How an induction motor turns

Reference notes

Use Next → on the narrator above to step through six configurations: from a static rotor and stator, to a fully running induction motor with steady-state slip.

The six-line elevator pitch

1. Three-phase currents in the stator produce a rotating magnetic field at synchronous speed n_s (see the synchronous chapter's first lesson).
2. The rotor is a short-circuited cage of bars — there's no external connection to it.
3. The stator's rotating field sweeps past the stationary rotor bars, cutting them and inducing an EMF in each bar.
4. Because the bars are short-circuited at the ends, currents flow in them — the magnitude of those currents depends on the relative speed between the field and the bars.
5. Each bar carries a current in the stator's flux, so it experiences a force F = i·L × B. The forces all act tangentially on the rotor, in the same sense as the field's rotation.
6. The result is a net torque that accelerates the rotor in the direction of the rotating field. By Lenz's law, the rotor's motion reduces the relative speed → reduces the induced EMFs → reduces the current → reduces the torque — until the system settles at a steady-state slip.

Why the rotor can never quite catch the field

If the rotor ever reached synchronous speed, the field would no longer sweep past the bars at all — no flux cutting, no induced EMF, no current, no torque. The rotor would coast down due to friction, fall behind the field, and re-accelerate. So in steady state the rotor settles at a speed n slightly below n_s, with a small slip:

Typical full-load slip is 2–6 %. At standstill, s = 1; at no-load, s ≈ 0 (just enough to overcome friction).

Squirrel-cage vs slip-ring rotors

• Squirrel-cage rotor: solid copper or aluminium bars short-circuited by end rings. Simple, rugged, cheap, maintenance-free. The default for industrial drives — pumps, fans, compressors, conveyors.
• Slip-ring (wound) rotor: rotor has a real 3-phase winding brought out to slip rings. External resistance can be inserted at the slip rings to control starting torque and provide limited speed control. Used in cranes, hoists, mills, and where high starting torque is needed without a VFD.

Why "asynchronous"

Synchronous machines lock to n_s exactly. Induction machines can't — they need slip to function. Hence the name. The slip is what generates the rotor EMF that drives the rotor current that produces the torque. No slip, no party.

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