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Ratio error of isolation transformer

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Freddybaby

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Not having a lot of experience with isolation xfmrs I'm puzzled with the performance.

I have 3 different 115/115 transformers and with about a 50% load I'm getting about 10-15% higher output voltage between primary and secondary.

Measuring the resistance of the coils I get a significant difference too (5 to 7 ohms)

Am I missing something or is this normal ?
 

Hi freddybaby,
I belief this is normal, because your 2 windings are probably all over the other, this is the reason for 2 different resistances too.
The measured good 10% higher Uout with 50 % load is OK: (usually)you should apply 100% load for nominal specifications...
Remember: what is your main voltage if you do chek the transformer_maybe it was some higher too:)
K.
 

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    Freddybaby

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Yes. The coils resistances would be different anyway for most transformers due to the different mean winding diameter
between primary (usually inner) and secondary (outer) winding.

But most transformers are designed to achieve nominal output voltage at nominal (full) load , so the unloaded or partially
loaded output voltage can be expected higher. 10-15% difference at 50% load is a lot, however. There should be a
specification, I think.
 

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    Freddybaby

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Thank you for the replies..

My nominal V is whatever is coming out of the power line. My problem was my line voltage runs about 123 and the extra 12-15V on the secondary is causing my contactor to heat up (+100 C)

I would expect the windings to be fairly similar in a 1:1. An extra 2-5 ohms might mean more windings on one side. (Using a 4 wire kelvin sense btw)

Would they really need or use different wire diameters for an isolation xfmr ?

Added after 5 minutes:

Oh ya, in reference to specifications, I scoured a bunch of data sheets and they only spec regulation and output impedance. No specification for ratio error (except on instrument xfmrs)
 

It must have an short in primary: this can be the reason for your practically 50% delta between winding resistances...
I belief, that 100C is too much, this can not be coming from +10-12% higher main voltages_ this is the other resultat of an short in primary winding.
K.
 

If the transformer itself isn't heating up, it should be O.K.
My nominal V is whatever is coming out of the power line.
Click to expand...
The usual specification is different. Standard transformers don't have 1:1 windings ratio for 1:1 input to output ratio.
They have an extra on the secondary to compensate for the voltage drop with full load. This can be of course a problem in
some cases. You need a variable or tapped transformer to adjust the output voltage in this case. As a workaround, you
can reverse primary and secondary of the transformer, then it should sligthly step down the input voltage.
 

FvM said:
If the transformer itself isn't heating up, it should be O.K.
My nominal V is whatever is coming out of the power line.
Click to expand...
The usual specification is different. Standard transformers don't have 1:1 windings ratio for 1:1 input to output ratio.
They have an extra on the secondary to compensate for the voltage drop with full load. This can be of course a problem in
some cases. You need a variable or tapped transformer to adjust the output voltage in this case. As a workaround, you
can reverse primary and secondary of the transformer, then it should sligthly step down the input voltage.
Click to expand...

This is what I ended up doing and now my contactor will not pull in below 111VAC. Ok for my area but If the line drops I'm sunk.. I think the low spec for utility is 105 ??? Guess I'll keep my fingers crossed. Should have went with a DC coil and step down+rectifier...

Thanks again for all the comments and help.
 

Isolation transformators used for industrial automation mostly have additional taps for +5% and -5% output, they can handle most of these cases.
 

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