# transformer in reverse



## brian john (Mar 11, 2007)

Only if there is a low or high voltage issue.


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## Magnettica (Jan 23, 2007)

Triple phase?

Single phase?


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## waco (Dec 10, 2007)

Interesting question. First, the primary wiring of a step down transformer is often too small to carry the current required to allow the transformer to effectively function in reverse. As for taps, I don't see any purpose for them if the transformer is being used in reverse.


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## brian john (Mar 11, 2007)

waco said:


> Interesting question. First, the primary wiring of a step down transformer is often too small to carry the current required to allow the transformer to effectively function in reverse. As for taps, I don't see any purpose for them if the transformer is being used in reverse.


 
Amps is amps? A 75 KVA 480-208/120 delta = 90 amps primary, 208 amps secondary at full load reversed fed at full loads 208 primary, 90 amps secondary.

Waco if you have more information on this please post.

What can happen is the inrush is excessive.


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## nolabama (Oct 3, 2007)

brianjohn and waco arguing transformers ... I will use Brian's answers.


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## bobelectric (Feb 24, 2007)

delta-wye wye -delta? Whats the application?


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## william1978 (Sep 21, 2008)

Anybody got any wiring diagrams for this installation?


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## brian john (Mar 11, 2007)

nolabama said:


> brianjohn and waco arguing transformers ... I will use Brian's answers.


I am not disagreeing I am questioning I had never heard this previously.


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## JayH (Nov 13, 2009)

Taps are still adjustable on a reverse fed step down transformer but may not be enough to compensate for the voltage drop that will occur.

Inrush could cause nuisance tripping.


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## waco (Dec 10, 2007)

There are many types of step down transformers besides those used in power distribution systems. The rule-of-thumb is current capacity is inverse of the voltage change. For example, a battery charger plugged into a 120 volt outlet pulling ten amps can theoretically deliver 100 amps at 12 volts (10:1 ratio). Of course, the secondary and its leads can no more handle 100 amps than fly.

Probably important to recall that electronics and electrical are related, but not the same things.


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## brian john (Mar 11, 2007)

Waco:
My inquire relates to a dry type standard off the shelf built as a step down transformer, have you heard of or have any documentation regarding the current issue you noted.


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## waco (Dec 10, 2007)

I doubt I understand your question, but the original post proposes using a step-down transformer in reverse, which means (to me) the primary of the step-down will be used as the secondary and vice-versa.

My experience with transformers does not include power distribution transformers beyond the simple theories governing their use to alter transmission dynamics by managing voltage.

Would you care to express your concern in electronic/electrical terms?


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## brian john (Mar 11, 2007)

waco said:


> I doubt I understand your question, but the original post proposes using a step-down transformer in reverse, which means (to me) the primary of the step-down will be used as the secondary and vice-versa.
> 
> My experience with transformers does not include power distribution transformers beyond the simple theories governing their use to alter transmission dynamics by managing voltage.
> 
> Would you care to express your concern in electronic/electrical terms?


That was not the question was it?


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## brian john (Mar 11, 2007)

waco said:


> There are many types of step down transformers besides those used in power distribution systems. The rule-of-thumb is current capacity is inverse of the voltage change. For example, a battery charger plugged into a 120 volt outlet pulling ten amps can theoretically deliver 100 amps at 12 volts (10:1 ratio). Of course, the secondary and its leads can no more handle 100 amps than fly.
> 
> Probably important to recall that electronics and electrical are related, but not the same things.


Depends on the charger and the efficiency of the device. A typical battery charger utilized in a UPS or Telco system will operate in in a ratio of input,output, BUT the efficiency plays into this and the efficiency is not linear from light to full load.

Lastly converting AC to DC is an inverter not a transformer, though in some systems transformers are part of the device.


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## Lz_69 (Nov 1, 2007)

brian john said:


> Lastly converting AC to DC is an inverter not a transformer, though in some systems transformers are part of the device.


Technically AC to DC is a converter DC to AC is an inverter.


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## brian john (Mar 11, 2007)

Lz_69 said:


> Technically AC to DC is a converter DC to AC is an inverter.


 
YOU ARE RIGHT Rectifier AC to DC inverter DC to AC, my apologies.


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## Lz_69 (Nov 1, 2007)

brian john said:


> YOU ARE RIGHT Rectifier AC to DC inverter DC to AC, my apologies.


Don't worry you can thank me later for wasting your brain space with rather useless knowledge


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## waco (Dec 10, 2007)

who said anything about rectifying AC or inverting DC? The point is, a step down transformer meant to convert 120 VAC to 12 VAC has the turns ratio needed to have ten amps in the secondary be 1 amp in the primary. What is difficult about that?

Of course, the reactance of both the primary and secondary windings play into the dynamics as impedance effecting the current flow which also influences the design size of the primary and secondary windings wiring.

And so on.

Now, what was the question?


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## brian john (Mar 11, 2007)

waco said:


> who said anything about rectifying AC or inverting DC?
> 
> Now, what was the question?



Look I do not want to argue with you, which always appears to be your method of operation BUT 

*YOU DID*


> a battery charger plugged into a 120 volt outlet pulling ten amps can theoretically deliver 100 amps at 12 volts (10:1 ratio). Of course, the secondary and its leads can no more handle 100 amps than fly.


Explain to me how you are going to plug a battery charger into a 120 outlet and charge a battery at 12 (typically 13.5-15 vdc) and not rectify the voltage?

Just curious.


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