The complexity and engineering challenges of the electric motor
It looks as if the electric motor is poised to replace the internal combustion engine in the automobile.
Britain and France have put an end date on the sale of passenger vehicles with gasoline or diesel engines, and China is contemplating the same move.
Several automakers have said their current generation of internal combustion engines may be the last they develop.
And last week at the Frankfurt auto show, just about every automaker amped up its electrification plans.
The change from internal combustion engines to electric motors will be one of the biggest the industry has ever faced. More than a century of development and investment in the internal combustion engine will now start to slow as automakers begin designing, developing and, most likely, building electric motors in-house.
Production engineers are probably already thinking about how engine plants will need to be retooled from producing internal combustion engines to spinning out electric motors.
While the improvements in the automobile internal combustion engine are well documented, the state of the electric motor is less well known — at least among those of us who focus on the auto industry. At Automotive News, we’ve rarely written about the internal workings of the electric motor and the engineering challenges it poses as it moves into high volume production and use. That seems likely to change.
For now, I’ve checked in with Larry Nitz, executive director of General Motors’ global propulsion systems. Nitz most recently led the engineering team that designed the electric motors for the Chevrolet Volt and Bolt.
These motors are not off-the-shelf components bought from suppliers, but were designed in-house at GM with the goal of optimizing them for their specific duty cycles. GM builds some motors in-house and buys others that have been designed by GM engineers.
Compared with a typical four-cylinder internal combustion engine, which has hundreds of moving parts, the electric motor is very simple. Motors have a rotor, stator, armature, commutator, windings and bearings. While they may be far less complex than gasoline or diesel engines, electric motors pose their own challenges. Some, such as cost, weight and smoothness, are the same as those facing internal combustion engines.
And, Nitz says, even though electric motors are about twice as efficient as gasoline engines, there is room for improvement. “Incremental improvements are possible in the reduction of rare earths in permanent magnet motors,” he said. “Incremental improvements are also possible in induction motors.”
Engineers are looking at traditional ways to improve the electric motor, too.
“In a car, size matters. Mass matters. Cost, performance, efficiency, and [noise, vibration and harshness] matter. That’s the beautiful thing about an automobile. It brings together almost every cross-functional element,” Nitz told me. “Whether it is a combustion engine or an electric motor, they all have to be optimized. The objective is to move people and not take a lot of space and mass and have tremendous efficiency.”
Efficiency is the key word. For now, SAE International lacks a standard way to measure electric motor efficiency.
That will soon change, says John Tintinalli, director of the global product group for ground vehicles at SAE International. He said SAE is developing a standard way to measure efficiency of electric motors, as well as torque and horsepower.
The Chevrolet Volt has two electric motors — one is 96 percent efficient, while the other is 94 percent efficient. Nitz says electric motor efficiency at GM is measured in two ways.
One way is DC (direct current) from the battery to mechanical power out. At GM, Nitz said, that measurement is taken by including the inverter, which changes DC to AC (alternating current).
“We like that because it is system efficiency,” Nitz said. “That’s energy stored in the battery and delivering it to the road.”
Another way to gauge the efficiency of an electric motor is to measure the AC current going in vs. the shaft power of the motor.
As with an internal combustion engine, electric motors also have what Nitz calls “islands of efficiency” — certain speeds and loads at which the motors operate most efficiently. And engineers can tune that based on the vehicle.
GM has created an electric motor product development system that Nitz says is just as thorough as the one it has for internal combustion engines. He said engineers tested between 30,000 and 50,000 iterations of the motors’ electromagnetics before choosing the production version. Most internal combustion engines have a maximum of about 6,000 rpm. But an automobile’s electric motor might spin as fast as 9,000 rpm.
Nitz said that speed presents engineering challenges. Centrifugal force inside the engine can cause the small air gap between the rotor and stator to change, and so there must be a mechanical system that controls that movement.
“Geometric dimensioning and tolerancing of the bearing systems to hold the rotor and keep it concentric, and how the stator is mounted to the rotor is probably one of the most challenging jobs that I can think of,” Nitz said. “It’s as challenging as anything we do in transmissions, maybe a little more.”
Listening to Nitz describe the inner workings of electric motors dovetails perfectly with how the conversation about automotive powertrains is going to change in the coming years. There will be less talk of turbochargers, 10-speed transmissions, variable valve timing, pistons, fuel injection and ignition systems and more about traction motors, inverters, magnets and windings.
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