Permanent magnet assisted motor coil designs

[gyulasun]

Is there a puffer capacitor right across the supply rail of the coils + the MOSFET switch, which may be able to maintain the supply voltage for a few moments the rotor spins down? Maybe the internal puffer cap of the power supply? (unless you unplug one of its DC output connections)  EDIT check with a separate DC voltmeter across the DC rails of the circuit whether there is some supply voltage remaining after the switch off.

IF there is no such cap, then something may trigger on the MOSFET switch? (the spikes occur at the falling side of the induced voltage just like in the powered case)   
To check this, after you switch off the power supply, get ready with a piece of wire to short circuit the gate and source pins of the MOSFET, to prevent any unwanted drive for the MOSFET. 

[captainpecan]

Is there a puffer capacitor right across the supply rail of the coils + the MOSFET switch, which may be able to maintain the supply voltage for a few moments the rotor spins down? Maybe the internal puffer cap of the power supply? (unless you unplug one of its DC output connections)  EDIT check with a separate DC voltmeter across the DC rails of the circuit whether there is some supply voltage remaining after the switch off.

IF there is no such cap, then something may trigger on the MOSFET switch? (the spikes occur at the falling side of the induced voltage just like in the powered case)   
To check this, after you switch off the power supply, get ready with a piece of wire to short circuit the gate and source pins of the MOSFET, to prevent any unwanted drive for the MOSFET.

The power supply I am using does use those huge caps. So what I did this time was run it up. And then completely disconnect it from the circuit. And I got the same results. Here is a quick jot down of the circuit. I will make one showing the mosfet connections instead of just a switch. But basically, I am triggering the gate using a reed switch for each one. There are no other power sources or capacitors in the circuit yet. I haven't added the recovery circuit yet either. I have the coils on the source side of the n channel mosfets instead of on the drain side? Something is causing that spike with only spinning magnets in the circuit? You think the mosfet gate could be triggering? And if so, what would cause the spike? There should be no way for a short coil situation I don't think? After looking at the mosfets, I cam probably get by with just 1 resistor, but shouldn't matter I don't think here.

[captainpecan]

I am trying to understand this. Again, this is after power is off to circuit and rotor is still spinning. It looks to me like because the magnets are opposite they are attracting on the way in so the wave looks normal. Maybe it is even pulling the magnetic field from the inner PM outside of the core. But just after the peak it begins to decline. This decline seems to cause that internal magnetic field to flutter and something happens. Maybe this is where it flips it back inside the core causing a negative spike? But it's technically an open coil situation? Thoughts?

[captainpecan]

This is wierd, so I dicided to spin it up. The probes are across the coils still. But this time I have BOTH the positive and negative disconnected so there is no way a gate should trigger on a mosfet. It's still happening. I went ahead checked with a test signal to make sure something isn't just interfering with stuff randomly I can't think of. And the square wave is perfect with no spike. Very interesting.


I went ahead and shot a quick video to show what's happening.
https://youtu.be/tCHc9wYdwsE

[gyulasun]

Thanks for the circuit drawings.  Considering 2 coils in series with the source of the MOSFET and the 10 k resistor towards the gate of the MOSFET,  the induced voltage appears across the gate-source I suppose. And the reed switches still operate from the small control magnets and they short the drain of the MOSFET to the gate, right?   And there is the body diode between the drain and the source, which may start rectifying the induced coil voltage and the horizontal line you indicate with a questionmark in the scopeshot above may be the indication of a DC component, what may be the supply voltage for a moment?  maybe this is what is happening.

You could use one scope probe across the gate-source when the other scope probe would remain across the same two coils and see the waveforms. (Both scope channel ground clips should be on the common negative of the circuit.) 
Temporarily you could remove the wire coming from the reed switch to the gate the moment you switch off the supply voltage and the rotor starts decelerating from the earlier full speed, to see if the spike disappears. I am watching the video now.

Edit: for the time being, no more comment, try to check the above things.