Electric Motors Without the Equations

Nissan Leaf Cutaway

Earlier this year, Bloomberg published an interesting article on the future of electric vehicles in relation to oil demand. It points out that global EV sales increased 60% in 2015, the same rate of growth seen by the Ford Model T in its early years. Though EVs still only command about 0.1% of the worldwide auto market—and the current glut of cheap oil has kept many people behind the wheels of their favorite crossovers and trucks—more affordable batteries, growing cultural acceptance, and the looming threat of global warming will most likely only improve EV sales from here on out. Bloomberg itself predicts “the 2020s will be the decade of the electric car,” anticipating that at some point EV demand will surpass even demand for oil.

Probably, then, EVs are vehicles worth knowing a little more intimately. Most of the major automakers have developed their own electric or plug-in hybrid models, and many are investing heavily in improved EV technology. Though we may feel familiar enough with the most basic parts of an EV—battery, drivetrain, charging port, some kind of regeneration system—odds are few of us know much about how an EV’s motor actually moves the car. And, frankly, it’s a pretty complicated process. But it’s still possible to grasp in a basic way that isn’t overwhelmingly technical.

Most EVs use some kind of induction motor. These are essentially composed of two parts: a stator and a rotor.

Stator

Stator—the conducting wire is on the inside of the center tube.

Rotor

Rotor—the conducting rods can be wrapped around the cylinder or inserted into a kind of cage.

The stator runs electrical current through coils of conductive wire, turning them into electromagnets. These coils, insulated from one another, can be split up into “phases,” and the phases direct the movement of the electromagnetic field in a rotating fashion around the interior of the stator. The rotor fits into the middle of the stator and has conducting rods of its own. Through the process of induction—brought on by proximity to the stator’s electromagnetic field—the rotor also turns into an electromagnet and tries to align itself with the magnetic poles as determined by the stator’s phases. Since these phases are staggered and powered by an alternating current (AC), the rotor begins to rotate as it follows the electromagnetic field around the stator.

Image courtesy of SaveOnEnergy

The rotor fitting into the stator. This and the above detail images courtesy of SaveOnEnergy

The rotor never quite catches up to the stator’s magnetic field, but in attempting to do so it produces the torque that spins the gears to drive the wheels. Pressing the accelerator directs more current into the stator and speeds up the rotor’s rotational speed. When you take your foot off the accelerator, the magnetic field shorts out but the rotor continues to spin freely; it’s possible to reclaim power from the spinning motor, which the control unit can then convert to DC and store in the battery. This is why EVs don’t need an alternator to charge the battery—the powerplant itself can do the charging.

It’s possible to fine-tune the interaction between stator and rotor in order to create a smoother engine feel, but that’s a more complicated matter than we’re concerned with here. Many people like to talk about the “instant torque” you get with an EV. This is because all the voltage is available to the electric motor immediately, and the strength of the current determines the torque. With an internal combustion engine, you have to wait until the engine is running fast enough to allow an optimum amount of air and fuel to enter the combustion chamber and thus produce maximum power. Electric motors also tend to have fewer moving parts overall, meaning less friction—which is a good thing.

EVs generally employ electric gauge clusters.

EVs generally feature electric gauge clusters…

Automakers don’t seem particularly eager to share details about their electric drivetrains. Usually, they make a passing mention of the motor and then go into some detail about the battery. This makes sense given most people are probably more concerned about battery power and range. But as EVs grow more popular, perhaps manufacturers will be more interested in showing off. And if the technology becomes more common, keeping it a competitive secret won’t be quite as big of a deal. Seeing as the world at present is just about drowning in computers, we can probably all agree it’s important to know the basics of how a computer works. A lesser version of this same logic also applies to discerning car buyers looking toward an EV-filled future. Not to mention there can be kind of unexpected interest in EV mechanics, even if you’re nowhere near getting that engineering degree.

Though less advanced on the Leaf compared to the Model S.

…though the Leaf’s gauge isn’t quite as advanced as the one on the Model S.

Of course, it’s still more important as a consumer to know about EV batteries: their capacities, their ranges, how long it takes them to charge. But if you’re interested in learning more about electric motors, there are countless places to go. SaveOnEnergy, a shopping site for utilities like electricity and gas, has a good overview of electric motors, and some of the images in this post are drawn from its website. AllAboutCircuits, an online electrical engineering community, goes into a little more detail in its article on induction motors. And if you want to see some helpful presentations, you can check out Engineering Explained on YouTube, specifically the playlist on EVs and hybrids.

What’s your take on the difference between driving an EV and a conventional car?

-Chase Hammond

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2 Comments

  1. I’m definitely curious to see where this technology goes in the semi-near future. I think it can really be the answer when oil starts phasing itself out.

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