Magnetic Breakdown in Ferromagnetic Core: How Does It Happen?

[Passionate Eng]

In a ferromagnetic core of a transformer
why could breakdown happen if we reach "over_saturation"
in other words:
magnetic flux could make breakdown in some material just like electric current
how does that happen?

 

[berkeman]

In a ferromagnetic core of a transformer
why could breakdown happen if we reach "over_saturation"
in other words:
magnetic flux could make breakdown in some material just like electric current
how does that happen?

What kind of "breakdown"? Saturation of a magnetic core just decreases the effective inductance. What do you expect to break down physically?

 

[CWatters]

Saturation can cause overheating and the breakdown of some types of insulation.

 

[jim hardy]

...

how does that happen?

One must keep in mind firstly that it's not flux's magnitude that creates voltage, it is flux's rate of change.
Rate of change is slope when plotted as a graph.

Secondly that power transformers almost universally use sine wave voltage excitation (so what? you probably ask ...)
.. well sinewaves have this peculiar trait that their rate of change is just another sine shaped wave.
So, flux and voltage have the same smooth shape. No drastic rate of change anyplace on the wave.

UNLESS
you over-excite the transformer to the point its iron can no longer support the flux required to oppose primary voltage.
Then flux is no longer a smooth sine shaped wave but a truncated one with sharp transitions between its positive and negative flux levels. Those sharp transitions can induce enough voltage pierce the insulation

This picture comes from a tutorial on "current transformer" - which is just a transformer with a large turns ratio . The chart shows primary current instead of voltage...
Observe flux is a truncated sinewave
and voltage spikes occur at the transitions between flux levels. That's because slope is greatest there.
That's why in the days of vacuum tube hi-fi amplifiers with transformer output stage we were cautioned to never let the speaker wires fall off. Better designs included a resistor across secondary to absorb those spikes in the event a wire did come loose...

http://www.electrical4u.com/knee-point-voltage-of-current-transformer-ps-class/

old jim