ECN Forum Forums General Topics of the Trade General Discussion Area Open Delta

 
Open-delta transformer banks always consist of two 1ø transformers, where delta banks {sometimes called ‘closed-delta’} use three 1ø transformers. See the two compared in Scott35’s Technical Reference Area under https://www.electrical-contractor.net/ubb/Forum15/HTML/000054.html

It’s important to understand that [in elecbob’s sketch] that there should almost always be very close to 240V across the two shown transformer-secondary windings. But also, there should be 240V across the open-delta ‘ends’ at 12 o’clock and 4 o’clock in elecbob’s sketch.

For most utilities, open-delta configurations are reserved for smaller motor loads, on the order of 7½-20hp. Voltage imbalance is the biggest problem—and 3ø induction motors are sensitive to stator heating—with reducing the motor’s mechanical load to prevent overheating.

Note that one way to illustrate open-delta imbalance is using two 10kVA transformers and a 10kW 240V 1ø 2-wire resistive load. If the 10kW load is connected across the lower or left winding in elecbob’s sketch, voltage drop will be about this same. But if the same load is connected across the open ends of the two windings [still at 240V] somewhat greater voltage drop will be produced.

I would take exception to that statement. The only way that would be true is if the load were to be large enough to collapse the delta a bit and thus lowering the voltage. That doesn't happen in a normally loaded transformer bank. If we have permitted the bank to have too much load that does indeed happen. What happens is that the voltage is lower to start with if the bank is overloaded.

Don't lose sight of the fact that the rated voltage output of a transformer will only happen when no load is on the transformer and the rated voltage is on the primary side. As soon as load is applied, the impedance will cause voltage drop through the transformer.

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Charlie Eldridge, Indianapolis, Utility Power Guy

 
A high-leg-to-ground reading of 265V almost sounds like there may be a misplaced 277V-secondary “power” transformer, with a correct 240V unit for the “lighting” pot.

CharlieE, pardon me, but that’s the idea of having other than a theoretical zero-impedance transformer. Very roughly, the change in voltage on secondary terminals decreases by the per-unit impedance when nameplate loaded versus zero load—e.g., a 3% Z, nominal 240V [or ~247V] transformer terminal voltage drops to about 97% ~233V [or ~240V] when fully loaded.

Equal loading across the ‘open’ terminals [versus across one of the two individual windings] on the open-delta bank approximately doubles. While a 3ø induction motor can succesfully operate with a lower overall [balanced] voltage like a 5% reduction, it will typically experience much greater stator heating if terminal-voltage imbalance is 5% per ANSI/NEMA standard C84.1-1995 Annex D. Discussed at www.joliet-equipment.com/voltage_unbalance.htm

I have a feeling we are saying the same thing. I normally think of voltage drop as just on the conductors and I was pointing out that more voltage drop will be experienced through transformation. I thought you were talking about the drop on just the conductors and you were intending to use the transformation in the calculations. With that, I agree.

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Charlie Eldridge, Indianapolis, Utility Power Guy

Although I haven't measured it myself, Bjarney, my understanding is that the 265 volt measurement was phase-to-phase, not phase to ground. After spending a few hours online I'm wondering if this isn't a power quality problem. It's a tool & die shop with CNC machines and possibly a fair computer load with CAD for blueprints. I'm wondering if there are grounding problems, neutral problems, harmonics, and service load imbalance. I also read about installing a zig-zag transformer close to the load. Any thoughts?

By the way, if you're not an electrical engineer and want to feel like an idiot, check out www.eng-tips.com


Dave

 
Dave55 — Zigzag transformers from drytype transformer producers are intended for triplen-harmonic mitigation and neutral ‘stiffening’ in {symmetrical} four-wire wye circuits. They would be a misapplication on the described 4-wire delta system, even it they were available for 240V. One type is the Acme ‘I-Trap’ with a nominal 208V rating— www.acmepowerdist.com/pdf/Page%2041.pdf See also www.electrical-contractor.net/ubb/Forum15/HTML/000072.html




[This message has been edited by Bjarney (edited 10-30-2004).]

Thanks for the info, Bjarney. When they talk about close to the load, I'm assuming this means next to the macnine? These sizes could cover the whole shop...(2) 200 amp panels, but there would be some 150' runs to the machines.

Dave

 
Dave55, these particular zigzag transformers are intended for use only with 208Y/120V (remote) panelboards, and not 240/120V ∆ service.

Are the ø-ø voltages all high [A-B, B-C and C-A]?




[This message has been edited by Bjarney (edited 10-31-2004).]

Dave55, check out American Power Conversion:
http://www.apc.com

They make systems up to several kva for mainframes, etc. and I'm sure they have someting to help you with the machine you're working on...

[This message has been edited by mxslick (edited 10-31-2004).]

Thanks, guys. Sorry about not understanding the first time about the zig-zag transformer, Bjarney. I thought the link was to something other than a zig-zag transformer.

Am I understanding correctly that elecbobs diagram is correct, except the 12 o'clock position to ground is 208 volt? I believe this is a typical configuration for our POCO with two pole mounted transformers.

I'll check out that APC site some more, but I belive it's for 480 volt. The more I read on this, the more I think this should be going to an electrical engineer. This is over my head and I like working from experience. It has, however been very educational.

Dave