer with bizarre outputs

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1) you have an open neutral, either in your wiring or in the transformer itself, or you perhaps have somehow picked up the line-side neutral connection in the transformer rather than the load-side neutral.

2) USA design power transformers always hum, larger ones hum more loudly. Victron transformers use a toroidal design that is quieter and lighter but that does not have a barrier layer between line and load. The lack of a barrier layer makes them non-ABYC compliant. Whether the barrier layer provides a meaningful improvement in safety is debatable. The idea behind it is that it will cause a breaker trip in the event of an insulation breakdown while the toroidal designs theoretically could have an insulation breakdown that would go undetected.
I'm looking at Victron now. But it looks like I'd need two of their 7kw units, and it's unclear if they can be paralleled easily to get 7kw output on L1 and the same on L2.

Bridgeport Magnetics' 6kw Marine-Pucks are ground-free (the case is non-metalic) and two can easily be wired for 6kw output on L1 and L2. But they're ~$600 more for a set than Victron.
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Old 27-09-2022, 09:13   #13
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Re: Noisy isolation transformer with bizarre outputs

I should maybe clarify that I'm getting these strange 120v readings with the power selector switch OFF. So no juice is getting through to the distribution panel. They're no-load readings.

And the humming isn't subtle. I've read that they should be ~40 decibels. I don't have a meter, but decibel comparative charts make me think it's closer to 60. When I flip the breaker on the pedestal ON, I can hear the humming start from outside the boat. Even if I square away the odd voltage problem, the humming is bad enough to make me think modern toroidal transformers would be the way to go.
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Old 27-09-2022, 09:14   #14
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Noisy isolation transformer with bizarre outputs

Quote:
Originally Posted by q240z View Post
I'm looking at Victron now. But it looks like I'd need two of their 7kw units, and it's unclear if they can be paralleled easily to get 7kw output on L1 and the same on L2.



Bridgeport Magnetics' 6kw Marine-Pucks are ground-free (the case is non-metalic) and two can easily be wired for 6kw output on L1 and L2. But they're ~$600 more for a set than Victron.


Nothing wrong with the bridgeport. The issue is the problem of a unbalanced split phase floating neutral output.

Really if you want 120 and 240 and the benefits of a floating neutral you would be Better with two transformers ie 240 and 120
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Old 27-09-2022, 09:27   #15
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Re: Noisy isolation transformer with bizarre outputs

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Here’s whatI recommend you do:
...
Post the values, we’ll go from there
I'd love to! But I stupidly assumed the original wiring was right and maintenance access for the transformer would be pretty much unnecessary. I built cabinetry around it that will require a chainfall to hoist it up to a level where I can take the bottom off and get at the cable connections.

But several commenters have said that humming is what these transformers do. If the humming isn't going to quiet down even if I solve the odd voltage problem, then I'm inclined to just mothball it in place, cut the cables, buy quiet new transformers, and relocate them.

The humming really is obnoxious.
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