The Inside Story on Rotary Position Sensors for Electric Vehicle Traction Motors

Wells Vehicle Electronics manufactures more than 1,000 part types for fuel delivery, ignition, and emissions and now is expanding on its expertise in control systems for internal-combustion engines to diversify into electrification for vehicle propulsion. In an interview with SAE’s Automotive Engineering, Greg Burneske, Director, Wells Engineered Products, discusses the advances and advantages behind the company’s first-ever rotary position sensor for electric vehicle (EV) traction motors.

October 1, 2020

AE: Position sensors for EV motors are crucial — but common. Why is Wells’ new two-axis sinusoidal magnet design such an advantageous upgrade over typical rotary-position sensing technology?

Burneske: In automotive traction motors, you always want the magnetic field that you’re generating with the stator to be orthogonal [at right angles] to the magnets in the rotor. That’s an over-simplification, but it gets more complicated as there are more pole pairs in the motor.

In an electric vehicle application, you really want to optimize the efficiency. And smooth torque throughout the whole range of drivability is important, too, because you want to minimize vibration and resonances in the motor. The way most people achieve that is with variable-reluctance resolvers and transformer type resolvers.

Those work pretty well — but we’re replacing the resolver with something that’s simpler, lighter and smaller. And it’s more accurate. We achieve the accuracy by creating a proprietary magnetization pattern on a ring magnet mounted to the rotor.

AE: How is the pattern created?

Greg Burneske, Director, Wells Engineered Products

Burneske: We use a through-shaft magnet, a ring magnet that goes on the shaft. Now imagine that the pattern on this magnet has a B-field vector; as you go around the circumference of the ring, we’re going to rotate that angle sinusoidally. By measuring the proprietary magnetization on the ring magnet, we always know the position of the rotor. Our sensor can output the position in any format — analog, sin/cos, digital, etc.

AE: In almost any kind of measurement, enhanced accuracy is a good thing. What does it offer for an EV traction motor?

Burneske: With our sensor, all you need is a simple pickup and our proprietary ring magnet. With that, we can create output signals of a resolver without the excitation, without all the copper, without all the mechanical challenges, the weight, the bulk, the expense.

And it is very accurate: accuracy is measured in arc minutes of the electrical angle. Accuracy is somewhat of a function of the diameter of the magnet and how many poles you have — but when you’re driving a motor, you don’t really care about the mechanical accuracy. The accuracy you care about is the angle relative to the poles of the motor.

AE: How does your rotary position sensor compare in terms of performance and cost?

Burneske: Parts count definitely is fewer. Basically, it’s two components: the ring magnet [on the motor shaft] and a simple, compact pickup. We anticipate offering a 25 to 30% cost reduction over resolvers in some applications, and it could be more in others. In addition, our technology is lighter and smaller, with inherent redundancy.

AE: What about applicability to existing traction motors or those currently in the design phase?

Burneske: We can customize to any rotor size or any number of poles. We can go as small as 30 or 40 millimeters in diameter. We can do any number of poles — we’ve got a 12-pole system operating right now in our lab.