Electronic Design

Roadster Charges Up The Electric Car

Electric cars aren't a joke anymore. Tesla Motors will make you forget all about their boxy styling, sluggish acceleration, and limited range with its sleek, environmentally friendly Roadster (Fig. 1). Founded by Martin Eberhard and Marc Tarppening in 2003, the company set out to build a green car that doesn't compromise on looks, performance, or efficiency.

Previous electric cars only had a range of 60 to 80 miles. Tesla Motors countered that limit with a proprietary lithium-ion (Li-ion) battery pack that provides up to 250 miles on a single charge and lasts up to 100,000 miles (Fig. 2). After all that distance, the pack won't end up in a junkyard either, since it's recyclable. It includes 6831 batteries and weighs about 1000 lb. A liquid cooling circuit, a sensor array, and a dozen microcontrollers maintain its efficiency. An onboard, high-power charging system "tops off the tank" in about 3.5 hours.

A three-phase, four-pole ac induction motor provides 248 hp peak (185 kW) and a 13,500 redline. Tesla says that it does 0 to 60 mph in about 4 seconds with a top speed of 130 mph. The company also says it uses the lowest-loss conductors and the highest quality magnetic steel possible. An array of air-cooling fins on the aluminum housing keeps the motor running at around 120°C. Measuring just 250 mm (diameter) by 350 mm (length), the motor also operates with 85% to 95% efficiency.

The transmission only has two forward gears. There's no clutch, so drivers can quickly and easily start from a stop or shift gears on the highway. Regenerative braking recovers and stores energy, extending the vehicle's range even further during first gear. And unlike gasoline engines, the Roadster's peak torque begins at 0 rpm and stays powerful beyond 13,000.

In addition to twin airbags and seat-belt retractors, the Roadster offers four-wheel antilock braking and traction control. An air pressure sensor system keeps tabs on the car's four Yokohama Neova tires. During charging, power won't begin transferring until computers on board the car and in the charging unit agree that the cable is correctly attached. Programmed to prevent overcharging, the self-regulating battery box also will shut itself down if it's immersed in water or if it detects smoke or airbag deployment.

The car's Visual Display System reports all the vital information drivers need. Drivers enter their PIN on the VDS to start the car, eliminating the possibility of hotwiring. Its Valet Mode limits the car's range, acceleration, and speed when someone else has to drive it. Also, the Roadster comes with a Blaupunkt radio with an iPod connector. Satellite navigation and satellite radio are available as well.

While electric cars have zero emissions, the power plants that provide their power when they're recharging consume energy and can emit plenty of pollution themselves. So, Tesla Motors conducted a "well-to-wheel" study including these factors and found that the Roadster still has double the energy efficiency of popular hybrid cars while generating a third of the carbon dioxide. Compared to other sports cars, it's six times as efficient while producing a tenth of the pollution.

Tesla, which unveiled the Roadster at a star-studded event in San Carlos, Calif., on July 19, expects the car to be available in early 2007 with a starting price around $100,000 (Fig. 3). "We at Tesla Motors love cars. We love to drive," said Eberhard. "We appreciate beautiful and fun cars. And Tesla cars are built for people who love to drive. So our optimization is not for ultimate low cost, but rather for performance, aesthetics, and sex appeal."

Richard Gawel
Tesla Motors

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