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Development Survey of Modern AC Frequency Conversion Speed ​​Regulation Technology

Release time:2020-06-07 17:19Popularity:

The development of modern AC motor variable frequency speed regulation system has mainly gone through the following stages: Early general-purpose inverters, most of which are open-loop constant voltage-frequency ratio control methods. Its advantages are simple control structure and low cost. The disadvantage is the system The control and speed regulation performance is not high, and it is more suitable for applications such as fans and water pumps. The control curve will change with the change of the load, the torque response is slow, the electromagnetic torque utilization rate is not high, and the stator resistance and reverse The existence of the dead zone effect of the converter reduces the performance and the stability. In the early XNUMXs, Japanese scholars proposed the voltage space vector method based on the magnetic flux trajectory (or called the magnetic flux trajectory method).This method is based on the premise of the overall generation effect of the three-phase waveform, and aims to approximate the ideal circular rotating magnetic field trajectory of the air gap of the motor, and generate the three-phase modulation waveform at one time.This method is called voltage space vector control. It eliminates the steady-state error of speed control by introducing frequency compensation control.electric motorIn the steady-state model, the DC current signal is used to reconstruct the phase current, from which the flux linkage amplitude is estimated, and the influence of the stator resistance on the speed regulation performance at low speed is eliminated through feedback control.Realize the output voltage and current closed-loop control to improve the voltage control accuracy and stability under dynamic load, and at the same time obtain the improvement of the current waveform to a certain extent.Another advantage of this control method is that it has obvious suppression of regenerative overvoltage and overcurrent, which can realize rapid acceleration and deceleration.

The AC motor is a multi-variable, non-linear and complex controlled object. The above-mentioned methods study its control characteristics, and the static and dynamic effects are not ideal. In the above-mentioned methods, because torque control adjustment is not introduced, the system performance is not Get a fundamental improvement.In view of the shortcomings of the above-mentioned control methods, some foreign scholars have simplified the mathematical model of the motor to realize the widely used vector control theory, also called field-oriented control.It was first proposed by F. Blasschke and others in West Germany in the early 1985s. This principle was analyzed and explained by the method of comparing DC motors and AC motors, thus creating a precedent for the equivalent DC motor control of AC motors.It makes people see that although the control of AC motors is complicated, they can also realize the intrinsic nature of independent control of torque and magnetic field].The basic principle of vector control is to control the flux linkage vector of the motor, by decomposing the stator current into two components, torque and magnetic field, and achieving orthogonal decoupling control through coordinate transformation.However, it is difficult to accurately observe the rotor flux linkage and the complexity of the vector transformation.The actual control effect is often difficult to achieve the effect of theoretical analysis, which is the shortcoming of vector control technology in practice.In addition, it must directly or indirectly obtain the position of the rotor flux linkage in space to realize the stator current decoupling control, which makes it necessary to configure the rotor position or speed sensor in this vector control system, which brings inconvenience to many applications.During the operation of the motor, due to the influence of the external environment such as temperature and the change of the motor's magnetic field on the parameters such as the motor rotor time constant, the accuracy of the control system is greatly reduced. Many scholars at home and abroad have done a lot of work in the identification of the rotor time constant. Following the vector control method, in XNUMX, Professor Depenbrock of Ruhr University in Germany first proposed the Direct Torque Control Theory (Direct Torque control referred to as DTC).Direct torque control is different from vector control. It does not indirectly control the torque by controlling the current, flux linkage, etc., but the torque is directly controlled as the controlled quantity. The advantage of torque control lies in the fact that torque control is Controlling the stator flux does not essentially need speed information. Direct torque control directly analyzes the mathematical model of the AC motor in the stator coordinate system to control the motor chain and torque.It does not need to equate AC motor control to DC motor control mode, thus avoiding many complicated calculations in vector rotation transformation, it does not need to imitate DC motor control, and it does not need to simplify the mathematical model of AC motor for decoupling.In addition to the stator resistance, the control has good robustness to all other motor parameter changes; the introduced stator flux observer can easily estimate the synchronous speed information, so it can easily realize the speed sensorless control.This control method is used in the design of general frequency converters. It is especially important for some occasions where it is inconvenient to install speed sensors, and improves the stability of the system. This control is called speed sensorless direct torque control.However, this kind of control relies on an accurate motor mathematical model and automatic identification of motor parameters (ID). The actual stator impedance mutual inductance, motor inertia and other important parameters are established through ID, and then the motor is estimated based on the accurate motor model. The actual torque, stator flux linkage and rotor speed are generated by Band-Band control of flux linkage and torque to control the switching state of the inverter.AC drive and control technology is currently one of the most rapidly developing technologies, which is closely related to the rapid development of power electronic device manufacturing technology, converter technology, control technology, and microcomputers and large-scale integrated circuits.

The development of frequency conversion technology is based on the development of power electronics technology.In the transmission control of low-voltage AC motors, the most used power devices are GTO, GTR, IGBT and IPM. IGBT and IPM combine the low saturation voltage characteristics of GTR and the high-frequency switching characteristics of MOSFET, which are currently the most widely used in general inverters. A wide range of mainstream power devices. The IGBT collector-emitter-collector voltage can be less than 3V, the switching frequency can reach 20KHz, and the built-in super-high-speed diode T between the collector and emitter.Up to 150ns.The application of the fourth-generation IGBT has greatly improved the performance of the inverter.One is the reduction in heat generation of IGBT switching devices, which reduces the heat generation of the devices that once accounted for 50-70% of the main circuit heating to 30%; the second is high carrier control, which significantly improves the output current waveform; the third is the increase in switching frequency, which makes It surpasses the human ear's perception range, that is, it realizes the silent operation of the motor; the fourth is that the driving power is reduced and the volume tends to be smaller.The application of IPM is about two years later than IGBT. Because IPM includes IGBT chip and peripheral driving and protection circuits, some even integrate optocouplers, so it is a more economical and applicable integrated power device.At present, in the range of module rated current 10-600A, general-purpose inverters have a trend of adopting IPM. Its advantages are: (1) fast switching speed, low drive current, simpler control and drive; (2) built-in current sensor , Can detect overcurrent and short-circuit current efficiently and quickly, and can provide adequate protection to the power chip, and the failure rate is greatly reduced; (3) Due to the optimization of the wiring design of the internal power circuit and drive circuit of the device, the wave Problems such as surge voltage, gate oscillation, and interference caused by noise can be effectively controlled; (4) The protection functions are relatively rich, such as current protection, voltage protection, and temperature protection. With the advancement of technology, the protection function will be further improved. Improving day by day; (5) The price of IPM has gradually approached IGBT, and after designers adopting IPM, the switching power supply capacity, the reduction of drive power capacity, the saving of devices and the improvement of comprehensive performance, etc. have become more cost-effective in many occasions. Higher than IGBT, it has good economic efficiency.The development of control technology also benefits from the development of microprocessor technology. Since INTEL launched the 1991XI8MC series in 196, the chips dedicated to motor control have developed greatly in terms of variety, speed, function, and cost performance.For example, the M37705 and M7906 single-chip microcomputers developed by Japan's Mitsubishi Electric for motor control and the TMS320C240 series of Texas Instruments are quite representative products.Compared with single-chip microcomputers, DSP devices have a higher level of integration, faster CPU, larger memory, provide high-speed, synchronous serial ports and standard asynchronous serial ports, and some integrated analog-to-digital converters and sample-and-hold circuits , Can provide PWM output, its assembly instruction set is in C language or algebraic language format, all instructions can be completed in one machine cycle, and through parallel processing technology, one machine cycle can complete multiple instructions. TI and AD’s DSP adopts an improved Harvard structure, with independent program space and data space, allowing simultaneous access to programs and data. At the same time, the program space and data space also have special channels for data exchange, thus avoiding certain A waste of space prepares for some applications.Built-in high-speed hardware multiplier and enhanced multi-stage pipeline make DSP devices have high-speed data computing capabilities.The single-chip microcomputer is a complex instruction system computer (CISC), and most instructions take 2-3 instruction cycles to complete.The single-chip microcomputer adopts the Neumann structure, the program and data are accessed in the same space, and can only access instructions or data individually at the same time. ALU can only be used for addition, and multiplication needs to be implemented by software, so it takes up more instruction cycles and the operation speed is relatively slow.Therefore, the difference in structure makes DSP devices 16-8 times faster than the single instruction execution time of 10-bit single-chip microcomputers, and 16-30 times faster to complete a multiplication operation.

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