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1 AC motor working principle and frequency control
1. 1 Asynchronous motor working principle
When a three-phase AC voltage is applied to the stator winding of a three-phase asynchronous motor, this voltage will produce a rotating magnetic field whose speed is determined by the frequency of the stator voltage. When the magnetic field rotates, the rotor windings in the magnetic field will cut the magnetic lines of force and generate corresponding induced electromotive forces and induced currents in the rotor windings, and the induced currents will be subjected to the action of the rotating magnetic field to generate electromagnetic forces, ie, torque. The rotor is rotated following the rotating magnetic field. When any two phases of the three-phase asynchronous motor winding are exchanged, the direction of the generated rotating magnetic field will change. Therefore, the steering of the motor will also change.
1. 2 variable frequency speed regulation of asynchronous motor
The speed of the stator magnetic field of the asynchronous motor is called the synchronous speed of the asynchronous motor, and the speed of the asynchronous motor is always less than its synchronous speed. The synchronous speed is determined by the number of magnetic poles of the motor and the power frequency, ie: Ns=120 f / p .
Where: Ns synchronous speed; f power frequency; p pole number.
If N is the speed of the asynchronous motor and the synchronous motor speed Ns is used as the reference, the slip of the asynchronous motor is defined as: S = ( Ns - N) / Ns , (1) the actual speed N of the asynchronous motor can be given by Out: N = 120f ( 1- S ) / p , (2)! ! ! ! ! !
Where: N motor speed; S slip.
When the motor is unloaded, the slip S is substantially zero, and when the motor is at full load, the slip is generally in the range of 1% 10%. It can be known from equation (2) that changing the speed of any one of the parameters f, S, p can change the speed of the motor, that is, the speed control of the asynchronous motor. Therefore, the speed control of the asynchronous motor can be realized by changing the frequency of the power source. In a sense, the frequency converter is an AC power source that can change the frequency arbitrarily.
2 The composition and working principle of the inverter
2. 1 Basic structure of the inverter 2. 2 Working principle of the internal circuit of the inverter
The general three-phase inverter rectifier circuit consists of a three-phase full-wave rectifier bridge.
Its main function is to rectify the external power supply of the power frequency and provide the required DC power to the inverter circuit and the control circuit. The rectifier circuit can be a DC voltage source or a DC current source according to its control mode.
The role of the DC intermediate circuit is to smooth the output of the rectifier circuit to ensure that the inverter circuit and the control power supply can obtain a higher quality DC power supply. In addition, braking resistors and other auxiliary circuits are sometimes included in the DC intermediate circuit due to the need for motor braking.
The inverter circuit is one of the most important parts of the inverter. Its main function is to convert the DC power output of the smoothing circuit into an AC power source with arbitrary adjustable frequency and voltage under the control of the control circuit. The output of the inverter circuit is the output of the inverter, which is used to achieve speed control of the asynchronous motor.
The control circuit of the inverter includes the main control circuit, the signal detection circuit, the gate drive circuit, the external interface circuit and the protection circuit. It is the core part of the inverter. The advantages and disadvantages of the control circuit determine the performance of the inverter. The main function of the control circuit is to send various signals obtained by the detection circuit to the operation circuit, so that the operation circuit can provide the necessary gate drive signal for the main circuit of the inverter according to requirements, and provide necessary protection for the inverter and the asynchronous motor.
In addition, the control circuit also receives and transmits various forms of external signals through external interface circuits such as A/O, O/A and gives the internal working status of the system, enabling the inverter to cooperate with external devices for various high-performance control.
3 Classification of frequency converter
3. 1 Classification by frequency conversion
According to the different types of frequency conversion, the frequency converter can be divided into two categories: direct delivery and handover. The former is an inverter that converts the power frequency AC power into a direct current through a rectifier, and then sends it to a high-power triode through a filter. The triode in the inverter is controlled by the microcomputer to turn on and off according to a certain program, and the direct current sent to the inverter is converted into alternating current of a desired frequency, thereby realizing frequency conversion. The AC frequency converter directly converts the power frequency AC power into the AC power of the required frequency through two sets of anti-parallel high-power triode rectifier bridges and filters. However, since its maximum frequency generally does not exceed 1/ 2 1/3 of the grid frequency, the control system is complicated and currently has few applications.
3. 2 Classification according to the working mode of the main circuit
According to the working mode of the main circuit, the inverter can be divided into voltage type and current type. The filter part of the voltage type inverter is mainly capacitor, the power supply impedance is small, equivalent to the voltage source, the output voltage is square wave, the output current is approximate sine wave, which contains more harmonics, the frequency conversion efficiency is lower than the current type, dynamic performance And the reliability is poor, suitable for multiple motors running at the same speed. The current-type inverter filter is mainly an inductor. The power supply impedance is large, which is equivalent to the power supply. Its output voltage is close to sine wave, the output current is square wave, and it contains less harmonic current. It has better reliability and dynamics. It can be used. Current inner loop control, suitable for multiple motor operation, especially suitable for single machine acceleration and deceleration operation.
3. 3 Sort by switch
When classifying the inverter according to the switching mode of the inverter circuit, the inverter can be divided into three types: pulse amplitude modulation control mode, pulse width modulation control mode and high carrier frequency control mode.
3. 4 Classification by working principle
When classifying the frequency converter according to the working principle, according to the development process of the frequency converter technology, it can be divided into v / f control mode, slip frequency control mode and vector control mode.
4 inverter control method and basic principle
When classifying the inverter according to the control method, the inverter can be divided into three types: v / f control inverter, slip frequency and vector control inverter.
4. 1 v / f Control the basic principle of the inverter
When the frequency of the motor power source is changed, the internal impedance of the motor also changes, causing a change in the field current, causing the motor to be under-excited or too strong. The former motor will have difficulty giving enough torque, which will cause a drop in motor power factor and efficiency. In order to obtain the ideal torque speed characteristics, while changing the power frequency for speed regulation, necessary measures must be taken to ensure that the air gap flux of the motor is in an efficient state. By controlling the supply voltage and frequency to keep it constant, this is the v/f control inverter.
4. 2 The basic principle of vector control inverter
It controls the magnetic current and torque current of the motor by controlling the amplitude and phase of the stator current of the motor, so as to achieve the purpose of controlling the torque current characteristics of the motor.
Since the inverter of the slip frequency control mode has been basically replaced by the frequency conversion of the vector control mode, the slip frequency control inverter is not described in this paper.
4. 3 Comparison of characteristics of various control methods
The v/f control inverter adopts open-loop control. Without speed feedback, it can not give satisfactory control performance in speed control. However, such a frequency converter has a high performance-price ratio and is widely used in various applications for energy saving purposes and for applications where speed accuracy is not critical.
Vector control is an ideal control method for asynchronous motors and it has many advantages. It can control the speed from zero speed, wide speed range, precise control of torque, fast system response, good acceleration and deceleration characteristics.
In short, AC variable frequency speed control technology is the main development direction of modern electric drive technology. With the application of power electronics technology, microelectronic technology and modern control theory in AC variable frequency speed control system, as the inverter of the core of variable frequency speed control system, Its performance has also been greatly improved, and it has been widely used in many fields of industrial production and daily life, and has produced good economic benefits.
Inverter technology is a comprehensive technology. It is based on control technology, electronic power technology, microelectronics technology and computer technology. Compared with the traditional AC drive system, it uses the frequency converter to control the speed of the AC motor. There are many advantages, such as power saving, it is easy to realize the speed control of the existing motor, and it can realize high-efficiency continuous adjustment in a wide range. Speed ​​control makes it easy to switch between positive and negative motor. It can start and stop at high frequency. It can be electrically braked. It can drive the motor at high speed. It can adapt to various environments. You can use one inverter to multiple units. The motor performs speed control, the power factor is large, the required capacity is small, and a high-performance control system can be formed.