Rotating Magnetic Field's and Inductors.

[gyulasun]

Hi webby,

If may 'whisper' a little...       

Consider the positioning of either the magnet with respect to the coil or vice versa, in one position you would receive the waveform Luc shows... 

Gyula

[MileHigh]

The issue about the waveforms that is almost never discussed around here is the geometry of the magnet/rotor, the geometry of the coil, and the relative placement between the two.  That overall geometry determines the EMF waveform of the coil.

With Brad's eight-pole alternating N-S rotor, you can assume that most of the flux produced by each magnet will opportunistically split off to the neighbouring magnets on each side.   Therefore the effective radial range of the flux is probably quite short and hugs the surface of the rotor.  Even with the ferrite core in the coil, you can still assume most of the flux will stay close to the rotor.   Therefore most likely the half of the coil nearer the rotor sees changing flux, and the other half of the coil sees almost no changing flux and mostly acts like a dumb resistor.  That is probably part of the explanation for the poor overall performance of his motor - the rotor's flux does not really pass through the coil from one end to the other end.

[tinman]

Argue with an oscilloscope, why don't you.

Explain this trace, which I'm sure you have seen many times in your own work.

You stated-Quote: that increases up to the nearest approach, then flips sign and decreases as the magnet goes away again.

At(or very close to) the nearest approach,the voltage will be zero,so the voltage starts to decrease close to the nearest approach(TDC),not increase. Peak voltage is reached when total flux linkage has been achieved from magnet to core--Which i see you corrected in your next post-Quote: The zero-crossing happens at the instant that the flux in the coil changes from increasing to decreasing, as the magnet moves past the "magnetic dead center"

[TinselKoala]

You stated-Quote: that increases up to the nearest approach, then flips sign and decreases as the magnet goes away again.

At(or very close to) the nearest approach,the voltage will be zero,so the voltage starts to decrease close to the nearest approach(TDC),not increase. Peak voltage is reached when total flux linkage has been achieved from magnet to core--Which i see you corrected in your next post-Quote: The zero-crossing happens at the instant that the flux in the coil changes from increasing to decreasing, as the magnet moves past the "magnetic dead center"

No, the peak voltage happens when the _rate of change_ of the flux linked by the core is at maximum. As you can see from the scopeshots I've presented, and as you can see from examining the equation for Faraday's Law.
Since the "magnetic center" of the core of the coil isn't a point but is extended, there is a small portion of rotation where the magnitude of the linked flux is constant. This means that the  positive peak and negative peak have some time in between them. The sign of the peak depends on whether the magnet is approaching or receding. Just where in that "dead spot" the voltage reverses is a moot point. The real point is that it does reverse.

[tinman]

No, the peak voltage happens when the _rate of change_ of the flux linked by the core is at maximum. As you can see from the scopeshots I've presented, and as you can see from examining the equation for Faraday's Law.
Since the "magnetic center" of the core of the coil isn't a point but is extended, there is a small portion of rotation where the magnitude of the linked flux is constant. This means that the  positive peak and negative peak have some time in between them. The sign of the peak depends on whether the magnet is approaching or receding. Just where in that "dead spot" the voltage reverses is a moot point. The real point is that it does reverse.

The real point is that it does reverse

That is dependent on magnet orientation--see next post.