Asynchronous Motor

Induction motors are alternating current motors widely used in industry, consisting of two main parts—the stator and the rotor—and operating at variable speeds with low cost and minimal maintenance requirements. The most commonly used motor type today, Induction Motors, are manufactured in single-phase and three-phase configurations. Due to their durability, low cost, and high efficiency, induction motors are extensively employed in industrial applications such as conveying systems, fans, pumps, reducers, and compressors. Because induction motors are used across a broad spectrum of industrial applications, they account for a significant portion of global energy consumption. Consequently, motor efficiency plays a critical role in global energy conservation. The efficiency of induction motors has been significantly improved across a wide range of areas through advances in design, maintenance, repair, and control methods. Generally, the efficiency of induction motors varies depending on the application and operating conditions. Therefore, proper sizing and operation of motors are crucial to achieving maximum efficiency.

The most important feature distinguishing induction motors from synchronous motors is their non-constant rotational speed. In terms of operating principle, induction machines are also referred to as induction machines. Their low cost and absence of brushes and commutators result in fewer faults and less maintenance, making them widely used in industry. The production of induction motors is cheaper than that of direct current machines, but their control is more complex. An induction motor consists of two main parts: the stator and the rotor. The stationary part is called the stator, while the rotating part is called the rotor. The stator’s function is to generate a magnetic field, while the rotor’s function is to produce the force that drives motion. The rotor is supported by end caps fitted on both sides of the stator, which contain bearings, and rotates smoothly with an appropriate air gap.

Three-Phase Induction Motor - (Gamak)

Stator

The stator is the stationary part of the induction motor. It consists of windings and a laminated core. The laminated core is formed by stacking special insulated sheets with a thickness of 0.4–0.8 mm. The stator windings are created by winding copper wire around the core for a number of turns determined by the motor’s design. The winding configuration determines the number of poles in the motor.

Rotor

The rotor is the rotating part of the induction motor. Induction motors are classified into two types based on rotor construction: squirrel-cage induction motors and wound-rotor induction motors. The rotor of a squirrel-cage induction motor consists of a series of conductive bars placed in slots on the rotor surface and short-circuited at both ends by large end rings. This design is known as a squirrel-cage. The other rotor type is the wound-rotor induction motor. A wound rotor has a complete three-phase winding set similar to the stator windings. The three phases of the rotor winding are typically connected in a Y (star) configuration, and the ends of the three rotor wires are connected to slip rings mounted on the rotor shaft. The rotor windings are short-circuited via brushes that slide on the slip rings. This allows additional resistance to be added to the rotor circuit, enabling control of the rotor current. This feature can also be used to modify the motor’s torque-speed characteristic.

Wound-rotor induction motors are more expensive than squirrel-cage motors. Additionally, due to wear on the brushes and slip rings associated with the wound-rotor structure, they require significantly more maintenance. As a result, wound-rotor motors are less commonly preferred in industry compared to squirrel-cage motors.

Operating Principle of Induction Motors

When an AC (alternating current) voltage is applied to the stator of an induction motor, alternating current flows through the stator windings. This alternating current generates a rotating magnetic field in the magnetic circuit. This rotating field can be expressed using a Fourier series. Each harmonic of the alternating field can be separated into two rotating fields rotating at the same angular velocity. If one of these fields rotates clockwise, the other rotates counterclockwise. In three-phase induction motors, currents with a 120° phase difference between them produce three alternating fields in the stator. If only the first harmonic of these three alternating fields is considered, six rotating fields are generated. Of these six, three rotate clockwise and three rotate counterclockwise, all at the same synchronous angular speed (ωs). When the three clockwise rotating fields overlap, the three counterclockwise fields, which have a 120° phase difference among them, cancel each other out, resulting in a net zero value. Therefore, the motor rotates in the direction of the resultant torque produced by the overlapping three clockwise rotating fields. At the instant voltage is applied to the motor, the rotor is stationary. The rotating stator field cuts the stationary rotor conductors at synchronous speed (ns), inducing an alternating voltage in the rotor. If the rotor were to rotate at synchronous speed, the stator field would no longer cut the rotor conductors, and no voltage would be induced in them. Consequently, no current would flow in the rotor, and torque would be zero. Therefore, induction motors cannot operate at synchronous speed. During operation, the rotor rotates at a speed slightly lower than the synchronous speed, and this speed varies with load. This speed difference is called slip. Slip is proportional to the difference between the rotor speed and the speed of the stator’s magnetic field and is typically expressed as a percentage. As the load on the induction motor increases, the slip also increases. With higher load, the rotor requires more energy to maintain alignment with the magnetic field, causing the rotor speed to decrease. As a result, the slip ratio increases.