An induction motor turns its rotor without a single wire ever reaching it. Three-phase current in the stator sets up a magnetic field that sweeps around the bore; that moving field induces currents in the rotor and drags it along, always just a little behind.
An induction motor is an AC motor whose rotor has no electrical connection to the outside world. The stator — the fixed outer part — is wound with coils and fed with alternating current, and those coils create a magnetic field. The rotor, sitting inside, is turned not by wires but by the field reaching across the gap and inducing currents in it.
Because there are no brushes, commutators or sliding contacts, an induction motor is rugged, cheap and almost maintenance-free. That is why it drives so much of the world's machinery, from pumps and fans to conveyors and compressors. The same idea, run in reverse, is also how many generators are built.
The grey ring is the stator, with its slots holding the three sets of windings coloured for phases A, B and C. Inside the narrow air gap sits the rotor, whose cage bars make up the inner ring. The blue rotating ring with its N and S poles is the resultant magnetic field, sweeping steadily around the machine and pulling the rotor after it.
The three windings are connected to the three lines of a three-phase supply. Each line carries an alternating current, but the three currents do not peak at the same instant: they reach their maximum one after another, spaced evenly around the cycle.
Each winding makes its own magnetic field, and because the peaks follow one another, the combined field does not simply pulse — it sweeps steadily around the bore. The stator now behaves as if a bar magnet were spinning inside it.
The moving field cuts across the conducting bars of the rotor cage and induces a voltage in them. Because the bars are shorted together at the ends, this voltage drives currents around the cage, strongest when the field is moving fastest relative to the rotor.
Those induced currents set up the rotor's own magnetic field. Now there are two magnetic fields — the stator's and the rotor's — and while the rotor lags behind, the forces between opposite poles pull the rotor around in the direction the field is turning.
If the rotor ever caught up and matched the field exactly, the field would stop moving relative to the bars, no current would be induced and the pull would vanish. So the rotor always turns slightly slower than the field — a difference called slip — and that is what carries torque out along the shaft.
An induction motor is an AC motor with no electrical connection to its rotor. Alternating current in the stator windings produces a rotating magnetic field, which induces currents in the rotor and drags it around. It is the most common type of motor in industry because it is simple and robust.
By induction. The rotating field sweeps past the conducting cage bars in the rotor and induces a voltage and current in them. Those currents create the rotor's own magnetic field, and the pull between that field and the stator's field turns the rotor — all without any sliding contacts.
If the rotor caught up and turned at exactly the field's speed, the field would no longer be moving relative to the bars, so no current would be induced and no force would be produced. Some slip is essential, which is why the rotor always turns a little slower than the field.
A single AC winding would only produce a field that pulses back and forth. With three windings fed three currents that peak one after another, the combined field sweeps smoothly around the bore — a genuinely rotating field that can start the motor by itself.
The rotor winding. Instead of coils connected to the outside, the rotor has a set of conducting bars near its surface, joined at both ends by shorting rings. It looks like a hamster's exercise wheel, hence the name, and it needs no maintenance contacts.