Transformer question

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kwired said:
Yes.

Now if the 208/120 system were separately derived from say 480 volts, might be best to just put in 480 x 120/240 single phase transformer to begin with, of whatever rating the vessel needs. Add unless it is determined the vessel will be just fine with 208 instead of 240 and the 208 was already existing (which is what I think we have from OP).
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I agree. No sense transforming the 480 to 208 and then back to 240. A 480/240-120 transformer should be readily available IF the vessel truly must have 240.
 
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LarryFine said:
Not necessarily. A while back, I came up with the idea of wiring the two boost secondaries at opposite ends of the primary, effectively boosting each half of 208v (104v to the neutral) up to (approx) 120v.

Added: See: https://forums.mikeholt.com/threads/ev-charger-voltage-range.2565807
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jim dungar said:
The resultant L-N voltages would not be 120V.
The starting L-N is 120V given a 120/208 3 wire source.
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I agree with Jim. The L-N voltages would be approximately 134V and 127° apart from each other.
 
Etanuye said:
120/208V Transformer is feeding 100A, 240V 1phase vessel at marina. They use 2 phases of conductor which is 208V plus or minus voltage drop while the vessel needs 240V.
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I do see a potential safety advantage in using a 120//208V feeder vs. having a 120/240V split-phase one, because with 120/208V the L-G leakage current on the two phases cannot cancel out within the CT of the GFI since they are 120° apart.
For example, leakage currents of 100mA L1-G and 100mA L2-G would be measured as a net leakage currrent of 100mA by a GFI when using 120/208V, but it would be measured as 0 mA with 120/240V.
 
LarryFine said:
Not necessarily. A while back, I came up with the idea of wiring the two boost secondaries at opposite ends of the primary, effectively boosting each half of 208v (104v to the neutral) up to (approx) 120v.
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I'll accept that, but kinda sorta consider separate secondaries as EFFECTIVELY FUNCTIONALLY separate transformers. But I know you are correct.
 
synchro said:
I agree with Jim. The L-N voltages would be approximately 134V and 127° apart from each other.
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I did mention the calculated voltage:

"A 16/32v secondary on 208v will develop a 27.7v boost, for a 235.7v output. (all numbers theoretical)."

As for your point #2:

How can a 1ph transformer put out anything other than a single (180° for conversation sake) wveform? :unsure:
 
GeorgeB said:
I'll accept that, but kinda sorta consider separate secondaries as EFFECTIVELY FUNCTIONALLY separate transformers. But I know you are correct.
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Electrically, yes, as isolated, independent sources, which is what makes my suggestion possible to wire.

But since the windings all share a single core, there can never be anything but a single waveform (phase) coming from it, neutral or not.
 
If the proposal is to take 208V 2-wire from a grounded 208/120Y system, and connect it to an autotransformer with a center tap to get 120/240V, then the center tap will not be at ground potential. It will be at 60V to ground. But the two output legs 120V from the center tap will be 180 degrees apart.

If you take 208 3-wire from a 208/120Y system, and then connect it to that same autotransformer (now the center tap has a connection from the 208/120Y system neutral), the output center tap is now at ground potential. The two output legs will still be 120 degrees apart. The voltage to ground will be (240/208)*120 = 138V.

Cheers, Wayne
 
wwhitney said:
If the proposal is to take 208V 2-wire from a grounded 208/120Y system, and connect it to an autotransformer with a center tap to get 120/240V, then the center tap will not be at ground potential. It will be at 60V to ground. But the two output legs 120V from the center tap will be 180 degrees apart.
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You lost me. An auto-transformer, not a buck-boost? (I know buck-boosts are wired as auto-transformers.)

I'd have to see a diagram of what you're saying.

wwhitney said:
If you take 208 3-wire from a 208/120Y system, and then connect it to that same autotransformer (now the center tap has a connection from the 208/120Y system neutral), the output center tap is now at ground potential. The two output legs will still be 120 degrees apart. The voltage to ground will be (240/208)*120 = 138V.
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You can not connect the neutral from 120/208v 3-wire to the center tap of a 1ph transformer primary.

It will quickly overheat and over-current because the single core can not "bend" to accommodate 120°.
 
LarryFine said:
You lost me. An auto-transformer, not a buck-boost? (I know buck-boosts are wired as auto-transformers.)
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If you're wiring it as an auto-transformer, it doesn't matter what other configurations it can do. It ends up one long coil with various taps.

LarryFine said:
I'd have to see a diagram of what you're saying.
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OK, below is a terrible drawing I made in Paint. The black lines are the voltage vectors of the 208Y/120V system (with the neutral point grounded). The red line(s) are the single coil of the autotransformer. The red dots are connection points. And the blue lines and text are voltage measurements.

LarryFine said:
You can not connect the neutral from 120/208v 3-wire to the center tap of a 1ph transformer primary.
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Hmm, I'm not sure what restriction you're thinking of (there may be one I'm unfamiliar with), but I don't believe it would apply to an autotransformer. But maybe in the second drawing, that red bent line would actually require two separate coils on two separate cores? Rather than one long coil on a single core? I'm not clear on that.

Cheers, Wayne

TerribleDrawing.jpg
 
wwhitney said:
If you're wiring it as an auto-transformer, it doesn't matter what other configurations it can do. It ends up one long coil with various taps.


OK, below is a terrible drawing I made in Paint. The black lines are the voltage vectors of the 208Y/120V system (with the neutral point grounded). The red line(s) are the single coil of the autotransformer. The red dots are connection points. And the blue lines and text are voltage measurements.


Hmm, I'm not sure what restriction you're thinking of (there may be one I'm unfamiliar with), but I don't believe it would apply to an autotransformer. But maybe in the second drawing, that red bent line would actually require two separate coils on two separate cores? Rather than one long coil on a single core? I'm not clear on that.

Cheers, Wayne

View attachment 2559354
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Isn't that poor of a drawing and shows what is happening.

"It ends up one long coil with various taps." is exactly what is going on. each 120 volt leg of the wye is getting 16-17 volts added to it making it~ 137 to the neutral point of the wye. At same time you increased the distance between the ends to ~240.

If you connect as shown in your first drawing there is a second neutral point but it is not at same potential as the neutral point of the wye, if you try to bond it to the wye neutral you will have high current flow. If you leave it unbonded it technically would be another ungrounded conductor and would need overcurrent protection if you connect loads to it.
 
kwired said:
If you connect as shown in your first drawing there is a second neutral point but it is not at same potential as the neutral point of the wye, if you try to bond it to the wye neutral you will have high current flow.
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So that's the question that Larry and I were discussing. Because the only difference between the top and the bottom diagram to my understanding is that you connect the black wye neutral point to the red center point.

If a single coil has to be represented as a straight line on a diagram like I drew, then that will be a problem, and lots of current will flow. But I don't see the difference between a single coil with a center tap, and two individual coils where you connect one end of each coil together. Perhaps this is my mistake. And you certainly could implement the second diagram with 2 separate red coils, so why not with one coil with a center tap?

Hopefully someone more familiar with transformer theory will answer this question. A related question I have is that when thinking about transformers, for the voltage vector theory, is it enough to just think about the coils? Or do I have to pay attention to how they are wound around iron cores, possibly jointly?

Cheers, Wayne
 
The new neutral point, lets call it N' between the 240V terminations would need to be grounded per the NEC. However, as shown above, this neutral point is not electrically the same as the grounded neutral point of the 208Y/120V source supplying the buck-boost transformer. The 60V difference will cause problems, regardless if it is from a center tapped winding or the junction of two windings.
 
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