How an electric motor works

Why the commutator is the whole trick

Take the commutator out and you still have a motor — for about a quarter of a turn. The coil swings until its north face is against the stator’s south, and there it sits. Push it past that point and the field pushes it straight back. What you have built is not a motor but a compass needle with extra steps.

The commutator fixes that by reversing the current at the one moment when reversing costs nothing. As the coil crosses the neutral plane the forces on its two sides point straight out from the shaft: full strength, no turning effect. That is where the gaps in the split ring are aimed. Each brush steps off one segment and onto the other, the current through the coil turns around, its north and south faces swap, and the push it was about to receive backwards arrives forwards instead.

Notice what does not reverse. Outside the brushes the current runs the same way all the time — it is a battery, it has no other setting. The alternation is manufactured inside the machine, by a mechanical switch made of two pieces of copper and the shaft they are already bolted to. That is the engineering to admire here: a rotating switch that needs no timing signal, because the thing being switched is what drives it.

What the model leaves out

One loop is the smallest motor you can explain and the worst one you could build. Watch the green torque arc in the scene: it swells, collapses to nothing twice a revolution, and swells again. A real motor would stall on the neutral plane if it ever stopped there, and it would shudder rather than pull.

Real armatures carry many coils at different angles, each on its own pair of commutator segments — a dozen or more, not two. While one coil is crossing its neutral plane the others are near their best angle, so the torque adds up to something nearly constant and the motor starts from any position. The armature is also wound on a laminated iron core, which concentrates the field into the gap and multiplies the force by far more than the extra turns alone would. The ghost bar drawn across the loop in the model is not a part either: it is a stand-in for the coil’s magnetic moment, drawn so the push has something recognisable to act on.

The brushes get glossed over too. They spark as they cross the gap, they shed carbon dust, and they set a ceiling on speed and life. The flash in the model is the cheerful version of a real and unwelcome event.

From this to the motor in an electric car

Nothing in an electric car looks like this, and everything in it works like this. The physics is unchanged — a field, a current at right angles to it, and a force — but the switching moved out of the copper and into silicon. In a brushless motor the coils are bolted to the stator where they can be cooled and never need sliding contacts, the magnets ride on the rotor, and an inverter energises the coils in sequence, thousands of times a second, using sensors or the coils’ own back-EMF to know where the rotor is.

That is the same job the commutator does here, done by transistors that never wear out and can also decide how much current to send and when. Which is why the same hardware that drives the car also brakes it: reverse the timing and the motor becomes a generator, the back-EMF pushes current back into the battery, and the car slows down. The commutator could never do that. It only ever knew one direction.