Could Mechanics Best Power Electronics in EVs?

Could smarter mechanical transmissions knock power electronics out of wind turbines, providing a cheaper and more efficient means of coupling the variable energy from ever-shifting winds to the regular waveform of AC power on the grid? They could according to my reporting in MIT's TechReview today on Viryd Technologies' bid to exploit continuously variable transmissions (CVTs). If mechanics reclaiming territory ceded to electronics sounds like a technological step backwards, here's an more heretical corollary: the same CVTs could also squeeze the power electronics out of electric vehicles (EVs).

That's the argument put forward by Rob Smithson, CTO for Viryd parent company Fallbrook Technologies and one of the inventors of its clever CVT (dubbed NuVinci in a tip-of-the-hat to the Italian polymath who first dreamed up the CVT concept). "If you look at cost in large car-replacement type EVs today, the cost gets dominated by the battery pack and the motor controls. There’s an opportunity to knock out one of those two with an infinitely variable transmission," insists Smithson.

Most EV elaborations today, says Smithson, rely on the electric motor to meet the entire dynamic performance envelope of the vehicle, from vehicle speed to torque demand -- a feat made possible by hefty power electronics. Swap in a CVT to handle the vehicle speed, however, and the electric motor can operate as a fixed speed variable torque device. "When that happens there’s a tremendous opportunity there to simplify your power electronics and a lot of the attendant cost that goes with that," he says. For more details, see Fallbrook's white paper on increased power, speed and range observed in a NuVinci-equipped electric scooter.

Smithson is well aware that his proposition will sound heretical to many EV designers ("I’m looking forward to my turn at being burned at the stake," he told me with a chuckle). But an EV source I trust says Smithson could be pardoned. Ed Benjamin, an expert in light electric vehicle and bicycle technology and managing director of Benjamin Consulting, agrees that CVTs have great potential in EVs. "If a CVT was light, had a wide range, worked well and did not lose much energy - it couldgreatly improve the performance of Light Electric Vehicles, extending the capability of the drive system and extending battery / range," says Benjamin.

Benjamin adds that Fallbrook's NuVinci CVT is the best CVT he has seen. "It is an impressive device. Ingenious, clean, works well. Not too heavy, does not lose a lot of energy," he says. At the same time, he notes that many attempts to engineer CVTs have failed in the past and, "often they are just a hair away from being right."

I'd call it a story worth following. Fallbrook has already commercialized its CVT in high-end bicycles, and says it is developing applications for power transmission in electric vehicles as well as auxiliary power generation in military vehicles and optimization of vehicle air conditioning, which accounts for nearly one-tenth of U.S. annual fuel consumption. And then there are the wind turbines I covered for TechReview today, which are arguably the toughest application of all. Automobiles are designed for something like 5,000 hours of lifetime operation, whereas wind turbines must run more like 80,000 hours.

For those who still doubt the feistyness of mechanical engineers (and their EE sympathisers!) to challenge the trend towards digital power control and transmission, check out the back-to-the-future example of GE's variable frequency transformers. We covered this adaptation of transformers for coupling non-synchronous power grids in 2007 in "Power Transmission Without the Power Electronics".

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