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Old 30th May 2010, 07:27 AM   #1
ECM is offline ECM  United States
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Default Two Transformer Balanced Power Unit

Hi All, I've built a few balanced power units before and I'm looking to wire up my next unit a little differently.

I'll be using two transformers, each are 115+115 input and 60+60 outputs, 1000VA.

Originally I was going to parallel them to get a 2000VA unit, however, I am running them as two independent 1000VA outputs within the same chassis.

My question is this: Can I back to back the two transformers, like a "Felicia" conditioner on steroids. Basically, one transformer will feed the second transformer with balanced power.

I understand I will limit the power to 1000VA. I've seen commercial units wired this way with talk about "extra reserves" of power being stored in the coils, etc. I can always add additional filtering between the two units as well.

Thanks.
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Old 30th May 2010, 04:41 PM   #2
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Using the transformers 'back to back' won't do anything except waste power. The energy stored in a descent transformer is neglegible.
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Old 30th May 2010, 06:35 PM   #3
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The "talk about extra reserves of power" is just that, talk. A transformer doesn't store any energy. It transforms volt-amperes at any power factor, and consumes a little power in the process.

There are other devices that are able to store small amounts of power for a brief period of time, such as a ferroresonant transformer, but the device you are referring to stores absolutely no power whatsoever.

Placing the two units in series (step-down to step-up) will work, but provide a higher source impedance to your load, which means poor supply regulation. Best bet is to parallel the primaries and series the secondaries, for an effective 1:1 isolation transformer.

Grounding the center tap of the secondary (balanced power) is a violation of the National Electrical Code; you need to ground one end, which becomes your neutral.
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Old 31st May 2010, 03:52 AM   #4
ECM is offline ECM  United States
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Thanks for the replies. I'll leave them as is.

zigzagflux, if grounding the center tap is a violation, how are companies able to sell balanced power units to the general public?
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Old 31st May 2010, 04:14 AM   #5
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Quote:
Originally Posted by ECM View Post
Thanks for the replies. I'll leave them as is.

zigzagflux, if grounding the center tap is a violation, how are companies able to sell balanced power units to the general public?
As long as you're not routing high voltage through building wiring, NEC does not apply. It is an isolated circuit. (UL, however, does apply.)

Note that balanced outputs are very common on power inverters since it simplifies the design.

And in the US, 240v is always balanced by design.
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Old 31st May 2010, 05:15 AM   #6
ECM is offline ECM  United States
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Quote:
Originally Posted by star882 View Post
As long as you're not routing high voltage through building wiring, NEC does not apply. It is an isolated circuit. (UL, however, does apply.)

Note that balanced outputs are very common on power inverters since it simplifies the design.

And in the US, 240v is always balanced by design.
Thanks for the clarification.
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Old 31st May 2010, 02:35 PM   #7
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We've been here before. Lots of misunderstanding on the subject.

Question about how to wire an Isolation Transformer
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Old 31st May 2010, 09:40 PM   #8
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ZigZag has covered balanced AC power in the US, but I would like to add:
Permanent balanced 120V (60V / 60V) AC power is permitted in commercial or industrial occupancies. The system's use is restricted to areas under close supervision by qualified personnel. This is per NEC 647 (2008 and soon 2011).
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