"So there’s a few things at play here. For context, I run the Product Security team at Tesla and I’m safety-trained on the HV systems - I’m also working hands-on with a small drive inverter on a hobby project right now.
First and foremost, our large drive unit pulls about 1000A at full load, and switching that with silicon is tough. We use a bank of custom IGBTs on each of the high/low sides of each of the 3 rotor phases in order to handle the power, and that’s with active fluid cooling. You can switch that much current with silicon but it ain’t cheap, and you’ll need either active cooling or a bunch of thermal mass if you want the thing to switch more than once. http://www.teslamotorsclub.com/attachment.php... is a decent pic, the object on the left is a single-phase switch, you can see 6x transistors laying flat at the front for one side of the phase (the other bank is behind).
Secondly, Model S is an AC induction motor so the current through the winding ramps up more-or-less linearly over time until the phase switches off (or changes direction). You’re at high power but you’re not switching the load at zero-crossing as you would in a resonant load such as a Tesla coil, instead you have to switch at an increasing current depending on how much power you want to the wheels. You now don’t just have to switch a lot of power, you have to switch it FAST so that the resistive losses in the FETs don’t blow out the power channel due to ohmic losses. Your switch is now not just big and bulky, it’s complicated (since you need an additional HV supply) and pretty sensitive to things like stray capacitances. On the previous pic the big black brick on top of the PCB is the capacitor that dumps into the IGBT gates to make them switch fast enough.
Finally, I believe there’s a regulatory issue. I think I’m right in saying that automotive standards around the world require that all electrical systems are fused, and considering that there’s multiple separate power rails it’s not inconceivable that an event could take place that leaves the HV drive rail powered on but kills the 12V accessory rail that powers a lot of the CAN systems. You could end up disabling your active fuse while the HV system is still energized, and considering the amperage our lithium packs can deliver (P85D draws up to 1.5kA) that’s not going to end well.
Woz: I would LOVE to put you under a Tesla NDA and then give you a _real_ tour of the vehicle - ping me at kpaget@teslamotors.com if you’re interested. I’m curious, do you still have one of my RFID cloners on your shelf somewhere?"