How an Induction Motor Works: Stator, Rotor and Star-Delta Configurations Explained

This video provides a detailed explanation of how a three-phase induction motor operates. It covers the anatomy of the motor including the stator windings, the squirrel cage rotor, and the housing with bearings and cooling fan. It explains how a rotating electromagnetic field is generated by three-phase currents, why the rotor cannot catch up to the rotating field, and how a second set of windings and phase shift drive the rotor to rotate. The clip also compares delta and star configurations, showing how line and coil voltages and currents differ in each arrangement, with numerical examples to illustrate the differences. The content is a thorough guide to motor operation and practical wiring configurations for three-phase motors.

Introduction to Induction Motors

The video opens with an introduction to induction motors as essential devices that convert electrical energy into mechanical work. It outlines common applications such as pumps, fans, and cranes, and notes why cooling is necessary due to heat generated during operation.

Motor Anatomy and Cooling

The main housing contains the shaft, front and rear shields, bearings, and a cooling fan connected to the shaft. The fins on the enclosure increase surface area for heat dissipation, preventing insulation damage that could lead to shorts and motor failure.

Stator Windings and the Rotating Field

Inside the housing lies the stator, composed of copper windings wrapped into enamel-insulated coils arranged around the inner perimeter. In a 3-phase motor, three separate winding sets form a rotating electromagnetic field when fed by three-phase power. The rotor, a squirrel cage type, sits inside this field and begins to rotate as the magnetic field interacts with the rotor bars via induced currents.

Squirrel Cage Rotor and Lamination

The rotor uses laminated steel sheets to concentrate the magnetic field and minimize eddy currents that would waste energy. Shorted end rings connect the rotor bars, creating multiple loops that induce currents as the rotating stator field sweeps past.

How a Rotating Magnetic Field Causes Rotation

Electric current in a wire creates a magnetic field. When wrapped into coils, the coil behaves like an electromagnet with distinct north and south poles. Alternating current causes the field to collapse and reverse, creating a dynamic interaction with a nearby closed loop rotor that experiences opposing forces, resulting in rotation. The rotor cannot perfectly keep up with the rotating field, a phenomenon called slip, which is essential for torque to be produced.

Three-Phase Operation and Phase Shifting

Three coils connected to separate phases produce a magnetic field that rotates. The phases are spaced 120 degrees apart so that the overall field appears to rotate, pulling the rotor along. When multiple windings are present, the electromotive forces in the coils combine to form the rotating field, while the rotor responds to the changing magnetic field with induced currents that generate torque.

Delta vs Star Connections

The video explains two ways to connect the six terminals in the electrical box: delta and star (Y). In delta, windings connect across pairs of phases, exposing each coil to full line-to-line voltage (for example 400 V). In star, windings connect to a common neutral point, resulting in coil voltage of line-to-neutral (for 400 V line-to-line, coil voltage is 230 V). The currents also differ: delta yields higher line currents due to two coils per phase, while star uses lower coil voltages and currents. The video provides a numerical example to illustrate how a 400 V supply and 20 ohm coil impedance yield 20 A per coil in delta, but 11.5 A per coil in star, with corresponding line currents calculated using the square root of 3 when appropriate.

Wiring Configurations and Terminal Box

The terminal box presents U1, V1, W1, and U2, V2, W2 terminals. In delta, connections are U1 to W2, V1 to U2, and W1 to V2; in star, coil ends are tied together at a common point (the neutral), with only one end of each coil connected to a phase through the supply.

Takeaway

Understanding the rotor-stator interaction, the role of slip, laminated rotor construction, and the delta and star configurations is essential to appreciating how induction motors deliver reliable torque in a wide range of industrial and consumer applications.