Poly-phase induction motors, often simply referred to as induction motors, are one of the most common types of electric motors used in various industrial and commercial applications. They are widely used due to their simplicity, robustness, and efficiency. The theory of operation of poly-phase induction motors is based on electromagnetic principles and the interaction of magnetic fields.
Here's a brief overview of the theory of operation of a poly-phase induction motor:
Rotating Magnetic Field: Induction motors rely on the concept of a rotating magnetic field. When a set of three-phase currents (usually three sinusoidal currents that are 120 degrees apart in phase) are applied to the stator windings of the motor, a rotating magnetic field is generated in the air gap between the stator and rotor.
Rotor Induced Currents: The rotor of an induction motor is typically made of conducting material (often aluminum or copper) and is not electrically connected to an external power source. As the rotating magnetic field sweeps across the rotor, it induces currents in the rotor bars due to the principle of electromagnetic induction.
Rotor Motion: The currents induced in the rotor interact with the magnetic field and create a torque that drives the rotor to start rotating. This is based on Lenz's law, which states that the induced currents will oppose the change in magnetic field that produced them. As a result, the rotor tries to "catch up" with the rotating magnetic field.
Slip: The difference between the speed of the rotating magnetic field (synchronous speed) and the actual speed of the rotor is called slip. Slip is necessary for the motor to produce torque. At startup, slip is high, allowing the motor to accelerate, and as the motor speeds up, slip reduces.
Torque Production: The torque produced in an induction motor is proportional to the product of the magnetic field strength, the rotor current, and the sine of the angle between them. This torque causes the rotor to accelerate and reach a speed where the slip is minimized.
Synchronous Speed: The synchronous speed of an induction motor is determined by the frequency of the applied power supply and the number of pole pairs in the motor. It is given by the formula: Synchronous Speed (in RPM) = (120 * Frequency) / Number of Pole Pairs.
Rotor Construction: There are two common types of rotor construction: squirrel-cage rotor and wound rotor. In a squirrel-cage rotor, the rotor consists of short-circuited bars or conductors that resemble a squirrel cage. In a wound rotor, the rotor windings are externally connected through slip rings and brushes, allowing for external resistance control and improved starting characteristics.
Poly-phase induction motors are designed to be self-starting and require no additional starting mechanisms. Their operation is based on the interaction between the rotating magnetic field and the induced currents in the rotor, which generates the necessary torque for motion. These motors are used in a wide range of applications, from pumps and fans to industrial machinery and household appliances.