Discussion on Several Technologies for Improving the Performance of VFD (Variable-frequency Drive)

1 Introduction At present, most of the VFD (Variable-frequency Drive) produced by Chinese VFD (Variable-frequency Drive) manufacturers adopt the ordinary V/f control method, and only a few of them claim to use the speedless vector control method. Control Technology. In foreign brands, the control idea of open-loop, closed-loop and speedless control has basically been reached. It is especially worth noting that ABB's DTC control method has been successfully applied in products. China's domestic academic circles have done a lot of corresponding work in the research of frequency conversion speed regulation systems, and achieved certain results, but compared with foreign countries, there is still a certain gap in engineering practice and accumulation. If the theoretical knowledge is transformed into the practice of China's existing industrialized products, and the performance and quality of China's frequency conversion speed regulation products are improved, it is a relatively urgent problem worthy of research. This article attempts to compare and explain the current V/f control and implementation technology from two aspects of control strategy and modulation technology, for everyone to communicate and discuss. Usually, the system structure of V/fVFD (Variable-frequency Drive) is composed of three parts: control, modulation and main loop, among which the control part and pulse width modulation part are all realized by software algorithms. This control method is obtained for the steady-state model of the AC motor, and does not depend on the motor parameters and their changes, so the control is simple and easy to implement. However, the range of speed regulation is relatively narrow, and it is only suitable for loads such as fans and water pumps that do not require high speed regulation performance. In order to improve the loading capacity and dynamic performance of the system at low speed and meet the needs of the actual industrial site, the existing control method and pulse width modulation strategy must be improved and improved accordingly. Several key technologies are discussed below. 2. SOME TECHNIQUES IN CONTROL STRATEGY 2.1 Compensation Technology Compensation technology is indispensable in open-loop control. It includes torque compensation, slip compensation and dead zone effect compensation. At low frequencies, the voltage drop of the stator resistance can no longer be ignored compared to the VFD (Variable-frequency Drive) output, and must be compensated, otherwise the output voltage is insufficient, and the motor does not move or the speed drops significantly at low frequencies. Slip compensation is mainly designed for the fact that the actual output speed of the motor will be lower than the set speed when the load is heavy. These two compensation methods can use simple fixed values for compensation in practice. The improved method uses the three-phase motor current to calculate compensation, but it is only based on the compensation of the current amplitude. In fact, this method is scalar compensation; more accurate The compensation method is to decompose the AC current of the three-phase motor into vector, and at the same time, the loss of the motor is involved in the calculation, so that the compensation effect is better. However, the calculation of this method is relatively complicated, and at the same time, it has certain dependence on some parameters of the motor, and there are some difficulties in the implementation process. Dead-zone effect compensation technology plays an important role in open-loop control. It can effectively improve the smoothness of the output current waveform and reduce harmonics. At the same time, it can increase the effective value of the output voltage and reduce the oscillation of the motor current. Especially in the environment where silence is required, the carrier frequency is artificially increased. If there is no dead zone compensation, the motor may not be able to run at low frequencies even with no load. At present, the commonly used dead zone compensation techniques include direct current zero-crossing compensation method, current decomposition method based on stator magnetic field orientation, dead zone voltage pulse width compensation method, dead zone time prediction compensation method without current sensor, etc. The compensation method for judging the current zero-crossing point is simple and easy to implement, but due to the large noise component in the current waveform, at the same time, the fluctuation of the load and any external interference will cause the judgment error of the zero-crossing point. There is a dead zone platform at the zero-crossing point that affects the low-frequency compensation effect, especially Especially when the carrier frequency is relatively high. The method based on the stator magnetic field orientation does not directly judge the current zero crossing point, but decomposes the stator current in the rotating coordinate system to obtain the relationship between the current vector angle and the dead zone voltage vector for corresponding compensation. If the method and the dead zone voltage pulse Combined with width compensation, the effect is more prominent. The dead time compensation method of phase angle prediction is a fixed compensation method that saves the current sensor. This method first predicts the current phase angle, and then makes corresponding compensation for the dead time. The predicted angle can be based on VFD (Variable -frequency Drive) The difference in output capacity is set in the software, or set by external modification. The advantage of this method is that the current sensor can be omitted, and the cost and system size can be reduced, but the compensation is not adjusted according to the external load change, so the accuracy and dynamic performance will be reduced accordingly. 2.2 Current oscillation suppression technology Under the condition of PWM power supply, when the motor is light-loaded or no-loaded, due to some reasons, the motor will appear local instability in a relatively wide frequency range, and the current amplitude value fluctuates at this time If the output frequency is very large, the output frequency will also change to a certain extent. The oscillation of the current may cause the system to falsely trigger an alarm due to over-current, making the system unable to work stably and reliably. There are many reasons for the oscillation. The more common view is that the motor and VFD (Variable-frequency Drive) are caused by the energy exchange process, and its appearance is also closely related to the dead zone effect. Compensating for the dead zone effect can effectively reduce the amplitude of the vibration, but it cannot fundamentally suppress the vibration. An effective method is to change the actual output frequency or voltage accordingly when the oscillation occurs, and form a simple negative feedback system through the current to achieve the purpose of suppressing the oscillation. But this method also has certain limitations. Oscillation frequency ranges of different motors are different, ranging from about 5Hz to 30Hz, and the current amplitude control is only a scalar, which makes the control effect not good and the robustness of the system is reduced. If the stator current is decomposed to directly control the magnetic flux excitation current component that affects energy exchange, the suppression effect will be greatly improved. A more accurate and effective method is to adopt the method of intelligent control, but the algorithm is complicated, and it is difficult to realize it on the complex V/f control platform. 2.3 Simple magnetic flux vector control method Ordinary V/f control is based on the steady-state motor model, ignoring the voltage drop of the stator resistance, so it cannot control the state of the motor during the dynamic process. Since it is an open-loop control, the load It is not sensitive to fluctuations or changes in motor parameters, and its dynamic performance is not high. The simple magnetic flux vector control method is to control the motor current on the basis of ordinary V/f control. The specific performance is that the torque current component and the excitation current component are obtained by vector decomposition calculation of the current output by VFD (Variable-frequency Drive) , and then adjust the voltage to match the motor current and load torque, thereby improving the low-speed torque characteristics. This method can provide a rated torque of 200 at 6Hz. Some motor parameters used in vector calculation are pre-stored in the RAM of the controller, and these parameters are basically constants for a certain type of motor. 2.4 Vector control technology based on speed sensorless For high-performance AC speed control system, speed closed-loop is essential. The speed closed-loop needs real-time motor speed. At present, the detection of speed feedback mostly adopts photoelectric pulse encoder, rotary Transformer or tachogenerator. The speed sensor is relatively expensive, which significantly increases the hardware cost of the system; the adaptability to the environment is not strong, which is not conducive to use in high temperature or vibration situations; the signal transmission distance is limited and cannot work reliably in long-distance lines. Therefore, the research on the AC speed control system without speed sensor is of great significance to improve the reliability of the system, the adaptability of the environment, and further expand the application range of the AC speed control system. . The ultimate goal of speed sensorless control is to accurately estimate the motor speed, rotor flux linkage and motor parameters at the same time. There are many methods for estimating motor speed and flux linkage. The observation and estimation methods based on the ideal model include: open-loop flux estimation and flux estimation with compensation; model reference adaptive method (MRAS); closed-loop observer method. Methods based on non-ideal characteristics include: speed identification method using tooth harmonic signal; rotating high frequency injection rotor salient pole detection method; leakage inductance pulse detection method; dq impedance difference orientation method; saturated salient pole detection method; There are two methods of detection, offline detection and online detection. There are several aspects worthy of special attention in the realization of the speed sensorless vector control technology, which have a very important impact on the system control performance and control accuracy. These aspects are: (1) Current and voltage signal detection and signal processing technology Among them, the signal processing technology is mainly how to filter the detected current and voltage signals with effective precision, which can reproduce the effective signal without generating amplitude. value decay and phase lag. The more practical methods are simplified extended Kalman filter, morphological filter and so on. (2) On-line adjustment of stator resistance The resistance value of the stator resistance changes greatly with the increase of temperature when the motor is running, and the maximum change can reach 150% of the rated value. How to detect the stator resistance online during operation and adjust the corresponding The amount of control that affects system performance is important. (3) Compensation technology for dead zone effect (4) Establishing an accurate dynamic motor model The motor parameters measured online or offline are only obtained at a certain moment. If the parameters change during operation, the motor model should also be corresponding Change to achieve the best control effect. At present, the model reference is mostly used in practical research.