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From my understanding, a bad transformer may have high internal resistance. This causes a lower output voltage than approx. 120V. Say it has an internal resistance of 1ohm and supplies 10 amps, then the voltage drop across its internal resistor will be 10 V, and the actual output voltage is 120 - 10 = 110.

Most of the articles I found online just say this increase in internal resistance is from age, overloading, and impure oil.

My first question is, does age and overloading somehow increase overall internal resistance over time, and how? Do the wires heat up and have carbon build-up that slowly increases resistance (just thinking out loud)

So I would assume some smooth drop in output voltage as the current slowly increases. Looking at actual data from a bad transformer, the voltage does decrease as more current is supplied, but the out voltage has these deep sharp sags that increase in size. I can't find anywhere explaining these fast chaotic drops in voltage. Any explanation for these deeper more random drops? Here is a plot.

enter image description here

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    \$\begingroup\$ The usual failure modes of a transformer are open circuit in the windings or a shorted turn where the insulation has failed and either it shorts causing a decrease in output and greater loss or it occasionally breaks down causing random decrease in output. \$\endgroup\$
    – Kartman
    Commented Jun 2, 2021 at 23:40
  • \$\begingroup\$ Current dropouts to me indicate thermal issues from weak /broken gaps, oxidation , open arc and rejuvenated bad contacts repeating. Weird. Oil filled is different , bad oil means partial discharge on HV side but unlikely load dependent. \$\endgroup\$
    – D.A.S.
    Commented Jun 2, 2021 at 23:43
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    \$\begingroup\$ If it hums then it’s intermittent shorts on HV side \$\endgroup\$
    – D.A.S.
    Commented Jun 3, 2021 at 0:13

2 Answers 2

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Based on my experience, I would say:

  • transformer resistance does not increase significantly with time. Most of the resistance comes from the resistivity of the conductors (aluminum or copper) and this does not change over time, however there are some internal contacts that can become loose over time;
  • These bad internal contacts could justify this fluctuating behavior;
  • Another explanation could be a partial short circuit between turns, where the insulation was broken by a small arc, and eventually it ends up forming a new momentary arc (partial discharges). This defect, over time, tends to evolve into a complete short circuit.

But, I emphasize, this is not trivial behavior.

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  • \$\begingroup\$ So to clarify, a poor transformer may have an internal resistance that is too large. As it supplies more current it has a decline in output voltage. This part you agree with, correct? That explains the "smooth" decline in V out. The drastic sharp drops may happen from small arcs inside the coils? \$\endgroup\$
    – cj91
    Commented Jun 3, 2021 at 5:17
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    \$\begingroup\$ @Krits I don't believe the transformer can have a very high resistance: - If any bad contact increases the resistance too much, this indicates a large heat generation in a single point and the transformer would fail in moments; - An open circuit wouldn't generate those sharp edges either. \$\endgroup\$
    – Luiz Oliveira
    Commented Jun 4, 2021 at 9:50
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I'd like to expend on Luiz answer.

The transformer is made of turns of wire around a magnetically conductive core. You have the primary and secondary winding.

As Luiz mentioned, the insulation of the wire may degrade over time, especially with the heat of an overloaded transformer.

This causes some winding to be short-circuited, this will reduce the output voltage, the final circuit drawing more current, increasing the heat further, causing more windings to fail, and so on.

If no fuse is present, it can lead to a fire.

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  • \$\begingroup\$ Ok great, I would like to find a research article that confirms the data I'm looking at. If some of the windings are short-circuited and the transformer is supplying increasing current then you can observe sharps drops in output voltage (high voltage over transformer?) If I'm understanding correctly. \$\endgroup\$
    – cj91
    Commented Jun 3, 2021 at 17:32
  • \$\begingroup\$ Also, I'm wondering if my plot can be explained by a saturated core. \$\endgroup\$
    – cj91
    Commented Jun 3, 2021 at 17:37
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    \$\begingroup\$ @Krits, a saturated core would drain a much higher excitation current, even without load in the secondary. \$\endgroup\$
    – Luiz Oliveira
    Commented Jun 6, 2021 at 21:19
  • \$\begingroup\$ great thank you. I have no practical knowledge of transformers. But a saturated core cause issues more on the primary side correct? My issue is the output voltage \$\endgroup\$
    – cj91
    Commented Jun 6, 2021 at 22:08
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    \$\begingroup\$ The core is pretty much like a wire, but instead of electricity flowing through is magnetic flux. Saturated means it is beyond it's maximum magnetic flux capability. (Which creates more eddies current and more heat). Don't forget to upvote if that answer is useful for you. @Krits \$\endgroup\$
    – Damien
    Commented Jun 7, 2021 at 4:57

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