chainsaw generator

The cells I plan to use in the quickly-taking-shape DerpBike are A123 26650s.  A huge pile (ha) of manufacturing line rejects was donated to a nearby student group over a decade ago, and many of them are still in good usable condition.  While not at all unobtainium IRL, they are quite expensive at between $5 and $10 per cell in small quantities.  In the spectrum of lithium cell compositions, these are “power” cells, meaning that they're optimized for dumping lots of energy quickly without damage.  On the other end of the spectrum are “energy” cells, like those in a laptop, cell phone, or consumer electric motorcycle.  They are optimized for capacity.  They store more energy per unit volume than power cells, but are less capable at high discharge rates.  There is a pack size threshold in EVs above which it makes sense to use energy cells.  If you can connect enough parallel cells so that peak current draw is still not enough to make any individual cell unhappy due to high discharge current, then you can get a significant bonus in overall pack energy density.  Zero and Tesla both do this.

Derpbike, however, is not above that capacity threshold.  The plan is to make a whopping 120s2p pack.  It will weigh about 40 lbs and have a capacity of 1.6 kWh.  This is a bit less than twice the energy that Brems-Chopper carried around.  Brems-Chopper, in all of its brushed motor, low-side-chopper terribleness excellence got about a 6 mile range at about 130 Wh/mile.  That's rather awful considering that entire cars like the Chevy Volt and the Tesla Model S get 200-300 Wh/mile at similar speeds.

So, Derpbike's range will be pretty awful.  That's why we're going to stick a generator on it.  A couple of kW will be enough to keep us moving at 30 or so.  The plan is: small engine into three phase permanent magnet “melon style” motor into three phase bridge rectifier, through some filtering and on to the bus.

Jaguar gave me a melon husk and charles donated the stator.  I turned a shaft and the bearing carrier.

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I bought a running Poulan 42cc chainsaw head on ebay for $50.  I removed the centrifugal clutch and unnecessary plastics.  It is a pretty good example of remarkably cheap manufacturing.  The fuel and bar-oil tanks are designed into the two injection molded chassis halves.



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The melon assembly attaches where the bar used to.  It is in direct drive configuration through a flexible shaft coupling.  I spun it up without winding the stator just to see what it felt like.

fail.  With the melon attached, I noticed much more vibration than before.  The injection molded chassis was overmolded around the M8 mounting screws for the bar.  They felt pretty stiff at zero frequency, but the mode shapes were quite apparent after revving up the saw.  The shaft coupling was unhappy with this rough duty and fractured after a few minutes of runtime.  The melon noticed the vibration also, dislodging a couple of magnets in the process.



The melon in generator duty certainly needs better vibration isolation.