【Daily Question 20090220】What is the difference between an AC-AC inverter and an AC-DC-AC inverter?

AC-DC-AC voltage-type frequency converters are relatively expensive. There are also two technical issues with them: first, the presence of an intermediate rectification and filtering stage results in low efficiency; second, when the motor is in power generation mode, it is difficult to return energy to the power grid, and usually a resistance circuit is used to dissipate that energy. This not only increases the size of the equipment but also represents a significant waste as the energy is not utilized. To enable the use of this energy, an active inversion circuit can be added, but this in turn increases costs and complicates the circuit design. The working principle of the AC-AC frequency converter is to convert a three-phase mains power supply into the desired voltage and frequency by using several sets of phase-controlled switches. Its advantage is high efficiency, as energy can be easily returned to the power grid. Its biggest drawback is that the highest output frequency must be less than 1/3 or 1/2 of the input power frequency; otherwise, the output waveform becomes poor, causing the motor to vibrate and fail to operate. Old-fashioned frequency converters are still limited to applications requiring speed control at low speeds, which **limits their range of use.

AC-AC inverters use thyristor-based natural commutation, ensuring stable and reliable operation; they are suitable as power supplies for the rotor windings of doubly-fed motors. The maximum output frequency of AC-AC inverters is 1/3 to 1/2 of the grid frequency, giving them a significant advantage in high-power, low-frequency applications. AC-AC frequency conversion does not have a DC link, resulting in high conversion efficiency. The main circuit is simple, as it lacks DC circuits and filtering components; thus, it is easy to handle reactive power and to recycle active power with the power supply. Although AC-AC variable-frequency double-fed systems are widely used, their application is limited to some extent due to their low power factor, numerous high harmonics, low output frequency, narrow frequency range, and the large number of components required. A matrix-type inverter is a type of AC-AC direct converter, consisting of nine switches directly connected between the three-phase input and output. Matrix converters do not have an intermediate DC link; their output consists of three levels, and they have a low level of harmonics ； Its power circuit is simple and compact, and it can generate a sinusoidal load voltage with controllable frequency, amplitude, and phase ； The input power factor of the matrix converter is controllable, and it can operate in all four quadrants. Although matrix converters have many advantages, it is difficult to implement them since two switches cannot be turned on or off simultaneously during the commutation process. The low maximum output voltage capability of matrix converters, along with the high voltage stress on the components, is also a significant drawback of such converters. When applied in wind power generation, since the input and output of the matrix converter are not decoupled, asymmetries on either the load side or the power supply side will affect the other side. Furthermore, a filtering capacitor must be connected to the input side of the matrix converter. Although its capacitance value is smaller than that of the intermediate energy storage capacitor in the AC-DC-AC conversion system, since it is an AC capacitor that has to handle alternating current at the switching frequency, its size is not small. AC-AC frequency conversion technology was the main form of medium-voltage frequency conversion in the early days; its operating principle meant that it could only function at low frequencies, making it suitable only for applications with low speeds and large capacities. Since the switching devices in the main circuit are in a naturally closed state, there is no issue of forced commutation; therefore, the first generation of power electronic devices – thyristors (SCRs) – were sufficient to meet the requirements. As a result, this type of medium-voltage frequency conversion technology is quite mature

The principle of an inverter is a power control device that converts alternating current power supply into electrical energy of another frequency by utilizing the on-off operation of power semiconductor devices. An AC-DC-AC inverter first converts alternating current into direct current using a rectifier; the DC intermediate circuit smooths and filters the output of the rectifier circuit, and then an inverter transforms this direct current into alternating current with variable frequency and voltage. AC-DC-AC inverters can be further divided into voltage-type and current-type types. Due to various factors such as control methods and hardware design, voltage-type inverters are more widely used. Traditional current-type AC-DC-AC inverters use thyristors with natural commutation as power switches. The inductor on the DC side is relatively expensive, and when used in doubly-fed speed control, a commutation circuit is required at speeds above synchronous speed; moreover, their performance is poor at low slip frequencies, which is why they are not widely used in doubly-fed asynchronous wind power generation. The voltage-type AC-DC-AC inverter, which is a type of rectification and frequency conversion device, boasts excellent advantages such as a simple structure, low harmonic content, and the ability to adjust the power factors of both the stator and rotor. It can significantly improve the operating conditions of doubly-fed generators as well as the quality of the electrical energy they produce. Moreover, this design enables complete separation between the grid side and the rotor side through the capacitors on the DC bus side. The stator flux orientation vector control system for the doubly-fed generator in voltage-type AC-DC-AC inverters enables decoupled control of the active and reactive powers of the generator based on maximum power point tracking of the wind turbine, and it represents a prominent approach in variable-speed constant-frequency wind power generation. In addition, there is also a parallel AC-DC-AC inverter topology. The main idea of this structure is to combine an AC-DC-AC current-type inverter with an AC-DC-AC voltage-type inverter in parallel; the current-type inverter serves as the main inverter responsible for power transmission, while the voltage-type inverter acts as a secondary inverter to compensate for the harmonics generated by the current-type inverter. In this structure, the main inverter has a lower switching frequency, while the auxiliary inverter has a lower switching current. Compared with the aforementioned AC-DC-AC voltage-source inverter, this topology features low switching losses, resulting in a higher efficiency for the entire system. Its disadvantages are also obvious: the use of a large number of power electronic devices leads to increased costs and more complex control algorithms, and moreover, this type of structure has a relatively low voltage utilization efficiency. High-voltage frequency converters can be divided into AC-DC-AC frequency converters and AC-AC frequency converters; currently, AC-DC-AC frequency converters are widely used in China. Its features: high efficiency, with no additional losses during speed regulation ； It has a wide range of applications and can be used in cage-type asynchronous motors ； Wide speed control range, firm characteristics, high precision ； It is technically complex, costly, and difficult to maintain and repair. This method is suitable for applications that require high precision and good speed regulation performance. Features of AC-DC-AC inverters: Since the 1980s, self-turning-off power semiconductor devices that challenged the dominance of thyristor components have been introduced, such as high-power transistors GTR, turn-off thyristors GTO, and field-controlled devices like insulated gate bipolar transistors IGBT. This marked the beginning of a new era centered around self-turning-off power semiconductor devices. Compared to traditional semi-controlled thyristor devices, electrical drives that use these self-turning-off power semiconductor devices offer significant advantages, including reduced use of raw materials, simpler circuit structures, smaller size, lower weight, a higher power factor, and less harmonic distortion. Problems with AC-DC-AC inverters: Although AC-DC-AC inverters have advantages such as high output frequency and high power factor, they still have many issues that need to be improved: (1) Current high-power, high-voltage power electronic devices are still in the development stage; GTO components are on the verge of being phased out, while IGBTs and IGCTs are not yet fully mature ； (2) For converters using IGCTs (or GTOs) and IECTs, providing protection against direct short circuits caused by device failures remains a challenge ； If a direct short circuit occurs in the converter on the power supply side, it can cause a short circuit in the power grid; therefore, the converter must use an input transformer with high leakage inductance, typically 15%, or even as high as 20% ； (3) The overload capacity of AC-DC-AC inverters decreases when operating at low frequencies; generally, the overload capacity is reduced by half when operating below 5Hz ； (4) The voltage variation rate du/dt of the PWM-modulated voltage waveform output by the AC-DC-AC inverter is very high, which can easily cause insulation fatigue damage to motors and electrical devices ； When the output conductors are long, common-mode reflection voltage can generate a very high voltage on the motor side. For a two-level converter, the peak value of this voltage is twice the DC voltage; for a three-level converter, it is three times the intermediate half-voltage ； (5) The PWM modulation of AC-DC-AC inverters can cause problems such as harmonics, noise, and shaft current.

Low-voltage frequency converters primarily use the AC-DC-AC approach: the mains AC power is first converted into DC power using a rectifier, and then that DC power is transformed into AC power whose frequency and voltage can be controlled, in order to supply the motor. The circuit of an inverter generally consists of four parts: rectification, intermediate DC link, inversion, and control. The rectification section is a three-phase bridge-type uncontrolled rectifier, the inversion section is an IGBT three-phase bridge-type inverter, and the output is in PWM waveform form; the intermediate DC link serves for filtering, storing DC energy, and buffering reactive power.

Differences between AC-AC inverters and AC-DC-AC inverters: The principle of an inverter is to use the on-off operation of power semiconductor devices to convert mains electricity into electrical energy at another frequency; it is a control device for this purpose. An AC-DC-AC inverter first converts alternating current into direct current using a rectifier; the DC intermediate circuit smooths and filters the output of the rectifier circuit, and then an inverter transforms this direct current into alternating current with variable frequency and voltage. AC-DC-AC inverters can be further divided into voltage-type and current-type types. Due to various factors such as control methods and hardware design, voltage-type inverters are more widely used. Traditional current-type AC-DC-AC inverters use thyristors with natural commutation as power switches. The inductor on the DC side is relatively expensive, and when used in doubly-fed speed control, a commutation circuit is required at speeds above synchronous speed; moreover, their performance is poor at low slip frequencies, which is why they are not widely used in doubly-fed asynchronous wind power generation. The voltage-type AC-DC-AC inverter, which is a type of rectification and frequency conversion device, boasts excellent advantages such as a simple structure, low harmonic content, and the ability to adjust the power factors of both the stator and rotor. It can significantly improve the operating conditions of doubly-fed generators as well as the quality of the electrical energy they produce. Moreover, this design enables complete separation between the grid side and the rotor side through the capacitors on the DC bus side. The stator flux orientation vector control system for the doubly-fed generator in voltage-type AC-DC-AC inverters enables decoupled control of the active and reactive powers of the generator based on maximum power point tracking of the wind turbine, and it represents a prominent approach in variable-speed constant-frequency wind power generation. In addition, there is also a parallel AC-DC-AC inverter topology. The main idea of this structure is to combine an AC-DC-AC current-type inverter with an AC-DC-AC voltage-type inverter in parallel; the current-type inverter serves as the main inverter responsible for power transmission, while the voltage-type inverter acts as a secondary inverter to compensate for the harmonics generated by the current-type inverter. In this structure, the main inverter has a lower switching frequency, while the auxiliary inverter has a lower switching current. Compared with the aforementioned AC-DC-AC voltage-source inverter, this topology features low switching losses, resulting in a higher efficiency for the entire system. Its disadvantages are also obvious: the use of a large number of power electronic devices leads to increased costs and more complex control algorithms, and moreover, this type of structure has a relatively low voltage utilization efficiency. High-voltage frequency converters can be divided into AC-DC-AC frequency converters and AC-AC frequency converters; currently, AC-DC-AC frequency converters are widely used in China. Its features: high efficiency, with no additional losses during speed regulation ； It has a wide range of applications and can be used in cage-type asynchronous motors ； Wide speed control range, firm characteristics, high precision ； It is technically complex, costly, and difficult to maintain and repair. This method is suitable for applications that require high precision and good speed regulation performance. AC-AC inverters use thyristor-based natural commutation, ensuring stable and reliable operation; they are suitable as power supplies for the rotor windings of doubly-fed motors. The maximum output frequency of AC-AC inverters is 1/3 to 1/2 of the grid frequency, giving them a significant advantage in high-power, low-frequency applications. AC-AC frequency conversion does not have a DC link, resulting in high conversion efficiency. The main circuit is simple, as it lacks DC circuits and filtering components; thus, it is easy to handle reactive power and to recycle active power with the power supply. Although AC-AC variable-frequency double-fed systems are widely used, their application is limited to some extent due to their low power factor, numerous high harmonics, low output frequency, narrow frequency range, and the large number of components required. A matrix-type inverter is a type of AC-AC direct converter, consisting of nine switches directly connected between the three-phase input and output. Matrix converters do not have an intermediate DC link; their output consists of three levels, and they have a low level of harmonics ； Its power circuit is simple and compact, and it can generate a sinusoidal load voltage with controllable frequency, amplitude, and phase ； The input power factor of the matrix converter is controllable, and it can operate in all four quadrants. Although matrix converters have many advantages, it is difficult to implement them since two switches cannot be turned on or off simultaneously during the commutation process. The low maximum output voltage capability of matrix converters, along with the high voltage stress on the components, is also a significant drawback of such converters. When applied in wind power generation, since the input and output of the matrix converter are not decoupled, asymmetries on either the load side or the power supply side will affect the other side. Furthermore, a filtering capacitor must be connected to the input side of the matrix converter. Although its capacitance value is smaller than that of the intermediate energy storage capacitor in the AC-DC-AC conversion system, since it is an AC capacitor that has to handle alternating current at the switching frequency, its size is not small. AC-AC frequency conversion technology was the main form of medium-voltage frequency conversion in the early days; its operating principle meant that it could only function at low frequencies, making it suitable only for applications with low speeds and large capacities. Since the main circuit switching devices are in a naturally off state and there is no issue of forced commutation, the first generation of power electronics devices – thyristors (SCRs) – were sufficient to meet the requirements; as a result, this type of medium-voltage frequency conversion technology is quite mature.

AC-AC frequency conversion technology: AC-AC frequency converters use thyristor-based natural commutation, which ensures stable and reliable operation; they are suitable as power supplies for the rotor windings of doubly-fed motors. The maximum output frequency of AC-AC frequency conversion is 1/3 to 1/2 of the grid frequency, giving them a significant advantage in the high-power, low-frequency range. AC-AC frequency conversion does not have a DC link, resulting in high conversion efficiency. The main circuit is simple, as it lacks DC circuits and filtering components; thus, it is easy to handle reactive power and to recycle active power with the power supply. Although AC-AC variable-frequency double-fed systems are widely used, their application is limited to some extent due to their low power factor, numerous high harmonics, low output frequency, narrow frequency range, and the large number of components required. A matrix-type inverter is a type of AC-AC direct converter, consisting of nine switches directly connected between the three-phase input and output. Matrix converters do not have an intermediate DC link; their output consists of three levels, and they have a low level of harmonics ； Its power circuit is simple and compact, and it can generate a sinusoidal load voltage with controllable frequency, amplitude, and phase ； The input power factor of the matrix converter is controllable, and it can operate in all four quadrants. Although matrix converters have many advantages, it is difficult to implement them since two switches cannot be turned on or off simultaneously during the commutation process. The low maximum output voltage capability of matrix converters, along with the high voltage stress on the components, is also a significant drawback of such converters. When applied in wind power generation, since the input and output of the matrix converter are not decoupled, asymmetries on either the load side or the power supply side will affect the other side. Furthermore, a filtering capacitor must be connected to the input side of the matrix converter. Although its capacitance value is smaller than that of the intermediate energy storage capacitor in the AC-DC-AC conversion system, since it is an AC capacitor that has to handle alternating current at the switching frequency, its size is not small. AC-AC frequency conversion technology was the main form of medium-voltage frequency conversion in the early days; its operating principle meant that it could only function at low frequencies, making it suitable only for applications with low speeds and large capacities. Since the switching devices in the main circuit are in a naturally closed state, there is no issue of forced commutation; therefore, the first generation of power electronic devices – thyristors (SCRs) – were sufficient to meet the requirements. As a result, this type of medium-voltage frequency conversion technology is quite mature

Low-voltage frequency converters primarily use the AC-DC-AC approach: the mains AC power is first converted into DC power using a rectifier, and then that DC power is transformed into AC power whose frequency and voltage can be controlled, in order to supply the motor. The circuit of an inverter generally consists of four parts: rectification, intermediate DC link, inversion, and control. The rectification section is a three-phase bridge-type uncontrolled rectifier, the inversion section is an IGBT three-phase bridge-type inverter, and the output is in PWM waveform form; the intermediate DC link serves for filtering, storing DC energy, and buffering reactive power. AC-AC inverters are less commonly used; their technology is complex and not yet fully mature, with applications mainly focusing on high-voltage and high-power scenarios. The disadvantages of the AC-DC-AC frequency conversion method are a low input power factor, large harmonic currents, the need for large energy storage capacitors in the DC circuit, and the inability to feed back the regenerated energy into the power grid, meaning it cannot operate in all four quadrants. For this reason, matrix-type AC-AC frequency conversion was developed. Since matrix-type AC-AC frequency conversion eliminates the intermediate DC link, it eliminates the need for electrolytic capacitors, which are large in size and expensive. It can achieve a power factor of 1, the input current is sinusoidal, and it can operate in all four quadrants, resulting in a high power density for the system. Although this technology is not yet mature, it continues to attract many scholars to conduct in-depth research on it. Its essence is not to indirectly control quantities such as current and flux, but rather to directly use torque as the quantity to be controlled. The specific method is: —— Control the stator flux and introduce it into the stator flux observer to achieve a sensorless approach ； ——Automatic identification (ID) relies on an accurate motor mathematical model to automatically identify motor parameters ； ——Calculate the stator impedance, mutual inductance, magnetic saturation factors, inertia, etc., corresponding to the actual values; thereby determine the actual torque, stator flux, and rotor speed for real-time control ； ——Band-Band control is implemented; this type of control generates PWM signals based on flux and torque in order to regulate the switching state of the inverter. Matrix-type V/F conversion offers a fast torque response (2ms), high speed accuracy (±2%, without PG feedback), and high torque accuracy (+3%) ； It also features high starting torque and high torque accuracy; especially at low speeds (including 0 speed), it can deliver 150% to 200% of its torque.

AC-AC frequency conversion technology: AC-AC frequency converters use thyristor-based natural commutation, which ensures stable and reliable operation; they are suitable as power supplies for the rotor windings of doubly-fed motors. The maximum output frequency of AC-AC frequency conversion is 1/3 to 1/2 of the grid frequency, giving them a significant advantage in the high-power, low-frequency range. AC-AC frequency conversion does not have a DC link, resulting in high conversion efficiency. The main circuit is simple, as it lacks DC circuits and filtering components; thus, it is easy to handle reactive power and to recycle active power with the power supply. Although AC-AC variable-frequency double-fed systems are widely used, their application is limited to some extent due to their low power factor, numerous high harmonics, low output frequency, narrow frequency range, and the large number of components required. A matrix-type inverter is a type of AC-AC direct converter, consisting of nine switches directly connected between the three-phase input and output. Matrix converters do not have an intermediate DC link; their output consists of three levels, and they have a low level of harmonics ； Its power circuit is simple and compact, and it can generate a sinusoidal load voltage with controllable frequency, amplitude, and phase ； The input power factor of the matrix converter is controllable, and it can operate in all four quadrants. Although matrix converters have many advantages, it is difficult to implement them since two switches cannot be turned on or off simultaneously during the commutation process. The low maximum output voltage capability of matrix converters, along with the high voltage stress on the components, is also a significant drawback of such converters. When applied in wind power generation, since the input and output of the matrix converter are not decoupled, asymmetries on either the load side or the power supply side will affect the other side. Furthermore, a filtering capacitor must be connected to the input side of the matrix converter. Although its capacitance value is smaller than that of the intermediate energy storage capacitor in the AC-DC-AC conversion system, since it is an AC capacitor that has to handle alternating current at the switching frequency, its size is not small. AC-AC frequency conversion technology was the main form of medium-voltage frequency conversion in the early days; its operating principle meant that it could only function at low frequencies, making it suitable only for applications with low speeds and large capacities. Since the switching devices in the main circuit are in a naturally closed state, there is no issue of forced commutation; therefore, the first generation of power electronic devices – thyristors (SCRs) – were sufficient to meet the requirements. As a result, this type of medium-voltage frequency conversion technology is quite mature

The disadvantages of the AC-DC-AC frequency conversion method are a low input power factor, large harmonic currents, the need for large energy storage capacitors in the DC circuit, and the inability to feed back the regenerated energy into the power grid, meaning it cannot operate in all four quadrants. For this reason, matrix-type AC-AC frequency conversion was developed. Since matrix-type AC-AC frequency conversion eliminates the intermediate DC link, it eliminates the need for electrolytic capacitors, which are large in size and expensive. It can achieve a power factor of 1, the input current is sinusoidal, and it can operate in all four quadrants, resulting in a high power density for the system. Although this technology is not yet mature, it continues to attract many scholars to conduct in-depth research on it. Its essence is not to indirectly control quantities such as current and flux, but rather to directly use torque as the quantity to be controlled. The specific method is: —— Control the stator flux and introduce it into the stator flux observer to achieve a sensorless approach ； ——Automatic identification (ID) relies on an accurate motor mathematical model to automatically identify motor parameters ； ——Calculate the stator impedance, mutual inductance, magnetic saturation factors, inertia, etc., corresponding to the actual values; thereby determine the actual torque, stator flux, and rotor speed for real-time control ； ——Band-Band control is implemented; this type of control generates PWM signals based on flux and torque in order to regulate the switching state of the inverter. Matrix-type V/F conversion offers a fast torque response (2ms), high speed accuracy (±2%, without PG feedback), and high torque accuracy (+3%) ； It also features high starting torque and high torque accuracy; especially at low speeds (including 0 speed), it can deliver 150% to 200% of its torque.

The maximum output frequency is 1/3 to 1/2 of the grid frequency, giving it a significant advantage in the high-power low-frequency range. AC-AC frequency conversion does not have a DC link, resulting in high conversion efficiency. The main circuit is simple, as it lacks DC circuits and filtering components; thus, it is easy to handle reactive power and to recycle active power with the power supply. Although high-power AC-AC frequency converters are widely used, their application is limited to some extent due to their low power factor, numerous higher harmonics, low output frequency, narrow frequency range, and the large number of components required. It is widely used in traditional high-power motor speed control systems. The matrix inverter is a new type of AC-AC direct converter, consisting of nine switches directly connected between the three-phase input and output. Matrix converters do not have an intermediate DC link; their output consists of three levels, and they have a low level of harmonics ； Its power circuit is simple and compact, and it can generate a sinusoidal load voltage with controllable frequency, amplitude, and phase ； The input power factor of the matrix converter is controllable, and it can operate in all four quadrants. Although matrix converters have many advantages, it is difficult to implement them since two switches cannot be turned on or off simultaneously during the commutation process. The low maximum output voltage capability of matrix converters, along with the high voltage stress on the components, is also a significant drawback of such converters. It is used in wind turbine excitation power supplies. AC-DC-AC inverters are quite common; they consist of three components: a rectifier, a filtering system, and an inverter. The rectifier is either a diode-based three-phase bridge uncontrolled rectifier or a fully controlled rectifier made up of high-power transistors. The inverter, on the other hand, is a three-phase bridge circuit composed of high-power transistors; its function is the opposite of that of the rectifier – it converts constant direct current into alternating current with adjustable voltage and frequency. The intermediate filtering stage uses capacitors or inductors to filter the rectified voltage or current. Depending on the intermediate DC filtering circuit, AC-DC-AC inverters can be further divided into voltage-type and current-type types. Due to various factors such as control methods and hardware design, voltage-type inverters are more widely used. It refers to frequency converters in the field of industrial automation (which use variable voltage and frequency VVVF control, etc.), as well as uninterruptible power supplies in the IT and power supply sectors (i.e., UPS, which utilize

Low-voltage frequency converters primarily use the AC-DC-AC approach: the mains AC power is first converted into DC power using a rectifier, and then that DC power is transformed into AC power whose frequency and voltage can be controlled, in order to supply the motor. The circuit of an inverter generally consists of four parts: rectification, intermediate DC link, inversion, and control. The rectification section is a three-phase bridge-type uncontrolled rectifier, the inversion section is an IGBT three-phase bridge-type inverter, and the output is in PWM waveform form; the intermediate DC link serves for filtering, storing DC energy, and buffering reactive power. AC-AC inverters are less commonly used; their technology is complex and not yet fully mature, with applications mainly focusing on high-voltage and high-power scenarios. The disadvantages of the AC-DC-AC frequency conversion method are a low input power factor, large harmonic currents, the need for large energy storage capacitors in the DC circuit, and the inability to feed back the regenerated energy into the power grid, meaning it cannot operate in all four quadrants. For this reason, matrix-type AC-AC frequency conversion was developed. Since matrix-type AC-AC frequency conversion eliminates the intermediate DC link, it eliminates the need for electrolytic capacitors, which are large in size and expensive. It can achieve a power factor of 1, the input current is sinusoidal, and it can operate in all four quadrants, resulting in a high power density for the system. Although this technology is not yet mature, it continues to attract many scholars to conduct in-depth research on it. Its essence is not to indirectly control quantities such as current and flux, but rather to directly use torque as the quantity to be controlled. The specific method is: —— Control the stator flux and introduce it into the stator flux observer to achieve a sensorless approach ； ——Automatic identification (ID) relies on an accurate motor mathematical model to automatically identify motor parameters ； ——Calculate the stator impedance, mutual inductance, magnetic saturation factors, inertia, etc., corresponding to the actual values; thereby determine the actual torque, stator flux, and rotor speed for real-time control ； ——Band-Band control is implemented; this type of control generates PWM signals based on flux and torque in order to regulate the switching state of the inverter. Matrix-type V/F conversion offers a fast torque response (2ms), high speed accuracy (±2%, without PG feedback), and high torque accuracy (+3%) ； It also features high starting torque and high torque accuracy; especially at low speeds (including 0 speed), it can deliver 150% to 200% of its torque.

It seems that the answers are more or less the same, haha. AC-DC-AC voltage-type frequency converters are relatively expensive. Additionally, there are two major technical issues with these converters: first, they have an intermediate rectification and filtering stage, which results in low efficiency; second, when the motor is in power generation mode, it’s difficult to return the energy to the power grid. Usually, a resistance circuit is used to dissipate this energy, which not only increases the size of the equipment but also represents a significant waste of energy since it isn’t utilized. To make use of this energy, an active inversion circuit can be added, but this in turn increases costs and complicates the circuit design. The working principle of the AC-AC frequency converter is to convert a three-phase mains power supply into the desired voltage and frequency by using several sets of phase-controlled switches. Its advantage is high efficiency, as energy can be easily returned to the power grid. Its biggest drawback is that the highest output frequency must be less than 1/3 or 1/2 of the input power frequency; otherwise, the output waveform becomes poor, causing the motor to vibrate and fail to operate. Old-fashioned frequency converters are still limited to applications requiring speed control at low speeds, which **limits their range of use.

Low-voltage frequency converters primarily use the AC-DC-AC approach: the mains AC power is first converted into DC power using a rectifier, and then that DC power is transformed into AC power whose frequency and voltage can be controlled, in order to supply the motor. The circuit of an inverter generally consists of four parts: rectification, intermediate DC link, inversion, and control. The rectification section is a three-phase bridge-type uncontrolled rectifier, the inversion section is an IGBT three-phase bridge-type inverter, and the output is in PWM waveform form; the intermediate DC link serves for filtering, storing DC energy, and buffering reactive power. AC-AC inverters are less commonly used; their technology is complex and not yet fully mature, with applications mainly focusing on high-voltage and high-power scenarios. Here is some information on plagiarism for your reference: The disadvantages of AC-DC-AC frequency conversion systems include a low input power factor, high harmonic currents, the need for large energy storage capacitors in the DC circuit, and the inability to feed back regenerated energy into the power grid; as a result, they cannot operate in all four quadrants. For this reason, matrix-type AC-AC frequency conversion was developed. Since matrix-type AC-AC frequency conversion eliminates the intermediate DC link, it eliminates the need for electrolytic capacitors, which are large in size and expensive. It can achieve a power factor of 1, the input current is sinusoidal, and it can operate in all four quadrants, resulting in a high power density for the system. Although this technology is not yet mature, it continues to attract many scholars to conduct in-depth research on it. Its essence is not to indirectly control quantities such as current and flux, but rather to directly use torque as the quantity to be controlled. The specific method is: —— Control the stator flux and introduce it into the stator flux observer to achieve a sensorless approach ； ——Automatic identification (ID) relies on an accurate motor mathematical model to automatically identify motor parameters ； ——Calculate the stator impedance, mutual inductance, magnetic saturation factors, inertia, etc., corresponding to the actual values; thereby determine the actual torque, stator flux, and rotor speed for real-time control ； ——Band-Band control is implemented; this type of control generates PWM signals based on flux and torque in order to regulate the switching state of the inverter. Matrix-type V/F conversion offers a fast torque response (2ms), high speed accuracy (±2%, without PG feedback), and high torque accuracy (+3%) ； It also features high starting torque and high torque accuracy; especially at low speeds (including 0 speed), it can deliver 150% to 200% of its torque.

Traditional AC-AC frequency converters use thyristor-based natural commutation, ensuring stable and reliable operation. The highest output frequency of AC-AC frequency conversion is 1/3 to 1/2 of the grid frequency, giving it a significant advantage in the high-power low-frequency range. AC-AC frequency conversion does not have a DC link, resulting in high conversion efficiency. The main circuit is simple, as it lacks DC circuits and filtering components; thus, it is easy to handle reactive power and to recycle active power with the power supply. Although high-power AC-AC frequency converters are widely used, their application is limited to some extent due to their low power factor, numerous higher harmonics, low output frequency, narrow frequency range, and the large number of components required. It is widely used in traditional high-power motor speed control systems. AC-DC-AC inverters are quite common; they consist of three components: a rectifier, a filtering system, and an inverter. The rectifier is either a diode-based three-phase bridge uncontrolled rectifier or a fully controlled rectifier made up of high-power transistors. The inverter, on the other hand, is a three-phase bridge circuit composed of high-power transistors; its function is the opposite of that of the rectifier – it converts constant direct current into alternating current with adjustable voltage and frequency. The intermediate filtering stage uses capacitors or inductors to filter the rectified voltage or current. Depending on the intermediate DC filtering circuit, AC-DC-AC inverters can be further divided into voltage-type and current-type types. Due to various factors such as control methods and hardware design, voltage-type inverters are more widely used. It is used in frequency converters in the field of industrial automation (which employ VVVF control, etc.), as well as in uninterruptible power supplies in the IT and power supply sectors (i.e., UPS, which use CVCF control).

Traditional AC-AC frequency converters use thyristor-based natural commutation, ensuring stable and reliable operation. The highest output frequency of AC-AC frequency conversion is 1/3 to 1/2 of the grid frequency, giving it a significant advantage in the high-power low-frequency range. AC-AC frequency conversion does not have a DC link, resulting in high conversion efficiency. The main circuit is simple, as it lacks DC circuits and filtering components; thus, it is easy to handle reactive power and to recycle active power with the power supply. Although high-power AC-AC frequency converters are widely used, their application is limited to some extent due to their low power factor, numerous higher harmonics, low output frequency, narrow frequency range, and the large number of components required. It is widely used in traditional high-power motor speed control systems. The matrix inverter is a new type of AC-AC direct converter, consisting of nine switches directly connected between the three-phase input and output. Matrix converters do not have an intermediate DC link; their output consists of three levels, and they have a low level of harmonics ； Its power circuit is simple and compact, and it can generate a sinusoidal load voltage with controllable frequency, amplitude, and phase ； The input power factor of the matrix converter is controllable, and it can operate in all four quadrants. Although matrix converters have many advantages, it is difficult to implement them since two switches cannot be turned on or off simultaneously during the commutation process. The low maximum output voltage capability of matrix converters, along with the high voltage stress on the components, is also a significant drawback of such converters. It is used in wind turbine excitation power supplies. AC-DC-AC inverters are quite common; they consist of three components: a rectifier, a filtering system, and an inverter. The rectifier is either a diode-based three-phase bridge uncontrolled rectifier or a fully controlled rectifier made up of high-power transistors. The inverter, on the other hand, is a three-phase bridge circuit composed of high-power transistors; its function is the opposite of that of the rectifier – it converts constant direct current into alternating current with adjustable voltage and frequency. The intermediate filtering stage uses capacitors or inductors to filter the rectified voltage or current. Depending on the intermediate DC filtering circuit, AC-DC-AC inverters can be further divided into voltage-type and current-type types. Due to various factors such as control methods and hardware design, voltage-type inverters are more widely used. It is used in frequency converters in the field of industrial automation (which employ VVVF control, etc.), as well as in uninterruptible power supplies in the IT and power supply sectors (i.e., UPS, which use CVCF control).

AC-AC inverters use thyristor-based natural commutation, ensuring stable and reliable operation; they are suitable as power supplies for the rotor windings of doubly-fed motors. The maximum output frequency of AC-AC inverters is 1/3 to 1/2 of the grid frequency, giving them a significant advantage in high-power, low-frequency applications. AC-AC frequency conversion does not have a DC link, resulting in high conversion efficiency. The main circuit is simple, as it lacks DC circuits and filtering components; thus, it is easy to handle reactive power and to recycle active power with the power supply. Although AC-AC variable-frequency double-fed systems are widely used, their application is limited to some extent due to their low power factor, numerous high harmonics, low output frequency, narrow frequency range, and the large number of components required. A matrix-type inverter is a type of AC-AC direct converter, consisting of nine switches directly connected between the three-phase input and output. Matrix converters do not have an intermediate DC link; their output consists of three levels, and they have a low level of harmonics ； Its power circuit is simple and compact, and it can generate a sinusoidal load voltage with controllable frequency, amplitude, and phase ； The input power factor of the matrix converter is controllable, and it can operate in all four quadrants. Although matrix converters have many advantages, it is difficult to implement them since two switches cannot be turned on or off simultaneously during the commutation process. The low maximum output voltage capability of matrix converters, along with the high voltage stress on the components, is also a significant drawback of such converters. When applied in wind power generation, since the input and output of the matrix converter are not decoupled, asymmetries on either the load side or the power supply side will affect the other side. Furthermore, a filtering capacitor must be connected to the input side of the matrix converter. Although its capacitance value is smaller than that of the intermediate energy storage capacitor in the AC-DC-AC conversion system, since it is an AC capacitor that has to handle alternating current at the switching frequency, its size is not small. AC-AC frequency conversion technology was the main form of medium-voltage frequency conversion in the early days; its operating principle meant that it could only function at low frequencies, making it suitable only for applications with low speeds and large capacities. Since the main circuit switching devices are in a naturally off state, there is no issue of forced commutation; therefore, the first generation of power electronic devices – thyristors (SCRs) – were sufficient to meet the requirements. As a result, this type of medium-voltage frequency conversion technology is quite mature. The disadvantages of AC-DC-AC frequency conversion include a low input power factor, large harmonic currents, the need for large energy storage capacitors in the DC circuit, and the inability to feed back the regenerated energy into the power grid, meaning it cannot operate in all four quadrants. For this reason, matrix-type AC-AC frequency conversion was developed. Since matrix-type AC-AC frequency conversion eliminates the intermediate DC link, it eliminates the need for electrolytic capacitors, which are large in size and expensive. It can achieve a power factor of 1, the input current is sinusoidal, and it can operate in all four quadrants, resulting in a high power density for the system. Although this technology is not yet mature, it continues to attract many scholars to conduct in-depth research on it. Its essence is not to indirectly control quantities such as current and flux, but rather to directly use torque as the quantity to be controlled. The specific method is: —— Control the stator flux and introduce it into the stator flux observer to achieve a sensorless approach ； ——Automatic identification (ID) relies on an accurate motor mathematical model to automatically identify motor parameters ； ——Calculate the stator impedance, mutual inductance, magnetic saturation factors, inertia, etc., corresponding to the actual values; thereby determine the actual torque, stator flux, and rotor speed for real-time control ； ——Band-Band control is implemented; this type of control generates PWM signals based on flux and torque in order to regulate the switching state of the inverter. Matrix-type V/F conversion offers a fast torque response (2ms), high speed accuracy (±2%, without PG feedback), and high torque accuracy (+3%) ； It also features high starting torque and high torque accuracy; especially at low speeds (including 0 speed), it can deliver 150% to 200% of its torque.

The principle of an inverter is a power control device that converts alternating current power supply into electrical energy of another frequency by utilizing the on-off operation of power semiconductor devices. An AC-DC-AC inverter first converts alternating current into direct current using a rectifier; the DC intermediate circuit smooths and filters the output of the rectifier circuit, and then an inverter transforms this direct current into alternating current with variable frequency and voltage. An AC-AC inverter, on the other hand, performs the conversion directly, without going through a DC stage