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Old 18th July 2003, 05:17 AM   #1
lgreen is offline lgreen  United States
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Default Transformers...In reverse

If I wanted to use a transformer backwards as a step-up transformer, would there be any problem or change in rating? For example, will a 300 VA transformer with dual 20 V secondaries still be 300 VA if driven backwards as a step up transformer? Anything to be concerned with if driving in reverse like this?
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Old 18th July 2003, 05:47 AM   #2
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Just don't go over the rated voltage on the winding and you'll be fine. By definition a transformer will work as well forward as reverse, and in reality there's loss, but it shouldn't be more than say 10%.

Tim
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Old 21st July 2003, 02:12 AM   #3
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Default Please be careful!

The answer to your question is as follows. A 300 VA transformer with 20 volt dual secondaries can be driven "backwards" only if you limit the voltage to a value based on the volts per turn of the primary. Suppose the primary is designed for 120 volts rms. A 20 volt secondary has one sixth the number of turns as the primary. If you drive the transformer backwards, the voltage placed across the 20 volt winding originally intended as secondary must not exceed 20 volts rms. The winding originally intended to be the primary will now output 120 volts rms. The VA rating, in this case, remains at 300 VA. Do not attempt to apply more than 20 V rms to the lower voltage winding. If you had intended to step up from 120 V rms to a larger value, the answer is no, you cannot. The magnetic core will saturate, very large current will be drawn, and the transformer winding could be severely damaged, not to mention potential safety hazards. I hope this helps. Best regards.
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Old 21st July 2003, 03:22 AM   #4
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And, if you intend to operate it at lower than design voltages (say 10:60V), VA goes down proportionally, since the wire can only handle so much current.

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Old 21st July 2003, 04:14 AM   #5
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Quote:
Originally posted by Sch3mat1c
Just don't go over the rated voltage on the winding and you'll be fine. By definition a transformer will work as well forward as reverse, and in reality there's loss, but it shouldn't be more than say 10%.

Tim
To second Sch3mat1c's warning of exceeding the voltage rating of the winding, do not put 120 in to the secondaries because you'll get over 700VAC on the outputs. This will surely exceed the voltage rating of the insulating varnish.
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