Frequency converter is an electric energy control device that uses the on-off function of power semiconductor devices to convert the power frequency power supply to another frequency, which can realize soft start, variable frequency speed regulation, improve operation accuracy, change power factor, over-current/over-voltage/overload protection and other functions.
Common control modes in frequency converter:
1. Non-intelligent control mode:
(1) voltage/frequency control
V/f control is put forward in order to obtain ideal torque-speed characteristics, based on the idea of changing the frequency of power supply to speed regulation, and at the same time ensuring the magnetic flux of the motor unchanged. This control method is basically available in general frequency converters. The structure of V/f control inverter is very simple, but this kind of inverter adopts open-loop control mode and cannot achieve high control performance. Moreover, at low frequency, torque compensation must be carried out to change the torque characteristics at low frequency.
(2) slip frequency control
Slip frequency control is a control method to directly control torque. Based on V/f control, by knowing the power frequency corresponding to the actual speed of asynchronous motor, the output frequency of inverter is adjusted according to the expected torque, so that the motor has corresponding output torque. This control method needs to install speed sensor in the control system, and sometimes it needs to add current feedback to control frequency and current. Therefore, it is a closed-loop control method, which can make the inverter have good stability and good response characteristics to rapid acceleration and deceleration and load changes.
(3) Vector control
Vector control is to control the magnitude and phase of motor stator current through vector coordinate circuit, so as to control the excitation current and torque current of the motor in D, Q and 0 coordinate systems respectively, and then control the motor torque. By controlling the action sequence and time of each vector and the action time of zero vector, various PWM waves can be formed to achieve various control purposes. For example, a PWM wave with the least switching times is formed to reduce switching loss. At present, there are two kinds of vector control methods actually applied in inverter: vector control method based on slip frequency control and vector control method without speed sensor. It is called "closed loop" to set a speed detector (PG) for the used motor device and feed back the actual speed to the control device for control, and it is called "open loop" without PG operation. Most of the general frequency converters are in open-loop mode, and some models can carry out PG feedback by using options. The speed sensorless closed-loop control mode is to calculate the actual speed of the motor according to the established mathematical model, which is equivalent to forming closed-loop control with a virtual speed sensor.
The vector control method based on slip frequency has the same steady-state characteristics as the slip frequency control method, but the vector control method based on slip frequency also needs to control the phase of motor stator current through coordinate transformation to meet certain conditions to eliminate the fluctuation in the torque-current transition process. Therefore, the vector control method based on slip frequency can greatly improve the output characteristics than the slip frequency control method. However, this control mode belongs to closed-loop control mode, and speed sensors need to be installed on the motor and frequency converter with PG feedback function, so the accuracy is improved. But the value of speed accuracy depends on the accuracy of PG itself and the resolution of inverter output frequency.
Speed sensorless vector control is to control the excitation current and torque current through coordinate transformation, and then identify the speed by controlling the voltage and current on the stator winding of the motor, so as to control the excitation current and torque current. This control method has the advantages of wide speed range, large starting torque, reliable operation and convenient operation, but the calculation is complex and generally requires a special processor to calculate. Therefore, the real-time performance is not ideal, and the control accuracy is affected by the calculation accuracy.
(4) Direct torque control
Direct torque control is to use the concept of space vector coordinates to analyze the mathematical model of AC motor in stator coordinate system, control the flux linkage and torque of the motor, and observe the stator flux linkage by detecting the stator resistance. Therefore, the complicated transformation calculation such as vector control is omitted, and the system is intuitive and concise, and the calculation speed and accuracy are improved compared with vector control. Even in the open-loop state, it can output 100% rated torque, and has the function of multi-load balancing.
(5) Optimal control
The application of optimal control in practice is different according to different requirements, and individual parameters can be optimized according to optimal control theory. For example, in the control application of high voltage inverter, two strategies, time period control and phase shift control, are successfully adopted, and the optimal voltage waveform under certain conditions is realized.
(6) Other non-intelligent control methods
In practical application, there are also some non-intelligent control methods in frequency converter control, such as adaptive control, sliding mode variable structure control, difference frequency control, circulating current control, frequency control and so on.
2 Intelligent control mode:
Intelligent control methods mainly include neural network control, fuzzy control, expert system, learning control and so on. There are some successful examples of intelligent control mode in the specific application of inverter control.
(1) neural network control
Neural network control method is applied to the control of frequency converter, which is usually a complex system control. At this time, little is known about the system model, so the neural network should not only complete the function of system identification, but also control it. Moreover, neural network control mode can control multiple inverters at the same time, so it is more suitable for control when multiple inverters are cascaded. However, too many layers of neural network or too complicated algorithm will bring many practical difficulties in specific applications.
(2) Fuzzy control
Fuzzy control algorithm is used to control the voltage and frequency of the inverter, so as to control the acceleration time of the motor, and avoid the influence of too fast acceleration on the service life of the motor and too slow acceleration on the working efficiency. The key of fuzzy control lies in the division of universe, membership and fuzzy hierarchy, which is especially suitable for multi-input single-output control systems.
(3) Expert system
Expert system is a control method using the experience of so-called "experts". Therefore, an expert database is generally established in the expert system to store certain expert information, and there is also a reasoning mechanism to seek the ideal control result according to the known information. The design of expert database and reasoning mechanism is particularly important, which is related to the quality of expert system control. The application of expert system can control the voltage and current of frequency converter.
(4) learning control
Learning control is mainly used for repetitive input, and regular PWM signals (such as center modulation PWM) just meet this condition, so learning control can also be used for inverter control. Learning control does not need to know too much system information, but it needs 1~2 learning cycles, so the rapidity is relatively poor. Moreover, the algorithm of learning control sometimes needs to realize the lead link, which cannot be realized by analog devices. At the same time, learning control also involves a stability problem.
Frequency conversion drive, the main circuit is the power conversion part that provides voltage regulation and frequency modulation power supply for asynchronous motor. The main circuit of the frequency converter can be roughly divided into two types: the voltage type is a frequency converter that converts the DC of the voltage source into alternating current, and the filtering of the DC loop is a capacitor. Current mode is a frequency converter that converts DC of current source into AC, and its DC loop filter is inductance. It consists of three parts, namely, a rectifier that converts power frequency power supply into DC power supply, a smoothing circuit that absorbs voltage pulsation generated by converter and inverter, and an inverter that converts DC power supply into AC power supply.
(1) rectifier: diode converter is widely used recently, which converts power frequency power supply into DC power supply. Two groups of transistor converters can also be used to form a reversible converter, which can be regenerated due to its reversible power direction.
(2) Smoothing circuit: The DC voltage rectified by the rectifier contains a pulsating voltage 6 times the power frequency, and the pulsating current generated by the inverter also changes the DC voltage. In order to suppress voltage fluctuation, the pulsating voltage (current) is absorbed by inductance and capacitance. When the device capacity is small, if the power supply and the main circuit are redundant, the inductance can be omitted and a simple smoothing circuit can be used.
(3) Inverter: Contrary to rectifier, inverter converts DC power into AC power with required frequency, and three-phase AC output can be obtained by turning on and off six switching devices within a certain time. Taking voltage source pwm inverter as an example, the switching time and voltage waveform are given. The control circuit is a circuit that provides control signals for the main circuit that supplies power (adjustable voltage and frequency) to the asynchronous motor. It consists of frequency and voltage arithmetic circuit, voltage and current detection circuit of main circuit, speed detection circuit of motor, driving circuit for amplifying control signal of arithmetic circuit, and protection circuit of inverter and motor. (1) operation circuit: compare external commands such as speed and torque with current and voltage signals of detection circuit to determine the output voltage and frequency of inverter. (2) Voltage and current detection circuit: it is isolated from the main loop potential and detects voltage and current. (3) Driving circuit: the circuit that drives the main circuit device. It is isolated from the control circuit to turn on and off the main circuit device. (4) Speed detection circuit: signals from speed detectors (tg, plg, etc.). ) as a speed signal, sent to the operation circuit, according to the instruction and operation, make the motor run at the instruction speed. (5) Protection circuit: detect the voltage and current of the main circuit. When there are abnormalities such as overload or overvoltage, in order to prevent the inverter and asynchronous motor from being damaged, stop the inverter or suppress the voltage and current values.