Working Principle of Three Phase Induction Motor

How does a three-phase induction motor work? In short, it works based on the principle of electromagnetic induction. When the stator windings are supplied with three-phase alternating current, a rotating magnetic field is generated between the stator and the rotor. The rotating magnetic field cuts the rotor windings to generate induced electromotive force and current in the rotor circuit. The current in the rotor conductor forces the rotor to rotate under the effect of the rotating magnetic field. Below, we will specifically analyze the generation of the rotating magnetic field, its direction and speed, as well as the slip.

How does it generate a rotating magnetic field?

For three-phase induction motors, U / V / W windings with the same three-phase structure are placed in the stator core. Each phase of the winding differs spatially from an electrical angle of 120 degrees, as shown below, and the three-phase windings are provided with symmetrical three-phase AC, as shown in Figures (b) and (c) below. Here, take a 2-pole induction motor as an example to illustrate the location of the magnetic field in space like the current at different times.

three-phase winding in the star connection of the induction motor As shown in Figure (b) above, it is assumed that when the instantaneous current value is positive, it flows from the first ends of each winding and flows through the ends of the tail. On the contrary, it is when the current is a negative value.

As Figure (c) shows, when ωt = 0, iu = 0, the value of iv is negative and iw is positive. Then, the current of phase V flows from V2 and leaves V1, while the current of phase W flows from W1 and W2. According to the Ampere right-handed rule, the direction of the composite magnetic field produced by the three-phase current can be confirmed at the time ωt = 0, as shown in figure (d) ① below. It can be seen that the composite magnetic field is a pair of poles and the direction of the magnetic field is consistent with the direction of the longitudinal axis, that is, the top is the North Pole and the bottom is the South Pole.

symmetric three-phase current waveform diagrams ωt = π / 2, after a quarter cycle, the value of u changes from zero to maximum, and current flows from the first end U1 and exits the end U2. The iv value is still negative; therefore, the current direction of phase V is the same as shown in figure ①. iw also becomes negative and therefore the current of phase W is input and output W2. The direction of the compound magnetic field is shown in figure (d) ② that the direction of the magnetic field rotates clockwise by 90 ° compared to when ωt = 0.

Using the same analytical method, magnetic fields can be plotted when ωt = π, ωt = 2/3 * π and ωt = 2, as shown in (d) ③ ④ ⑤ respectively. Obviously, it is seen by the figure that the direction of the magnetic field gradually rotates in a clockwise direction, fully 360 °, that is, a rotation cycle.

2-pole winding rotating magnetic field diagram

It can be concluded as follows: the three-phase windings are placed in the stators of the three-phase motor in the same structure, but in the spatial position with an electrical angle difference of 120 degrees between them. As they are supplied separately with the three-phase AC, the composite magnetic field generated between the stator and the rotor rotates along the stator's inner circle, called the rotating magnetic field.

The direction of the rotating magnetic field

It is shown in the figure above that three-phase AC changes in the sequence of U-V-W phases, so the generated rotating magnetic field rotates clockwise in space. If the current phase sequence of the two-phase motor windings, such as U-W-V, is arbitrarily switched, it is practically proven that the rotating magnetic field generated must rotate counterclockwise. In conclusion, the direction of the rotating magnetic field depends on the phase sequence of the three-phase AC power supply in the winding. As long as the motor phase sequence is switched arbitrarily, the direction of the rotating magnetic field can be changed.

Magnetic field rotation speed and sliding

The example above is based on the 2-pole motor for illustration. If you want to obtain a 4-pole magnetic field, the number of coils will be doubled, as shown in figures (a) and (b) below. According to the analytical method above, the 4-pole rotating magnetic field diagram in space is shown in figure (c). Comparing the speed of rotation of the magnetic field in figure (c) with that of figure (d), as mentioned above, it is not difficult to discover that the speed of the magnetic field is not only related to the frequency of the power but also the number of poles.

Rotating magnetic field of 4-pole induction motor

Therefore, the speed of the rotating magnetic field is calculated by n1 = 120f1 / P, where it is:

n1 - the speed of the rotating magnetic field in rev / min

f1 - the frequency of the three-phase AC supply in Hertz

P - the number of poles

The speed of rotation of the magnetic field (n1) is also known as synchronous speed. The rotor speed of the three-phase induction motor (n) will not be accelerated to the speed of the rotating magnetic field (n1). Only in this way will there be a relative movement between the magnetic field of the winding and the rotating one to cut the magnetic lines. Thus, the induced electromotive force and current can be generated in the winding conductor of the rotor, producing the electromagnetic torque to make the rotor rotate continuously along with the direction of the rotating magnetic field. It can be seen that n ≠ n1 and n <n1 are a necessary condition for the operation of the induction motor, from which the name “asynchronous motor” derives. The difference between them is called "slip", which is expressed by the ratio between the difference and the synchronous speed: s = (n1-n) / n1.