Tired of hearing about business deals, “stonks,” and the personal life of Elon Musk? Me, too (hopefully it’s not just me). While those things can be important at times, that’s the kind of stuff bean counters love. Me? I’m an old-school car girl. While the bankers and accountants among us were doing whatever they did in high school, I was spending my spare time turning wrenches and modifying air intakes and such with my friends and cousins. In college, I got into photography and journalism, but since then, I’ve still done most of my own automotive work, helped friends modify stuff, and most recently, helped my brother and dad convert a classic Ford Bronco to electric power.
If you’re not like me, and you’re into all the business stuff, that’s cool. We need business people in the world and I appreciate you guys. I don’t have the personality for that, and I know you all have my back. But, aren’t you a little curious about how your Tesla works? If that’s you, stick around and we’ll watch some fun videos about that!
Back in 2019, I shared a video from WeberAuto, the YouTube channel of Weber State University’s automotive program in Layton, Utah. In it, the instructor gave us a deep, deep dive on what makes the Chevy Bolt’s drive unit run. Since then, they’ve torn down a bunch of other vehicles, and have made equally fascinating and informative videos. Today, I’m going to share their video on the modular drive units that propel the Tesla Model 3 and Model Y (article/commentary continues after embedded video).
The really cool thing about Weber’s videos is that Professor John Kelly makes the inner workings of EVs extremely accessible to anybody interested in how they work. Instead of going through a bunch of boring math and diagrams, he literally lays the parts out on the bench with wooden supports and shows you how they work together. He does get into important math, but you don’t have to be a math major to see for yourself the things that make us go.
One thing I noticed about the modular Tesla motors right away is that they’re pretty similar to what’s in the Bolt. While some EVs use planetary gears to reduce the gearing (make the motor turn faster than the wheels) and multiply the torque, both the Bolt and Tesla drive units use a simpler set of gears to get the differential moving.
Once he has it all laid out and moving, he even shows the basic thing that makes a permanent magnet motor turn: magnetic fields. He takes a powerful magnet and pushes the rotor manually, and then uses the motor to make the magnet tumble in his hand. When we were all kids, we played with magnets and felt the force of magnets, and how they’re pushing us into our seats when we stomp on that skinny pedal. (there are lots of good videos on YouTube that explain how an electric motor uses electro-magnets to make the rotor turn if you’re curious about this on a deeper level)
Let’s Talk Differentials
The final step in the gear set is the differential (inside the biggest gear). He doesn’t get into how that works in this video, but in short, it sends the power out to the axles and to the wheels. You can learn more in a short video about how differentials work here, or watch Weber’s much longer video here, but an important thing the professor mentions is that Tesla drive units are equipped only with a simple open differential.
This might sound like they’re cheaping out, but modern traction control systems largely negate the need for a limited-slip differential (they can hit the brake on a slipping wheel as needed), and Tesla is actually engineering a simpler and more reliable drive unit this way. Limited-slip differentials are awesome, but they have clutch packs or other additional parts in them like an automatic transmission, and they can wear out over time. Plus, as our brave Technoking says, “The best part is no part.” Having more parts means there’s more to go wrong. Also, brake jobs are a lot easier than tearing a drive unit apart to fix a busted differential, so traction control wins on that count, too.
It’s worth noting that when people use salvaged Tesla drive units to convert classic vehicles to electric, there are aftermarket suppliers selling limited-slip differentials. Classic cars don’t have traction control systems that can hit the brake on the slipping side, making these extra parts the best approach in that niche application. Also, for some off-roading applications (and driver preferences), the more proactive approach of the limited-slip differential or a locking differential is superior to the reactive approach of traction control, but the Model 3 and Model Y are not intended for off-road driving, so Tesla again wasn’t in the wrong to skip it.
Anyways, back to the Weber video!
Why They Call Them Modular Drive Units
Another cool thing the professor points out is that most of the parts in a Tesla 3 or Y drive unit are the same. The only thing that changes is the motor itself. This helps save Tesla money, because it can order the parts for the drive units in larger quantities, and there’s really no downside.
It’s possible to put a permanent magnet motor, an induction motor (uses electr0-magnets for both rotor and stator), and motors of different sizes and power outputs in what’s basically otherwise the same drive unit. Both the rotor (the part that moves) and the stator (the part that doesn’t spin) can built into the same basic case, making for fewer parts that Tesla needs to build or buy to make the different drive units. Different inverters can be bolted to the outside of the same case, too.
For the business majors, this saves Tesla considerable money and simplifies business operations. But, DIY swappers probably wouldn’t find it to be a simple job, as the stators and cooling drain are different for front and rear drive units.
Cooling & Lubrication, Housing
The whole assembly is cooled and lubricated by automatic transmission fluid. This is pumped around the case and cooled or warmed by a heat exchanger (via the multi-way valve in the coolant system). It also has an external filter the fluid is pumped through, which should protect the gears inside from any loose metal that collects as the drive unit wears.
This is a much simpler system than the professor shows us for the Bolt family of vehicles, which should mean more cost savings for Tesla. It might also be easier to rebuild some drive units that fail, because the main part of the case might not have to be opened if something is wrong with a rotor or stator.
All in all, the whole system is very simple, and it’s part of a larger effort to keep things simple within Tesla. Costs are lower, support is easier, and changes are easier to make to production lines. All of this makes for a more nimble and efficient automotive production process, which is what Tesla is known for.
Featured image: a screenshot from the above video, showing some of the guts of a Tesla drive unit.
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