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:

DO NOT TRY THIS AT HOME...OR ANYWHERE.

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.

On Simplicity

I love simple systems. I think somewhere in the process of building a 300-amp motor controller from scratch, we forgot how remarkably simple the electric go-kart really is. It took a while to get there, but in what I think is a good example of engineering system design, the individually complex pieces (most of which I would have preferred to buy off-the-shelf if they actually existed) together make something so easy to understand that it must work.

When the kart engages its regenerative braking circuit, there are only really two places for energy to be stored: in the moving mass, or in the capacitor. The batteries are out of the equation, since the main controller, in an act of almost absurd simplicity, cuts them off and shorts the motor terminals across the capacitor. Of course there is friction, but let's say we idealize by using a 54lb steel flywheel instead. ("That is one of the scariest things you've ever built." -Matt R.)

The only way to move energy from the mass to the capacitor is by energizing the motor's magnetic field, forcing current to flow as it acts like a generator. The flywheel slows down, the capacitor fills up, and along the way a sizable portion of the energy is dissipated in the resistance of the motor. Since we can measure current the whole time, the energy dissipated can be calculated. And guess what? It almost exactly matches the energy difference between the final value of the capacitor and the initial value of the flywheel, every time. The data is up on the site.


I'm not sure why I was surprised by this. There's a huge spinning disk, a huge capacitor, and a huge copper motor winding. There's no where else for 10kJ to go in a matter of seconds without destroy something, so that must be all there is to it. I'm pretty sure I thought this all through and came to a similar conclusion before we even started building. But after months of fiddling with power electronics nuances, it was wonderful to see the conceptual simplicity come to life and to see the data confirm that physics does indeed work.

Take-away: If I draw a resistor and a capacitor and ask people if that's a simple thing, they say yes. If I draw a go-kart with regenerative braking, they say no. Hopefully I've taught a few people to say they're the same.

Large Spinning Disks of...Aluminum?!

I was wondering how UPS had managed to alter the material properties of steel to make it so light. A minor setback, steel disks are now on their way. I hope somebody out there is designing something really cool that can be cut out of the inside material of left-over 14" aluminum disks.

Large Spinning Disks of Doom (or Steel)

An annoyingly blog-like title to start with. Aside from just testing out how this blog thing works, I will attempt to catch my readers (hah!) up on one of my current fun projects, an electric go-kart with a 110F ultracapacitor boost. (Think railgun-you-can-ride.) Working with a crazy - but very competent - group of high school students from the MIT Edgerton Center Summer Engineering Workshop, we built it over the summer and have had some fun driving it around campus:



By the way, I do this kind of stuff because I sometimes get bored with business-as-usual in my life as an engineering student. But this particular project is interesting because I think, as fun as it is, it also has significant serious research potential. Since we've already proven it is a fun ride, I'm thinking it might be time to satisfy academia with some more controlled experimentation. Hence, the spinning disks of doom:

These 11lb, 14" steel disks will provide rotational inertia directly to the electric motor on the kart, enough to simulate the effect 500lbs of kart + driver. And all without leaving the table in our shop. Not as much fun, but with the wireless data acquisition system it should rake in results fast enough to meet some paper deadlines. And even if the papers get rejected, guess what, now we have a sweet electric go-kart and five massive steel plates.

If you are wondering what magic MIT technology we use to produce custom-cut steel disks, it is called an abrasive waterjet. But in fact you can use one yourself. I sent the order out at 2:45PM and they were cut and shipped by 5:00PM. Can't beat that.