5.4.7 Induction Motors

In previous lessons, we examined direct current (DC) and synchronous alternating current (AC) motors, where electric current is supplied directly to the rotating armature through mechanical commutators or slip rings.

An AC induction motor operates on a fundamentally different principle: there is no physical electrical connection to the rotor. Instead, the rotor receives its electrical energy entirely through electromagnetic induction, making it highly reliable, efficient, and virtually maintenance-free.

Core Principles & Operation

The operation of an induction motor relies on the interaction between a stationary stator and a free-spinning rotor (typically a "squirrel cage" conductor assembly):

• Rotating Magnetic Field (Stator): Multi-phase AC current supplied to the stationary electromagnets (stator) produces a magnetic field that rotates smoothly at a constant synchronous speed (n_s).
• Induced Current (Rotor): As the stator's magnetic field sweeps past the stationary rotor bars, it creates a changing magnetic flux (^ΔΦ⁄_Δt). By Faraday's Law, this induces an electromotive force (EMF) and strong circulating currents within the rotor bars.
• Lorentz Force & Torque: By Lenz's Law, the induced current in the rotor bars creates its own magnetic field that reacts with the stator's rotating field. This produces a mechanical torque (F = BIl) that drags the rotor in the same direction as the rotating magnetic field.

The Concept of "Slip"

Crucially, an induction motor can never rotate at the exact same speed as the stator's magnetic field (synchronous speed). If the rotor ever caught up with the field, there would be zero relative motion (ΔΦ⁄Δt = 0), zero induced EMF, zero rotor current, and zero torque.

The difference between the synchronous field speed (n_s) and the actual rotor speed (n_r) is known as slip:

Watch the videos below to explore how multi-phase AC supplies create a rotating magnetic field and see how squirrel-cage induction motors are engineered.