A three-phase induction motor is a type of AC (alternating current) electric motor widely used in various industrial and commercial applications. It is the most common type of motor due to its simplicity, ruggedness, and reliability. The primary components of a three-phase induction motor include:
Stator: The stator is the stationary part of the motor and consists of a cylindrical core made of laminated steel sheets. It contains three sets of windings, spaced 120 degrees apart, to create a rotating magnetic field when supplied with three-phase AC power.
Rotor: The rotor is the rotating part of the motor and is also made of laminated steel sheets. It can be of two types: squirrel cage rotor or wound rotor. The squirrel cage rotor consists of copper or aluminum bars short-circuited at the ends, while the wound rotor has three-phase windings connected to slip rings for external connections.
Bearings: The motor has bearings that support the rotor shaft and allow it to rotate freely with minimal friction.
How does it work?
When three-phase AC power is supplied to the stator windings, it generates a rotating magnetic field. The rotating magnetic field induces currents in the rotor, according to Faraday's law of electromagnetic induction. The direction of the rotor current opposes the rotating magnetic field, causing a torque to be exerted on the rotor.
In the squirrel cage rotor, the induced currents flow through the bars and create their own magnetic fields. The interaction between the rotating magnetic field and the rotor's magnetic fields causes the rotor to start rotating in the same direction as the stator's rotating magnetic field. The rotor will never catch up with the rotating magnetic field; hence, it will keep rotating as long as the stator is supplied with the three-phase AC power.
In the case of a wound rotor, the slip rings allow external resistors to be connected to the rotor windings. By adjusting the resistance, the speed-torque characteristics of the motor can be modified for specific applications.
The speed of the induction motor depends on the frequency of the AC power supply and the number of poles in the motor design. The synchronous speed (Ns) of the motor is given by the formula:
Ns = (120 * f) / P
Ns = Synchronous speed (in RPM)
f = Frequency of AC power supply (in Hz)
P = Number of poles
The actual speed of the motor is slightly less than the synchronous speed due to a phenomenon called slip. Slip is the difference between the synchronous speed and the actual rotor speed. The slip is necessary for the rotor to generate the torque required to overcome the mechanical load.
Overall, the three-phase induction motor's simplicity, robustness, and ability to operate in various industrial environments have made it an essential workhorse in industries worldwide.