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I'm trying to learn more about transformers that are failing. From my understanding transformers create heat. From too much load or current on the secondary side of a step-down transformer, the heat can damage the insulation and cause a short on some of the secondary side turns.

Now that we have a short in one or a few of the windings. How does the transformer behave under normal rated conditions? Say the secondary had 90 turns and now has 88 turns from two shorts. It seems the transformer should almost behave the same as when it had 90 windings. This is the part I think I'm wrong. Do the shorted windings increase current heat loss? Do the shorted windings increase eddy current/hysteresis loss? Or do the two shorted windings have really minimal effect?

If it is true that a few shorts can have large negative effects, they would cause more heat and could potentially lead to more insulation damage? My other assumption is that extra heat from the damaged secondary would be noticed in a decrease of output voltage because the transformer is getting very hot then its internal resistance will increase.

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So the key here is the difference between shorts on the primary winding and the secondary, which each behave very differently. Your intuition about shorting only 2 windings being negligible makes some sense if we are talking about the primary, in fact we do that with inductors all the time to adjust its inductance, just short out part of the inductor.

Consider that in a secondary however that if you short the entire secondary, as one would expect, you will get thermal runaway just as shorting any power source directly might cause. You will have huge currents running through that will burn everything up as its limited only by the resistance of the wire.

But consider now a secondary with 2 turns shorted and 98 behaving normally. In this case you have the 98 acting as a normal secondary, nothing special there. But the 2-turns that are shorted acts like a second secondary (for lack of a better term) but this secondary is shorted and just like shorting any secondary will induce a large current flow limited only by the wire resistor and ultimately thermal runaway.

So in short, the reason a short on the secondary will be so devastating so quickly is because the shorted portion will act like an independent secondary that has its outputs shorted and thus induce high current and glowing red wire.

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  • \$\begingroup\$ Thank you for clarifying! I posted this above but maybe you can help too... I experienced a failing transformer. Utility company came out and said it was bad and had to replace it. I had very low chaotic voltage during high use times (dinner/hot days) for weeks. I'm wondering if the transformer they replaced had shorted windings or if it was something else? \$\endgroup\$
    – Krits
    Jun 25 at 1:32
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    \$\begingroup\$ @Krits Hard to really say but in that case if i had to guess it would be that the oil in the transformer had leaked. This could cause chaotic arcing, and would likely get worse during high-use periods, but its just a guess. When things fail if its predicable its probably catastrophic, otherwise there is usually many failure modes and I can only speculate. \$\endgroup\$ Jun 25 at 1:37
  • \$\begingroup\$ "Thermal runaway" is not just when something melts, it's a specific thing where heating up a thing causes the thing to dissipate more power. (Maybe that happens here, but I wouldn't think so...? It doesn't happen when you short any power source directly) \$\endgroup\$
    – user253751
    Jun 26 at 7:09
  • \$\begingroup\$ @user253751 Yes thermal runaway happens here. As the shorted turn heats up it burns off the enamel from additional turns causing additional shorts. Or at least that has been my expiernce. \$\endgroup\$ Jun 26 at 16:15
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A shorted turn is usually catastrophic for a transformer. Very high currents flow in the shorted turn, leading to further insulation breakdown and dramatic temperature rise.

This keeps getting worse until the safety thermal fuse embedded in the winding opens, or the shorted turn melts open.

After that, that transformer must be discarded.

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  • \$\begingroup\$ Discarded, or in some cases, rewound. \$\endgroup\$
    – Hearth
    Jun 25 at 0:23
  • \$\begingroup\$ If the transformer outside a house had a short in the windings could it, in theory, last a few months, or would it fail/blow a fuse much sooner than that? \$\endgroup\$
    – Krits
    Jun 25 at 0:36
  • \$\begingroup\$ To the OP I don't think you are thinking about what elchambro said correctly yet. For a moment, imagine that you strip the secondary copper out, then put a single copper band around the secondary. This is a shorted turn. How much current would flow in the shorted turn, according to transformer theory? Now, go back to a normal secondary. If two turns next to each other short out, you basically create this same problem. It is as if a copper band is around the secondary. \$\endgroup\$
    – mkeith
    Jun 25 at 1:12
  • \$\begingroup\$ Thanks you're right maybe I don't have the right picture of a shorted winding in my head \$\endgroup\$
    – Krits
    Jun 25 at 1:24
  • \$\begingroup\$ I'm interested in this because I experienced a failing transformer. Utility company came out and said it was bad and had to replace it. I had very low voltage during high use times (dinner/hot days) for weeks. I'm wondering if the transformer they replaced had shorted windings or if it was something else. \$\endgroup\$
    – Krits
    Jun 25 at 1:25

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