The core of the frequency conversion technology is the frequency converter, which realizes the automatic adjustment of the motor running speed rate by converting the power supply frequency, and changes the fixed frequency of 50 Hz to the changing frequency of 30-130 Hz.
Frequency conversion overview frequency conversion is to change the power supply frequency
The core of the frequency conversion technology is the frequency converter, which realizes the automatic adjustment of the motor running speed rate by converting the power supply frequency, and changes the fixed frequency of 50 Hz to the changing frequency of 30-130 Hz. At the same time, the power supply voltage adapts to the range of 142-270V, which solves the problem that the electrical equipment voltage is unstable due to the instability of the grid voltage.
The technology for realizing AC power control by changing the frequency of the AC power is called frequency conversion technology.
Frequency conversion technology was born in response to the need for stepless speed regulation
In the second half of the 1960s, power electronics from SCR (Thyristor), GTO (Gate Turn-Off Thyristor), BJT (Bipolar Power Transistor), MOSFET (Metal Oxide Field Effect Transistor), SIT (Static Induction Transistor) ), SITH (electrostatic induction thyristor), MGT (MOS control transistor), MCT (MOS control product thyristor) developed to today's IGBT (insulated gate bipolar transistor), HVIGBT (high voltage insulated gate bipolar thyristor), device The update has prompted the continuous development of power conversion technology. Since the 1970s, the research on pulse width modulation and variable voltage frequency conversion (PWM-VVVF) speed regulation has attracted people's attention. In the 1980s, the PWM mode optimization problem as the core of the frequency conversion technology attracted people's strong interest, and many optimization modes were obtained, among which the saddle-wave PWM mode was the best. Since the second half of the 1980s, VVVF inverters from developed countries such as the United States, Japan, Germany and Britain have been put into the market and widely used.
The frequency converter is generally a power control device that converts the power frequency power source into another frequency by using the on/off function of the power semiconductor device. The control of VVVF inverter is relatively simple, and the mechanical characteristics are also good in hardness. It can meet the requirements of smooth speed regulation of general transmission, and has been widely used in various fields of the industry. However, in the low frequency mode, since the output voltage is small, the influence of the stator resistance voltage drop is significant, so that the maximum output torque is reduced. In addition, its mechanical characteristics are not as hard as DC motors, dynamic torque capability and static speed regulation performance are not satisfactory, so people have studied vector control frequency control.
Vector control frequency control
The vector control frequency conversion speed regulation method is: the stator alternating current Ia, Ib, Ic of the asynchronous motor in the three-phase coordinate system is equivalent to the alternating current Ia1Ib1 in the two-phase stationary coordinate system by the three-phase-two-phase transformation, Then, according to the rotor field oriented rotation transformation, it is equivalent to the direct current Im1, It1 in the synchronous rotating coordinate system (Im1 is equivalent to the excitation current of the DC motor; It1 is equivalent to the armature current proportional to the torque), and then mimics the DC The control method of the motor obtains the control amount of the DC motor, and realizes the control of the asynchronous motor through the inverse inverse transformation of the corresponding coordinates.
The proposed vector control method has epoch-making significance. However, in practical applications, because the rotor flux linkage is difficult to accurately observe, the system characteristics are greatly affected by the motor parameters, and the vector rotation transformation used in the equivalent DC motor control process is complicated, making the actual control effect difficult to achieve the ideal analysis. result. In 1985, Professor DePenbrock of Ruhr University in Germany first proposed the direct torque control frequency conversion technology. This technology largely solves the above-mentioned shortcomings of vector control, and has been rapidly developed with novel control ideas, simple and clear system structure, and excellent dynamic and static performance. At present, this technology has been successfully applied to high-power AC drives for electric locomotive traction. And the frequency conversion technology is applied to more and more industries, and can be used in energy-related industries. Examples: living air conditioners, refrigerators, washing machines, etc., industry: cranes, etc.
The direct torque control directly analyzes the mathematical model of the AC motor in the stator coordinate system to control the flux linkage and torque of the motor. It does not require the conversion of an AC motor into an equivalent DC motor, thus eliminating many of the complex calculations in vector rotation transformation; it does not require the control of a DC motor, nor does it need to simplify the mathematical model of the AC motor for decoupling. VVVF frequency conversion, vector control frequency conversion, direct torque control frequency conversion are all one of AC-DC-AC frequency conversion. The common disadvantage is that the input power factor is low, the harmonic current is large, the DC loop requires a large storage capacitor, and the regenerative energy cannot be fed back to the grid, that is, the four-quadrant operation cannot be performed. To this end, matrix-type AC-AC frequency conversion came into being. Since the matrix type AC-AC frequency conversion eliminates the intermediate DC link, the bulky and expensive electrolytic capacitor is omitted. It can achieve a power factor of l, the input current is sinusoidal and can operate in four quadrants, and the system has a high power density. Although the technology is not yet mature, it still attracts many scholars to study in depth.
Frequency conversion technology and household appliances In the 1970s, household appliances began to gradually change frequency, and electromagnetic cookers, variable frequency lighting appliances, inverter air conditioners, inverter microwave ovens, inverter refrigerators, IH (induction heating) rice cookers, inverter washing machines, and the like appeared.
In the second half of the 1990s, household appliances relied on frequency conversion technology, mainly aimed at high functions and power saving. For example, it is required to have high speed and high output, good control performance, small size and light weight, large capacity, high comfort, long life, safety and reliability, quietness, and power saving.
The first is the refrigerator. Since it is working all day, after the inverter is used for refrigeration, the compressor is always in the low-speed running state, which can completely eliminate the noise caused by the compressor starting, and the energy-saving effect is more obvious.
Secondly, the air conditioner uses the frequency conversion to expand the working range of the compressor, and the cold and warm control can be realized without the compressor operating in the intermittent state, thereby reducing the power consumption and eliminating the discomfort caused by the temperature fluctuation. In recent years, the new type of air conditioner has adopted the brushless DC motor to achieve variable frequency speed regulation, and its energy saving effect is increased by about 10%-15% compared with the AC asynchronous motor frequency conversion. In order to further improve the performance of the device, in recent years, Japanese air conditioners have gradually changed from simple PWM control to PWM ten PAM hybrid control. That is, PWM control is adopted at a lower speed to keep U/f constant; when the rotation speed is greater than a certain value, the modulation degree is fixed near the maximum value, and the inverter input is improved by changing the conduction duty LL of the DC chopper. The DC voltage value, so as to keep the inverter output voltage proportional to the speed, this area is called the PAM area. After adopting the hybrid control mode, the input power factor, motor efficiency and overall efficiency of the inverter are greatly improved compared with the PWA4 control alone.
New type of frequency conversion in recent years
The new type of inverter refrigerator not only reduces power consumption, but also achieves quietness, and it can realize rapid freezing at a large speed by using high-speed operation. In the washing machine, variable frequency control is used in the past to improve the cleaning performance, and the new popular washing machine is not only energy-saving. In addition to quieting, it also introduces new control content in ensuring soft washing of clothes; electromagnetic cooking uses high-frequency induction heating to directly heat the pot, without the hot part of gas and electric heating, so it is not only safe but also greatly improved. Heating efficiency, its working frequency is higher than the hearing, thus eliminating the noise caused by the vibration of the rice cooker; IH electric rice is more powerful than the electric heater, and the frequency can be used to fine-tune the fire, as long as the heating induction coil is properly designed Any heating layout can be obtained, and the performance of risotto is up to a grade; the inverter microwave oven uses high-frequency electric energy to provide the necessary boosting drive for the magnetron, the power supply structure is small, the furnace space is more spacious, and the new microwave oven can adjust the power arbitrarily, and Choose the best heating method according to different foods, shorten the time and reduce the power consumption; Aspect, the use of high-frequency fluorescent lighting, can improve the luminous efficiency, energy saving, flicker-free, easy dimming frequency adjustable, compact and lightweight ballast.
Frequency conversion technology is bringing a new revolution to a wide range of home appliances, and will bring greater gospel to users. In the future, the frequency conversion technology will also develop with the advancement of power electronic devices, new power conversion topology circuits, filtering and shielding technologies. Home solar power systems will also add new energy to home appliances.
Harm and Countermeasures of Power Electronic Devices Phase-controlled rectification and uncontrollable diode rectification in power electronic devices cause severe distortion of the input current waveform, which not only greatly reduces the power factor of the system, but also causes serious harmonic pollution. In addition, the sharp changes in voltage and current in the hardware circuit cause the power electronics to withstand large electrical stresses and cause severe electromagnetic interference (EMl) to surrounding electrical equipment and electric waves, and the situation is becoming increasingly serious. Many countries have established national standards for limiting harmonics. The International Institute of Electrical and Electronics Engineers (IEEE), the International Electrotechnical Commission (IEC) and the International Conference on Large Power Grids (CIGRE) have introduced their own harmonic standards. The Chinese government also enacted regulations on limiting harmonics in 1984 and 1993 respectively.
1. Countermeasures for Harmonics and Electromagnetic Interference
(1) Harmonic Suppression In order to suppress harmonics generated by power electronic devices, one method is to perform harmonic compensation, that is, to set a harmonic compensation device so that the input current becomes a sine wave.
The traditional harmonic compensation device uses an lC tuned filter that compensates for both harmonics and reactive power. The disadvantage is that the compensation characteristics are affected by the impedance and operating state of the power grid, and it is easy to parallel resonance with the system, resulting in harmonic amplification, which causes the LC filter to be overloaded or even burned. In addition, it can only compensate for harmonics at a fixed frequency, and the effect is not ideal. However, such a compensation device has a simple structure and is still widely used.
After the popularization of power electronic devices, the use of active power filters for harmonic compensation has become an important direction. The principle is that a harmonic current is detected from the compensation object, and then a compensation current having the same polarity as the harmonic current is generated, so that the grid current only contains the fundamental component. This filter can track and compensate harmonics with varying frequency and amplitude, and the compensation characteristics are not affected by the impedance of the grid. It has received people's attention and will gradually be applied.
Another method is to reform the working mechanism of the converter to suppress both harmonics and power factor. This converter is called a unit power factor converter. The main method for reducing harmonics in large-capacity converters is to use multiple techniques: superimpose multiple square waves to eliminate lower harmonics, resulting in a nearly sinusoidal stepped wave. The more the number, the closer the waveform is to the sine, but the more complicated the circuit structure.
High-power factor converters ranging from a few kilowatts to hundreds of kilowatts mainly use PWM rectification technology. It directly performs sinusoidal PWM control on each power electronic device on the rectifier bridge, so that the input current is close to a sine wave, and its phase is the same as the phase of the power phase voltage. In this way, the input current contains only the higher harmonics related to the switching frequency. These harmonics are high in number and easy to filter, and also make the power factor close to 1. A PWM inverter is used as an AC/DC converter PWM inverter, which is a so-called dual PWM inverter. It has an input voltage, a fixed current frequency, a sinusoidal waveform, a power factor close to 1, a variable output voltage, a variable current, and a sinusoidal current waveform. This frequency converter enables four-quadrant operation for bidirectional transmission of energy.
In order to achieve low harmonics and high power factor, small-capacity converters generally use diode rectification plus PWM chopping, often referred to as power factor correction (PEC). Typical circuits are boost type, step-down type, buck-boost type, and the like.
(2) Electromagnetic interference suppression The EMI measures are to overcome the excessive current rise rate di/dt and voltage rise rate du/dt when the switching device is turned on and off. The current comparison is focused on the zero current switch (ZCS). And zero voltage switching (ZVS) circuits. The method is: 1 series inductor on the switching device, which can suppress the di/dt when the switching device is turned on, so that there is no voltage and current overlapping region on the device, and the switching loss is reduced; 2 the parallel capacitance of the switching device is turned off when the device is turned off. After the suppression of du/dt rise, there is no voltage and current overlap region on the device, which reduces the switching loss; 3 the anti-parallel diode on the device, during the diode conduction period, the switching device exhibits zero voltage and zero current state. Turning on or off can achieve ZVS and ZCS actions.
The more commonly used soft switching technologies are:
1 part resonant PWM. In order to make the efficiency as close as possible to the hard switching, it is necessary to prevent an increase in the effective value of the device current. Therefore, in one switching cycle, the circuit is resonated only when the device is turned on and off, which is called partial resonance. 2 lossless buffer circuit. When the energy on the series inductor or shunt capacitor is released, it does not pass through the resistor or the switching device, and is called a lossless buffer circuit, and the anti-parallel diode is often not used. In the motor control, the main switching device adopts IGBT, and the tail current flows when the IGBT is turned off, which has an impact on the turn-off loss. Therefore, it is more appropriate to use a ZCS with a short zero current for the turn-off.
2. Early methods of power factor compensation
It is a synchronous camera, which is a synchronous motor specially used to generate reactive power. Different sizes of capacitive or inductive reactive power are generated by overexcitation and underexcitation respectively. However, since it is a rotating electrical machine, the noise and loss are large, the operation and maintenance are complicated, and the response speed is slow. Therefore, in many cases, it is unable to meet the requirements of fast reactive power compensation.
Another method is to use a static var compensator for a saturable reactor. It has the advantages of static type and fast response speed, but because its core needs to be magnetized to saturation, the loss and noise are very large, and there are some special problems of the nonlinear circuit, and it cannot be phase-phase adjusted to compensate the load imbalance. Therefore, it failed to occupy the mainstream of the static var compensator.
With the continuous development of power electronics technology, static var compensators using SCR, GTO and IGBT have been greatly developed, among which static var generators are the most advantageous. It has the advantages of fast adjustment speed and wide operating range, and can greatly reduce the harmonic content in the compensation current after taking measures such as multiplexing, multi-level or PWM technology. More importantly, the reactor and capacitor components used in the static var generator are small, which greatly reduces the size and cost of the device. The static var generator represents the development direction of the dynamic var compensator.
Radio frequency conversion principle
The so-called "frequency conversion" is to convert the received station signal into a "intermediate frequency" with a relatively low frequency but the same program content through a circuit called "inverter", and then amplify and "detect" the "intermediate frequency" ( Take out the audio signal carried in the radio high-frequency signal ["Electric signal indicating sound"] for listening).
Because the intermediate frequency is lower than the radio signal frequency (some intermediate frequency of some machines is higher than the radio signal frequency, it is another matter), the amplification is easy, it is not easy to cause self-excitation, the sensitivity is high, and many "tuning loops" can be made for the fixed intermediate frequency. Good selectivity. With automatic gain (magnification) control circuit (so-called AGC), the volume difference between strong and weak stations is reduced.
Frequency conversion series resonance test device for variable frequency series resonance test set is based on the principle of series resonance
The series resonant circuit is excited by the excitation transformer, and the output frequency of the variable frequency controller is adjusted, so that the loop inductance L and the sample C are in series resonance, and the resonance voltage is the voltage applied to the sample. The frequency conversion resonance test device is widely used in electric power, metallurgy, petroleum, chemical and other industries, and is suitable for the handover and preventive test of large-capacity, high-voltage capacitive samples.
BPXZ series resonant withstand voltage device is mainly composed of frequency conversion controller, excitation transformer, high voltage reactor, high voltage divider and so on. The frequency conversion controller is divided into two categories, the console type is 20KW and above, and the portable box type is below 20KW; it is composed of a controller and a filter. The main function of the variable frequency controller is to convert the 380V or 200V power frequency sinusoidal alternating current with fixed amplitude and frequency into a sine wave with adjustable amplitude and frequency. And provide power for the entire set of equipment. The function of the excitation transformer is to raise the voltage output from the variable frequency power supply to a suitable test voltage. The high-voltage reactor L is an important component of the resonant circuit. When the power supply frequency is equal to 1/(2π√LCX), it is in series resonance with the test object CX.
The device is suitable for AC withstand voltage test of 10KV, 35KV, 110KV, 220KV, 500KV polyhexene power cables. Applicable to 60KV, 220KV, 500KV GIS AC withstand voltage test. Applicable to large transformers, generator set power frequency withstand voltage test; power transformer induction withstand voltage test; grounding resistance measurement.
Product name of frequency conversion resonance test device:
Frequency conversion resonance, variable frequency series resonance, series resonance, series resonant transformer, series resonance test equipment, series resonance principle, series resonance application, series resonance
Frequency conversion resonance product features:
1. High stability and reliability. The system adopts imported power components as the core of power conversion, stable voltage output and frequency output, reasonable electromagnetic compatibility design and perfect protection function. After many high-voltage direct ground short-circuit tests, the system remains intact and the system also has a strong overload. ability.
2. Automatic tuning is powerful. When the system is automatically tuned, the frequency is automatically swept from 30Hz to 300Hz, and the sweep curve is displayed. The user can visually see the system tuning process. After the sweep is completed, the system finds the resonant frequency based on the frequency sweep, within ±5Hz. The frequency is finely scanned at a resolution of 0.01 Hz, and finally the resonant frequency is accurately locked.
3. Support multiple test modes. The system supports "automatic tuning + manual voltage regulation", "automatic tuning + automatic voltage regulation", "manual tuning + manual voltage regulation" and other test modes, it is recommended to use "automatic tuning + manual voltage regulation" mode, which can quickly find the resonance point. Moreover, the test process can be controlled by manual pressure regulation, and the safety is higher.
4. The system human-computer interaction interface is friendly. The test parameter setting, test control and test result are equivalent to the screen display, which is intuitive and clear, and has automatic timing and operation prompt function. Full touch screen operation and display, with test data saving and query function.
5. The protection function is perfect. With zero protection (when the voltage output control knob is not in the zero position, the system is disabled), overvoltage protection, overcurrent protection, flashover protection and other functions ensure the reliability of the system.
Http://news.chinawj.com.cn Editor: (Hardware Business Network Information Center) http://news.chinawj.com.cn
Frequency conversion overview frequency conversion is to change the power supply frequency
The core of the frequency conversion technology is the frequency converter, which realizes the automatic adjustment of the motor running speed rate by converting the power supply frequency, and changes the fixed frequency of 50 Hz to the changing frequency of 30-130 Hz. At the same time, the power supply voltage adapts to the range of 142-270V, which solves the problem that the electrical equipment voltage is unstable due to the instability of the grid voltage.
The technology for realizing AC power control by changing the frequency of the AC power is called frequency conversion technology.
Frequency conversion technology was born in response to the need for stepless speed regulation
In the second half of the 1960s, power electronics from SCR (Thyristor), GTO (Gate Turn-Off Thyristor), BJT (Bipolar Power Transistor), MOSFET (Metal Oxide Field Effect Transistor), SIT (Static Induction Transistor) ), SITH (electrostatic induction thyristor), MGT (MOS control transistor), MCT (MOS control product thyristor) developed to today's IGBT (insulated gate bipolar transistor), HVIGBT (high voltage insulated gate bipolar thyristor), device The update has prompted the continuous development of power conversion technology. Since the 1970s, the research on pulse width modulation and variable voltage frequency conversion (PWM-VVVF) speed regulation has attracted people's attention. In the 1980s, the PWM mode optimization problem as the core of the frequency conversion technology attracted people's strong interest, and many optimization modes were obtained, among which the saddle-wave PWM mode was the best. Since the second half of the 1980s, VVVF inverters from developed countries such as the United States, Japan, Germany and Britain have been put into the market and widely used.
The frequency converter is generally a power control device that converts the power frequency power source into another frequency by using the on/off function of the power semiconductor device. The control of VVVF inverter is relatively simple, and the mechanical characteristics are also good in hardness. It can meet the requirements of smooth speed regulation of general transmission, and has been widely used in various fields of the industry. However, in the low frequency mode, since the output voltage is small, the influence of the stator resistance voltage drop is significant, so that the maximum output torque is reduced. In addition, its mechanical characteristics are not as hard as DC motors, dynamic torque capability and static speed regulation performance are not satisfactory, so people have studied vector control frequency control.
Vector control frequency control
The vector control frequency conversion speed regulation method is: the stator alternating current Ia, Ib, Ic of the asynchronous motor in the three-phase coordinate system is equivalent to the alternating current Ia1Ib1 in the two-phase stationary coordinate system by the three-phase-two-phase transformation, Then, according to the rotor field oriented rotation transformation, it is equivalent to the direct current Im1, It1 in the synchronous rotating coordinate system (Im1 is equivalent to the excitation current of the DC motor; It1 is equivalent to the armature current proportional to the torque), and then mimics the DC The control method of the motor obtains the control amount of the DC motor, and realizes the control of the asynchronous motor through the inverse inverse transformation of the corresponding coordinates.
The proposed vector control method has epoch-making significance. However, in practical applications, because the rotor flux linkage is difficult to accurately observe, the system characteristics are greatly affected by the motor parameters, and the vector rotation transformation used in the equivalent DC motor control process is complicated, making the actual control effect difficult to achieve the ideal analysis. result. In 1985, Professor DePenbrock of Ruhr University in Germany first proposed the direct torque control frequency conversion technology. This technology largely solves the above-mentioned shortcomings of vector control, and has been rapidly developed with novel control ideas, simple and clear system structure, and excellent dynamic and static performance. At present, this technology has been successfully applied to high-power AC drives for electric locomotive traction. And the frequency conversion technology is applied to more and more industries, and can be used in energy-related industries. Examples: living air conditioners, refrigerators, washing machines, etc., industry: cranes, etc.
The direct torque control directly analyzes the mathematical model of the AC motor in the stator coordinate system to control the flux linkage and torque of the motor. It does not require the conversion of an AC motor into an equivalent DC motor, thus eliminating many of the complex calculations in vector rotation transformation; it does not require the control of a DC motor, nor does it need to simplify the mathematical model of the AC motor for decoupling. VVVF frequency conversion, vector control frequency conversion, direct torque control frequency conversion are all one of AC-DC-AC frequency conversion. The common disadvantage is that the input power factor is low, the harmonic current is large, the DC loop requires a large storage capacitor, and the regenerative energy cannot be fed back to the grid, that is, the four-quadrant operation cannot be performed. To this end, matrix-type AC-AC frequency conversion came into being. Since the matrix type AC-AC frequency conversion eliminates the intermediate DC link, the bulky and expensive electrolytic capacitor is omitted. It can achieve a power factor of l, the input current is sinusoidal and can operate in four quadrants, and the system has a high power density. Although the technology is not yet mature, it still attracts many scholars to study in depth.
Frequency conversion technology and household appliances In the 1970s, household appliances began to gradually change frequency, and electromagnetic cookers, variable frequency lighting appliances, inverter air conditioners, inverter microwave ovens, inverter refrigerators, IH (induction heating) rice cookers, inverter washing machines, and the like appeared.
In the second half of the 1990s, household appliances relied on frequency conversion technology, mainly aimed at high functions and power saving. For example, it is required to have high speed and high output, good control performance, small size and light weight, large capacity, high comfort, long life, safety and reliability, quietness, and power saving.
The first is the refrigerator. Since it is working all day, after the inverter is used for refrigeration, the compressor is always in the low-speed running state, which can completely eliminate the noise caused by the compressor starting, and the energy-saving effect is more obvious.
Secondly, the air conditioner uses the frequency conversion to expand the working range of the compressor, and the cold and warm control can be realized without the compressor operating in the intermittent state, thereby reducing the power consumption and eliminating the discomfort caused by the temperature fluctuation. In recent years, the new type of air conditioner has adopted the brushless DC motor to achieve variable frequency speed regulation, and its energy saving effect is increased by about 10%-15% compared with the AC asynchronous motor frequency conversion. In order to further improve the performance of the device, in recent years, Japanese air conditioners have gradually changed from simple PWM control to PWM ten PAM hybrid control. That is, PWM control is adopted at a lower speed to keep U/f constant; when the rotation speed is greater than a certain value, the modulation degree is fixed near the maximum value, and the inverter input is improved by changing the conduction duty LL of the DC chopper. The DC voltage value, so as to keep the inverter output voltage proportional to the speed, this area is called the PAM area. After adopting the hybrid control mode, the input power factor, motor efficiency and overall efficiency of the inverter are greatly improved compared with the PWA4 control alone.
New type of frequency conversion in recent years
The new type of inverter refrigerator not only reduces power consumption, but also achieves quietness, and it can realize rapid freezing at a large speed by using high-speed operation. In the washing machine, variable frequency control is used in the past to improve the cleaning performance, and the new popular washing machine is not only energy-saving. In addition to quieting, it also introduces new control content in ensuring soft washing of clothes; electromagnetic cooking uses high-frequency induction heating to directly heat the pot, without the hot part of gas and electric heating, so it is not only safe but also greatly improved. Heating efficiency, its working frequency is higher than the hearing, thus eliminating the noise caused by the vibration of the rice cooker; IH electric rice is more powerful than the electric heater, and the frequency can be used to fine-tune the fire, as long as the heating induction coil is properly designed Any heating layout can be obtained, and the performance of risotto is up to a grade; the inverter microwave oven uses high-frequency electric energy to provide the necessary boosting drive for the magnetron, the power supply structure is small, the furnace space is more spacious, and the new microwave oven can adjust the power arbitrarily, and Choose the best heating method according to different foods, shorten the time and reduce the power consumption; Aspect, the use of high-frequency fluorescent lighting, can improve the luminous efficiency, energy saving, flicker-free, easy dimming frequency adjustable, compact and lightweight ballast.
Frequency conversion technology is bringing a new revolution to a wide range of home appliances, and will bring greater gospel to users. In the future, the frequency conversion technology will also develop with the advancement of power electronic devices, new power conversion topology circuits, filtering and shielding technologies. Home solar power systems will also add new energy to home appliances.
Harm and Countermeasures of Power Electronic Devices Phase-controlled rectification and uncontrollable diode rectification in power electronic devices cause severe distortion of the input current waveform, which not only greatly reduces the power factor of the system, but also causes serious harmonic pollution. In addition, the sharp changes in voltage and current in the hardware circuit cause the power electronics to withstand large electrical stresses and cause severe electromagnetic interference (EMl) to surrounding electrical equipment and electric waves, and the situation is becoming increasingly serious. Many countries have established national standards for limiting harmonics. The International Institute of Electrical and Electronics Engineers (IEEE), the International Electrotechnical Commission (IEC) and the International Conference on Large Power Grids (CIGRE) have introduced their own harmonic standards. The Chinese government also enacted regulations on limiting harmonics in 1984 and 1993 respectively.
1. Countermeasures for Harmonics and Electromagnetic Interference
(1) Harmonic Suppression In order to suppress harmonics generated by power electronic devices, one method is to perform harmonic compensation, that is, to set a harmonic compensation device so that the input current becomes a sine wave.
The traditional harmonic compensation device uses an lC tuned filter that compensates for both harmonics and reactive power. The disadvantage is that the compensation characteristics are affected by the impedance and operating state of the power grid, and it is easy to parallel resonance with the system, resulting in harmonic amplification, which causes the LC filter to be overloaded or even burned. In addition, it can only compensate for harmonics at a fixed frequency, and the effect is not ideal. However, such a compensation device has a simple structure and is still widely used.
After the popularization of power electronic devices, the use of active power filters for harmonic compensation has become an important direction. The principle is that a harmonic current is detected from the compensation object, and then a compensation current having the same polarity as the harmonic current is generated, so that the grid current only contains the fundamental component. This filter can track and compensate harmonics with varying frequency and amplitude, and the compensation characteristics are not affected by the impedance of the grid. It has received people's attention and will gradually be applied.
Another method is to reform the working mechanism of the converter to suppress both harmonics and power factor. This converter is called a unit power factor converter. The main method for reducing harmonics in large-capacity converters is to use multiple techniques: superimpose multiple square waves to eliminate lower harmonics, resulting in a nearly sinusoidal stepped wave. The more the number, the closer the waveform is to the sine, but the more complicated the circuit structure.
High-power factor converters ranging from a few kilowatts to hundreds of kilowatts mainly use PWM rectification technology. It directly performs sinusoidal PWM control on each power electronic device on the rectifier bridge, so that the input current is close to a sine wave, and its phase is the same as the phase of the power phase voltage. In this way, the input current contains only the higher harmonics related to the switching frequency. These harmonics are high in number and easy to filter, and also make the power factor close to 1. A PWM inverter is used as an AC/DC converter PWM inverter, which is a so-called dual PWM inverter. It has an input voltage, a fixed current frequency, a sinusoidal waveform, a power factor close to 1, a variable output voltage, a variable current, and a sinusoidal current waveform. This frequency converter enables four-quadrant operation for bidirectional transmission of energy.
In order to achieve low harmonics and high power factor, small-capacity converters generally use diode rectification plus PWM chopping, often referred to as power factor correction (PEC). Typical circuits are boost type, step-down type, buck-boost type, and the like.
(2) Electromagnetic interference suppression The EMI measures are to overcome the excessive current rise rate di/dt and voltage rise rate du/dt when the switching device is turned on and off. The current comparison is focused on the zero current switch (ZCS). And zero voltage switching (ZVS) circuits. The method is: 1 series inductor on the switching device, which can suppress the di/dt when the switching device is turned on, so that there is no voltage and current overlapping region on the device, and the switching loss is reduced; 2 the parallel capacitance of the switching device is turned off when the device is turned off. After the suppression of du/dt rise, there is no voltage and current overlap region on the device, which reduces the switching loss; 3 the anti-parallel diode on the device, during the diode conduction period, the switching device exhibits zero voltage and zero current state. Turning on or off can achieve ZVS and ZCS actions.
The more commonly used soft switching technologies are:
1 part resonant PWM. In order to make the efficiency as close as possible to the hard switching, it is necessary to prevent an increase in the effective value of the device current. Therefore, in one switching cycle, the circuit is resonated only when the device is turned on and off, which is called partial resonance. 2 lossless buffer circuit. When the energy on the series inductor or shunt capacitor is released, it does not pass through the resistor or the switching device, and is called a lossless buffer circuit, and the anti-parallel diode is often not used. In the motor control, the main switching device adopts IGBT, and the tail current flows when the IGBT is turned off, which has an impact on the turn-off loss. Therefore, it is more appropriate to use a ZCS with a short zero current for the turn-off.
2. Early methods of power factor compensation
It is a synchronous camera, which is a synchronous motor specially used to generate reactive power. Different sizes of capacitive or inductive reactive power are generated by overexcitation and underexcitation respectively. However, since it is a rotating electrical machine, the noise and loss are large, the operation and maintenance are complicated, and the response speed is slow. Therefore, in many cases, it is unable to meet the requirements of fast reactive power compensation.
Another method is to use a static var compensator for a saturable reactor. It has the advantages of static type and fast response speed, but because its core needs to be magnetized to saturation, the loss and noise are very large, and there are some special problems of the nonlinear circuit, and it cannot be phase-phase adjusted to compensate the load imbalance. Therefore, it failed to occupy the mainstream of the static var compensator.
With the continuous development of power electronics technology, static var compensators using SCR, GTO and IGBT have been greatly developed, among which static var generators are the most advantageous. It has the advantages of fast adjustment speed and wide operating range, and can greatly reduce the harmonic content in the compensation current after taking measures such as multiplexing, multi-level or PWM technology. More importantly, the reactor and capacitor components used in the static var generator are small, which greatly reduces the size and cost of the device. The static var generator represents the development direction of the dynamic var compensator.
Radio frequency conversion principle
The so-called "frequency conversion" is to convert the received station signal into a "intermediate frequency" with a relatively low frequency but the same program content through a circuit called "inverter", and then amplify and "detect" the "intermediate frequency" ( Take out the audio signal carried in the radio high-frequency signal ["Electric signal indicating sound"] for listening).
Because the intermediate frequency is lower than the radio signal frequency (some intermediate frequency of some machines is higher than the radio signal frequency, it is another matter), the amplification is easy, it is not easy to cause self-excitation, the sensitivity is high, and many "tuning loops" can be made for the fixed intermediate frequency. Good selectivity. With automatic gain (magnification) control circuit (so-called AGC), the volume difference between strong and weak stations is reduced.
Frequency conversion series resonance test device for variable frequency series resonance test set is based on the principle of series resonance
The series resonant circuit is excited by the excitation transformer, and the output frequency of the variable frequency controller is adjusted, so that the loop inductance L and the sample C are in series resonance, and the resonance voltage is the voltage applied to the sample. The frequency conversion resonance test device is widely used in electric power, metallurgy, petroleum, chemical and other industries, and is suitable for the handover and preventive test of large-capacity, high-voltage capacitive samples.
BPXZ series resonant withstand voltage device is mainly composed of frequency conversion controller, excitation transformer, high voltage reactor, high voltage divider and so on. The frequency conversion controller is divided into two categories, the console type is 20KW and above, and the portable box type is below 20KW; it is composed of a controller and a filter. The main function of the variable frequency controller is to convert the 380V or 200V power frequency sinusoidal alternating current with fixed amplitude and frequency into a sine wave with adjustable amplitude and frequency. And provide power for the entire set of equipment. The function of the excitation transformer is to raise the voltage output from the variable frequency power supply to a suitable test voltage. The high-voltage reactor L is an important component of the resonant circuit. When the power supply frequency is equal to 1/(2π√LCX), it is in series resonance with the test object CX.
The device is suitable for AC withstand voltage test of 10KV, 35KV, 110KV, 220KV, 500KV polyhexene power cables. Applicable to 60KV, 220KV, 500KV GIS AC withstand voltage test. Applicable to large transformers, generator set power frequency withstand voltage test; power transformer induction withstand voltage test; grounding resistance measurement.
Product name of frequency conversion resonance test device:
Frequency conversion resonance, variable frequency series resonance, series resonance, series resonant transformer, series resonance test equipment, series resonance principle, series resonance application, series resonance
Frequency conversion resonance product features:
1. High stability and reliability. The system adopts imported power components as the core of power conversion, stable voltage output and frequency output, reasonable electromagnetic compatibility design and perfect protection function. After many high-voltage direct ground short-circuit tests, the system remains intact and the system also has a strong overload. ability.
2. Automatic tuning is powerful. When the system is automatically tuned, the frequency is automatically swept from 30Hz to 300Hz, and the sweep curve is displayed. The user can visually see the system tuning process. After the sweep is completed, the system finds the resonant frequency based on the frequency sweep, within ±5Hz. The frequency is finely scanned at a resolution of 0.01 Hz, and finally the resonant frequency is accurately locked.
3. Support multiple test modes. The system supports "automatic tuning + manual voltage regulation", "automatic tuning + automatic voltage regulation", "manual tuning + manual voltage regulation" and other test modes, it is recommended to use "automatic tuning + manual voltage regulation" mode, which can quickly find the resonance point. Moreover, the test process can be controlled by manual pressure regulation, and the safety is higher.
4. The system human-computer interaction interface is friendly. The test parameter setting, test control and test result are equivalent to the screen display, which is intuitive and clear, and has automatic timing and operation prompt function. Full touch screen operation and display, with test data saving and query function.
5. The protection function is perfect. With zero protection (when the voltage output control knob is not in the zero position, the system is disabled), overvoltage protection, overcurrent protection, flashover protection and other functions ensure the reliability of the system.
Http://news.chinawj.com.cn Editor: (Hardware Business Network Information Center) http://news.chinawj.com.cn
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