EV Motors Explained

From the April 2022 issue of Car and Driver.

Automobile lovers have been steeped in the language of the internal-combustion engine for so long that the inexorable switch to electrification requires a tune-up of our knowledge base. Many of us are familiar with the suck-squeeze-bang-blow rhythm of the four-cycle engine that powers the majority of today's daily drivers, while the snowmobilers and outboard enthusiasts among us can probably explain the inner workings of a two-stroke. A few nerds may even have a handle on the epitrochoidal machinations of the Wankel rotary, but the average gearhead's experience with electric motors might just begin and end with the last time their starter conked out.

All types of electric-vehicle motors share two major parts. The stator is the motor's stationary outer shell, whose housing is mounted to the chassis like an engine block. The rotor is the lone rotating element and is analogous to a crankshaft in that it feeds torque out through the transmission and onto a differential.

Most EVs rely on a direct-drive (single-ratio) unit to step down the rotating speed between the motor and the wheels. Like internal-combustion engines, electric motors are most efficient at low rpm and higher load. While an electric car might enjoy an acceptable driving range with a single gear, heavier pickups and SUVs designed to pull trailers will increase range with a multi-speed transmission at highway speed. Today only the Audi e-tron GT and Porsche Taycan use a two-speed transmission. Multi-gear (spin) losses and development costs are reasons why EVs with more than one gear are uncommon, but we predict that will change.

EV Motor Commonality

All three major EV motor types use three-phase alternating current to set up a rotating magnetic field (RMF), the frequency and power of which are controlled by the power electronics that respond to the accelerator. Stators contain numerous parallel slots stuffed with interconnected loops of copper windings. These can be bulky looms of round copper wire or tidy hairpin-shaped copper insertions with square cross-sections that increase both fill density and direct wire-to-wire contact within the grooves. Denser windings improve torque capability, and tidier interlacing at the ends amounts to less bulk and a smaller overall package.

Batteries are direct-current (DC) devices, so an EV's power electronics include a DC-AC inverter to provide the stator with the AC current necessary to create the all-important variable RMF. But it's worth pointing out that these electric motors are also generators, which means that wheels will back-drive the rotor within the stator to induce an RMF in the other direction that feeds power back through the now AC-DC converter to send power into the battery. This process, known as regenerative braking, creates drag that slows the vehicle. Regen is not only central to extending an electric car's range, it's pretty much the whole ball of wax when it comes to highly efficient hybrids because lots of regen improves the EPA fuel-economy numbers. But in the real world, regen is less efficient than coasting, which avoids the losses each time the energy passes through the motor and converter when harvesting kinetic energy.