Two Motors in One: Intelligent Controls Efficiently Deliver Torque and Speed
Tom Lombardo posted on July 22, 2020 |
Exro optimizes motors for both torque and speed by changing motor wiring on the fly.

When engineers design an electric motor for a given application, the motor is optimized for either torque or speed, as the two factors are often mutually exclusive. That’s not a problem if the motor is only subjected to a constant load or speed, but what if the product needs a lot of torque under certain conditions and high speed under different conditions? The conventional solutions are to add a gearbox or to use multiple motors—both of which add cost, complexity and weight to a product. Alternatively, engineers may simply compromise and design the motor for torque, speed or something in the middle, accepting inefficient operation everywhere outside the motor’s “sweet spot.”

Exro Technologies takes a different approach—an electronic gearbox of sorts—and the company is gaining traction in the electric mobility market. I spoke with Exro CEO Sue Ozdemir and CCO Josh Sobil, who gave me a rundown of the technology and its applications.

Sue Ozdemir and Josh Sobil. (Image courtesy of Exro Technologies.)
Sue Ozdemir and Josh Sobil. (Image courtesy of Exro Technologies.)

Incidentally, Ozdemir has been in this industry her entire life. Her parents owned a motor repair shop where Sue spent her formative years diagnosing and fixing motors. Prior to joining Exro, Ozdemir spent nine years as CCO and then CEO of General Electric’s Small Industrial Motors Division.

Coil Driver Technology

Exro’s patented technology, which is applicable to both motors and generators, effectively provides two machines in one—a high-torque motor and a high-speed motor. Ozdemir said that others have tried to make this kind of technology, but Exro was the first to succeed. The method uses a combination of switching circuitry and a microcontroller running a dynamic power management algorithm to determine the optimal configuration of coils needed to produce the desired torque-speed profile. Exro refers to it as a “Coil Driver” and says that it “enables two separate torque profiles within a given motor. The first is calibrated for low speed and high torque, while the second provides expanded operation at high speed. The ability to change configuration allows efficiency optimization for each operating mode, resulting in overall reductions in energy consumption. The controller automatically and seamlessly selects the appropriate configuration in real time so that torque demand and efficiency are optimized.”

In a standard motor, each pair of poles has a coil, and when that pair is energized, the magnetic field pulls the rotor. Exro takes each coil and breaks it into two or more coils. The switching circuitry puts the coils in series, parallel, or a combination, depending on load and speed demands at that moment.

Two configurations of motor windings. (Image courtesy of Exro.)
Two configurations of motor windings. (Image courtesy of Exro.)

Sobil said that when the coils are in series, the motor generates high torque at low speeds. In parallel, the lower inductance reduces the reactance and the stator impedance, and with less voltage dropped across the stator, more voltage is available to drive the machine faster at a higher torque, as seen in the torque profile below.

Series and parallel torque-speed profiles. (Image courtesy of Exro.)
Series and parallel torque-speed profiles. (Image courtesy of Exro.)

New Designs and Retrofits

These electronic gearboxes can be applied to new motor designs or retrofitted to existing motors. It doesn’t change the fundamental design of the motor at all—in many cases, rewiring the windings is all that’s needed. In fact, in late 2019, Exro released its first proof of concept, a retrofit for Motorino e-bikes. It was a fairly easy retrofit, as the original motor was mechanically sound—only the coil wiring needed to change.

Motorino e-bike retrofitted with Exro technology. (Image courtesy of Exro.)
Motorino e-bike retrofitted with Exro technology. (Image courtesy of Exro.)

Exro’s engineers helped Motorino to rewind its coils, adjusting the turns ratio and wiring the motors so they could be reconfigured on the fly. The result was a 25 percent increase in both torque and acceleration.

Exro recently agreed to collaborate with Zero Motorcycles to incorporate coil switching technology into Zero’s electric motors. In addition, Exro is partnering with SEA Electric to produce advanced powertrains for cargo vans and heavy-duty trucks, with Aurora Powertrains to enhance motors in its electric snowmobiles, and with Clean Seed to develop electric farm equipment.

Applications

Ozdemir says that the design is scalable and can be applied to motors in devices such as skateboards, bikes, motorcycles, cars, trucks and buses, as well as industrial equipment. Although she didn’t mention any specific brands, the Tesla Model 3 employs a dual-motor all-wheel-drive—one motor for low-end torque and another for speed. Seems to me they could shave a few kilograms off of the car’s weight, not to mention a few thousand dollars off of the price, by switching to a single motor with an electronic gearbox. Furthermore, the fact that it works for both motors and generators is a double win for an electric vehicle’s (EV’s) range, as it improves motor efficiency (thus using less energy per unit of distance) and increases energy capture during regenerative braking. Exro will have an opportunity to pitch its product to the EV industry, as the company was invited to present a white paper, “The Coil Switching Inverter—A Step Change in Powertrain Design,” at the Car Training Institute (CTI) Symposium in October 2020.

Designed for EV powertrains and other electric motors. (Image courtesy of Exro.)
Designed for EV powertrains and other electric motors. (Image courtesy of Exro.)

Since the same concept also applies to generators, I asked if the company plans to break into the wind turbine market. Ozdemir said that Exro does plan to work with various turbine manufacturers (not just wind), but wouldn’t go into detail on the issue. I wouldn’t be surprised if these discussions were already underway. Considering Ozdemir’s background with GE, I’m guessing she has more than a few contacts in the turbine industry.

As Exro gains momentum, it’ll be interesting to see how many companies decide to take its technology for a spin.

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