Engines · Electric · Demo
Three coils that never move, three currents out of step — and the magnetism between them spins.
Same stator, same rotating field — only the rotor changes. The cage runs on slip; the magnet locks in step (and is started pre-synchronised, as a real one must be).
Slowed ~50×: a 1 500 rpm field crosses the screen twice a second. Restarting the cage replays a direct-on-line start — full torque from the first instant.
Torque needs only field × current, and both exist before the shaft ever moves — that is why an electric motor pulls hard from 0 rpm.
In plain terms
Every AC motor is built on one trick, spotted by Galileo Ferraris and Nikola Tesla in the 1880s. Wind three coils into a ring of iron, 120° apart in space. Feed them three alternating currents, 120° apart in time — which is exactly what the three wires of a three-phase supply carry. Each coil only pulses along its own axis, but the three pulses always add up to a magnetic field of constant strength pointing in a direction that rotates, smoothly, at the supply frequency. No moving parts make it spin. Drop in a magnet and it locks on and turns in step; drop in a cage of copper bars and it gets dragged around slightly slower. That is the entire secret of the synchronous and induction motor.
Each winding alone makes a field that just throbs back and forth along its axis. But sum three throbbing fields, spaced 120° in both space and time, and the wobble cancels perfectly: the result is a vector of constant length 1.5× the phase peak, gliding round at the supply frequency. It is a pure interference effect — the same mathematics as three overlapping waves making one travelling wave.
The field turns once per AC cycle divided by the number of pole pairs: n = 120·f/poles rpm. A 2-pole machine on 50 Hz spins its field at exactly 3 000 rpm, a 4-pole at 1 500 — which is why generators, pumps and old workshop motors all hum at those magic speeds. Change the speed and you must change the frequency: that is the whole job of the inverter in an EV.
A squirrel-cage rotor has no magnets and no wires to the outside — just shorted copper bars. It only feels a force while the field sweeps past it, because the sweeping is what induces its currents. So it can never quite catch up: it runs a few percent slower than the field, and that shortfall — the slip — is precisely where its torque comes from. Load it harder and it slips more, pulling more current, up to a breakdown point beyond which it stalls.
Give the rotor its own field — permanent magnets in most EV motors — and it snaps into lock with the rotating field, turning at exactly synchronous speed. Load it and it falls back by a fixed angle, like a magnetic spring being stretched; torque grows with that angle up to a pull-out limit, past which the lock breaks and the rotor slips poles. Zero slip means no rotor currents to heat things up — one reason permanent-magnet machines dominate EVs.