Hey, y'all. Just got done with the Edgerton Center Engineering Design workshop, where I mentored high school students on engineering principles. Check out the site at http://edw.scripts.mit.edu. Anyways, one of the projects during the program was a window opener, and their prototype used the standard motor used in the First Robotics Challenge, also known as the CIM motor.

Now, this motor has quite the amount of torque for its small size, designed to run at a maximum of 337 watts at 12 volts, plus, it costs less than $30—quite affordable for EVs. I also figured that the output power could easily be increased by raising the output voltage.

However, this motor has its drawbacks. Looking at the datasheet, one can see that at peak output power (at 12 volts), the efficiency is 41%, meaning more than half the power supplied to the motor is being dissipated as heat. To put that in perspective, the heat wasted in the motor is enough to heat half of my dorm room in the fall (which is quite a large room, by the way)! That much heat would destroy the motor in no time, so I needed a way to determine whether or not it would be feasible tossing this in an EV (in which I would probably put the motors in parallel), without wasting a serious amount of power. I needed to be able to estimate the efficiency.

Here's where MATLAB comes in, I could probably estimate the efficiency by hand, as several websites tell how to calculate peak efficiency in a motor as given at ~10% of rated power ^{[citation needed]}. However, I'm still learning about motors and power electronics, and I figured trying to calculate it given the motor parameters, not necessarily from generally used values should be a good learning experience.

First, we need to know what makes a motor inefficient. The main two causes of losses are conduction (resistive) losses, and core (hysteresis) losses.

Conduction losses come from the fact that all motors have some resistance in their coils, and as a result, some of the electric power flowing through the windings is dissipated as heat, which is known as joule heating. That power varies with the square of output torque, because current is directly proportional to torque, because the voltage drop in a resistor is current times resistance (ohms law), and because electrical power is voltage*current.

$\mathrm{P} = \left(\frac{\tau}{k}\right)^2*\mathrm{R}$

(k is the motor constant)

Core losses are a bit more complex. They come from the fact that magnets (electromagnets included) do not lose their all of their magnetic properties when there's no magnet field applied, but require a reverse magnet field to bring them back (this is how permanent magnets work).

The same happens in our motor, and you need extra energy to reverse the magnetic field to turn the motor. The energy required scales with frequency, and because at higher speeds, you have higher reversals, this power varies with the square of rotational velocity.

$\mathrm{P} = \omega^2*h$

(h is a constant relating to the core loss, $\omega$ is rotational velocity).

With that out of the way, we create our model.

This model takes in parameters, (max current, max voltage, motor constant, motor resistance, hysteresis loss coefficient), and generates power values for the possible torque and speed values. It then adds the conduction and core loss to determine the electrical power required to drive the motor at that point. It then calculates the efficiency with

$\eta = \frac{\mathrm{mechanical power}}{\mathrm{electrical power}}$.

It turns out, that at 48 volts, the maximum efficiency of a CIM motor is 78%, at 2.28 kW, with a torque of 1.323 Nm, and angular velocity of 1572 RPM. It's not great, but much better than the 65% at 12v.

That happened around late April 2014. What you didn't see was that the controller going up in smoke due to the back EMF, trying to reverse the motor while it was still running, and this controller doesn't support regenerative braking.

Rewind back a couple of days, and I finally get all the parts made of 1/4" aluminum, along with the brake rotor, cut on the CSAIL waterjet. This happened after trying for a week to get access to a waterjet, but the person who was available to help me was busy.

I finally got them cut, and got the gearbox assembled, along with the frame itself.

We now fast forward to late July, and I finally get the front wheel assemblies fabricated,

At this point, I was pushing to get the go kart finished, but I ran into a new problem.

Harbor freight wheels are cheap, and I planned on using for the drive wheels. However, if you turn them you notice that the wheel is notably out of true, i.e. they dont rotate in a plane. They wobble instead. There were also some issues with the rear brake not being able to fully rotate due to the brake caliper hitting the standoffs. To remedy this, I designed a new hub, so that I could keep the tire.

That hasn't been machined yet, so what we're left with is a 90% completed Megantereon, siting under a desk. Hopefully I'll finish it this summer.

Goals

• Electrify two bicycles for under $200 each
• Demonstrate axial-flux switched-Reluctance motor technology
  • Using hubmotor built into rear wheel
  • Using motor driving a chain or belt on rear wheel

The switched reluctance motor

Switched reluctance motors rely on the electromagnetic attraction of ferromagnetic materials (such as steel) to an applied magnetic field, instead of relying on permanent magnets or magnetic induction.

Advantages

• Can be made cheaper due to the lack of permanent magnets
• No detent torque (cogging) from permanent magnets

Challenges

• Axial flux-type motors are relatively rare for switched-reluctance motors
• Control technology is still somewhat undeveloped (leads to characteristic V8 sound)

Preliminary work

• Preliminary CAD of hubmotor:
  
  
  

Useful Technical Documents

• Switched reluctance motor control
• Axial flux type switched reluctance motor

So last time I covered the basic drivetrain design. Now I'll get to the CAD design of the go-kart itself.

While I was pondering the design of the gearbox, I went ahead and designed the frame. I knew I wanted a three-wheeled go kart (for simplicity and because I was somewhat inspired by the MIT EVT eBike which I helped design), and to use 80/20 framing because I'm not experienced with welding aluminium (and I wanted to avoid steel to minimize weight).

I went with a triangle-ish layout to ensure stiffness (triangles are the stiffest polygon).

I decided to adopt a steering layout similar to Charles Guan's BurnoutChibi, with the front brackets doubling as bearing holders for the steering kingpins, and using aluminum square extrusion as the steering spindles, and a generic (but modified) hex bolt as kingpin.

I went with a similar system to hold the rear axle and brakes. You can also see that I went with a belt drive for the rear wheel (quieter and cleaner, if not harder to design for).

For the sttering assembly and linkage, I decided to convert an old gaming sttering wheel to act as both steering wheel, throttle and electonic brake control, as well as serving as an emergency switch.

All that, plus a chair that I found in the Stata Center basement, makes a three-wheeled go kart.

I finished the basic design of Megantereon around mid-March, and proceeded to order the parts and materials necessary. Now it comes down to building the beast.

It seems like a rite of passage for many MIT engineering students to make their own powered vehicle to get across campus quickly.

I had these goals in mind:

• Be capable of going 35 miles per hour on a flat surface (not on the streets, though)
• Be as light as possible (use 80/20 aluminum extrusion for the frame)
• Have 3 wheels

So, to begin this ritual, in October, I purchased this:

Turnigy Trackstar 1/5 Sensorless BLDC

Now, if you look at the spec sheet for this motor, you come across a spec called Kv. This number specifies that maximum RPM per volt at which the motor will run. If you also look at the spec sheet you will also see that the motor is rated for 30 volts maximum, which gives a maximum RPM of around 22,000!

The downside to this is that although the motor will run really fast, the Kv of a motor is the inverse of the Kt of the motor which gives us:

$(\frac{760\cdot2 \pi}{\mathrm{V\cdot min}})^{-1} = 0.013\frac{\mathrm{N\cdot m}}{\mathrm{A}}$

Which times the maximum rated current (150 amps) gives us

$0.013\frac{\mathrm{N\cdot m}}{\mathrm{A}}\cdot 150:\mathrm{A} = 1.885:\mathrm{N\cdot m}$

That isn't a lot of torque, and since I'm using 8 inch wheels and if I were to direct drive I would only produce about 4 pounds of force at maximum current.

But wait! I said I wanted my maximum speed to be about 35 miles per hour (which is pretty fast when you're low), so I can find the gear ratio I need.

$\frac{22,000}{\text{min}}\div\frac{35\text{ mph}}{\pi\cdot 8\text{ in}} \approx \frac{22,000\text{ min}^{-1}}{1470\text{ min}^{-1}}\approx 15:1$

That's a high ratio.

So what do you do? You either design or buy a gearbox that can reduce the speed of the motor and generate the massive amounts of torque necessary.

So, what did I do? I designed two gearboxes: a planetary gearbox, and a more traditional gearbox.

Now, in actuality, the voltage I chose to use with the motor was a nominal 39.8 volts, or 12 A123 LiFePO$_\text{4}$ batteries in series, so the gear ratio is about 20:1.