Question

(a) Explain the working principle of a three-phase induction motor. What are the various types of rotors? (6)

(b) An 18.65kW, 6-pole, 50Hz, 3 phase slip ring induction motor runs at 960 rpm on full load with a rotor current per phase of 35A, allowing 1kW for mechanical losses, find the resistance per phase of 3-phase rotor winding. (10)

Answer 1

Part (a)

A three-phase induction motor operates on the principle of electromagnetic induction. A rotating magnetic field is created in the stator (stationary part) by supplying three-phase AC power to its windings. This rotating field induces currents in the rotor (rotating part), which in turn creates its own magnetic field. The interaction between the stator's rotating magnetic field and the rotor's magnetic field produces a torque that causes the rotor to rotate. The rotor speed is slightly less than the speed of the rotating magnetic field, a difference known as slip. The slip is necessary to induce the currents in the rotor that produce the torque.

Types of Rotors in Three-Phase Induction Motors:

Squirrel-Cage Rotor:

Consists of laminated steel sheets with parallel slots carrying heavy copper or aluminum bars. The ends of these bars are short-circuited by end rings, forming a closed loop resembling a squirrel cage.
The simplicity and ruggedness of the squirrel-cage rotor make it the most commonly used type in induction motors.

Wound Rotor (Slip-Ring Rotor):

Features a laminated iron core with three-phase windings placed in the slots. These windings are connected to external slip rings via brushes, allowing for external connections.
The wound rotor design allows for external resistances to be added to the rotor circuit, enabling control over the motor's starting torque and speed. This feature is particularly useful in applications requiring precise speed control and higher starting torque.

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