The working principle of AC motors

An AC motor is a device that converts electrical energy from alternating current into mechanical energy. It primarily consists of an electromagnetic winding or distributed stator winding used to generate a magnetic field, along with a rotating armature or rotor. The motor operates based on the principle that a current-carrying coil experiences a force in a magnetic field, causing it to rotate. AC motors are categorized into two types: synchronous AC motors and induction motors [1].

The stator windings of a three-phase AC motor are essentially three coils spaced 120 degrees apart, connected in a delta or star configuration. When three-phase current is applied, a magnetic field is generated in each coil, and the combination of these three fields produces a rotating magnetic field.

AC motors consist of a stator and a rotor, and are divided into two types: synchronous AC motors and induction motors. Both types of motors generate a rotating magnetic field by passing alternating current through the stator winding, but the rotor winding of synchronous AC motors usually requires a direct current (excitation current) supply from the exciter; Induction motors, on the other hand, do not require current to be passed through the rotor winding.

The stator winding of a three-phase AC motor is basically composed of three coils spaced 120 degrees apart from each other, connected in a triangular or star shape. When three-phase current is applied, a magnetic field is generated in each coil, and these three magnetic fields combine to form a rotating magnetic field. The current completes a full vibration, and the rotating magnetic field rotates exactly once. Therefore, the number of revolutions per minute of the rotating magnetic field is N=60f. In the formula, f is the power frequency.

AC motors can be divided into synchronous motors and asynchronous motors (also known as asynchronous motors) based on the speed of rotor rotation. Regardless of the load size, the rotor speed of a synchronous motor is always the same as the speed of the rotating magnetic field, so this speed is called synchronous speed. As mentioned above, it only depends on the frequency of the power supply. The speed of asynchronous motors is not constant, it depends on the size of the load and the power supply voltage. There are two types of three-phase asynchronous motors: those without rectifiers and those with rectifiers. The vast majority of asynchronous motors used in practical applications are induction motors without rectifiers (although parallel and series three-phase asynchronous rectifier motors have the advantages of adjustable speed over a large range and high power factor), and their speed is always lower than the synchronous speed.

 

Main purpose

The working efficiency of AC electric motors is high, without smoke, odor, environmental pollution, and low noise. Due to its series of advantages, it is widely used in various fields such as industrial and agricultural production, transportation, national defense, commercial and household appliances, medical electrical equipment, etc.

 

Working principle

Induction motor, also known as asynchronous motor, refers to the rotor being placed in a rotating magnetic field and obtaining a rotational torque under the action of the rotating magnetic field, causing the rotor to rotate.

The appearance and internal structure of an induction motor. The rotor is a rotatable conductor, usually in the shape of a squirrel cage. The stator is the non rotating part of an electric motor, whose main task is to generate a rotating magnetic field. Rotating magnetic fields are not achieved through mechanical methods. But instead, alternating current is applied to several pairs of electromagnets, causing their magnetic pole properties to cyclically change, thus equivalent to a rotating magnetic field. This type of motor does not have brushes or collector rings like DC motors. Depending on the type of AC power used, there are single-phase motors and three-phase motors. Single phase motors are used in devices such as washing machines and electric fans; Three phase electric motors are used as power equipment in factories. Through the relative motion between the rotating magnetic field generated by the stator (with a synchronous speed of n1) and the rotor winding, the rotor winding cuts the magnetic induction line and generates induced electromotive force, thereby generating induced current in the rotor winding. The induced current in the rotor winding interacts with the magnetic field to generate electromagnetic torque, causing the rotor to rotate. As the rotor speed gradually approaches the synchronous speed, the induced current gradually decreases, and the generated electromagnetic torque also decreases accordingly. When the asynchronous motor operates in the motor state, the rotor speed is lower than the synchronous speed. To describe the difference between the rotor speed n and the synchronous speed n1, a slip ratio is introduced

 

 

Control Strategy

With the development of power electronics technology, microelectronics technology, digital control technology, and control theory, the dynamic and static characteristics of AC drive systems can be fully comparable to DC drive systems. AC drive systems have been widely used, and the replacement of DC drive by AC drive has gradually become a reality.

Due to the fact that AC motors are inherently complex objects with nonlinearity, multi variables, strong coupling, time-varying parameters, and large disturbances, their effective control has always been a hot research topic both domestically and internationally, and various control strategies and methods have been proposed. Among them, classical linear control cannot overcome the influence of load, large-scale changes in model parameters, and nonlinear factors, resulting in low control performance; Vector control and direct torque control also have some problems: in recent years, with the development of modern control and intelligent control theory, advanced control algorithms have been applied to AC motor control and have achieved certain results [2].

Steady state model control method

The commonly used steady-state model control schemes include open-loop constant v/f ratio control (i.e. voltage/frequency=constant) and closed-loop slip frequency control.

(1) Constant voltage frequency ratio control

This method is an open-loop control method that starts from the basic control mode of variable voltage and frequency conversion and does not include speed feedback. Due to the fact that below the rated frequency, if the voltage remains constant and only the frequency is reduced, the air gap flux will be too large, causing magnetic saturation and, in severe cases, burning out the motor. In order to maintain the constant air gap magnetic flux, a constant ratio of induced potential to frequency is used for control.

 

 

Common Faults

AC motors are prone to malfunctions during operation due to friction, vibration, insulation aging, and other reasons. If these faults are checked, discovered, and eliminated in a timely manner, they can effectively prevent accidents from occurring.

Common fault inspection

1. Listen to the sound and carefully identify the fault point. During the operation of the AC asynchronous motor, if a faint "buzzing" sound is found without any fluctuations, it is a normal sound. If the sound is coarse and has sharp "buzzing" or "hissing" sounds, it is a precursor to a fault. The following reasons should be considered:

(l) The vibration and fluctuating temperature of a motor with loose iron core during operation can cause deformation of the fixing bolts of the iron core, resulting in loose silicon steel sheets and generating large electromagnetic noise.

(2) The sound produced by the rotation of the rotor, which is generated by the cooling fan, is a "wuwu" sound. If there is a "dongdong" sound like knocking a drum, it is caused by the loosening of the fit between the rotor iron core and the shaft due to the acceleration torque of the motor during sudden start, stop, reverse braking and other variable speed situations. Mild cases can continue to be used, while severe cases can be disassembled for inspection and repair.

(3) During the operation of the bearing noise motor, it is necessary to pay attention to the changes in the bearing sound. By touching one end of the screwdriver on the bearing cover and the other end on the ear, the internal sound changes of the motor can be heard. Different parts and faults have different sounds. The sound of "creaking" is caused by the irregular movement of the rolling gun inside the bearing, which is related to the clearance of the bearing and the state of the lubricating grease. The "sizzling" sound is a metal friction sound, usually caused by a lack of oil in the bearing due to wear. The bearing should be disassembled and lubricated with grease, etc.

2. Use the sense of smell to analyze that the faulty motor does not have any odor during normal operation. If you smell any odor, it is a fault signal, such as burnt smell, which is emitted by insulation grilling and can even smoke as the motor temperature increases; If there is a burnt oil smell, it is mostly due to the lack of oil in the bearing, and the odor caused by the evaporation of oil and gas when approaching the dry grinding state.

3. Use the tactile sensation to check for faults. By touching the casing of the TV with your hand, you can roughly determine the temperature. If you feel very hot and the temperature value is high when touching the motor casing with your hand, you should check the cause, such as excessive load or high voltage, and then troubleshoot the problem based on the cause.