Shorting coil gives back more power

[darkspeed]

romerouk..

keep in mind your reed switch is drawing an arc at this power level.
your magnets are flying by this reed gap and there may be a quenching effect provided by them to the arc duration.

[romerouk]

@Arthurs
That is the core as it comes from the microwave fan.

[gyulasun]

Hi Laurent,

I appreciate your kind words, thank you very much!

Very good informative video again.
I attached a corrected schematic on your double FET switch and Hall sensors because I think there is a drawing error in it: I show you a blue wire which is needed to connect the 9V negative battery point to the common source electrodes of the MOSFETs) and you need to cut the wire where I show a gray line and text, ok? Also, I added black dots where connections are needed. Please check how your actual circuit is wired, I believe it is wrong as you show in the video.
When you short the generator coil by the FETs (where, at what circuit point do you activate it? at the 9V battery?) so when you activate it then all the nice 80-90V_{peak to peak} sine wave voltage seems to be shunted to a lower than 10V or so. I cannot see any spike when you activate the two FETs, some spikes appear later when the gen coil (i.e. rotor's RPM) starts to recover (video time 6:03 - 6:05 or so).  Please switch on and use the Trigger cursor to stop the displayed waves running, this way the spikes will be seen much better.
(Your first or second video on this coil shorting topic showed very nicely the big spikes at the induced voltage peaks when the reed switch was activated at the right time.) 

Thanks,  Gyula

EDIT: The 22 kOhm resistor between the gates and sources is a bit high: a bit speedier switching should happen with 4.7 to 5.6 KOhm or so (also depends on the max output current ratings of the Hall switches). This way the switch-OFF time of the FETs improves. The best would be to use a dedicated MOSFET driver IC for the job, Doug Konzen just showed such schematic and tested it successfully 1-2 days ago).

...
let's go on and
my last video

http://www.youtube.com/watch?v=L0l_zAC-m90

[gyulasun]

Hi joefr,

I know you did not ask me but probably you do not mind my humble answer... 

Most of the power MOSFETs are fast enough for these pulse motors (even the P channel types has a less than 40-50 nanosecond rise and fall time or even better, for N channels this is under 20-25 ns or even better) so it is more important to choose very low R_DS types that also have at least a 200 - 250 Volt Drain Source breakdown voltage rating (so the two in series gives the possibility of switching safely a 400 - 500 V_{peak to peak} waveform across the coil, of course you wish to remain under the limits).
I suggest visiting either a MOSFET manufacturer web pages or a component seller web pages where you can search and pick easily. The goal is to have the minimum loss possible across the ON channel of the FET but no need for going to the extrems: if you have a generator coil with say 250 Ohm DC copper resistance and you use two MOSFETs with say 1 Ohm specified ON resistance for each, you will not have any reasonable improvement by using two MOSFETs with 0.05 Ohm ON resistance to justify the more expensive 0.05 Ohm MOSFET type, ok?  And in this example the loss is horrible in the gen coil if you wish to take out say 100mA, just calculate it. (P_loss=I²*R)  So using a dedicated generator coil for this job here is a way towards improvement, however you have to tinker again with the best placement for this coil to cause the minimal drag on the rotor!

rgds,  Gyula

Hi Laurent very good work and video

I working on replicating your circuit design for coil shorting with mosfet you showed in the latest video.

I have to order some mosfets and I like to ask which is the most important parameter of mosfet for coil shorting circuit

Drain Source Resistance RDS on
or
Turn-On Delay Time td ON
or
Turn-Off Delay Time td OFF

to get best result.

and can you specify model of Hall Effect Sensor you used

[joefr]

Thanks Gyula for great answer