Abnormal Voltages in Transformer

[EEngineer91]

Hi,

Please have a look at this paper:

"Abnormal Voltages in Transformers" L.F. Blume, 1919 AIEE (someone had mentioned this prior)

I tried attaching the paper, did not work. The most interesting this is that abrupt pulses make transformers react as a condenser rather than an inductor? What is the mechanism behind this? Obviously, coil-to-coil capacitance and mutual inductance plays huge role. Seems to be a non-linear phenomenon, and could have some interesting implications. Anyone have additional references that are similar?

 

[Averagesupernova]

There is a point where most inductors will start to behave like capacitors as the frequency increases. When the component goes through self resonance you have reached this point. The same applies for capacitors as the frequency increases. The lead length becomes significant and the capacitor now appears inductive.

 

[EEngineer91]

What is the reason or mechanism behind this? Seems like it works in transformers because of coil-to-coil capacitance and mutual inductance. How would you define the frequency of an abrupt pulse? Excitation mode? Relaxation time? Or time between impulses? Seems like a perfect impulse would have infinite frequency...

 

[Baluncore]

I attached the paper to this post. https://www.physicsforums.com/showpost.php?p=4808587&postcount=11

A coil will have one or more self resonant frequencies, SRF.
Below that frequency it appears inductive. Immediately above it appears capacitive.

If you imagine a perfect impulse, it will have a harmonic frequency spectrum from it's repetition frequency to infinity.

Any harmonic frequency components below the SRF will react inductively.
Components above the SRF will react capacitively and so be attenuated by what is effectively a low-pass filter.

 

[EEngineer91]

Any equations or references to explain this cross-over? What about the voltage amplification (the alpha factor)?

 

[Baluncore]

A coil can be modeled as a network of L, C and R. The L is reasonably obvious, the R can be calculated, but the C is not actually present as an obvious component of the coil. The apparent lump of C is due to many effects such as the length of the wire and the interaction of n factorial electric fields.

So take the coil model to be a simple LCR tuned circuit. Then study the reactance of the network as it crosses the SRF.
At resonance, if the circuit is lightly loaded, (high Q), the circulating energy may result in very high voltages.

http://en.wikipedia.org/wiki/RLC_circuit

 

[Windadct]

If you think about how a coil is made, particularly a large one, is built as a number of windings of insulated conductor - and consider you can not instantaneously change the current in an inductor, then there is a V developed across the windings, basically at sufficently high frequency - the windings look like a series of capacitors. ( conductor-insulator-conductor-insulator-conductor-insulator - etc)