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Old 10th June 2012, 02:01 AM   #1
srinath is offline srinath  United States
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Default Power transformer question.

Back in the day when I was awake in electrical engineering class, I seem to recall that transformers can be run backwards and the limit is the current on the high voltage side cannot be exceeded ... Like if its got 500v and its a 500va rated trafo, and a say 100v primary, feeding 110v into the secondary side will give you a 20v output in the secondary but you're limited to 1 amp on that 110v. Sounds confusing ... but is the concept right ... or did my teachers lie to me ... again ... wont be the first time ... that's my story and I'm sticking to it.

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Srinath.
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Old 10th June 2012, 02:28 AM   #2
Jay is offline Jay  Indonesia
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Quote:
Originally Posted by srinath View Post
or did my teachers lie to me ... again ... wont be the first time ... that's my story and I'm sticking to it.
What You played truant and now accusing your teachers (was that including the Principal also?)???!
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Old 10th June 2012, 02:34 AM   #3
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You would look at it as power in = power out x efficiency. Larger transformers tend to be more efficient. You might loose 5% or so in power rating going backwards with 500 va, because the primary is made to support the extra current to make up for the transformer loss in efficiency. I can not follow your example with 3 secondary’s and a 100 volt primary but i will give a different example.

Transformer
500va
120vac primary
500vac secondary.

Assuming a resistive load and perfect power factor.

500va / 120vac = 4.167A input current.

500va / 500vac = 1 amp secondary.

When going from the 120vac primary to the 500vac secondary. The primary winding’s are sized to give you your rated output with % efficiency calculated in.

When going the other way, you can put 500vac at 1 amp in to the secondary (because that is the maximum the secondary is rated at) and you will get 120vac out at 4.167A with a slight drop in voltage and probably a 5% or so drop in current. The slight loss in voltage is because when going primary to secondary the loss is made up for in a few extra turns on the secondary. When reversing the primary and secondary you loose this advantage.

If you do not believe me, try measuring an isolation transformer 120vac to 120vac. Then turn it around and measure it and you will get a lower voltage going secondary to primary.
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Old 10th June 2012, 02:50 AM   #4
srinath is offline srinath  United States
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Aaaah yes powerbob, why would I not believe someone with "power" in their name ... he he ... anyway I get it ... but wont the whole thing being run @ the lower voltage let it run cooler and hence not get as warm and therefore as high resistance ... good comeback eh ...

Jay: OK guilty as charged ... except like I went to school for civil engineering, so I had 2 courses of electrical basics and no electronics ... I guess there were too many civil engineers dropping dead touching high voltage lines just cos they were building the towers that hold em up ...

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Srinath.
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Old 10th June 2012, 03:10 AM   #5
Jay is offline Jay  Indonesia
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Originally Posted by srinath View Post
Jay: OK guilty as charged ... except like I went to school for civil engineering, so I had 2 courses of electrical basics and no electronics ... I guess there were too many civil engineers dropping dead touching high voltage lines just cos they were building the towers that hold em up ....
Then be careful with your 500v transformer. I guess I'm quite paranoid about safety that so far I have no problem with high voltage. The only thing that I often miss is the capacitor. When I turn off the power I often forget that the capacitor is still holding the charge
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Old 10th June 2012, 04:07 AM   #6
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Seems you got the voltage and the VA thing all figured out. But I'd like to second the notion about using caution. In fact, be very careful with anything near line voltages. Be careful with grounding, make sure everything is fused, make sure everything is turned off when making or breaking connections, and make sure all connections are properly insulated.

Lastly, when working with transformer ratios, make sure nothing gets stepped up beyond 500 or so volts AC. Transformer insulation is typically rated 2,500VAC or higher, but if a voltage is accidentally stepped up too high, there is the potential for arcing/insulation breakdown. That could get ugly and hot.

Sorry if this is too elementary or insulting, but I always like to err on the cautious side.
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Old 10th June 2012, 02:07 PM   #7
srinath is offline srinath  United States
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Its just the same as using the trafo forwards ... the same cautions will be observed. I am turning a 480 volt one into a 30 volt one in fact. Not the other way around.
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