An AC motor is a machine used to convert mechanical energy and AC electrical energy into each other. Due to the tremendous development of AC power systems, AC motors have become the most commonly used motors. Since the AC motor has no commutator, it has a simple structure, is convenient to manufacture, and is relatively strong. It is easy to make a high-speed, high-voltage, high-current, and large-capacity motor. AC motor power covers a large range, from a few watts to hundreds of thousands of kilowatts, or even millions of kilowatts. AC motor speed regulation With the development of power electronics technology, microelectronics technology, digital control technology and control theory, the dynamic and static characteristics of the AC drive system can be compared with the DC drive system, the AC drive system has been widely used, and the AC drive has gradually replaced the DC drive. become reality. Since the AC motor is essentially a complex object with nonlinearity, multi-variables, strong coupling, time-varying parameters, and large disturbances, its effective control has always been a hot topic of research at home and abroad, and a variety of control strategies and methods have been proposed. Among them, the classical linear control cannot overcome the large-scale changes of load and model parameters and the influence of nonlinear factors, and the control performance is not high; there are also some problems in vector control and direct torque control: in recent years, with the modern control and intelligent control theory Development, advanced control algorithms have been applied to AC motor control, and achieved certain results. (1) Constant voltage frequency ratio control This method starts from the basic control method of variable voltage and frequency conversion and is an open-loop control method without speed feedback. Because below the rated frequency, if the voltage is constant and the frequency is only reduced, then the air gap flux will be too large, causing the magnetic circuit to be saturated, and in serious cases, the motor will be burned. In order to keep the air gap magnetic flux constant, the ratio of induced potential to frequency is constant for control. The advantages of this method are: simple structure, reliable operation, and low requirements for control operation speed. Disadvantages of this method: the speed regulation accuracy and dynamic performance of the open-loop control are poor; only the air gap flux is controlled, but the torque cannot be adjusted, and the performance is not high; since it does not contain current control, it must have a given integral link when starting, To suppress the current impact; the torque is insufficient at low frequency, and torque compensation is required to change the low frequency torque characteristics. (2) Closed-loop slip frequency control This method is a control method that directly controls the torque. When the motor is running stably, within a small variation range of the slip rate, as long as the motor flux linkage remains unchanged, the motor torque is approximately proportional to the slip angular frequency, so the motor torque can be controlled by controlling the slip angular frequency . Advantages of this method: Basically, the motor torque is controlled, and the dynamic performance and steady-state accuracy of speed regulation are improved. Disadvantages of this method: the torque of the dynamic process cannot be really controlled, and the dynamic performance is not ideal. Speed regulation method of AC motor speed regulation 1. Listen to the sound and carefully find the fault point. During the operation of the AC asynchronous motor, if you find a thin "hum" sound, which does not change from high to low, it is a normal sound. If the sound is thick and sharp "Buzzing" and "hissing" sounds are harbingers of failures, and the following reasons should be considered: (l) Iron core loosening The vibration of the motor during operation and the sudden change in temperature will deform the iron core fixing bolts, cause the silicon steel sheet to loosen, and generate large electromagnetic noise. (2) Rotor noise The sound of the rotor rotating is produced by the cooling fan. It is a "whining" sound. If there is a "boom" sound like a drum, it means that the motor is suddenly starting, stopping, and reversed. In the case of speed change such as braking, the acceleration torque causes the loosening of the rotor core and the shaft. If it is light, it can continue to be used, and if it is serious, it can be disassembled for inspection and repair. (3) Bearing noise When the motor is running, you must pay attention to the change of the bearing sound. Touch one end of the screwdriver to the bearing cover and the other end to your ear, you can hear the sound change inside the motor, different parts, different faults , with different sounds. For example, the "creaking" sound is produced by the irregular motion of the inner rolling of the bearing, which is related to the clearance of the bearing and the state of the grease. The "hissing" sound is the sound of metal friction, which is generally caused by the lack of oil in the bearing. The bearing should be disassembled and greased. 2. Use the sense of smell to analyze that the faulty motor has no peculiar smell during normal operation. If it smells strange, it is a fault signal, such as burnt smell, which is emitted by the insulation. There will also be smoke; such as oil burnt smell, most of the bearings are short of oil, and the peculiar smell that occurs when the oil vapor evaporates when it is close to the dry grinding state. 3. Use the hand feel to check the fault. Touch the TV shell with your hands to roughly judge the temperature. If you touch the motor shell with your hand, you will feel very hot, and the temperature value is high. You should check the reason, such as: overload, overvoltage Advanced, and then troubleshoot the cause. What are the three methods of AC motor speed regulation? 1. The purpose of changing the pole logarithm speed regulation is to control the synchronous speed of the magnetic field to control the rotor speed. The slip rate remains unchanged, and its slip loss is small. It is a step-by-step speed regulation, and the application occasions are limited. 2. Change the slip rate to adjust the speed, keep the synchronous speed unchanged, and change the excitation current of the rotor return difference. The disadvantage is that the slip power of the motor cannot be used, its power factor is low, and all the slip power is consumed in the form of heat energy. 3. Changing the frequency speed regulation, the slip rate and the slip power consumption remain unchanged. From the point of view of energy conversion of the above three speed regulation methods, changing frequency speed regulation has the highest efficiency. Because of the high power and efficiency of frequency conversion speed regulation, the speed regulation accuracy is also quite high. And it has very hard mechanical characteristics and a wide range of speed regulation, and it is easy to realize closed-loop automatic control. It is also the most energy-saving speed regulation method for AC motor speed regulation. It represents the current level of speed control of AC drives, and is also the main reason why frequency converters are used more and more widely. (1) Pole-changing speed regulation; this speed-changing method is only suitable for specially produced pole-changing multi-speed asynchronous motors, and three speeds of 2, 3 and 4 poles can be obtained through different connection methods of windings. Only a wide range of speed regulation can be realized. (2) Rotor series resistors; only suitable for winding rotor asynchronous motors, changing the resistance value of the resistors connected in series on the rotor circuit, thereby changing the slip rate to achieve speed regulation. Various speed adjustments can be realized. However, the resistance consumes power, the efficiency is low, and the mechanical properties become soft, so it is only suitable for occasions with low speed regulation requirements. (3) Cascade speed regulation; only suitable for wound-type asynchronous motors, which feed back the slip power to the grid through certain electronic equipment. It is widely used in transmission systems such as air pumps. (4) Super-synchronous cascade speed regulation; access to AC-AC frequency converter, control the working state of the frequency converter, and make the motor speed up and down at the synchronous speed. The speed range is large, and it can generate braking torque. (5) Voltage regulation and speed regulation: Use thyristors to form an AC speed regulation circuit, change the trigger angle, and change the terminal voltage of the asynchronous motor to regulate the speed. Low efficiency, only suitable for motors with special rotors; AC motor speed regulation method 1. Variable pole logarithmic speed regulation method Change the connection way of the stator winding to change the number of pole pairs of the cage motor stator to achieve speed regulation. 2. Frequency conversion speed regulation method The frequency converter is used to change the frequency of the motor stator power supply, thereby changing the speed regulation method of its synchronous speed. Three, cascade speed regulation method Cascade speed regulation means that an adjustable additional potential is connected in series in the rotor circuit of a wound motor to change the slip of the motor and achieve the purpose of speed regulation. Most of the slip power is absorbed by the additional potential connected in series, and then used to generate additional devices to return the absorbed slip power to the grid or convert energy for use. According to the way of slip power absorption and utilization, cascade speed regulation can be divided into motor cascade speed regulation, mechanical cascade speed regulation and thyristor cascade speed regulation, and thyristor cascade speed regulation is mostly used. 4. Speed regulation method of winding motor rotor series resistance The rotor of the linear asynchronous motor is connected in series with an additional resistor, which increases the slip rate of the motor, and the motor runs at a lower speed. The larger the resistance connected in series, the lower the speed of the motor. This method has simple equipment and convenient control, but the slip power is consumed on the resistor in the form of heat. It belongs to stepped speed regulation, and its mechanical characteristics are relatively soft. 5. Stator voltage regulation and speed regulation method When the stator voltage of the motor is changed, different speeds can be obtained. Because the torque of the motor is proportional to the square of the voltage, the torque drops a lot, and its speed regulation range is small, which makes it difficult to apply the general cage motor. In order to expand the range of speed regulation, a cage motor with a large rotor resistance value should be used for voltage regulation and speed regulation, such as a torque motor specially used for voltage regulation and speed regulation, or a frequency-sensitive resistor connected in series with a wound motor. 6. Speed regulation method of electromagnetic speed regulating motor The electromagnetic speed regulating motor consists of a cage motor, an electromagnetic slip clutch and a DC excitation power supply. The power of the DC excitation power supply is small, and it is usually composed of a single-phase half-wave or full-wave thyristor rectifier. Changing the conduction angle of the thyristor can change the magnitude of the excitation current. 7. Speed regulation method of hydraulic coupling The hydraulic coupling is a hydraulic transmission device, generally composed of a pump wheel and a turbine. When the pump wheel rotates under the drive of the prime mover, the liquid in it is driven by the blades to rotate, and under the action of centrifugal force, it moves along the outer ring of the pump wheel. When entering the turbine, the turbine blades are given thrust in the same direction to make it drive the production machinery to run. Speed regulation is achieved through different hydraulic forces (lube oil and turbine).
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