Analysis of the Rotation Principle of Three phase Asynchronous Machine in Hengli Motor

Today, Hengli Motor focuses on the principle of asynchronous motors, explaining the two interdependent situations of "asynchronous" and "induction" in asynchronous motors, and exploring the working characteristics of three-phase asynchronous motors.

Fundamental wave of three-phase synthesized magnetic potential - rotating magnetic potential

In the article "How does the fundamental wave of three-phase synthesized magnetic potential rotate?", the constant force motor mathematically demonstrates the "rotating magnetic field" in the AC armature, that is, when a three-phase symmetrical winding with a spatial difference of 120 degrees in electrical angle flows through a three-phase symmetrical current with a temporal difference of 120 degrees in phase, the fundamental wave of three-phase synthesized magnetic potential is a rotating magnetic potential wave, and the rotational speed of the rotating magnetic potential wave is

N1=60f/p (RPM)... (1)

In the formula, f represents the power frequency in Hz

P - polar logarithm

Asynchronous and Inductive

Many people who have been exposed to imported motors have the impression that the English name for three-phase asynchronous motors is often three-phase induction motors. The reason for this is that the induced current drives the asynchronous machine to rotate, and the asynchronous phenomenon of the rotor speed always lagging behind the fundamental wave of the synthesized magnetic potential generated by the AC armature current is the prerequisite for generating the induced current.

Principle and characteristics of three-phase induction motor

Three phase asynchronous or induction motors are mainly composed of two electrical components: stator and rotor. Three phase windings with spatially symmetrical distribution are embedded on the stator; There are two types of rotors: one with three-phase symmetrical windings embedded like the stator but short circuited during operation, and the other consisting of squirrel cage windings with conductive bars and short-circuit rings.

As mentioned above, when a three-phase symmetrical current passes through the stator, a rotating magnetic potential wave and a stator rotating magnetic field will be generated, with the magnetic field rotating at a synchronous speed n1.

The short-circuit rotor winding cuts the stator rotating magnetic field, inducing current. The induced current is in the stator magnetic field and is driven by electromagnetic force to rotate the rotor, turning in the direction of the stator rotating magnetic field. Similar to the stator, a symmetrical rotor current flows through the symmetrical rotor winding to generate a rotating magnetic field. The relationship between the magnetic field and the rotor speed n2, induced current frequency f2, and pole pairs p is exactly the same as the relationship between n1, f, and p in the stator

N2=60f2/p (RPM)... (2)

Assuming the rotor speed is n, the speed at which the rotor winding cuts the rotating magnetic field of the stator is n1-n, and the number of pole pairs of the rotor winding is the same as that of the stator. Therefore, the induced current changes p times per revolution and p times per (n1-n) revolution. Therefore, the frequency of the induced current is f2=(n1-n) p/60. Substituting f2 into equation (2), the rotational magnetic field of the rotor relative to its rotational speed is obtained

N2=60f2/p=60 · (n1-n) p/60 ·/p=n1-n (revolutions per minute)... (3)

The rotational speed of the rotor's rotating magnetic field relative to the stator is

N2+n=n1-n+n=n1 (RPM)... (4)

Equation (4) indicates that the magnetic field speed of the stator and rotor is equal (both are n1), relatively stationary, and this relationship remains unchanged regardless of the motor speed n.

Usually, the difference (n1-n) between the rotor speed n and the synchronous speed n1 of a three-phase asynchronous motor is very small. When s=(n1-n)/n1, s generally fluctuates within the range of 0.01 to 0.05. Therefore, the speed of the three-phase asynchronous motor is almost equal to the synchronous speed, and the speed regulation characteristics can be analyzed according to the following formula

N=60f/p (RPM)... (5)

From equation (5), it can be seen that there are two methods to control the speed of a three-phase asynchronous motor: 1) changing the magnetic pole method; 2) Frequency conversion method. In the past, the first method of variable pole speed regulation was commonly used. Nowadays, with the rapid development of power electronics technology, high-power variable frequency technology is widely applied, and variable frequency continuously variable transmission has become an inevitable choice for speed regulation applications.