High-Torque AC-Induction Motor Makes Electric Racer PossibleBy Susi | Published on Dec 03,2015
If you fire up the Formula race car that Raser Technologies recently built, don't expect to hear the roar of the engine. Because there isn't one. Raser's racer instead runs solely on electric power from a bank of more than 30 12V batteries and a proprietary drive system based on an ac-induction pancake motor. In dynamometer testing, this drive system has generated as much as 420 ft-lb of torque, enough to produce 500 horsepower at 6,250 rpm. Raser even tested the car at the Grand Prix course in Monaco in early April. Later that month, at the SAE World Congress, Indy 500 winner Danny Sullivan put the car through its paces.
Yet despite its outward appearance and the company it keeps, the Raser car should not be confused with a real race car. In fact, all of its trials so far have intentionally been conducted below highway speeds. "It's not really engineered for speed," says Timothy Fehr, Raser's senior vice president and chief technologies officer. What it is engineered for is showing the world that ac-induction motors can take on pricier permanent magnet motors in transportation applications that need to pack lots of torque in a small space without turning up the heat.
And in this regard, Raser's race car serves as a powerful demonstration. According to Fehr, the 67-kg ac-induction "P2" pancake motor that powered the car has a torque density of 35 Nm/, when coupled with Raser's proprietary "Symetron" drive. "With conventional design practice, you would expect less than 25 Nm/, for an ac-induction motor," he says.
Looked at from a different perspective, these motors achieve such a high torque density because they run efficiently while the motor generates high peak torque levels. And this, in Fehr's view, represents the biggest selling point for Raser's technology. "We're able to extract a greater continuous torque at higher efficiencies from induction motors than was possible with current design practice," he says, explaining that the efficiency of traditional ac motors "quickly ramps off" when those motors are operated above their rated torque. At two times that rated torque, he says, the efficiency typically falls to about 85 percent from more than 90 percent. At three times that torque, efficiency usually drops even further, often to about 60 percent.
Raser's technology represents an attempt to preserve efficiency when the motor works its hardest. And Fehr says that Raser's motors, when run with the company's Symetron drives, lose less than two percent of their efficiency at twice their rated torque and lose only three to four percent at three times that torque. "We've even run motors at four or five times their design torque with acceptable results," he claims.
And every bit of efficiency really does count, especially for those engineers trying to squeeze motors into tight spaces. "A two percent improvement in efficiency typically equates to a 10 percent reduction in motor size for the same power out of the motor," notes George Holling, Raser's chief engineer.
How does Raser do it? The company's engineering executives treat the answer to that question something like a state secret-an understandable position given that the company has not yet commercialized its motor technology. Fehr will only say that Raser has made physical modifications to existing ac-induction motors based on the recognition of some "gaps in current design theory." For his part, Holling describes the Symetron controller technology as having attributes of VF and vector drives. "The technology has the advantages of a vector drive with the simplicity of a VF drive," he says, adding only that Raser has developed algorithms that provide the control of vector drive without the need for expensive high-resolution current sensors or microprocessors. "The Symetron hardware is very simple," he says.
Some tenuous clues to what Raser is doing might be found in the only patent it has been awarded so far (U.S. Patent 6,847,186). Issued not for a traditional ac-induction motor but for something the company calls a "resonant motor," this patent describes a motor with an air gap as much as 10 times larger than accepted design practice would dictate. According to the patent, this large air gap imparts a "pronounced magnetic inductance to the motor itself." Connected to a capacitor, the motor then functions as the inductor in a resonant (LCR) circuit. When powered, this circuit oscillates, and these oscillations are used to excite the motor, eliminating traditional drive electronics and PWM synthesis. "The inherent oscillatory characteristics of the resonant LCR circuit produce a sinusoidal ac voltage for self-commutation of power switches and efficient operation without resorting to PWM and the accompanying switching losses at high carrier frequency," the patent states.
None of this is to suggest that the P-2 motor and Symetron controller found in the race car are based on this particular patent. For example, the Symetron control technology does currently use "an efficient form of PWM," according to Holling. And he adds that all of Raser's physical modifications to traditional ac-induction motors "stay within the existing motor infrastructure, meaning that we maintain the same slot fill."
Fehr also cautions against reading too much into that single patent. "It won't tell you much by itself," he says. And he notes that the company has recently applied for at least 20 additional patents. Still, he does acknowledge that some of the counterintuitive design thinking on display in the patent does inform the company's ac-induction motor technology. And he says the company will reveal more about the inner workings of its ac-induction motors when the company's patent position solidifies.
Yet Gulalo doesn't rule out the possibility that Raser's technology could be the real deal. "It may all be true for a very specific performance profile and application area," Gulalo says. If Raser's claims do pan out, the technology could prove to be an important breakthrough. As Fehr points out, the extra torque could allow design engineers to downsize motors, saving space and weight. Likewise, it could allow engineers to get extra power from a given frame size.Raser's claims about all extra torque with such low losses will likely stir some skepticism among those who know motors. George Gulalo, president of Motion Tech Trends, has seen his share of bold performance claims during more than 20 years as a consultant to clients in the motor industry. "Every time I see claims about such a drastic technology improvement, it seems they never materialize," he says. And that makes sense particularly with ac-induction motors, a mature technology whose design inputs and construction methods are well understood. "AC induction motors have been around a long time," he says. "It's hard to believe anyone could make them that much better."
There are cost implications, too. A truly high-performing ac-induction motor could serve as an alternative to permanent magnet motors containing expensive rare-earth materials. Fehr's estimates that an ac-induction motor at commercial production volumes could cost as much as 25 percent less than a comparable PM motor. And Holling says the Symetron controller will offer its vector-drive-like performance at VF-drive prices.
Both the performance and cost issues were put into play in the two broad application areas targeted by Raser. The first, as might be guessed from the race car demo, consists of transportation applications. Fehr sees hybrid vehicle motors as the single most important application. "That's where our performance, packaging, and cost reduction advantages matter the most," he says. Aside from its race car, the company has other transportation demonstrations and design projects in the works, too. The company has already built and is now running trials with a small, all-electric utility vehicle. And the U.S. Army Research Laboratory recently contracted Raser to design two integrated starter generators/alternators (ISAs) for use in the drive trains of future military wheeled vehicles. These pancake-shaped, in-line motors will be designed to provide both acceleration support and mobile power generation.
The second group of applications involve industrial uses that now require or could benefit from variable output electric motors. Fehr cites drilling equipment and HVAC blowers as two applications that might increasingly incorporate variable output motors-if the price is right. A variety of machine tool applications could also employ Raser's technology.
So far, Raser has built demonstration motors in the 5 to 500 hp range-or on frame sizes ranging from NEMA 145 to 254. "The technology is scaleable," Fehr says. The technology may also extend into other types of motors, too. Raser has plans to develop switched and variable reluctance motors, according to Fehr. In all cases, though, the company has no plans to manufacture the motors. "We plan to commercialize them strictly through licensing agreements," Fehr says.