How Cars Work
The Motor
CHAPTER TEN
Last chapter, we built the battery. Hundreds of pounds of lithium-ion cells, sealed under the floor of the car, holding all the energy the car will use until somebody plugs it in again.
That energy is just sitting there. To make the car go, we need to turn the stored chemistry into motion.
The device that does that is the motor. And here is something surprising about the motor.
It is much, much simpler than a gasoline engine.
A gasoline engine, for all its cleverness, has hundreds of moving parts. Pistons. Connecting rods. Valves. Camshafts. A crankshaft. A timing belt. Spark plugs firing thousands of times a minute. Oil sloshing through every joint. An electric motor has, depending on how you count, somewhere between one and a few moving parts. There is a thing that spins, and a thing that doesn’t. That is mostly it.
This chapter is about how that thing spins.
Electricity in a coil makes a magnet
We have to take a short detour back to physics before getting to the motor.
In Chapter 3, we mentioned in passing that running electricity through a coil of wire makes the coil act like a magnet. We used this fact for the ignition coil — current through the inner coil made a magnetic field, and a sudden collapse of that field induced a giant voltage in the outer coil.
It is worth pausing on the first half of that sentence. Electricity in a coil makes a magnet. This is one of the most useful facts in physics. Wrap any old piece of wire around any old metal core (a nail, a finger, even nothing at all), hook the ends to a battery, and you have built a magnet. Disconnect the battery, the magnetism vanishes. Reverse the battery’s connections, and the magnet’s north pole becomes its south pole and vice versa.
This is what makes electric motors possible. If you can build a magnet that you can turn on, off, and flip, you can use it to make things move just by sending electricity in.
Pushing with magnets
If you have ever played with two magnets, you know the basic deal. North faces north, they push apart. North faces south, they snap together. The strength of the push depends on how close they are and how strong each magnet is. With strong magnets, the push is enough to throw them across a tabletop.
Imagine one magnet that you can spin freely on a pivot, with another magnet held still nearby. The free magnet will swing around to align itself — opposite poles facing each other — and then settle there. Once they line up, it stops moving. The free magnet has reached its happy place.
This is not yet a motor. To make it a motor, we need a way to keep the free magnet from settling — a way to make it keep wanting to rotate.