How Cars Work · The Motor

Why no transmission

The fact that the motor’s speed and force are entirely under the control of the controller has a remarkable consequence.

An electric motor produces full force from zero RPM.

A gasoline engine cannot do this. As we saw in Chapter 4, a gas engine has to be turning at at least a thousand RPM to make any meaningful power. Below that, it stalls. That is why a gasoline car has a clutch and a multi-speed transmission — to bridge the gap between an engine that wants to spin fast and wheels that are stopped.

An electric motor doesn’t have that gap. As soon as you press the pedal, the controller starts sending current through the stator. The rotor immediately wants to spin, even from a complete standstill, and it pushes against the wheels with full force right away. As you press the pedal further, the controller sends more current, and the rotor spins faster. The pedal directly controls how hard the rotor is pulling — and the pull is there from the very first moment you press.

This is why an electric car doesn’t need a transmission. There is no clutch, no gear shift, no neutral, no first or second or third gear. Just one fixed gear ratio between the motor and the wheels, and the controller takes care of everything else.

It is also why electric cars feel so quick to accelerate from a stop. A gas car at a green light has to spin up its engine, engage the clutch, and ease into first gear before the wheels get any real push. An electric car at a green light gives you full force the instant the pedal moves. No spinning up. No gear-engaging. Just the rotor going from still to fast in milliseconds.

Try It: Build a Simple Motor

This one takes a little more setup than most of the others, and might want a parent’s help. The materials are cheap and easy to find online or at a hardware store.

You will need:

• About 3 feet of enameled copper wire (also called “magnet wire” — it has a thin colored coating along its whole length)

• 1 fresh AA battery

• 2 large paper clips

• A small permanent magnet (a refrigerator magnet works; a small “rare earth” neodymium magnet works much better)

• A piece of fine sandpaper

• Tape or a rubber band

Wrap the copper wire into a tight coil around something cylindrical (the AA battery itself works) — make 15 to 20 loops. Slide the coil off, then wrap each loose end of the wire around the coil twice to hold the bundle together. Leave each end sticking straight out, like the axle of a wheel, about an inch on either side.

Now the trick. The enamel coating on the wire is what insulates it — current can only flow where the coating is removed. With the sandpaper, gently sand the coating off the BOTTOM HALF of each wire end. Leave the TOP HALF coated. Do the same on the other wire end, and make sure both wires are sanded on the same side (both undersides). This is the trickiest step. Get a parent to check your work before moving on.

Bend each paper clip so the top has a small loop you can rest a wire in. Tape one paper clip to each end of the AA battery, with the loops sticking up. Place the magnet on top of the battery, between the paper clips, with one of its faces pointing up.

Rest the two wire ends of your coil into the paper clip loops, so the coil is suspended above the magnet. Give the coil a small starting nudge with your finger.

It should keep spinning on its own.

Here is what is happening. When the bare side of each wire is touching the paper clip, current flows from the battery through the coil. The coil becomes a magnet. The permanent magnet on top of the battery pushes the coil — and the coil flips around. As it rotates so the coated side of the wire is touching the paper clip, current stops flowing (the coating insulates), but the coil keeps spinning from its own momentum. When the bare side comes back around, current flows again, and the magnet pushes again. The half-on, half-off pattern of your sanded wire is acting as a tiny commutator, switching the current at exactly the right moments to keep the coil spinning.

Sometimes it takes a few tries — the coil has to be balanced, the wire ends have to make good contact, the magnet needs to be strong enough. If your motor doesn’t spin, try a stronger magnet, more loops in the coil, or a fresh battery.

You just built a working electric motor. The one in a Tesla is bigger, fancier, and has a computer controlling it instead of sanded wire — but the principle is identical.

Did You Know? Electric cars are not new. The first one was built in the 1830s, around the same time as the first electric motors. By the late 1800s, electric cars and gasoline cars were both on the streets, competing for customers. In 1900, in New York City, electric cars actually outsold gasoline cars. What sank the early electric cars was the battery. A lead-acid battery from 1900 stored only a fraction of the energy of a modern lithium-ion pack — about a tenth — and it was just as heavy. Range was 30 to 40 miles between long, slow charges. A gasoline car could go 200 miles and refuel in five minutes. The choice was not close. Electric cars faded almost completely by the 1920s. The motor side of an electric car was already perfectly fine in 1900. The car your great-great-grandparents might have ridden in had an electric motor that, in principle, would still work today. It was the battery that needed another century of work. Once lithium-ion came along, electric cars came back.

Energy moving

So now we have the full architecture of an electric car. Battery in the floor, holding chemical energy. Motor at the wheels, turning electrical energy into rotation. A motor controller deciding how much current to send through the stator coils, based on how far the driver has pressed the pedal. Wires connecting all of it.

That is how an electric car drives.

There is one more clever thing — something a gas car can’t really do. When the driver lifts off the pedal or starts to brake, the motor doesn’t just stop pushing. It can briefly run backwards, pulling energy back out of the moving car and sending it into the battery. The same device that drives the wheels can act as a generator, putting energy back instead of taking it out.

That is the next chapter.

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