A new (contributed) hub motor

After a 3 week intensive sprint and with a little help from me, Pierre has finalized his own motor model and it is now available in the repository. He also contributed a couple of small tweaks to my common Lua “libraries”. The GPL wins again.

I helped out as much as I could (with just a few hints in magnetic design) in between my school exams and assignments, and after just a week he had my Lua/FEMM framework running with his new model. After two weeks he had a consistent motor and was running optimization scripts. Not bad at all, considering his haste and the sometimes flaky communication between two non-native English speakers! After 3 weeks he had the basic mechanic and cooling design too. So if anyone asks, you can tell them it is possible to completely design an electric wheel in under one month (I’m not so sure about the output quality, though). Anyway, hats off for a man who knows what he wants and works hard to get it.

I haven’t checked-in his optimization scripts yet, they are very redundant (extensive “copy-paste-modify” done in a hurry), but I intend to merge that feature into some “M-files” that I want to create for the project (FEMM has a nice integration level with Matlab & Octave). I did however spend some time cleaning up his model code and also refactoring some of mine to better allow multiple models in the framework. It’s all looking pretty usable now.

A map for my code

Pierre has told me it is difficult to understand my Lua scripts without a better explanation of what each variable means. He expressed a wish for a drawing that maps the variables to the geometry. I agree this is necessary, it just hadn’t been until now because nobody else dared to go into my code!

The braking problem – energy analysis

I’ve been just talking about regenerative braking for too long. It’s time to define the requirements.

When trying to dimension ultracapacitors or batteries for regen braking, we have to take into account the energy and power of the braking in it’s worst case: from top speed down to zero, and in the shortest time possible. Let’s start with the energy (I will post again later about the power).

So, I already set the top speed at 120 km/h, and that makes it easy to calculate the total energy generated by a full-stop braking: it is equal to the total accumulated kinetic energy of the car (well, not really, the aerodynamic drag helps the braking a little – but I’ll go for the worst case for now).

So the total energy of my car at 120 km/h is:

• E = 1/2 . m . V^2 [J]
  
• E @120 = 0,5 x 1230 kg x 33^2 m/s
• E @120 = 669.735,00 J = 670 kJ = 670 kWs
• E @120 = 669.735,00 Ws / 3600 s = 186,04 Wh
  

186 Wh is easily managed into a small battery pack, whether for acceleration or braking.

Back to school

I realized it is almost impossible for a family man with a full-time job to pull this project together in useful time, especially when trying to develop a crucial and complex component such as the hub motor.

So I’m changing my life in order to accommodate the project.

Model simulation results, part 1

At the request of many families, here are some results.

The following results where compiled like this:

1. The FEMM scripts simulated the “Halbach Rotor” (HR) and the “Sparse Rotor” (SR) motor models while exploring several variable variations (like I explained before), dumping the results to file; Used constants where 50 turns per stator tooth and 100 Amps per phase.
2. The results were parsed into a spreadsheet table (you can download it here in ODS format – change the extension to “.ods” before opening);
3. Then I filtered out the candidates that generated less than 500Nm torque;
4. Then I applied a “score” to each candidate according to my personal criteria: Score = 3 * (Nm/Euro) + 2 * (Nm/kW) + 1 * (Nm/kg). The ratios are weighed by their average before used. This means that, for me, a design’s cost-effectiveness is 3 times more important than it’s magnet weight, and the energy efficiency is 2 times more important than it’s magnet weight. If I were rich, this would be different. If you have alternative criteria to propose, I’d like to hear it.
5. Then I sorted the survivors by their score, and put them on the graph below. They are sorted decreasingly left-to-right according to their score (which does not show in the graph). The graph presents the most interesting data: variant name, max torque, Nm/Euro, Nm/kW, Nm/kg.

I reduced the size of the graph here to a thumbnail because it is huge. Click on it to get to the full-size thing (3686*1145 px).

The motor, in ASCII art

A fellow tinkerer has contacted me showing his own project of in-wheel motor.

His design is of Axial Flux with double rotor, and he shows me a series of details and shares a few enhancements over my original axial flux single rotor design.

I found this event quite gratifying; it’s the kind of confirmation I like to see that the smartest people on the planet are always the first to openly share what they know and have, in hope that their contribution will feed progress. Everybody wins.

But meta-remarks aside, I deeply enjoyed the simplicity and elegance with which he showed me his design: using ASCII art. It was very clean and explicit. And it also made me think that it’s been a long time since I posted the design I’m working on (and since people are still not sure if I’m working on an Axial or Radial Flux motor, I think it’s time to make things clearer).

Candidate selection process for motor prototype

In the past, I tried to create as many different motor simulation models as I could, in order to explore and learn about the possibilities. Halbach arrays or not, ironless or ferrous cores, one or two magnetic rotors, radial flux or axial flux, etc.

Lately I’ve been making practical decisions and tweaking my simulation scripts with the intention of selecting a specific model for the next phase: building a motor prototype.

To startup, or not to startup…

I’ve been quite distracted over the past 2 months, because after asking for an enthusiast’s help on building the motor, I was introduced to a couple of business men and the whole idea got blown out of proportion… I started thinking about a conversion business, or a conversion kit franchise, and a few other modes of making lots of money. Soon I forgot about motor modeling and HEV systems, all I could see was the business models, investor relationships, and patent lawyers crawling up my €$$.

No more.

I’m going to do what makes me happy: build the perfect car for me. If, after I do it, someone approaches me with commercial intentions, then it’s another issue entirely, and I’ll be happy to talk to them. But I don’t think that will happen, because the time-to-market is all wrong for me. I started this project on a dare: “I’m going to build what the car makers should have built already! I dare them to the finish line!“. Well, the finish line seems to be the year 2010, and they will probably win. As far as I am concerned, I feel vindicated! I’m happy to see that there will be serial hybrids coming into the market in 2 years.

So, on one hand, some new cars will have serial hybrid transmissions, and on the other hand the full-electric market seems to be taking off quite seriously, much before the hybrids. And as an added bonus, the old car conversion to electric is also kicking alive among the EV enthusiasts over here in Europe.

People are going in the right direction, and I’m glad to see I was just another one in a very large number of people who observed the tendency and started to move forward.

I may have lost the (market) battle, but we are all going to win the (energy) war.

No danger from obvious patents!

I’ve been worrying about patent infringement right from the beginning of this project. But now I can rest easy, because the two main distinctive traits of the project belong in the public domain and therefore cannot be used for legal prosecution: the Serial Hybrid Electric Vehicle and the Wheel Motor!!

Back in 1900, while working for Ludwig Lohner in Austria, Ferdinand Porsche built his first car, and it was an Electric Serial Hybrid Vehicle, with electric wheel motors!!! He won several competitions and set land speed records with it.

Fig. 1 – Two wheel drive hybrid version of the “Lohner-Porsche Mixte”, probably in competition.

(copyright Porsche)

Redoing the highway math

I haven’t had the time or the patience to repeat the roll-down test again. However, I found an old picture file I took out of the Web a few years ago that has all the measures of my car, and, most importantly, it has a perfect frontal area projection!

With a little help from “The Gimp” and some basic maths I was then able to get a very precise frontal area for my car: 2,30 m2.

Then I used the spreadsheet again and this time the values appear a little bit better:

Cd = 0,364

Crr = 0,013

I wouldn’t put my hand in fire for the trueness of these values, but they seem very likely.