In August of 2018, I moved into a house which had some space for me to work on a larger project. Up until then all of my projects had been “desktop scale,” but with some space in a garage I started thinking about larger projects that I’ve always wanted to do. Something I’ve always wanted to work on is a human-scale electric vehicle, something like a scooter or powered mountain board. I ended up settling on building a go-kart because they’re silly, impractical, childish, and I really wanted to make one.
Research
The starting place for my motorized vehicle was the motor. I needed to find a motor that could deliver enough power and torque to move a person and would be reliable enough to run at least half an hour at a time without burning up. I learned from a little research that racing karts are often around 5 Kw (at the low end) so I used that for target motor power.
Around the time that I was working through some options, my friend Isaak offered me an alternator from his car. The alternator in a car acts as a generator to keep the batteries topped up while the car is on, and most rotary generators are effectively motors working in reverse. As a bonus, alternators are intended to run continuously in hot and dirty environments, so it would probably be a reliable choice.
After pulling up the specifications for this alternator, I found it was rated for 110 A at 12 V. That’s about 1.3 Kw at the rated specifications (which I assume is its continuous rating, considering the safety requirements of car parts). From some past experience, motors can often be run at much higher voltages than they’re rated for without too many fires. So if I could run the motor at 48 V instead of 12 V, that means the alternator would have a power rating of almost exactly 5 Kw!
At this point I wasn’t actually sure if an alternator could actually be used as a motor. From quite a bit of internet research and some inspection of the alternator (which will be covered in the next section), I came to a pretty decent understanding of how alternators work and how one could be turned into a motor.
Tear Down
Here’s the alternator all torn apart:
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| Not pictured here is the diode pack that sits inside this sandwich, because I tore it apart and recycled it before I started taking photos. |
So what we have, from left to right is the following:
- The main casing of the alternator, and the claw-pole rotor.
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The stator windings (three phases unterminated, giving six wires).
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A middle piece of casing. Originally the diode pack sat on top of this. The diode pack (which I removed before taking photos) is a set of diodes whose purpose is to rectify the current from the three stator phases into one DC current to supply the car. I’ve labelled the places where the stator connections come out as phases: A, B and C.
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The brushes to excite the rotor. Originally there was a little power controller stuffed in here to manage the rotor current, raising and lowering it as needed to generate the right voltage out of the stator.
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The end cap of the casing.
Here’s some more details on the claw-pole rotor:
- The input shaft for the alternator. This is where the pulley would attach to bring in motion from the car’s engine. This is where I attached the chain sprocket to drive the kart.
- Fan blades to help cool the alternator.
- The design’s namesake claws. These two large pieces with claws help to direct the magnetic field from being axially aligned (as a donut sitting over the axle) to being radially aligned (as a series of humps between each claw).
- The field winding.
- Two copper contact rings. The graphite brushes ride here to provide current to the rotor.
- A bearing to support the rotor. There’s a similar one that lives in the casing to support the other end.
I’m very impressed with the way this alternator was constructed. The rotor is coated in an enamel to prevent corrosion and electrical shorts, the casing has heavy-duty mounting points, and the rotor and stator use very thick wires to handle high currents. The whole unit looks built to take physical blows, dirty water splashes, and high heat without failing for a long time... Which I suppose is what I should expect from an automotive component that’s bolted to an engine.
Modifications
There were a few modifications that had to be made to get the alternator running as a motor instead of a generator:
- Removed the diode pack and replaced the connections to the stator with wires to a motor controller
- Hijacked the brushes by removing the original current/voltage controller and replacing it with one I know how to use (I didn’t want to reverse engineer this system)
- Attached a chain cog to the output shaft to drive a chain. I used a rotary tool to grind a keyway into the shaft, and modified a small chain cog to fit on the shift.
First runs
Now I just hooked it up to a 3-phase brushless motor controller aaand it’s alive!
It was actually a little more work than that since I had to supply ~12 volts to the rotor to induce a field, and then I had to do a little trial and error to get the polarities of the three phases matched up right, since they weren’t labelled by the manufacturer. But once I figured that out it spun up surprisingly nicely, with smooth action, low noise, and excellent balance.
By varying the current to the rotor, I was able to observe that the torque and maximum speed were adjustable. Increasing the current to the rotor effectively increases the torque and lowers the maximum speed of the motor. I thought I would be able to utilize this to implement “field weakening” for the motor, which would theoretically allow me to increase the torque or top speed of the motor as needed. However, I had a hard time getting enough current into the rotor without making it too hot. Since I was running the stator at 48 V, I needed to run the rotor at high current to match, and it was running really hot. Roughly nearly-burn-me hot, which would likely turn into burned-enamel hot if it went on for long enough.
Adding sensors
The alternator basically worked like a 3-phase brushless motor now. The common name for a motor of this type is actually a separately-excited motor, because there are no permanent magnets and the rotor has to be excited separately from the stator. One of the common problems with brushless motors, especially in traction applications, is that they can get stuck in a stall when they first start. The most reliable way to fix this is to add position sensors to the motor so that the motor controller can know the position of the rotor as it starts up. I ended up having this stalling issue as I was working on my kart, so I added 3 Hall-effect sensors to sense the magnetic position of the rotor.
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| A little messy, but it runs smooth |
I used a rotary tool to grind three cavities into the stator and then glued in the Hall-effect sensors. I had to shave down the plastic casing of the sensors a little after gluing them in so they wouldn't rub on the rotor, but they ended up working perfectly! This solved my stalling problem entirely, and improved the off-the-line acceleration of the motor quite a bit.
