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Old 19th September 2007, 05:16 AM   #1
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Default Run a transformer backwards?

Can one run a transformer backwards, as in use the secondaries as the primaries?
I made a miscalculation and I need 240VAC for my mains and the wall voltage here is 220VAC....I found a transformer with the following specs.
190/200/208/220/240:220VAC, 500Watt.
If I run it backwards it will be a slight step-up. I know big jumps will bring up issues like insulation breakdown and such , but this is a slight increase. Are there other issues with operating transformers like this?
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Old 19th September 2007, 07:10 AM   #2
cerrem is offline cerrem  United States
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First thing ....I need to see a mucho bigger photo of your avatar

To answer your question.....Yes....
You can run this transformer either way...as long as you hook it to the proper voltages.....220 in and 240 out is fine...no difference...

Chris
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Old 19th September 2007, 07:40 AM   #3
AJT is offline AJT  Philippines
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Default Re: Run a transformer backwards?

Quote:
Originally posted by Richard Ellis
Can one run a transformer backwards, as in use the secondaries as the primaries?
I made a miscalculation and I need 240VAC for my mains and the wall voltage here is 220VAC....I found a transformer with the following specs.
190/200/208/220/240:220VAC, 500Watt.
If I run it backwards it will be a slight step-up. I know big jumps will bring up issues like insulation breakdown and such , but this is a slight increase. Are there other issues with operating transformers like this?
______________________________________Rick........ ......
by convention, primaries are connected to the power source and secondaries to load..... so as long as voltages are right, it should be no problem.


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Old 19th September 2007, 07:58 AM   #4
EC8010 is offline EC8010  United Kingdom
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Default Re: Re: Run a transformer backwards?

Quote:
Originally posted by Tony
By convention, primaries are connected to the power source and secondaries to load..... so as long as voltages are right, it should be no problem.
Not quite. A 1:1 transformer will have (should have) its primary wound from heavier gauge wire to allow for the magnetising current as well as the wanted current. Applying power to the secondary means you pass more current through a thinner wire and are in danger of overheating.
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Old 19th September 2007, 11:48 AM   #5
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_____________________________________Rick.......
I'll check the wire gauge on the Primaries vs. the secondaries.
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Old 19th September 2007, 01:27 PM   #6
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It will probably be fine with a slight derating. At 50 Hz, there may not be much margin though... check the current when unloaded - less than 20% of the full load current will be OK. If you get more than this, it's on the edge of saturation.

The rated voltage is under load, so the 220V winding is probably wound for about 230V to allow for resistive loss at full output. So you may not get as much step-up as you think...
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Old 19th September 2007, 03:38 PM   #7
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Have you actually measured your Mains Voltage
You may be surprised.....

And all transformers put out higher Voltages when unloaded or even half loaded (Voltage given are for fully loaded), so you may be able to use a 240VAC and still get the Voltage out you need...
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Old 19th September 2007, 03:57 PM   #8
john65b is offline john65b  United States
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Bigger pic please.
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Old 19th September 2007, 04:00 PM   #9
djQUAN is offline djQUAN  Philippines
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just curious, what device needs 240V and will not work at 220V?

depending on many factors, your mains voltage could vary a lot.
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Old 19th September 2007, 05:34 PM   #10
Tweeker is offline Tweeker  United States
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If there is built in fusing on the xfmr it may not provide the same protection it normally does.
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