Ferroresonance in a Typical Ferrite Core

I am beginning a series of tests into the ferroresonance phenomenon with off the shelf ferrite cores. Cores can be made from many different materials ranging from ferrite for high-frequency applications to high-flux for “big iron” applications to nanocrystalline for magnetic amplifies.

What is ferroresonance? Well it arises from magnetostriction which can expand and contract a magnetic material in response to a magnetic field. It is effectively a magnetic analog of the peizoelectric response and it is why we hear a hum coming from high voltage transformers. We are looking for a resonance where the mechanical expansion and contraction of the torid core is synchronized with the electromagnetic resonance of the circuit.

Why study it? Well it is quite possibly the key to understanding the Floyd Sweet VTA and a number of overunity magnet motors including the rotoverter.

We could hook up a signal generator directly to the core but this could be dangerous and ineffective because the load would be reflected back into the generator and it could burn it up. We  like the flexibility to amplfy the power as much as needed while providing bi-directional current so we can explore both ends of the hysterisis curve.

This is why I chose a push/pull design that provides bi-directional magnetic induction while providing enough protection so we don’t damage anything. I originally tried using a BJT for the inverter but it seems I get much better performance from an all MOSFET design.


The core I am testing with  (240-2526-ND) is designed for use as an RF choke and has a very high permeability of 5000. This makes it easy to work with because we only need about 10 turns of the wire for this test but it is probably not an ideal material for observing interesting phenomenon. This circuit includes a square wave generator and the transistor part numbers are listed in the diagram.


Here you can see the circuit in action with my homemade center tapped transformer. There are 3 separate coils around the core each with 10 turns. Here is a closer look:


I tuned the circuit until I heard the loudest hum coming from the core which is about 5.7 Khz square wave. Below is what I see on my scope when I measure the output of the coil.


What is perhaps instersting about this picture is that when the current shifts direction we have a capacitive spike followed by an inductive response. You can see the shift from the exponential decay of the peak to the linear decay of the inductor. Is this spike the result of ferroresonance? Is this the magic millisecond window where we can extract negative entropy?

Answering this question will require a fully resonant tuned LC circuit and it may likely require more exotic magnetic matierials like orthonol or vitroperm. Fortunatly I have some of these on hand that I intend to test with in the coming months.



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