Odd distribution voltage

[rlaft]

I need to install a distribution system to provide 138/80 vac 3 phase voltages into a vacuum test chamber. I am looking for alternatives for a method of producing this voltage from the more typical distribution voltages (208, 240 & 480).
Also, I am looking for an economical source of GFI circuit breakers for 80 vac single phase. I have confirmed that conventional 120 vac GFI's will not work at the 80 vac level.

 

[Dan76]

Sounds like 3:2 stepdown transformer is what you need
Maybe a 3:1 stepdown wired as a buck(opposing voltage)autotransformer, an appropriately sized unit that transforms 12.5 kV to 4160 should work wired to 208/120.
The High Voltage unit might be expensive but maybe you can find one surplus somewhere.
GE puts out a little book about their HV transformers that shows the connections.
As to the GFI's Those will probably have to be custom made but maybe you could try getting the parts from a Variable Frequency Drive since they use a lower voltage at lower frequencies.
Good Luck, -Dan76

 

[REDDOG]

Try Jefferson Transformer, Technical Support department at 1-800-892-3755 or 414-209-1620.

 

[jbartos]

Suggestions:
1. 240VAC 3phase may produce 138VAC 3phase over 240Vdelta/240Vstar transformer where 138VAC is phase to neutral voltage on the secondary 240Vstar.
2. Once you have 138V 3phase another transformer 138Vstar/138Vstar will produce 80V 3phase in the secondary star winding with neutral.

 

[jbartos]

Correction/clarification to points 1 and 2 in the above posting (I beg your pardon). The transformer is meant to be:
Point 1.
Primary 240Vdelta (preferred) or 240Vstar
Secondary 138Vstar (138Vdelta has less convenient grounding)
Point 2.
Primary 138Vdelta (preferred) or 138Vstar
Secondary 80Vstar (80Vdelta has less convenient grounding)

 

[rlaft]

Since writing this post, I have gotten estimating prices on a custom wound 480/138-80 vac 3 phase transformer. The prices per KVA are more than low enough to make this an economically attractive alternate for the use of variacs.
So that leaves me still with the challenge to find an economic source of GFI's. A company called Bender in Exton, PA has offered me several options not including anything comparable in price or simplicity to the "garden variety" of receptacles and circuit breakers seen in any supply house. I have soecifications from several manufacturers which state that their 120 vac units will work down to 80 - 85 % of nominal but that doesn't help me.

 

[busbar]

With 208Y/120 service, connecting three commercially available 240-to-48V buck-boost transformers in a delta/buck configuration (H1-to-X1) with primaries connected phase-to-phase, you'd get ~86V phase-to-neutral. Would that work, at least for your initial mockup? [Don't know what sorta kVA needs you have.]

Maybe you could open up a commercially available 120V GFCI receptacle or circuit breaker and replace the existing series-connected line resistor with a lower value to get satisfactory reduced-voltage operation.

Of course, doing anything like this would be absolute madness where life-safety or property-protection issues were involved.

 

[rlaft]

If I understand busbar's suggestion, the 240v winding would be connected to 208 v, and the 48 v secondary would then be 41.6v. If I connect the H1 and X1 togeter, it seems that I could buck the distribution voltage of 120 v. phase to neutral down to 78.4v which would be perfect. His post suggests 86v but I think he might have mis-calculated. Does anyone or you, busbar agree? But, I think the big question is how much money this would save vs. a custom wound 3 phase transformer which I have reasonable pricing for. I will check this with the transformer manufacturer.

On the GFI modifications, I am not permitted such an option. I need to find something that has the typical UL etc. approvals.

 

[busbar]

As I see it, the voltages do not subtract arithmetically. I'm not too good at phasors, but I think the 86(.5)V number comes from the third side of a triangle with one side being 120V, another side 41.6V, with an angle of 30 degrees between them.

If a custom transformer turns out to be the solution, and you don't need isolation from the source circuit, an autotransformer might be cost effective.

 

[jbartos]

Suggestion: The 480V/138delta-80star (I suggest 240V/138Vdelta-80Vstar since 240V is lower and potentially safer than 480V) may have a high-resistance grounded neutral system that can be made very sensitive to ground currents. However, this is more expensive scheme than off the shelf GFIs.

 

[busbar]

Here's another approach, but only uses about half of the buck-boost transformers' kVA rating. For each of the three phase-to-neutral legs of a 208Y/120 source, connect an appropriately sized 120/240-to-16/32V buck-boost transformer: H1 to source phase, H4 to circuit neutral, and tie H2-H3-X4 together/insulated, X2-X3 together/insulated, giving about 76V-to-neutral on each corresponding X1 terminal. This works out to be 60+16 volts, giving about 132V phase-to-phase.

For a custom transformer, specifying something like four 2½% taps FCBN and two 2½% taps FCAN should add only a small cost to the unit.

 

[rlaft]

This note is just to provide some closure. As I reported earlier, I found a tranformer vendor which will provide the custom wound tranformer with taps at a reasonable price. More recently, I finally also found a GFI manufacturer which will custom modify their 120 vac GFI's to accept 80 vac.
Thanks for all the interest.