Classification of frequency converters
There are several ways to classify frequency converters . According to the main circuit operating mode, they can be divided into voltage-source frequency converters and current-source frequency converters. According to the switching method, they can be divided into PAM control frequency converters, PWM control frequency converters, and high-carrier-frequency PWM control frequency converters. According to the working principle, they can be divided into V/f control frequency converters, slip frequency control frequency converters, and vector control frequency converters. According to the application, they can be divided into general-purpose frequency converters, high-performance special-purpose frequency converters, high-frequency frequency converters, single-phase frequency converters, and three-phase frequency converters.
VVVF: Change voltage, change frequency; CVCF: Constant voltage, constant frequency. The AC power supplies used in various countries, whether for household or factory use, typically have voltages and frequencies of 400V/50Hz or 200V/60Hz (50Hz), etc. Generally, a device that converts AC power with a fixed voltage and frequency into AC power with a variable voltage or frequency is called a " frequency converter ." To generate variable voltage and frequency, this device must first convert the AC power supply into DC power.
Inverters used for motor control can change both voltage and frequency.
1. Classified by the transformation process:
Frequency converters can be divided into AC-AC converters, which directly convert the power frequency AC into AC with adjustable frequency and voltage, also known as direct frequency converters; and AC-DC-AC converters, which first convert the power frequency AC into DC through a rectifier, and then convert the DC into AC with adjustable frequency and voltage, also known as indirect frequency converters, which are currently widely used general-purpose frequency converters.
2. Classification by DC power supply characteristics:
(1) Current-source frequency converter
A current-source inverter is characterized by using a large inductor as an energy storage element in its intermediate DC link to buffer reactive power, thus suppressing current changes and making the voltage approach a sine wave. Because of its relatively high DC internal resistance, it is called a current-source inverter (current-type). The advantage of a current-source inverter is its ability to suppress frequent and rapid changes in load current. It is often selected for applications with large load current variations.
(2) Voltage-type frequency converter
A voltage-type frequency converter is characterized by a large capacitor as the energy storage element in the intermediate DC link, which buffers the reactive power of the load, resulting in a relatively stable DC voltage and low internal resistance of the DC power supply. It is equivalent to a voltage source, hence the name voltage-type frequency converter. It is often selected for applications where the load voltage varies greatly.
In addition, frequency converters can also be classified according to output voltage regulation method, control method, main switch components, and input voltage level.
Development trend of frequency converters
Variable frequency drives (VFDs) are power converters in motion control systems. Modern motion control systems encompass multiple disciplines, and the general development trend is towards AC drives, higher frequency power converters, and digital, intelligent, and networked control. Therefore, VFDs, as crucial power conversion components in systems, have experienced rapid development in providing controllable, high-performance variable-voltage and variable-frequency AC power.
In the 21st century, the substrate for power electronics has shifted from Si (silicon) to SiC (silicon carbide), ushering in an era of high-voltage, high-capacity, high-frequency, modular, miniaturized, intelligent, and low-cost power electronic devices. Various new electrical devices suitable for variable frequency speed control are under development. The rapid development of IT technology and continuous innovation in control theory will influence the development trend of frequency converters.
With market expansion and diversified user demands, the functions of domestic frequency converter products are constantly being improved and expanded, with increasingly higher levels of integration and systematization. Furthermore, some dedicated frequency converter products have emerged. It is understood that in recent years, the Chinese frequency converter market has maintained a growth rate of 12-15%, and it is expected to maintain a growth rate of over 10% for at least the next five years. Currently, the growth rate of installed capacity (power) of frequency converters in the Chinese market is actually around 20%, and it is estimated that the frequency converter market will not reach saturation and gradually mature for at least another 10 years.
1. Intelligentization
Once installed in the system, intelligent frequency converters require minimal configuration and are easy to operate. They feature clear operating status displays, fault diagnosis and troubleshooting capabilities, and even automatic component switching. Remote monitoring via the internet allows for the联动 (interconnection/coordination) of multiple frequency converters according to process procedures, forming an optimized integrated management and control system.
2. Specialization
Specialized frequency converters are manufactured based on the characteristics of a particular type of load. This not only helps to control the motor of the load economically and efficiently, but also reduces manufacturing costs. Examples include frequency converters for fans and water pumps, frequency converters for hoisting machinery, frequency converters for elevator control, frequency converters for tension control, and frequency converters for air conditioning.
3. Integration
Frequency converters selectively integrate relevant functional components, such as parameter identification systems, PID controllers, PLC controllers, and communication units, into a single integrated unit. This not only enhances functionality and increases system reliability but also effectively reduces system size and external circuit connections. It has been reported that integrated units combining frequency converters and motors have now been developed, making the entire system smaller and easier to control.
4. Environmental protection
Protecting the environment and manufacturing "green" products are new concepts for humankind. Future frequency converters will focus more on energy saving and low pollution, that is, minimizing noise and harmonic interference to the power grid and other electrical equipment during use.
5. The main circuit power switching elements are self-turn-off, modular, integrated, and intelligent, with continuously increasing switching frequency and further reduction of switching losses.
6. Regarding the topology of the inverter's main circuit:
For low-voltage, small-capacity devices, grid-side inverters of frequency converters typically use 6-pulse inverters, while for medium-voltage, large-capacity devices, multi-pulse inverters with 12 or more pulses are used. For load-side inverters, low-voltage, small-capacity devices often use two-level bridge inverters, while medium-voltage, large-capacity devices use multi-level inverters. For four-quadrant drives, to achieve regenerative energy feedback from the frequency converter to the grid and save energy, the grid-side inverter should be a reversible inverter. Simultaneously, dual-PWM frequency converters with bidirectional power flow have emerged. With appropriate control of the grid-side inverter, the input current can be made close to a sine wave, reducing pollution to the grid. Currently, such products are available for both low- and medium-voltage frequency converters.
7. The control methods for pulse width modulation transformer frequency converters can include sinusoidal pulse width modulation (SPWM) control, PWM control to eliminate harmonics of a specified number, current tracking control, and voltage space vector control (magnetic flux tracking control).
8. The progress in AC motor frequency conversion adjustment and control methods is mainly reflected in the development from scalar control to vector control and direct torque control with high dynamic performance, and the development of sensorless vector control and direct torque control systems.
9. Advances in microprocessors have made digital control the development direction of modern controllers: Motion control systems are fast systems, especially the high-performance control of AC motors, which requires the storage of various data and the rapid real-time processing of large amounts of information. In recent years, major international companies have launched DSP (Digital Signal Processor) based cores, equipped with peripheral functional circuits required for motor control, integrated into a single chip, called a DSP single-chip motor controller. This significantly reduces price, shrinks size, improves compact structure, ease of use, and increases reliability. Compared to ordinary microcontrollers, DSPs have 10 to 15 times greater digital processing capabilities, ensuring superior system control performance.
Digital control simplifies hardware, and flexible control algorithms provide great flexibility in control, enabling the realization of complex control laws. This makes it possible to apply modern control theory to motion control systems, facilitates data transmission with upper-level systems, and enhances fault diagnosis, protection, and monitoring functions, making the system intelligent (e.g., some frequency converters have self-adjustment functions).
10. AC synchronous motors have become a rising star in AC adjustable drives, especially permanent magnet synchronous motors. These motors feature a brushless structure, high power factor, and high efficiency, with rotor speed strictly synchronized with the power supply frequency. Synchronous motor variable frequency speed control systems are broadly classified into two categories: externally controlled variable frequency and self-controlled variable frequency. Self-controlled variable frequency synchronous motors are very similar in principle to DC motors, using a power electronic converter to replace the mechanical commutator of a DC motor. When an AC-DC-AC converter is used, it is called a "DC commutatorless motor" or "brushless DC motor (BLDC)". Traditional self-controlled variable frequency synchronous motor speed control systems use rotor position sensors; systems without rotor position sensors are currently under development. Externally controlled variable frequency methods for synchronous motors can also employ vector control, which is simpler than that for asynchronous motors due to its rotor magnetic field orientation.
In summary, the development trend of frequency converter technology is towards intelligence, ease of operation, comprehensive functions, safety and reliability, environmental protection and low noise, low cost and miniaturization.
