A polyphase induction motor is a type of electric motor that operates on the principles of electromagnetic induction. It's one of the most common types of motors used in various industrial and commercial applications due to its simplicity, robustness, and efficiency. Polyphase induction motors are often referred to as simply "induction motors" or "asynchronous motors."
The term "polyphase" refers to the motor's operation with multiple phases of alternating current (AC) power. Polyphase systems, such as three-phase systems, are commonly used in electric power distribution because they provide efficient power transmission and allow for the development of rotating magnetic fields necessary for induction motor operation.
Here's an introduction to the key concepts of polyphase induction motors:
Rotating Magnetic Field: In a polyphase induction motor, the stator (the stationary part) contains multiple sets of windings, each connected to a different phase of AC power. When AC voltages of different phases are applied to these windings, they produce alternating magnetic fields that are displaced in space. The combination of these fields results in a rotating magnetic field that sweeps around the inside of the motor's enclosure. This rotating magnetic field is what drives the rotor (the moving part) to turn.
Rotor Construction: The rotor in an induction motor is typically made of laminated iron cores and conductive bars or loops. There are two main types of rotors: squirrel-cage rotors and wound rotor (slip ring) rotors. The squirrel-cage rotor consists of conductive bars embedded in the rotor's core, forming a cage-like structure. The wound rotor has coils of wire connected to slip rings, which allow external resistance or other devices to be connected for control purposes.
Induction Principle: The rotor of an induction motor is not directly connected to any external power source. Instead, it relies on electromagnetic induction. As the rotating magnetic field generated by the stator sweeps across the rotor, it induces voltage in the rotor conductors. According to Faraday's law of electromagnetic induction, this induced voltage causes current to flow in the rotor conductors. The interaction between the induced current and the magnetic field produces a torque, which causes the rotor to start rotating.
Speed: The speed of the rotor is not synchronous with the frequency of the applied AC power. It's slightly less than the synchronous speed, which is determined by the frequency of the power supply and the number of poles in the motor. The difference between the synchronous speed and the actual rotor speed is called the slip. The slip is necessary for torque generation and motor operation.
Polyphase induction motors are widely used in various applications, such as industrial machinery, pumps, fans, compressors, conveyor systems, and more. They are favored for their ruggedness, reliability, and relatively low maintenance requirements. Their efficiency, coupled with the simplicity of their design, makes them a fundamental component of modern industrial processes and technology.