My friend G, who repairs motors, was tasked with repairing an imported motor . Since he didn't understand English, he asked Ms. Can for help. After understanding Ms. Can's explanation, G asked her why an asynchronous motor was also called a motor. Ms. Can's subsequent explanation surprised G; he hadn't expected it to involve the principle of asynchronous motor rotation.
Today, Ms. Can will focus on the principle of asynchronous machines, explaining the inseparable relationship between "asynchronous" and "inductive" characteristics in asynchronous machines, and exploring the working characteristics of three-phase asynchronous motors.
Three-phase composite magnetomotive force fundamental wave—rotating magnetomotive force
In the article "How does the fundamental wave of the three-phase composite magnetomotive force rotate?", Ms. [Name] demonstrated through mathematical derivation that the "rotating magnetic field" in the AC armature, i.e., when three-phase symmetrical windings with a spatial phase difference of 120 degrees flow through three-phase symmetrical currents with a temporal phase difference of 120 degrees, the fundamental wave of the three-phase composite magnetomotive force is a rotating magnetomotive force wave, and the rotational speed of the rotating magnetomotive force wave is [missing information].
n1=60f/p(revolutions/minute)……（1）
In the formula, f is the power supply frequency (Hz).
p—Polar log number
"Asynchronous" and "Inductive"
Many people who have worked with imported motors have this impression: the English name for a three-phase asynchronous motor is often a three-phase induction motor. The reason for this is that the induced current drives the asynchronous motor to rotate, and the "asynchronous" phenomenon, in which the rotor speed always lags behind the fundamental wave of the synthetic magnetomotive force generated by the AC armature current, is the premise for generating the "induced" current.
Three-phase induction motor principle and characteristics
Three-phase asynchronous or induction motors mainly consist of two major electrical components: a stator and a rotor. The stator has three-phase windings arranged symmetrically in space. There are two types of rotors: one with three-phase symmetrical windings like the stator but short-circuited during operation, and the other a squirrel-cage winding composed of conductor bars and short-circuit rings. As mentioned above, when a three-phase symmetrical current flows through the stator, a rotating magnetomotive force wave and a rotating magnetic field are generated, with the magnetic field rotating at the synchronous speed n1.
When the short-circuited rotor winding cuts the stator's rotating magnetic field, an induced current is generated. This induced current is in the stator's magnetic field and is subjected to electromagnetic force, causing the rotor to rotate in the direction of the stator's rotating magnetic field.
Similar to the stator, symmetrical rotor currents flow in symmetrical rotor windings, generating a rotating magnetic field. The relationship between the magnetic field's rotational speed n2 relative to the rotor, the induced current frequency f2, and the number of pole pairs p is exactly the same as the relationship between n1, f, and p in the stator, i.e.
n2=60f2/p(revolutions/minute)……（2）
Let the rotor speed be n. Then the speed at which the rotor winding cuts the stator rotating magnetic field is n1-n. Since the number of pole pairs in the rotor winding is the same as that in the stator, the induced current changes p times per revolution, and (n1-n) times per (n1-n) revolutions. Therefore, the frequency of the induced current is f2 = (n1-n)p/60. Substituting f2 into equation (2), we get the rotational speed of the rotor rotating magnetic field relative to the rotor as:
n2=60f2/p=60·(n1-n)p/60·/p=n1-n(turns/minute)……（3）
The rotational speed of the rotor's rotating magnetic field relative to the stator is
n2+n=n1-n+n=n1(revolutions/minute)……（4）
Equation (4) shows that the stator and rotor magnetic fields have the same rotational speed (both are n1) and are relatively stationary. This relationship remains unchanged regardless of the motor speed n.
Typically, 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 using the following formula.
n = 60f/p (revolutions/minute)……(5)
As can be seen from equation (5), there are two methods to control the speed of a three-phase asynchronous motor: 1) changing the magnetic poles; 2) frequency conversion. In the past, the first method, namely pole changing speed regulation, was more commonly used. Nowadays, with the rapid development of power electronics technology, high-power frequency conversion technology is widely used, and stepless speed regulation by frequency conversion has become the inevitable choice for speed regulation applications.
