Asynchronous Motor Introduction There are many types of motors, and many ways to classify them. For example, according to the mode of motion, stationary motors include transformer motors, while moving motors include linear motors and rotary motors. Linear and rotary motors can be further classified according to the power supply type, into DC motors and AC motors. AC motors, based on the relationship between their operating speed and the power supply frequency, can be divided into two main categories: asynchronous motors and synchronous motors. The classification can be further subdivided. Given that linear motors are less commonly used, the above classification results can be summarized as follows:
An asynchronous motor is an AC motor, also called an induction motor, primarily used as an electric motor. Asynchronous motors are widely used in industrial and agricultural production, with capacities ranging from several kilowatts to several kilowatts. Increasingly common household appliances, such as washing machines, fans, refrigerators, and air conditioners, use single-phase asynchronous motors with capacities ranging from a few watts to several kilowatts. In high-tech fields such as aerospace and computers, control motors are widely used. Asynchronous motors can also be used as generators; for example, they are used in small hydroelectric power stations and wind turbines.
Asynchronous motors come in two stator phase types: single-phase and three-phase. Three-phase asynchronous motors have two rotor structures: squirrel-cage and wound-rotor, while single-phase asynchronous motors always have squirrel-cage rotors. An asynchronous motor mainly consists of a stationary stator and a rotating rotor, with an air gap between them. End caps at both ends of the stator support the rotor. The diagram below shows the structure of a three-phase squirrel-cage asynchronous motor.
The stator of an asynchronous motor consists of three parts: the stator core, the stator windings, and the frame. The stator core serves as part of the motor's magnetic circuit and houses the stator windings. To reduce losses caused by the alternating magnetic field in the core, it is typically made of 0.5mm thick low-silicon steel laminations (stamped laminations) with good magnetic permeability and low specific losses, as shown in the figure. To house the stator windings, several slots of the same shape are uniformly punched into the stator laminations. There are three slot shapes: semi-closed slots, semi-open slots, and open slots. Semi-closed slots are suitable for small asynchronous motors, where the windings are made of round wire. Semi-open slots are suitable for low-voltage medium-sized asynchronous motors, where the windings are formed coils. Open slots are suitable for high-voltage large and medium-sized asynchronous motors, where the windings are formed coils wrapped with insulating tape and impregnated with varnish.
The rotor of an asynchronous motor consists of a rotor core, rotor windings, and a shaft. The rotor core is part of the motor's magnetic circuit and is typically made of 0.5mm silicon steel sheets stamped and stacked. The shaft supports the rotor core and outputs mechanical torque, while the rotor windings induce electromotive force, carry current, and generate electromagnetic torque. There are two structural types: squirrel-cage and wound-rotor.
Squirrel cage winding: A conductor is placed in each slot evenly distributed within the rotor core. Two end rings are placed at both ends of the core, connecting the portions of all conductors extending beyond the slots to the end rings. If the core is removed, the remaining winding resembles a squirrel cage. This type of squirrel cage winding can be made by welding copper bars, as shown in the diagram, or by casting aluminum.
Cage rotor winding
Squirrel-cage cast aluminum rotor with wound winding: The wound winding is a symmetrical three-phase winding similar to the stator winding. It is generally connected in a star configuration. The three output terminals are connected to three slip rings on the shaft, and the current is drawn out through brushes. A characteristic of the wound rotor is that additional resistance can be connected in the rotor circuit through the slip rings and brushes to improve the starting performance of the motor and regulate its speed. Its wiring diagram is shown below.
Vibration and noise of asynchronous motors: Asynchronous motors are widely used mainly because they have the following advantages: simple structure, reliable operation, easy manufacturing, low price, robust and durable, as well as high efficiency and quite good working characteristics.
The main disadvantages of asynchronous motors are: currently, they cannot be smoothly speed-regulated over a wide range economically, and they must absorb lagging reactive power from the power grid. In the motor, the main magnetic flux enters the air gap roughly radially, generating radial forces in the stator and rotor, thus causing electromagnetic vibration and noise. Simultaneously, tangential torque and axial force are generated, causing tangential and axial vibrations. To calculate the electromagnetic noise of the motor and analyze and control this noise, it is necessary to know the sources of this noise and vibration, that is, the force waves that generate the vibration and noise. Currently, CAE simulation analysis methods can be used to calculate the magnetic field of the motor.
Motor vibration is currently the most concerning issue in motor structural design, and it consists of three parts: electromagnetic vibration, mechanical vibration, and gas vibration.
Electromagnetic vibration is caused by the interaction of magnetic fields in the air gap of the motor, which generates electromagnetic forces on the rotor and stator that vary with time and space, causing the motor to vibrate.
Mechanical vibration is vibration caused by rotor imbalance, bearings, or other mechanical structures or devices.
Gas vibration: Vibration caused by airflow in the motor's ventilation components or by aerodynamic forces.
Electromagnetic vibration is a major vibration source for many large and medium-sized motors. Since motor electromagnetic vibration is the result of the interaction between the motor's electromagnetic field and its structure, using magneto-solid coupling vibration theory is the most effective method for understanding the generation mechanism and resolving motor electromagnetic vibration issues. Because electromagnetic force is the excitation source of motor electromagnetic vibration, the accuracy of its calculation determines the accuracy of the motor electromagnetic vibration calculation; therefore, current research on motor electromagnetic vibration mostly employs numerical analysis methods to calculate the electromagnetic force of the motor.
ANSYS electromagnetic-thermal-structural vibration-noise coupling analysis for motors is applied in the calculation and analysis of motor structural vibration and noise. It mainly includes the following parts: dynamic analysis: including modal analysis, harmonic response analysis, rotor vibration analysis, rotor, stator, and frame coupled vibration analysis, stator and base vibration analysis, resonance and critical speed analysis, and transient response characteristics.
Noise analysis: vibration noise caused by motor vibration, aerodynamic noise caused by motor fan, etc.
Multiphysics coupling analysis: electromagnetic, thermal, fluid, and structural interactions of motors.
Applying finite element analysis software to motor structure design makes the calculation results for motor mechanics more accurate and intuitive. For complex motor structures and variable load conditions, the calculation results are much more accurate than traditional calculation methods.
Summary of Force Wave Analysis of Asynchronous Motors: The first part of the vibration frequency of an asynchronous motor is mainly the vibration at 2f1 (twice the power supply frequency), primarily generated by the fundamental wave of the air gap magnetic field. The second part of the vibration frequency is the force wave resulting from the interaction of stator and rotor tooth harmonics, which are the main components of electromagnetic vibration noise. To clearly understand the main vibration frequency components of an asynchronous motor, the following summary is provided: Main magnetomotive force wave in the air gap: where Z1 is the number of stator teeth, Z2 is the number of rotor slots, ω is the fundamental rotational angular velocity relative to the stator, and ω is the ω-th harmonic angular velocity of the rotor.
The main magnetic permeation wave in the air gap:
The main electromagnetic force waves in the air gap for k1, k2 = 1, 2, 3, ...
Where UZ = K1*z1 + p; VZ = K2 - z2 + p, S is the slip, k1, k2 = ±1, ±2, ..., force wave betweenness numbers n = uz + vz and n = uz - vz. Force waves of smaller orders are usually calculated using a table. The table of force waves will not be discussed here; interested readers can refer to Shubov's book, *Noise and Vibration of Electrical Machines*. The main force waves of vibration can be calculated using relevant motor parameters. CAE analysis or experiments can be used to analyze the natural frequencies of the motor rotor and stator, and even the cogging mesh. When designing a motor, the frequency of the force waves and the natural frequencies of the stator and rotor must be considered to avoid resonance.
