Take a good look at the following.
English name of impulse current Definition: aperiodic transient current. There are two commonly used waveforms: the first is that the current rises from zero to the peak in a short time, and then drops to zero with an approximate exponential law or damped sine wave. The waveform of the pulse current is represented by the wave front time T 1 and the half-peak time T2, which is denoted as T 1/T2, as shown in the following figure (a). The second waveform is approximately rectangular, which is called square wave pulse current (wave), as shown in the following figure (b). (1) (2) Applied discipline: electric power (first-class discipline); High Voltage Technology (two disciplines) This content is approved and published by the State Committee of Science and Technology Terminology. Impulse current generally refers to the large current generated at the moment when electrical appliances are energized. This is mainly reflected in inductive and capacitive loads. The inductive load represents the motor. The moment the motor starts, it is equivalent to a short circuit, and the current is quite large, because the resistance and inductance of the motor winding itself are very small. When the motor is turned off, reverse pulse current will also be generated. Capacitive loads such as capacitors are equivalent to short circuits at the moment of power-on, and theoretically the current is infinite. It also needs instantaneous high voltage and high current to ionize the mercury vapor inside the fluorescent lamp at the moment of starting.
Frequency converter/inverter
printing block
The English translation of frequency converter is VFD (variable frequency drive), which may be one of the few examples of modern technology from Chinese to English. (But VFD can also be interpreted as vacuum cathode fluorescent lamp, so this translation is not commonly used. ). Inverter is an electric drive component that uses frequency conversion technology and microelectronics technology to control AC motor by changing the frequency and amplitude of motor power supply. Due to the influence of Japanese manufacturers in China, North Korea and other Asian regions, the frequency converter was once called VVVF (Variable Voltage Frequency Converter).
catalogue
Basic principle knowledge of frequency converter
Overview of working principle of frequency converter
Selenium rectifier
Smoothing circuit
transverter
Function of frequency converter
Industrial application of frequency converter
Composition of frequency converter
Frequency converter control mode
History of frequency converter
Classification unit of frequency converter series frequency converter
According to the transformation link classification.
According to the nature of DC power supply
Accord to that working method of the main circuit
Classification according to working principle
Classification by switch mode
Classification by purpose
According to the voltage regulation method of the frequency converter
According to the working principle
Classification by international region
Classification by voltage class
Energy saving effect of frequency converter
Matters needing attention in using and maintaining the physical environment of frequency converter
Electrical environment
Harmonic control and reactive power compensation of inverter power supply system
Overview of working principle of frequency converter
Selenium rectifier
Smoothing circuit
transverter
Function of frequency converter
Industrial application of frequency converter
Composition of frequency converter
Frequency converter control mode
History of frequency converter
Classification unit of frequency converter series frequency converter
According to the transformation link classification.
According to the nature of DC power supply
Accord to that working method of the main circuit
Classification according to working principle
Classification by switch mode
Classification by purpose
According to the voltage regulation method of the frequency converter
According to the working principle
Classification by international region
Classification by voltage class
Energy saving effect of frequency converter
Matters needing attention in using and maintaining frequency converter
Harmonic control and reactive power compensation of inverter power supply system in physical and electrical environment
1, what is a frequency converter? 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. China's main brands are invigorating, Huichuan, Sanjing, Ziri Electric Technology Co., Ltd., Renault, Ourui (formerly Yantai HSBC) and Blue Ocean Hua Teng. 2. What's the difference between 2.PWM and PAM? PWM is the abbreviation of English Pulse Width Modulation, which changes the pulse width of pulse train according to certain rules to adjust the output and waveform. Frequency converter/inverter
PAM is the abbreviation of English Pulse Amplitude Modulation, which is a modulation method to adjust the output value and waveform by changing the pulse amplitude of pulse sequence according to certain rules. 3. What's the difference between voltage type and current type? The main circuit of frequency converter can be roughly divided into two categories: voltage type is a frequency converter that converts DC of voltage source into AC, and the filtering of DC loop is capacitor; Current mode is a frequency converter that converts DC of current source into AC, and its DC loop filter is inductance. 4. Why does the voltage of the inverter change in direct proportion to the frequency? The electromagnetic torque of any motor is the result of the interaction between current and flux, and the current is not allowed to exceed the rated value, otherwise it will lead to the motor heating. Therefore, if the magnetic flux decreases, the electromagnetic torque will also decrease, resulting in a decrease in current carrying capacity. From the formula E = 4.44 * k * f * n * φ, it can be seen that the magnetic circuit of the motor changes in a considerable range with the operating frequency fX during variable frequency speed regulation, which easily makes the magnetic circuit of the motor seriously saturated, resulting in serious distortion of the excitation current waveform and high peak current. Therefore, the frequency and voltage should be changed in proportion, that is, the output voltage of the inverter should be controlled while changing the frequency, so as to keep the magnetic flux of the motor constant and avoid the phenomena of weak magnetism and magnetic saturation. This control method is mostly used for energy-saving frequency converters such as fans and pumps. 5. When the motor is driven by the power frequency power supply, the current will increase when the voltage drops; For inverter drive, if the voltage drops when the frequency drops, will the current increase? When the frequency decreases (low speed), if the same power is output, the current will increase, but the current will hardly change under the condition of constant torque. 6. What is the starting current and starting torque of the motor when using the frequency converter? With the acceleration of the motor, the frequency and voltage increase correspondingly when the inverter is used, and the starting current is limited to below 150% of the rated current (125%~200% according to different models). When starting directly with power frequency power supply, the starting current is 6~7 times of rated current, so there will be mechanical and electrical shock. Using frequency converter drive can start smoothly (start time becomes longer). The starting current is 1.2~ 1.5 times the rated current, and the starting torque is 70% ~1.20% of the rated torque; For the inverter with automatic torque enhancement function, the starting torque is above 100%, and it can be started at full load. 7. What does V/F mode mean? When the frequency decreases, the voltage V also decreases proportionally, which has been explained in answer 4. Considering the characteristics of the motor, the proportional relationship between V and F is determined in advance. Usually, there are several features in the storage device (ROM) of the controller, which can be selected by switches or dials. 8. When V and F change in proportion, how does the torque of the motor change? When the frequency decreases, the voltage decreases proportionally, so the torque generated at low speed tends to decrease, because the AC impedance becomes smaller and the DC resistance remains unchanged. Therefore, given the V/f at low frequency, the output voltage should be increased to obtain a certain starting torque. This compensation is called enhanced starting. It can be realized by various methods, such as automatic method, selecting V/f mode or adjusting potentiometer. 9. The specification says that the speed range is 60~6Hz, that is, 10: 1, so there is no output power below 6Hz? Power can still be output below 6Hz, but according to the temperature rise and starting torque of the motor, the lowest frequency is about 6Hz. At this time, the motor can output rated torque without causing serious heating problems. According to the model, the actual output frequency (starting frequency) of the inverter is 0.5~3Hz. . 10.60Hz or above, is it possible to require constant torque? Under normal circumstances, it is not allowed. When the voltage is constant above 60Hz (there are modes above 50Hz), it is generally constant power. The same torque is needed at high speed, so we must pay attention to the selection of motor and inverter capacity. 1 1. What do you mean by open loop? 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. 12. What if the actual speed deviates from the given speed? In the open loop, even if the frequency converter outputs a given frequency, when the motor is running with load, the speed of the motor changes within the range of rated slip (1%~5%). For occasions that require high speed regulation accuracy and can run at a speed close to a given speed even if the load changes, a frequency converter with PG feedback function can be used (optional). 13. If the motor with PG is used, can the speed accuracy be improved after feedback? The frequency converter with PG feedback function improves the accuracy. But the value of speed accuracy depends on the accuracy of PG itself and the resolution of inverter output frequency. 14. What is the stall prevention function? If the given acceleration time is too short, the change of inverter output frequency will far exceed the rotational speed (electrical angular frequency), and the inverter will trip and stop running because of current, which is called stall. In order to prevent stall and keep the motor running, it is necessary to detect the current and control the frequency. When the acceleration current is too large, appropriately slow down the acceleration rate. The same is true when slowing down. The combination of the two is the stall function. 15. given model has acceleration time and deceleration time, given model has acceleration and deceleration time * * *, what's the point? Acceleration and deceleration can be given by different models, which is suitable for short-time acceleration and deceleration occasions, or occasions where small machine tools need to strictly give the production rhythm. But for occasions such as fan drive, the acceleration and deceleration time is long, and the acceleration and deceleration time can be given together. 16, what is regenerative braking? If the command frequency of the motor is reduced during operation, the motor will become an asynchronous generator and work as a brake, which is called regenerative (electric) braking. 17, can you get more braking force? Regenerative energy from the motor is stored in the filter capacitor of the frequency converter. Because of the relationship between capacitance and withstand voltage, the regenerative braking force of universal inverter is about 10%~20% of rated torque. If the optional brake unit is adopted, it can reach 50%~ 100%. 18, please explain the protection function of the frequency converter? Protection functions can be divided into the following two categories: (1) automatic corrective actions after detecting abnormal conditions, such as over-loss speed prevention and regenerative overvoltage stall prevention. (2) After the abnormality is detected, the PWM control signal of the power semiconductor device is blocked, thereby automatically stopping the motor. Such as overcurrent cut-off, regenerative overvoltage cut-off, overheating of semiconductor cooling fan, instantaneous power-off protection, etc. 19. Why does the protection function of the frequency converter act when the load is connected to the clutch? When the clutch is used to connect the load, at the moment of connection, the motor changes sharply from the no-load state to the area with large slip rate, and the large current flow causes the inverter to trip over-current and cannot run. 20. When large motors run together in the same factory, the frequency converter stops during operation. Why? When the motor is started, the starting current corresponding to the capacity will flow, and the transformer on the stator side of the motor will produce voltage drop. When the motor capacity is large, this pressure drop will also have a great impact. The inverter connected to the same transformer will make the judgment of undervoltage or instantaneous shutdown, so sometimes the protection function (IPE) will act, leading to the shutdown. 2 1. What is frequency conversion resolution? What's the point? For the numerical control inverter, even if the frequency instruction is an analog signal, the output frequency is given step by step. The smallest unit of this level difference is called frequency conversion resolution. The frequency conversion resolution is usually 0.0 15~0.5Hz. For example, if the resolution is 0.5Hz, the upper part of 23Hz can be changed to 23.5 Hz and 24.0 Hz, so the motor moves step by step. This brings problems to applications such as continuous winding control. In this case, if the resolution is about 0.0 15Hz, it can completely adapt to the 4-stage motor below 1r/min/step. In addition, some models have different given resolutions and output resolutions. 22. Are there any restrictions on the installation direction when installing the inverter? The structure of the inside and back of the inverter considers the heat dissipation effect, and the relationship between the top and bottom is also very important for ventilation. Therefore, for the unit type in the tray and hanging on the wall, adopt the vertical position and install it vertically as much as possible. 23, don't use soft start, just turn the motor directly into a fixed frequency inverter? It is possible at a very low frequency, but if the given frequency is high, the conditions for direct start-up of the same frequency power supply are similar. Excessive starting current (6~7 times the rated current) will flow, and the motor cannot be started because the inverter cuts off the overcurrent. 24. What problems should be paid attention to when the motor runs above 60Hz? When operating above 60Hz, the following matters should be noted: (1) Machines and devices should operate at this speed as much as possible (mechanical strength, noise, vibration, etc. ). (2) When the motor enters the constant power output range, its output torque should be able to maintain its work (the output power of shafts such as fans and pumps increases in direct proportion to the cubic speed, and attention should be paid when the speed increases slightly). (3) Bearing life should be fully considered. (4) For motors with medium capacity or above, especially 2-pole motors, it is necessary to discuss carefully with the manufacturers when operating at 60Hz or above. 25. Can the frequency converter drive the gear motor? According to the structure and lubrication mode of the reducer, some problems need to be paid attention to. In the structure of gears, 70~80Hz can usually be considered as the maximum limit. When oil lubrication is used, continuous low-speed operation is related to gear damage. 26, inverter can be used to drive single-phase motor? Can I use a single-phase power supply? Basically not. For the single-phase motor started by the governor switch, when the speed range is lower than the working point, the auxiliary winding will be burned; For capacitor starting or capacitor operation mode, capacitor explosion will be induced. The power supply of frequency converter is usually 3-phase, but for small capacity, there are also models running with single-phase power supply. 27. How much power does the inverter consume? It is related to the model, running state and frequency of the inverter, but it is difficult to answer. The efficiency of inverter is about 94%~96% below 60Hz, so the loss can be calculated. However, if the loss of the built-in regenerative braking (FR-K) inverter is taken into account, the power consumption will increase, so we must pay attention to the design of the operation panel. 28. Why can't it be used continuously in the whole area of 6-60hz? Usually, the motor is cooled by an external fan mounted on the shaft or blades on the rotor end ring. If the speed is reduced, the cooling effect will decrease, so it can't bear the same heat as high-speed operation. Therefore, it is necessary to reduce the load torque at low speed, or use the combination of large-capacity inverter and motor, or use special motor. 29. What should I pay attention to when using a motor with a brake? The power supply of the brake excitation circuit should be taken from the input side of the frequency converter. If the brake acts when the inverter outputs power, the overcurrent will be cut off. Therefore, it is necessary to activate the brake after the inverter stops outputting. 30. I want to drive the motor with capacitor and improve the power factor with frequency converter, but the motor won't move. Please explain the reason. The current of the inverter flows into the capacitor to improve the power factor, and the inverter cannot be started because of its charging current (OCT). As a countermeasure, please remove the capacitor and even increase the power factor. It is effective to connect the AC reactor to the input side of the inverter. 3 1, how long is the life of the inverter? Although the frequency converter is a static device, it also has consumption devices such as filter capacitor and cooling fan. If it is maintained regularly, its service life is expected to exceed 10 years. 32. There is a cooling fan in the inverter. What is the wind direction? What happens if the fan breaks down? Suitable for small capacity models with or without cooling fans. For models with fans, the wind direction is from bottom to top, so where the inverter is installed, don't place mechanical equipment that hinders the suction and exhaust. In addition, do not put parts that are afraid of heat above the inverter. When the fan fails, it is protected by the stop detection of the electric fan or the overheating detection of the cooling fan. 33. The filter capacitor is a consumable, how to judge its life? As a filter capacitor, its electrostatic capacity gradually decreases with the passage of time. Measure the electrostatic capacity regularly, and judge the service life according to reaching 85% of the rated capacity of the product. 34. Is there any restriction on the installation direction when installing the inverter? Basically, it should be stored in a tray, but the problem is that the tray with fully enclosed structure is large in size, takes up a lot of space and costs a lot. The measures are as follows: (1) The design of the disk should be aimed at the heat dissipation required by the actual device; (2) Use aluminum heat sink, heat sink coolant, etc. Increase the cooling area; (3) Use heat pipes. In addition, the back of the inverter can be exposed. 35, want to improve the speed of the original conveyor belt, running at 80Hz, how to choose the capacity of the inverter? The power consumed by the conveyor belt is proportional to the rotating speed, so if it is to run at 80HZ, the power of the inverter and motor should be increased to 80HZ/50HZ in proportion, that is, the capacity will be increased by 60%. Matters needing attention in maintenance and inspection are as follows: (1) After the input power is turned off, you can wait at least 5 minutes before starting the inspection (and the officially charged LED has gone out), otherwise you will get an electric shock. (2) Maintenance, inspection and parts replacement must be carried out by qualified personnel. (Remove all metal objects (watches, bracelets, etc.) before starting work. ) and use tools with insulation protection) (3) Don't modify the frequency generator without authorization, otherwise it will easily cause electric shock and damage the product. (4) Before frequency converter maintenance, it is necessary to confirm whether the input voltage is wrong. When the 380V power supply is connected to the 220V inverter, there will be machine explosion (explosion capacitor, explosion rheostat, explosion module, etc. ). The inverter is mainly composed of semiconductor components, so daily inspection is needed to prevent harsh working environment, such as temperature, humidity, dust and vibration, and to prevent other failures caused by the service life of components. Inspection items: (1) Daily inspection: Check whether the inverter works as required. Use a voltmeter to check the input and output voltages when the inverter is working. (2) Regular inspection: check all the places that can only be checked when the inverter is stopped. (3) Component replacement: The service life of components is largely related to the installation conditions.
In this section, edit the working principle of the frequency converter.
abstract
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.
Selenium rectifier
In recent years, diode converter is widely used, 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.
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.
transverter
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 in a certain period of 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.