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Old 07-04-2009, 04:21 AM
MileHigh MileHigh is offline
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Join Date: Nov 2008
Posts: 140
Magnetic flux "breaking"

I just want to mention a basic concept in electromagnetism because some people in the forum may not be aware of it and it should demistify the voltage waveforms that you observe from your convertor coils.

A coil will react to changes in the magnetic flux passing through the cross-sectional area of the coil and generate voltage across the unloaded coil terminals in response. If the coil is shorted then it will generate a current in response. The instant there are no changes in magnetic flux the response from the coil disappears and becomes zero.

To put it in more precise terms for the open-terminal voltage case: The voltage generated by a coil will be proportional to the inductance of the coil times the rate of change of magnetic flux going through the coil with respect to time. This is a very important concept and is worthwhile thinking about.

A week ago we discussed a converter waveform and how it was non-sinusoidal. Imagine a simplified case where the magnetic flux path has a cross-sectional area that is a perfect circle of uniform density and the coil corss-section is also a perfect circle the same size.

So you have a circular "pipe" of magnetic flux that is passing across a circular coil "receptor". You can imagine something akin to a total eclipse of the sun by the moon, where the moon is the magnetic flux "piple" and the sun is the coil. The area of the overlap would be equivalent to the amount of flux going through the coil.

When they first touch, only a small area overlaps, and therefore the rate of change of magnetic flux the coil "sees" at the beginning is very small. Since the amount of flux going through the coil is directly proportional to the overlap area, let's just use area to make it easier to visualize.

So the rate of change of overlap area with respect to time starts out slowly, and as the two circles continue to overlap, the rate of change of overlap area with respect to time starts to increase. At a certain point the rate of change of area reaches a peak, then after that it starts to decrease. When the two circles are perfectly lined up (i.e.; TDC or a "full eclipse") then for a brief moment the rate of change of overlap area with respect to time is zero. That's the zero cross point in the voltage waveform. Then as the "moon" continues moving past the "sun" the whole process repeats itself, but now the rate of change of area overlap with respect to time is negative.

If you go back to those scope waveforms, that is exactly what you are seeing. You really have to think hard and visualize it, to corelate the rate of change of overlap area with respect to time with the scope traces.

Some of you may be familiar with the equation for the voltage across a coil, v = L di/dt. This is essectially the same thing. "di/dt" is the rate of change of current with respect to time. For the generator (convertor) case, the equivalent to current is simply the magnetic flux density times the overlap area with the coil. Therefore the rate of change of overlap area with respect to time is the essentially the same as the rate of change of current with respect to time. In both cases you get voltage across the coil.

This might help: You are in a car with your eyes clossed. The car's speed is equivalent to the flowing current. If the current starts to increase, it's like the car is starting to accelerate. The force you feel on your body when the car is accelerating is equivalent to the voltage generated by the coil. (Force = mass x acceleraton, and acceleration is change in velocity with respect to time, therefore force = mass x change in velocity with respect to time. Voltage = Inductance x change in current with respect to time.)

v = L di/dt also explains the collapsing field spike that you get from a coil in a Bedini motor. When the transistor switches off abruptly, di/dt gets very very large, and therfore the voltage across the coil gets very very high.

Some of you may be tempted to say, "This doesn't apply for this unconventional setup" and I can assure you that would be a very unwise way of thinking. For every person that uploads scope shots of their waveforms and pictures of their setups, you should be able to see how the scope shots relate back to the actual physcial build. By the same token, like I said before, you can simply look at how a convertor is built, look at the cross-sectional shape of the magnetic flux path, look at the cross-sectional shape of the coils, and look at the speed of the "fly by" between the magnetic flux path and the coils to make an estimate of what the voltage waveform from the convertor will look like before you even see it.

In all likelyhood, every Kromrey convertor will react as desribed above without exception. Finally, if you understood all of the above then it should not be a surprise to observe increasing voltage output from the coils when the RPM increases.
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