Transformers; identifying which turn is shoted

[Silvertone Jockey]

Unfortunately, that won't work all that well. The problem is that windings are not really signal generators, the flux changes in the core is. The windings are only "sensors" of what's happening in the core, viewed through the distance of the leakage inductances. The windings will all have drastically diminished output.

A shorted turn anywhere in the winding window acts like an almost infinite load on the flux changes in the core. The way my motors professor would have put it is that, ignoring leakage for a moment, the volts per turn on all turns must be the same. So a shorted turn will have some milliohms of resistance, and the driving signal will force that turn to carry enough current to be limited by the driving source's impedance.

For shorted turns that are not heated to smoking and breakdown by the current, the volts per turn will be very low, probably best measured in millivolts. And for all the other turns coupled to that turn through the core, the volts per turn will be the same - millivolts.

That's only true to the extent that the other turns are coupled to the shorted turn by the core, and leakage inductance is literally a measure of how UN-coupled they are. So if a transformer winding has a large number of turns and only one or a few of them are shorted, the other turns will be carrying the current from the shorted turns, and will generate a leakage field outside the core material. Other secondaries will also share some amount of this leakage field and will show some signal, but vastly smaller than they would if the shared flux were in the main core, not the surrounding air.

This is actually a bit of expanding on the differences in measured leakage inductance I mentioned earlier. Directly measuring the leakage to all the other windings is probably simpler, although being able to measure the low signal voltages is another way of measuring the same thing.

How about if the entire winding were shorted, I'm grasping for straws here but wouldn't a scope be able to see it? and I get it that the potentials are difficult to measure. I'm asking these perhaps redundant question to expand my thinking and

Thanks for taking the time to explain the theory to me

I guess it's time to dust off that scope, hope the filter caps in it are still good and see what some of these questions look like for myself,

PS what if you used a small amp to apply the audio instead of relying on testing signal levels?

[Mick Bailey]

How about if the entire winding were shorted

Much easier if the entire winding is shorted - the resistance reading will be exceptionally low and can be measured with a DMM with a low resistance range. Hammond (for example) publishes DC resistance specs for its transformers so an entire winding is shorted it would be easy to measure.

You mention using a small amp as an audio source. The problem with a shorted transformer is the shorted turn(s) effectively kills any ac measurements due to energy absorption and excessive distortion across all windings. This can easily be seen if you have a transformer with a gap around the outside of the bobbin. Wrap a single shorted turn of wire around the bobbin and measure the output at audio levels with and without the short.

[Silvertone Jockey]

OK thanks I get it now, the shorted winding sucks all the energy leaving little to measure in the other good windings. Had to think about what you were saying a bit. Thanks I learned something I didn't know!