Wednesday, July 1, 2009

The epic journey of the spinning disks of doom.

Well, maybe not epic, but at least humorous. The spinning disks of doom were five "steel" plates I ordered from Big Blue Saw because they were cheaper than material + machining time at MIT even though we have six abrasive waterjets. Go figure. I say "steel" because the first set I got was aluminum. I had actually figured out this mistake before they arrived because the shipping weight was off by exactly the ratio of densities. Anyway, I eventually got the steel plates and installed them for motor testing:


This was an incredibly bad idea. 55lbs of steel dangling off the edge of a motor shaft with no other bearings and no enclosure is a disaster waiting to happen. It happened to be very well balanced due to the fact that the waterjet cuts the ID and OD at the same time, and it ran very quietly, but NO NO NO DO NOT DO IT. It stores as much energy as 500 lbs moving at 40mph. This makeshift inertial dynamometer provided some invaluable testing data for the kart regen system, and then I decided it must be destroyed. It was the most dangerous thing I've ever built, and I decided this while standing next to a 110F ultracapacitor on an electric go-kart...

So where do flywheels go to die? When I got involved with the MIT Electric Vehicle Team outreach project, they were looking to make a tabletop demonstration of regenerative braking. Sounds familiar. Turns out they had enough time to do it the right way...bearings, hubs, enclosure, etc. It was also much smaller...8" instead of 14" diameter disks. So, I gave them the plates and they produced a mini-flywheel out of the insides:

Much more sane.

So that takes care of the donut holes, but then what do you do with a bunch of 1/4" steel donuts? Pretty useless, right? Not if you believe in conservation of usefulness. After looking around for a microwave transformer or something to smooth out some extremely high battery charging current, also for an EVT project, I remembered these donuts. Toroidal inductor core!

Also surprisingly less dangerous than the previous usage.

Yes, I know this is not a great inductor, as a lot of field is wasted in the excessively-large steel ring. But according to some maths it is >200uH, which is good enough for the job. The job, btw, is insane-charging a motorcycle Li-Ion battery pack. (4-6C charge rates.) This will smooth out the 100A charge to about a 5A or 10A ripple current. Hopefully...

In any case, the spinning disks of doom have found new homes.

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