A 3-phase induction motor uses current delivered in three phases in a sequence into the coils of a stator to create a rotating magnetic field. This induces an electric field in a coil or squirrel cage to drive a rotor. The difference in speed between
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Induction motors working principles
An induction motor uses an alternating current in a stator, or stationary winding, to induce a magnetic field in a metal cage or wire winding as the rotor. The interaction of the electric and magnetic fields drives the motor without any connections between the moving parts via a slip ring, making it highly reliable.
Motor speed control
The motor speed control depends on the speed of rotation of the magnetic field, which depends on the frequency of the AC current and number of poles. The rotating magnetic field from the AC current in the stator results in a flux induced in the rotor, and the interaction of the electric and magnetic fields creates the rotation.
As the induced current in the rotor lags the flux current in the stator, the rotor will never reach the full rotating magnetic field speed, also called the synchronous speed.
A single-phase, constant-frequency AC current is used to drive smaller loads in household appliances, with the frequency producing a fixed speed.
Variable Frequency Drives (VFDs) are used more in fans, pumps and compressors to enable the motor speed control. A three-phase induction motor provides a smoother motion with more control and accuracy of positioning for the motor, and there are designs of motors with five or more poles. These polyphase designs give higher accuracy for positioning the motor, allowing more precise delivery of fluids through pumps or positioning of blades of all kinds, but require more complex control systems. These motor control systems are also evolving from simple scalar systems to various types of field-oriented control (FOC) or vector control algorithms.
Main types of induction motors
There are two main types of induction motor construction, one with a wound coil rotor and the other with a squirrel cage rotor. This squirrel cage rotor is a cylinder of steel laminations, with aluminum or copper conductors embedded in its surface where the magnetic field is induced.
Both the wound coil and squirrel cage induction motors can be driven by a single- or three-phase AC current with a constant or variable frequency, giving a wide range of performance from small motors in the home to large industrial motors driving large pumps or compressors. This also leads to a wide range of control systems.
A single-phase induction motor needs a starter configuration to provide the starting torque while a 3-phase motor can be inherently self-starting as the different phases can be manipulated to start the rotor moving. Power factor control and direct torque control can help to boost the performance of a three-phase induction motor drive.