[Technology Report]
Motor Control: More Than Just Switching MOSFETs
designing for certain motor-control systems can whet the engineer’s appetite more than others—and present the greatest challenges.
Displacement is now 1.8 liters, but the gearing was changed to lower RPM at high speeds, for a combined-cycle fuel economy of 72.4 mpg. Carbon-dioxide emissions are 89 g/km, beating the Euro 5 standard. Obviously, when operating as a purely electric vehicle, (speeds up to 30 mph), it produces zero exhaust emissions.
Toyota calls it a “synergy” drive because it can power the Prius exclusively from the gas engine, exclusively from the electric motor, or by powering the drive train simultaneously from both sources. Other hybrids use either a “parallel” arrangement, in which the electric motor is constantly in use as an auxiliary source of power for the gas engine, or a “series” topology, which uses the electric motor exclusively to power the drivetrain. Toyota says that its approach delivers the benefits of both.
The new version ups the displacement of the “variable-valve timing with intelligence” (VVT-i) gas engine to 1.8 from 1.5 liters. The engine generates 97 brake horsepower, or bhp (98 DIN hp) at 5200 RPM and 142 newton-meters (Nm) of torque at 4000 RPM, and it delivers higher torque at lower RPM—a reduction of 300 RPM at 75 mph. Toyota also says it’s quieter and delivers a 10% improvement in long-distance cruising fuel economy.
A new, more compact transaxle is 40 lb lighter and benefits from a 10% to 20% reduction in driveshaft energy losses compared to the current system. A new, cooled exhaust gas recirculation system boosts fuel efficiency and reduces emissions. Also, the new version of MG2 is bigger, rated at 60 kW/80 bhp. That’s 20% higher power, yet with a 33% size reduction.
The new Prius engine heat-management system improves fuel economy in cold weather as well as cabin comfort, employing a heatrecovery system and an electric water pump. Using an electric system in place of the waterpump drive belt reduces mechanical losses. As a result, the coolant flow rate can be controlled with greater precision for better fuel efficiency.
In the engine control unit, the new inverter is 37% smaller and 36% lighter, and it runs at a higher switching rate. Output voltage to the motors is now 650 V ac, up 150 V from generation 2. Similarly, the battery, still nickel-metal hydride (NiMH), has increased capacity up 2 kW to 27 kW. The battery also has been positioned to minimize its impact on cabin space.
The cooling system’s efficiency is improved with a significant increase in fan capacity. The controller has evolved, too. Drivers can now select EV, ECO, or POWER on the transmission control (Fig. 3). That’s electric-only (formerly available only as an aftermarket option in the U.S., though standard overseas), plus an economy/performance tradeoff option.
TESLA COURSE CORRECTION Mission does matter. The Tesla Motors Roadster is an entirely different beast from the Prius. To get some insights on Tesla motor control, I pulled some info from the extensive blogs maintained by firmware engineer Greg Solberg and chief technical officer J.B. Straubel.
Solberg notes that Tesla’s motor-controller algorithms handle both driving and regenerative braking. Throttle pedal position generates a torque command that’s applied to the motor, and the command can be either positive or negative, causing either acceleration or braking.
Interestingly, he says that since negative torque applied to the rear wheels can make the vehicle unstable (weight transfers from the rear to the front wheels), the Tesla controller uses the traction control system to limit regenerative braking if the rear wheels start to slip. In other ways, the control algorithms are like those in the Prius. For example, if the battery is so close to full charge that any additional charge from regenerative braking would cause a problem, the controller limits regenerative torque.
Solberg also discusses how market research affected the development of the algorithms. Most people “like the car to regen when you take your foot off of the throttle pedal, but [some] prefer the car to coast,” he says. “Ultimately, the Tesla Roadster is a sports car and the regen profile will be fine tuned for sports car driving.”
Straubel discusses the whole powertrain, which was redesigned to replace the original two-speed gearboxes with a shiftless, constantly engaged set of gears with an 8.2742:1 ratio (Fig. 4). That involved upgrading the power electronics module with more efficient IGBTs for switching and specifying a new motor, like the original, but with some changes so it can generate 850 A. All told, the new power train delivers 30% more torque (400 Nm at 14,000 RPM), but at the cost of a small hit in version 1’s foursecond 0- to 60 mph acceleration. The quarter mile still comes in at under 13 seconds.
I just purchased a 2010 Pruis over the weekend and the three way power options are vary nice to select the power needed for a given driving scenerio. The EV mode is very limited in range and I have yet to use it for more than a few hundred feet before the system is switched to gas engine mode. The ECO mode is the most usefull, this switches between EV and gas mode and adjusts for the power needed for acceleration or hills. The power mode switches to gas/electric boost and has more power all the time.
Anonymous -June 30, 2009
"It’s like a mechanical CVT that uses an electric motor instead of a cluster of gears."
This shows that the author does not fully understand the hybrid synergy drive. The "power split device" is a planetary gear set, which could be described as a "cluster of gears". It is the proper application or subtraction of torque by MG1, MG2, and the ICE to the sun, ring and planet carrier GEARS that creates the CVT in the Prius.
Anonymous -June 29, 2009
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