AC motor

AC motor
Name	AC motor
Caption	Cutaway view of a three-phase induction motor
Classification	Electric motor
Inventor	Nikola Tesla

AC motor. An AC motor is an electric motor driven by an alternating current. The fundamental operating principle relies on a rotating magnetic field generated by the stator to induce current in the rotor, producing torque. This technology, pioneered by inventors like Nikola Tesla and Mikhail Dolivo-Dobrovolsky, underpins the vast majority of industrial and domestic motor applications due to its robustness and efficiency.

Operating principle

The operation hinges on the generation of a rotating magnetic field. When polyphase AC, such as from a three-phase power supply, is applied to the stator windings, it creates a magnetic field that rotates at a speed determined by the utility frequency and the number of magnetic poles. In induction motors, the most common type, this rotating field induces a current in the conductive rotor, often a squirrel-cage rotor design; the interaction between the rotor's magnetic field and the stator's field produces electromagnetic force and rotation. This principle of electromagnetic induction was mathematically described by James Clerk Maxwell and practically demonstrated by Galileo Ferraris.

Types

The two primary categories are induction motors and synchronous motors. Induction motors, including single-phase induction motor and three-phase induction motor variants, operate at a speed slightly less than the synchronous speed, a difference called slip. Synchronous motors, such as those used in electric clocks and some industrial drives, rotate precisely at the synchronous speed of the stator's magnetic field. Other specialized types include the repulsion motor, the universal motor which can run on AC or direct current, and the brushless DC motor which is fundamentally an AC synchronous motor with electronic control.

Construction

The main stationary part, the stator, consists of a laminated electrical steel core with slots holding insulated windings, typically made of copper or aluminum. The rotor construction varies by type: induction motors commonly use a squirrel-cage rotor made of conductive bars short-circuited by end rings, while synchronous motors may use a wound rotor with field windings energized by direct current via slip rings or a permanent magnet assembly. The assembly is housed in a frame, often cast from iron or aluminum alloy, with bearing (mechanical) supporting the shaft. Key manufacturers in its development and production have included General Electric, Siemens AG, and Westinghouse Electric Corporation.

Speed control

Historically, AC motor speed was considered difficult to vary, but modern power electronics have revolutionized control. The most common method is variable-frequency drive, which varies the frequency and voltage of the power supplied to the motor using semiconductor devices like insulated-gate bipolar transistors. Other methods include changing the number of magnetic poles via pole-changing motor windings, adjusting slip with devices like the Kramer drive, or using wound rotor motors with external rotor resistance. For precise positioning, servomotors integrate the motor with encoders and controllers from companies like ABB Group and Yaskawa Electric Corporation.

Applications

AC motors are ubiquitous in industry and commerce. Three-phase induction motors drive pumps, fan (machine)s, compressors, and conveyor belt systems in facilities like refineries and manufacturing plants. They are the prime mover in machine tools, lathes, and rolling mills. In the domestic sphere, single-phase motors power refrigerators, air conditioners, washing machines, and furnace fans. Synchronous motors are found in applications requiring constant speed, such as recording devices like turntables, and in large installations like the Vatican Radio transmitter or the Jinan Waterworks.

Advantages and disadvantages

The primary advantages include simplicity, robustness, and low maintenance, especially for the squirrel-cage induction motor which has no brushes or mechanical commutator. They are generally less expensive and more reliable than comparable DC motors for most applications. High-power three-phase motors exhibit excellent efficiency and are directly compatible with the prevailing power grid infrastructure. Disadvantages include a more complex speed control requirement historically, though this is mitigated by modern drives. Single-phase motors typically have lower starting torque and require auxiliary starting windings or a capacitor to initiate rotation. Synchronous motors also require a separate DC excitation system or expensive permanent magnet materials.

Category:Electric motors