Buck Voltage Without Transformer?

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Dustin Foelber

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Good Afternoon,
We recently discovered an unfortunate issue on our build out. All existing equipment and transformers were labeled 440V. On our build out we had (3) new transformers that I had custom made 75KVA 440V Delta to 208/120Y As we began installation I discovered all equipment is currently labeled incorrectly. Actual voltage is standard 480V. And my transformers have no tap options. Is anyone aware of a cost effective way to buck voltage by 40-50V? Maybe an add an in line resistor rated at 100Amps pass through?

Any help would be greatly appreciated.
 
resistance is no go (unless load is constant)
load current will vary and so will Vdrop (and load V)
waste of power 4 kW x 3
 
resistor is no go because that is a constant dead load = $$$

can the custom xfmrs be returned at even a substantial (30%+) restocking cost? If not, a quick google search shows new 75kva 480V-208y/120V xfrmrs from ~$1900 ea.

A 440V xfmr with a 480 input is going to output 227/130V on the secondary, which if you have 208/240V motors/heat loads would be fine, but 130V L-N is a bit high for 120V loads; what about bucking just the 120V loads from 130V?

eta: if you have very long runs with minimally sized wire, and decent loads, 130V at the panel may even be tolerable for a fair amount of equipment. Not ideal ofc but you asked for inexpensive solutions. I'd look at the loads and loading to see if your custom xfmrs still have a chance of working.
 
Dustin Foelber said:
Thanks Gents,
Appreciate the feed back. Going to fire it and check secondary voltage then make the call.
Click to expand...

You're welcome. Checking the secondary voltage unloaded is not going to tell you much; it should be within a volt or two either way of what I posted, assuming the incoming voltage is 480V. I'd spend a few minutes going over what loads you are feeding from your 208y/120 panels.

As was pointed out to me before here, just because your incoming is say 480V now doesnt mean it will stay that way. Going over 480V could make your setup completely unworkable whereas under 480V would be more inline with what you need. As stated above, perhaps the utility has taps they can adjust to change your primary voltage closer to 440V.
 
You specifically asked about bucking voltage without a transformer, but you might consider the following options.

1) The primary coils on a transformer are generally wound on the outside; you may be able to get turns added to turn the transformer into 480 to 208/120 devices. I am guessing that you would need on the order of 10 turns added to each primary coil.

2) You only need to buck 40V, less than 10%. A 'buck-boost' transformer arrangement may be appropriate. Buck-boost transformers directly operate only on a portion of the load (the buck or boost fraction) with the bulk of the load being directly supplied by the source. So very roughly you would be able to use a 7.5kVA transformer as the 'front end' to your 75kVA transformers.

See: http://www.hammondpowersolutions.com/files/HPS_Catalog_BuckBoost_Section2.pdf

Per page 87 (bottom row) a pair of Q005DTCF transformers (rated 5kVA each) connected per diagram 7 (page 91) will take 480V and drop it to 436V and be able to serve an 86.5kVA load. If you can tolerate 'running hot' then a pair of Q003DTCF transformers (rated 3kVA each) would give you 457V and 109kVA capacity.

3) You probably paid a pretty penny for custom transformers, and that has got to hurt...but 480V to 208/120V transformers are blog standard and 75kVA is a standard size. Before going too far down the path of figuring out how to use the transformers that you have, you should probably price the standard transformers.

Good luck!

-Jon
 
The material and labor cost to hack something together will probably cost more than buying 3 new transformers.
 
Dustin Foelber said:
Good Afternoon,
We recently discovered an unfortunate issue on our build out. All existing equipment and transformers were labeled 440V. On our build out we had (3) new transformers that I had custom made 75KVA 440V Delta to 208/120Y As we began installation I discovered all equipment is currently labeled incorrectly. Actual voltage is standard 480V. And my transformers have no tap options. Is anyone aware of a cost effective way to buck voltage by 40-50V? Maybe an add an in line resistor rated at 100Amps pass through?

Any help would be greatly appreciated.
Click to expand...

You say the existing equipment was labeled 440 and come to find out it was actually 480v equipment?
Are you trying to boost the 440v side to 480v or buck the 120/208v side?

Jap>
 
From the utility connecting point up to your transformer secondary it could be+/- 5% voltage[423 to 463 V] drop -it depends on cable length and conductor cross section, from full load to about 30%.
You may use aluminum conductor instead of copper and single core cables and increased distance between phases.
Let's say the cable length up to transformer 1000 ft.
The rated current is 75/sqrt(3)/440*1000=98.4 A
Aluminum cable in conduit #1 [Table 310.15(B)(16) 75oC]
Table 9 [ch.9] Z=0.24/kft
Voltage drop=sqrt(3)*I*Z=40.9 V
On the other hand through transformer it is another voltage drop.
Let's say the transformer short-circuit impedance is 3% and X/R=1
X%=R%=3%/SQRT(2)=2.12%.
X=R=2.12%*480^2/75000=0.0651 ohm
cosfi=0.85 sinfi=sqrt(1-0.85^2)=0.5268
DVxfm=SQRT(3)*98.4*0.065*(0.85+0.5268)=15.25 V [considered at 480 V side].
480-15.25-40.1=424 V[424/440*100=96.4%]
However, the voltage drop depends upon load current.
For 30% full load DV=(15.25+40.9)*.3=16.84 V [463.2 V=105.3%].
For less current the voltage will rise up to 480 V.[109.1%] still tolerable.
You may insert a resistor in series in order to increase the cable impedance[for shorter length than 1000 ft.] but the voltage drop domain will stay the same.
However, the voltage supplied by utility may be 5.8% more [ANSI C84.1 Range B] then 15% overvoltage is not tolerable.:weeping:
 
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