Number of magnets vs wheel size

[Aware]

Hello, everyone!

First build based on SSG book 1, 2 and 3.

I built the stand, today. I found a new steel wheel, balanced and true. The manual says any wheel will work and John's original model is exactly 22" rim to rim. The chapter talking about the kits calls for a 20" wheel. Both indicate 24 C8 magnets. There is mention of 3" between magnets.

In light of the above: do I divide the circumference of my wheel by 24 and place 24 magnets spaced less than 3" apart or do I calculate how many magnets spaced 3" apart I can place on my 20" wheel?

Thank you in advance,
Eric

[Gary Hammond]

Hi Eric,

The SSG books 1 & 2 call for 24 magnets on a 26" bicycle steel wheel which actually measures 22.75" across the rim. However, at the energy science conference and also in EFTV videos 33 and 34 John said that 21 magnets work the best, and that is what he demonstrated at the conference. He also had two fans installed on that machine to get the exact speed he wanted.

Then in the SSG book #3, Peter dropped back to only 18 magnets in order to get a higher RPM for more output from the generator coil. The more magnets on the wheel, the slower it goes. And the fewer magnets, the faster it goes.

I've built wheels with 18, then 24, and then 21 magnets. They all work, but I get the best results from the one with 21 magnets. It all has to do with pulse width (off time relative to on time), coil saturation time, friction losses, mass of rotating parts, and best RPM for the best charging. John explained that it's like an internal combustion engine or any other rotating device, it has an optimum speed for best efficiency and drops off above or below that speed. It also seems to charge better while doing a little mechanical work as with fans or generator coils added.

[Aware]

Thanks, Gary!

I appreciate the effort and comprehensive reply, very much. The fewer magnets/higher speed seemed counter-intuitive at first, and at second thought it would make sense that more distance between magnets (to an extend) provides more time for full saturation to occur and therefore bigger spike when the field collapses.

In any case, I will go with the recommendations. My wheel being a 20" from rim to rim, putting a magnet every other spoke calls for 18 magnets. I will start with that. Of course 18 magnets on a 20" wheel might be more like 21 on the 22" wheel mentioned.

Thank you, again!
Eric

[Gary Hammond]

Hi Eric,

....... The fewer magnets/higher speed seemed counter-intuitive at first, and at second thought it would make sense that more distance between magnets (to an extend) provides more time for full saturation to occur and therefore bigger spike when the field collapses.

Assuming that you have a high enough wheel speed to get only one pulse per magnet pass at whatever base resistance and coil gap you are tuned to, the fewer the magnets (and therefore greater distance between) will give you more off time for each corresponding on time of the transistor. This then is a shorter duty cycle because of the fewer magnets.

The coil saturation time is controlled by several factors including coil impedance, coil gap, transistor gain, trigger signal voltage, base resistance, coil core makeup, and battery impedance. There may be other factors as well. But the saturation time for each coil charging event is a given length of time based on these factors. This is also the length of time the magnet is being attracted to the coil which gives a torque pulse to the wheel. This also determines the frequency in pulses per minute for a given set of parameters.

If there are then 18 pulses per revolution at a given frequency it takes less total time for one revolution of the wheel that it would for 21 or 25 pulses per revolution. Therefore, the fewer the number of magnets on the wheel, the higher the RPM and the lower the total torque.

[Aware]

Well put, Gary. Thank you!

I understand better John's insistence on having us all experiment as there are so many variables ( components selection and sourcing, build accuracy and tuning ) affecting results Thank you, again for the explanation.

Cheers,
Eric