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Safest way to test unkown transformers with wire taps
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David Hess:
I start by using a function generator to produce a 60 Hz sine wave for excitation so I can measure the transformer's turn ratios using an AC voltmeter.  If necessary, an oscilloscope can be used to measure the phasing although it is possible to do this with an AC voltmeter if you are clever.

Then I use calipers to measure the wire sizes which gives some idea of the current rating for each winding.
bob91343:
Yes the current rating can be deduced from wire gauge if you can measure it.  Figure perhaps 400 to 500 circular mils per Ampere.  (A circular mil is the square of the diameter of a round wire, measured in thousandths of an inch.)  So a 0.001 inch diameter wire can safely handle something around a half Ampere.  Transformers that run hot will run on the low side, maybe even as low as 300 CM/A and conservative units maybe 700.  These are steady state ratings; for units intended for low duty cycle you can push it.  Of course a limiting factor is winding resistance.

In cases of very low rated currents, the wire size may be larger due to cost considerations involved with winding very small wire.

The total power rating of a transformer can be deduced from its weight; look in transformer catalogs for other units of similar construction and frequency and see what comes close.  A transformer with many secondaries will give less power for a given weight due to insulation space overhead, etc.  High end (say, military or medical applications) will be more conservative.  Cheap commercial stuff the opposite.
rstofer:

--- Quote from: David Hess on May 13, 2019, 04:10:27 pm ---If necessary, an oscilloscope can be used to measure the phasing although it is possible to do this with an AC voltmeter if you are clever.

--- End quote ---

Or it can be done with a 'kick test'.  Section 7.2

https://nptel.ac.in/courses/108106071/pdfs/1_7.pdf

A small(ish) batter and an analog voltmeter.  I guess analog V-O-Ms are out of style...
I wonder if it would show up on the bargraph of a DMM.
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