Tula Technology has applied to electric motors some of the algorithmic ingenuity it originally developed to make internal-combustion engines save fuel. The Silicon Valley company is focusing on a motor design that’s not in many EVs now but may well be common a few years down the road.
It’s called a synchronous reluctance motor, in which the rotor carries a ferromagnetic material such as iron rather than a permanent magnet. The rotor moves under the influence of magnetic reluctance, induced in its poles by the electromagnetic force exerted by the poles of the rotor’s stationary counterpart, or stator. It’s synchronous because the rotor moves in step with the rotating magnetic field; in such a design, the rotor and stator have the same number of poles.
“Most of the motor companies are looking at them,” John Fuerst, Tula’s head of engineering, tells IEEE Spectrum.
This is because reluctance motors dispense with permanent magnets based on rare earths. Those materials come mainly from China, an increasingly important player in the EV industry, and competing manufacturers elsewhere understandably seek a degree of independence from it.
There are other flavors of reluctance motor. Tesla hides permanent magnets inside the rotor—what it calls a permanent magnet synchronous reluctance motor. As we reported a few weeks ago, Turntide Technologies is working on a non-synchronous, that is, a switched reluctance motor, in which the rotor has fewer poles.
This is Tula’s first foray into EV land. The company was founded in 2008 by Adya Tripathi, famed for being the first to introduce a class-D amplifier chip, the TA1101, in 1996. It was used in Apple’s Power Mac G4 Cube. The company went on to work on signal processing algorithms to optimize internal-combustion engines. The resulting management system, Dynamic Skip Fire, selectively switches off some of an engine's cylinders when the engine is operating under a low load. GM (an investor in Tula) has incorporated this system into more than a million of its vehicles so far.
Fuerst says that other companies have used selective cylinder switching, too, but he asserts that Dynamic Skip Fire can save more fuel—up to 15 percent, for a big V-8. Engines with fewer cylinders get smaller improvements.
You can use such optimization methods in any kind of power plant. “Skip Fire came out of a common control philosophy—finding the sweet spot for a machine, I don’t care what machine,” says R. Scott Bailey, Tula’s chief executive officer. “It’s the same thought problem. That underpins the entire company.”
Tula’s new system for synchronous reluctance motors, Dynamic Motor Drive, gets the motor to the sweet spot in a two-dimensional efficiency map that graphs engine speed on one axis against torque on the other. The main challenge is to provide a lot of torque to the rotor in short pulses, so that the overall torque stays sweet.
“By pulsing magnetic fields at a higher torque, you can keep the net effect on steady state torque at the desired low,” Fuerst says. “It provides the opportunity to operate the electric machine in a way that delivers the torque that’s demanded by the driver at an efficiency that could typically only be achieved at higher torque.
Most of the benefit comes from the efficient operation of the motor itself, but you also get savings in the power electronics, he adds. “Sometimes the benefit’s bigger there—which is why it could also be useful in nonsynchronous machines.”
Speaking of nonsynchronous machines, what about the switched reluctance motor touted by Turntide Technologies? That company claims 64 percent savings in energy usage. How much energy does Tula’s system save in a synchronous machine?
“We have to have our share of modesty relative to some numbers from Turntide,” Fuerst says. “We’re trying to look at the full cycle, and we’re projecting 2-3 percent savings. It doesn’t sound like much, but every percent really counts.
“If you switch off DMD capability, you could have a range of, say, 100 miles. Switch it on, and you get an extra 3 miles. And if you save a few percent on your US $10,000 battery, okay, that’s a few hundred bucks.”
The company is now testing its system in a converted Chevrolet Bolt.