Three Transformers Changing 3-phase to Single Phase?

TwoBlocked said:
You folks have been VERY helpful. After building the transformer skid (if we get the bid) I would like to test it in the shop with the 208V 3-phase we ave available. Of course the secondary voltage will be less than half of compared to energizing with 480V. Given the info on the attached nameplate, is there a way to know what the unloaded inrush current might be? Hopefully I can test this without tripping the main breaker!

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Maybe if I use like 50 ft of #10 wire as power leads to add some expendable resistance?
 
wwhitney said:
Case 1: the 480 delta generator supplies a 480V:120/240V single phase transformer on 480V phases A-B. If the currents on the 120/240V side are I1 and I2 (neutral current is I1-I2), then the 480V primary current is (I1+I2)/4. If the generator windings are equal impedance, then the generator sees (I1+I2)/6 current on its A-B winding, and (I1+I2)/12 current on both the B-C and C-A windings.

Case 2: the 480 delta generator supplies (3) 480V : 120V single phase transformers in a low zig-zag. Say the current I1 is on the leg provided by one transformer on A-B, and the current I2 is provided by the zig-zag on B-C and C-A. The current on the generator A-B coil is I1 / 4, while the current on the B-C and C-A coils are both I2/4.

So if the 120/240 load is balanced of current I = I1 = I2, in case 1 we have I/3 on one coil and I/6 on the other two, while in case 2 we have I/4 on each coil.

Cheers, Wayne
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I believe that the above analysis is correct for the transformer primary coils, but does not extend to the transformer input terminals.

In case 2, you have I2 flowing on both the B-C and C-A secondary coils, and thus I2/4 on both B-C and C-A primary coils (with the addition of magnetizing current). This agrees with your analysis.

However there is no connection to the secondary C terminal, so the current on the B-C and C-A secondary coils has to be strictly in phase; the two coils form a series circuit with the 120V load. The reflected load current on the primary coils is similarly in phase. Since there is _zero_ current flowing out on the secondary C terminal, there should be minimal current supplied to the primary C terminal.

IMHO your analysis makes it clear why it makes sense to have a generator connected in this fashion (or the 'double delta' fashion that someone mentioned), but it doesn't explain how inserting the transformer makes the balance better for the generator.

-Jonathan
 
TwoBlocked said:
You folks have been VERY helpful. After building the transformer skid (if we get the bid) I would like to test it in the shop with the 208V 3-phase we ave available. Of course the secondary voltage will be less than half of compared to energizing with 480V. Given the info on the attached nameplate, is there a way to know what the unloaded inrush current might be? Hopefully I can test this without tripping the main breaker!

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If you supply a transformer at less than half of its rated voltage, you should have virtually no inrush.

In normal steady state operation, the transformer flux oscillates from -max to +max. For economy of material, the transformer will be designed so that 'max' is pretty near the point of magnetic saturation. Inrush happens when the applied instantaneous voltage (where the supply is in the AC cycle) doesn't match the residual flux in the core, so the magnetic cycle tries to do something like 0 to 2*max rather than -max to +max. The core saturates and doesn't act to block current magnetically, so current jumps through the roof.

If you apply less than half rated voltage, then you pretty much can't drive the core into saturation, even if you apply voltage at the worst possible part of the AC cycle. You'd need lots of residual flux and bad cycle timing to drive the core into saturation and have inrush issues.

If you want to run the test with the primary configured for 240V, then my guess is you will still be okay unless you have a very small main breaker on a very low impedance service.

-Jonathan
 
winnie said:
If you supply a transformer at less than half of its rated voltage, you should have virtually no inrush.

In normal steady state operation, the transformer flux oscillates from -max to +max. For economy of material, the transformer will be designed so that 'max' is pretty near the point of magnetic saturation. Inrush happens when the applied instantaneous voltage (where the supply is in the AC cycle) doesn't match the residual flux in the core, so the magnetic cycle tries to do something like 0 to 2*max rather than -max to +max. The core saturates and doesn't act to block current magnetically, so current jumps through the roof.

If you apply less than half rated voltage, then you pretty much can't drive the core into saturation, even if you apply voltage at the worst possible part of the AC cycle. You'd need lots of residual flux and bad cycle timing to drive the core into saturation and have inrush issues.

If you want to run the test with the primary configured for 240V, then my guess is you will still be okay unless you have a very small main breaker on a very low impedance service.

-Jonathan
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Here is the transformer's wiring diagram. I suppose if you wanted to energize it with 240V and still get 240/120 out, you would jump the H1 tap to H2 and the H2 tap to H1 to put the coils in parallel. Since I won't be doing that, then each coil will have 104V instead of 240V and you don't believe the coil will saturate and trip the breaker. Cool! I've been sweating this part of it.

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TwoBlocked said:
It's gotta be this generator. It's a gas turbine with air bearings and so has no lubrication oil. It's super "green" and very quiet, too. The customer has a number of them and it seems a going thing in the gas industry. They learned by experience that these gennys will trip out on imbalance if the typical two leg 480 to 240/120 transformers are used. It may be more sensitive to imbalance. I don't know at what load they had problems. At one site I visited, it was using just two legs to a 480 to 240/120 transformer, but was drawing less than 10 amps 240v at full load. They are going to put, (now get this!) a Stirling engine genny with hydrogen gas as the "medium" if that's what you call it, rated at 5kw to replace the 65kw turbine, which then will be moved and require the transformer skid we've been discussing.

Oh, I love my job, but mostly because I got a boss I actually like, respect, and admire. :)
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connect three phase motor to output of this generator- it will always be balanced that way. then drive a single phase generator with it.

Might be super green but sounds possibly like overkill which possibly makes something less efficient still greener if not oversized? Some get hung up on that green or high efficiency term but ignore all other aspects of the situation. Efficiency specifications of such things often are only valid at a certain operating level and not necessarily the same across the possible operating levels.
 
winnie said:
I believe that the above analysis is correct for the transformer primary coils, but does not extend to the transformer input terminals.
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Good point.

I agree that in both cases the line currents between the generator and the transformer are the same, and those line currents alone should determine the generator loading. It is not true that the coil currents in the low zig-zag transformer primary match the coil currents in the generator, which I had assumed.

So it does seem that the choice of transformer configuration should have no bearing on the generator loading, at least for the idealized model. To balance the generator loading, you'd need to divide up the 120V loads and supply them with different out of phase voltages.

Cheers, Wayne
 
kwired said:
connect three phase motor to output of this generator- it will always be balanced that way. then drive a single phase generator with it.

Might be super green but sounds possibly like overkill which possibly makes something less efficient still greener if not oversized? Some get hung up on that green or high efficiency term but ignore all other aspects of the situation. Efficiency specifications of such things often are only valid at a certain operating level and not necessarily the same across the possible operating levels.
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Yes, "other aspects of the situation." The gas is virtually free, there's no oil in the generator to create an oil spill, and it's what the customer wants to do.
 
TwoBlocked said:
Yes, the generator also supplies an instrument air compressor set up. Also, they already have the generator.
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If the NEC calculated single phase load is 24kVA @ 240V then why use three 25kVA transformers?
Is the air compressor on the 480 side or also single phase 120/240?
 
tortuga said:
If the NEC calculated single phase load is 24kVA @ 240V then why use three 25kVA transformers?
Is the air compressor on the 480 side or also single phase 120/240?
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Compressor is on the 240/120V side. Might as well give the max power available by going with 25kw transformers in case they decide to increase the load. Our design is running twin 1/0 copper to the secondary breaker and then single 1/0 to their 100A manual transfer switch.

And, uh, that's also how our competitor did it at another site. But hey, the customer asked us for a second quote.

How's this for a quote: "When all is said and done, much more was said than done!"
 
tortuga said:
Is it a 240V motor?
How many HP is the compressor?
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The compressor system is 240V single phase, powering 2 VFDs. The compressor motors themselves are 3-ph. I don't know what HP. There's also a regenerative system, controls and lighting. It's a packaged unit, originally run by it's own 5kw genny, that no longer works. They got a 12 kw (?) standby genny providing power now. The breaker panel has a small heat trace load, but that is expected to increase. They'll be upping the size of the standby genny at some point.
 
Say, when getting a price for the transformers, I asked for bus bar jumpers for the secondary. They don't offer them so I thought I would just make some. Here's a page from the McMaster catalog. Can any of you fine folks tell me what size I would need for 300 amps?

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