Q1Electrical Machines II

A squirrel cage induction motor features a stationary stator and a rotating rotor. The stator contains a standard 3-phase distributed winding. The rotor consists of a laminated cylindrical iron core with slots. Thick, uninsulated aluminum or copper bars are placed in these slots and are permanently short-circuited at both ends by heavy conducting end-rings, forming a structure that looks like a hamster or squirrel cage. There are no slip rings or external connections.

When a 3-phase supply is applied to the stator, a Rotating Magnetic Field (RMF) is established, rotating at synchronous speed. This RMF sweeps across the stationary short-circuited rotor bars, inducing an electromotive force (Faraday's Law). Because the bars form a closed loop, heavy currents flow through them. These currents generate a rotor magnetic field. The interaction between the stator RMF and the rotor magnetic field produces a torque (Lorentz force) that drags the rotor in the same direction as the RMF.

The torque-slip curve plots torque against slip ($s$). - At $s=1$ (standstill), the motor provides starting torque. - As the rotor accelerates, slip decreases and torque rises to a peak maximum value known as the Pull-out torque or Breakdown torque. - After the peak, in the normal operating region near synchronous speed ($s\approx 0$), the curve is almost linear; torque is directly proportional to slip. The motor never reaches synchronous speed ($s=0$) because relative motion is required to induce rotor current.