voltage drop in autotransformer application

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franticEB

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Hello,
I mounted on a metallic plate 3 laminated autotransformers to convert a 50Hz 120Vac into 230Vac to turn on some light bulbs.

As you can see from the figure, when I power the input of the autotransformer in an isolated way from ground, with the multimeter I read the output voltage of 230Vac correctly but between pin 1 and the metal plate I read a 30Vac. Also in other points I read different voltages.

What could be the cause? The capacitance between the windings and the laminated core? Is this unexpected voltage dangerous?

Let me know.


 

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Hi,

I don´t know about the regulations in your country, but usually this metal plate needs to be EARTHed properly

On an autotransformer the output is not isolated at all .. so it is considered dangerous .. in any case.

And the voltage referenced to an ungrounded plate is just normal. Mainly capacitively coupled. But the current is rather low, not dangerous at all ... as long as the isolation barrier of the transformers is intact ...

And 30V AC would not be dangerous at all ... but in your case pin1 is considered dangerous ... so the voltage against the unEARTHed plate is not of interest.

Klaus
 

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If you put a resistor across the meter to drop the voltage a bit, you can estimate the stray impedance or the leakage current that will be diverted to PE gnd when you connect that to it.

When R drops the leakage voltage 50% that is your leakage reactance. You can estimate that by extrapolation after one test like any voltage divider as the leakage is high impedance and acts like a current source.
 

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