Can you explain how a three phase induction motor works and why it does not have a rotating magnetic field?

The working principle of three-phase induction motors (also known as asynchronous motors) depends on the electromagnetic force generated by the interaction of the rotating magnetic field generated by the stator windings with the induced current in the rotor. In fact, one of the key features of the three-phase induction motor is its ability to generate a rotating magnetic field, which is crucial for the start and operation of the motor. The following details the working principle of the three-phase induction motor and how it generates a rotating magnetic field.

The working principle of three-phase induction motor

• Stator winding: The stator is the stationary part of the motor that contains three sets of windings corresponding to each phase of the three-phase alternating current. The three sets of windings are spatially at 120° angles to each other. When three-phase alternating current is applied to each of the three windings, they generate a rotating magnetic field.

• Rotating magnetic field: Due to the phase difference of three-phase alternating current, the magnetic field generated by the stator winding presents a rotating effect in space. That is, when the current passes through the stator winding, the direction and position of the magnetic field constantly changes, forming a rotating magnetic field.The direction of this rotating magnetic field depends on the phase order of the current, that is, the A-B-C order or vice versa.

• Rotor: The rotor is the rotating part of the motor, usually composed of conductors (such as copper or aluminum bars) that form a closed loop in the rotor core. When the rotating magnetic field cuts through the rotor conductor, a current is induced in the rotor conductor (according to Faraday's law of electromagnetic induction).

• Electromagnetic force and torque: The induced current interacts with the rotating magnetic field to create a Lorentz force that drives the rotor to spin. Because the rotor speed is always lower than the synchronous speed, there is a slip rate (slip), which is the reason for the induction motor to produce continuous torque.

Why does a rotating magnetic field occur?

The rotating magnetic field is caused by the phase difference of the three-phase alternating current in the stator winding. To be specific:

• Phase difference: The phase difference between each phase of three-phase AC is 120°, which means that the peak and zero of the current are staggered in time.

• Spatial distribution: The stator windings are at a 120° Angle to each other in space, so that when the current passes through the windings, the magnetic field forms a rotating effect in space.

Why do you need a rotating magnetic field?

The importance of rotating magnetic field for three-phase induction motor is that:

• Starting capacity: The rotating magnetic field provides the starting torque that causes the stationary rotor to start rotating.

• Smooth operation: Once started, the rotating magnetic field continues to interact with the induced current in the rotor to produce continuous torque, making the motor run smoothly.

• Efficient transmission: The rotating magnetic field allows the motor to operate efficiently over a wide speed range while providing good speed control.

Sum up

The working principle of the three-phase induction motor is to generate torque through the interaction of the rotating magnetic field generated by the stator winding and the induced current in the rotor. The rotating magnetic field is caused by the phase difference and spatial distribution of three-phase alternating current in the stator windings. The rotating magnetic field is essential for the starting and continuous operation of the motor, as it provides the necessary starting torque and the continuous torque required for smooth operation. Therefore, three-phase induction motors do need and can generate a rotating magnetic field.