The invention of asynchronous motors has revolutionized human civilization. Today, let's explore the inner workings of asynchronous motors. Asynchronous motors are primarily composed of two components: the stator and the rotor. The stator is a coil with three windings that is powered by three-phase AC electricity.
The winding passes through the stator slots, which are made up of stacked thin steel sheets with high permeability. When three-phase current flows through this winding, it creates a rotating magnetic field, which is the cause of the rotor's rotation. To comprehend how the rotating magnetic field is generated and its characteristics, the stator can be simplified.
Three coils are connected at intervals of 120 degrees, creating a magnetic field around them when a current is passed through. When a three-phase power supply is applied to this special arrangement, the generated magnetic field changes direction with the alternating current at specific moments. By comparing these three examples, we can observe a rotating magnetic field with a uniform intensity. The speed at which the magnetic field rotates is known as the synchronous speed. Let's consider a closed conductor placed inside this rotating magnetic field.
According to Faraday's law, a changing magnetic field induces an electromotive force in a circuit, which in turn generates an electric current. This phenomenon is similar to a current-carrying loop in a magnetic field, which creates an electromagnetic force on the loop and causes it to start rotating. The same phenomenon occurs in an asynchronous motor, where instead of a simple loop, something resembling a squirrel cage is used. A rotating magnetic field is created by the three-phase alternating current passing through the stator.
In the previous example, the current will be induced in the squirrel cage bars of the short-circuited end ring. Consequently, the rotor will begin to rotate. This is the reason why this type of motor is referred to as an induction motor.
Instead of directly connecting to the rotor to generate electricity, electromagnetic induction is used. Insulation iron core sheets are filled inside the rotor to achieve this. By using these small-sized iron sheets, the induction motor minimizes eddy current losses and offers significant advantages. It is essentially self-starting, as both the magnetic field and the rotor are rotating. However, what is the speed at which the rotor is rotating?
To obtain the answer to this question, we need to consider different scenarios. Let's consider the case where the speed of the rotor is the same as the speed of the magnetic field. Since both are rotating at the same speed, the magnetic field will never cut off the loop. Therefore, no induced electromotive force or current will be generated. This will result in power being converted on the rotor bars. The rotor will gradually slow down, and as the speed decreases, the magnetic field will cut off the rotor circuit. As a result, the induced current and force will rise again. The rotor will then accelerate. In short, the rotor will never be able to catch up with the speed of the magnetic field.