Would (3) 120V-36V 1.5kVA buck-boost transformers (one per phase) be sufficient to supply a 100A 240V 3Ø panel? This has already been installed but it does not quite seem right to me. It seems that regardless of if the desired voltage can be obtained, that each leg would only be able to accomadate around 5A total. I am not fully understanding how the contractor came up with the values for these transformers, or how a 36V secondary would provide the desired 240V at the panel? Any help here would be appreciated.
Buck-Boost to raise 208 3Ø to 240 3Ø
- Thread starter aeagle
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wwhitney
Senior Member
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208V across the primary will give you 36V across the secondary and in phase with the primary. In the boost configuration, wiring the primary and secondary in series, you get the sum, or 244V. Only 36/244 of the circuit power actually flows through the secondary, so the balanced power available at 244V 3-phase is 3 transformers * 1.5 kVA * 244/36 = 30.5 kVA.
Cheers, Wayne
Cheers, Wayne
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What does your panel need?
I am usually leary of feeding L-N loads from a wye Buck-Boost.
I am usually leary of feeding L-N loads from a wye Buck-Boost.
wwhitney
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Ah, I misread the OP as 208V-36V boost transformers. Which could be used in a delta configuration.
For 120V-36V boost transformers, I believe you'd need the secondary to be split, so that it can be configured as a 120V-18V boost transformer at the full 1.5 kVA. Then they can be used in a wye configuration. The math is now that the available kVA at 240V delta is 3 * 1.5 kVA * 138/18 = 34.5 kVA.
L-N loads on a 240V 3 phase system would be 138V, which seems unlikely. But I'm curious what gives you pause about wye configured buck or boost?
Cheers, Wayne
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Many people forget the Line-Neutral voltage will change.
Joethemechanic
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There is a tanning salon a little down the street from me that has 208 wye. big new tanning bed was 240 3 phase. Manufacturer said to use boost bucks on two legs, and land the leg without the B/B on the terminal the control circuit is connected to. I know why they said to do that, but it seemed kind of unusual. I was meaning to amp clamp the phases with it on just to see the ballance. but I got distracted and forgot.
I have to wonder if it was designed for a 240 three high leg
I have to wonder if it was designed for a 240 three high leg
ptonsparky
Tom
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I've done two BB for specific equipment that utilized the neutral. Worked fine. I'd be hesitant about feeding a panel that way.
winnie
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The key feature of a 'buck/boost' setup is that the secondary of the transformer is placed in series with the supply voltage, so that the low voltage secondary is added to the high voltage supply, giving a new changed high voltage.
As Wayne described in post 2, only a portion of the total output power is actually 'transformed', so the load on the transformer is only a fraction of what is delivered to the load.
I think there is an error in your original description. You state that 120:36V transformers are being used. My _guess_ is that the secondary is really a dual voltage 18/36V design, wired for 18V. If this is the case, then the transformers would be wired in a wye boost configuration, taking the L-N voltage from 120V to 138V, and giving a 239V L-L.
A transformer kVA rating is based on its use as an isolation transformer, and can be used to calculate the maximum secondary current. The allowed secondary current doesn't change in a boost configuration.
So these transformers have a secondary current capacity of 1500/18 = 83.3A. In the wye boost configuration, 3 of these transformers can supply 83.3A at 239V, or 34.5 kVA (in agreement with Wayne's calculation)
Jon
(NB I've glossed over the way subtractive polarity is used for the buck connection)
S
If the existing load was really 100A, your transformer bank is operating at 104% capacity, which could be tolerable, IMO.
The short answer is you'll be short by a few amps. The possible current you can provide by using 3 X 1.5 kVA, 120/ 18V (36V halved) single-phase transformers will only be around 96A @ 239.2V, not 100A at 240V, 3-phase.
If the existing load was really 100A, your transformer bank is operating at 104% capacity, which could be tolerable, IMO.
LarryFine
Master Electrician Electric Contractor Richmond VA
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Welcome to the forum.
When you supply a 240v transformer with 208v, the 18/36v secondary voltage will be lower.
When you supply a 240v transformer with 208v, the 18/36v secondary voltage will be lower.
winnie
Senior Member
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This is true.
I was guessing that the transformers had 120/240V primaries, and were connected 120V L-N in a 'star' configuration. But for 3 phase buck-boost applications with 3 transformers, you also have the choice of a delta configuration, and then the 240V primary run at 208V comes into play.
Without further info from the OP we can't really know what the actual layout is.
-Jon
The assumption I used was that the 3 X single-phase transformers being mentioned are truly 120/18V X 1.5 kVA. There are two (2) primary windings rated 120V and two (2) secondary windings rated at 18V. The transformer bank connections will be wye-wye autotransformer connections, allowing a primary source of 208V (208/1.732 = 120V) to be used and the output will be 239.02V (I can't edit my mistake in my previous post anymore).
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The option you do not have, FWIW, is putting the primary of the buck-boost in wye configuration and trying to use the secondary to directly boost the delta phase. The problem there is the offset phase relationship between any wye phase voltage and the delta phase voltages.
wwhitney
Senior Member
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What does that even mean, what wiring configuration are you trying to describe?
The 3 autotransformers should be connected in a 3-fold symmetric way (under the permutation L1 -> L2, L2 -> L3, L3 -> L1), and one autotransformer primary is either connected N-L1 or L3-L1. Either way, that transformers secondary terminal say X1 is connected to L1 as well, and X2 is then the boosted version of L1.
I mean, I guess if the primary is connected L3-L1 you could connect X1 to L2 and get a less boosted new voltage system with a phase rotation, but that wouldn't be an efficient way to use your transformer capacity. It could work if you have the transformers already and the turns ratio (as a number greater than one) is slightly too low.
Cheers, Wayne
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