Tesla CTO provides update on Roadster Drivetrain 1.5

JB Straubel, Chief Technical Officer at Tesla Motors has just put up a new post at the Tesla Blog with an extensive update on the Roadster drivetrain updates that are coming later this year. As we reported at Tesla's recent schedule update, the Silicon Valley start up is currently planning a running change to the upgraded powertrain beginning with car #41 which should happen sometime in late summer. There aren't any huge surprises in Straubel's post, but there are some interesting details. When Tesla first told us about their plans for the 1.5 back in January, the plan was to modify the power electronics module to be able to supply more current to the motor thus increasing the output to a level that would allow meeting the vehicle performance targets without having to use a two speed gearbox.

Generally, pumping more current through an electrical circuit has the negative side effect of generating a lot more heat as the resistance increases. The indication we were given at the time was that the cooling capability of the motor would be increased to help dissipate that heat. Based on Straubel's description of the changes, it looks like they took a different approach instead, one that is all too uncommon in the auto industry. Instead of applying a band-aid and treating the heat symptom by adding cooling capacity, they chose to attack the disease itself. In electrical circuits heat is caused by resistance to current flow. Resistance is the electrical analog of mechanical friction. The more electrons you try to push against resistance in circuits the more heat you generate. Continue reading after the jump.

Unfortunately as the temperature of a circuit climbs, the resistance tends to go up creating a cascading effect of rising resistance and temperature making the problem even worse. In both the PEM and the motor, Tesla's engineers addressed the sources of resistance and temperature generation. Changing components and materials in both components has allowed them to reduce the resistance in both areas. The revised PEM has new transistors that help improve the overall efficiency, allowing more power with less heat generation.

The more substantial changes are in the motor. The terminal connectors have been redesigned and the high voltage cables that connect the motor to the PEM have been switched from a copper/aluminum composite to pure aluminum copper. The locations with the highest resistance were attacked directly allowing more current flow without increasing temperatures. The result is 33 percent more torque at the thermal limit than the existing motors. Accommodating that extra power required some mechanical updates to the motor shaft and spline. The motor housing was also reworked in order to mate up with the interface to the new single speed gearbox.

The gearbox is, of course, the main thing that prompted all of these changes in the first place. The idea of a two-speed gearbox has been abandoned in favor of a single speed constant mesh unit. Clutches, double-clutches and the need for any kind of shifting mechanism are all gone. A 12 percent shorter gear ratio going from 7.4:1 to 8.27:1 provides the 4 second 0-60 time when combined with the updated motor and electronics while keeping those parts within thermal limits. The simplified gearbox also drops about 17.6lbs compared to the original unit.

If it all works, this approach of improving efficiency will not only improve the dynamic performance of the Roadster, but also the range. The combination of less heat generation and the new gearbox is expected to improve the range of the car by about 10 miles. Over the coming months, Tesla plans to install the new hardware in all the prototypes as well as completing durability testing so that they can get upgraded cars into customer hands. We can't wait for a chance to try out a production Roadster with drivetrain 1.5!

[Source: Tesla Motors, thanks to Doug for the tip]

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