A variable-frequency drive (VFD) is a power control device that uses frequency conversion technology and microelectronics to control an AC motor by changing the frequency of the power supply. The VFD adjusts the voltage and frequency of the output power supply by switching its internal IGBTs, providing the required power voltage according to the actual needs of the motor, thereby achieving energy saving and speed regulation. In addition, VFDs have many protection functions, such as overcurrent, overvoltage, and overload protection. With the continuous improvement of industrial automation, VFDs have been widely used. So, what are the common types of VFDs, and what are their control methods?
In terms of control method, the most common types of frequency converters on the market are V/F control frequency converters and vector control frequency converters. From a voltage perspective, there are low-voltage frequency converters and high-voltage frequency converters. From a power supply perspective, there are single-phase frequency converters and three-phase frequency converters. Based on application, there are general-purpose frequency converters, frequency converters specifically for fans and pumps, frequency converters specifically for injection molding machines, frequency converters specifically for wire drawing machines, frequency converters specifically for elevators, frequency converters specifically for ball mills, and so on.
Common control methods for frequency converters
1. Non-intelligent control method
Non-intelligent control methods used in AC frequency converters include V/f coordinated control, slip frequency control, vector control, and direct torque control.
(1) V/f sinusoidal pulse width modulation (SPWM) control method
V/f control is proposed to achieve ideal torque-speed characteristics. It is based on the idea of adjusting the speed by changing the power supply frequency while keeping the motor's magnetic flux constant. Most general-purpose frequency converters use this control method. V/f control frequency converters have a very simple structure, but they use an open-loop control method, which cannot achieve high control performance. Furthermore, at low frequencies, torque compensation is necessary to change the low-frequency torque characteristics.
(2) Slip frequency control
Slip frequency control is a direct torque control method. Based on V/f control, it adjusts the inverter's output frequency according to the power supply frequency corresponding to the actual speed of the asynchronous motor and the desired torque, thus enabling the motor to achieve the desired torque.
The corresponding output torque. This control method requires the installation of a speed sensor in the control system, and sometimes current feedback is also added to control the frequency and current. Therefore, it is a closed-loop control method, which can give the frequency converter good stability and good response characteristics to rapid acceleration and deceleration and load changes.
(3) Voltage Space Vector (SVPWM) Control Method
It is based on the overall generation effect of three-phase waveforms, aiming to approximate the ideal circular rotating magnetic field trajectory of the motor air gap. It generates three-phase modulated waveforms in one step and controls them by approximating a circle using an inscribed polygon. After practical use, it has been improved by introducing frequency compensation to eliminate speed control errors; estimating the flux linkage amplitude through feedback to eliminate the influence of stator resistance at low speeds; and closing the output voltage and current loops to improve dynamic accuracy and stability. However, the control circuit has many components and lacks torque regulation, so the system performance has not been fundamentally improved.
(4) Vector Control (VC)
Vector control uses vector coordinate circuits to control the magnitude and phase of the motor's stator current, thereby controlling the excitation current and torque current in the d, q, and 0 coordinate systems, and ultimately controlling the motor torque. By controlling the sequence and timing of the vectors' actions, as well as the duration of the zero vector, various PWM waves can be generated to achieve different control objectives. For example, a PWM wave with the fewest switching operations can be generated to reduce switching losses. Currently, the vector control methods practically used in frequency converters are mainly two types: slip frequency-based vector control and sensorless vector control.
Both slip frequency-based vector control and slip frequency control have the same steady-state characteristics. However, slip frequency-based vector control requires coordinate transformation to control the phase of the motor stator current, ensuring it meets certain conditions to eliminate fluctuations during torque current transients. Therefore, slip frequency-based vector control offers significantly improved output characteristics compared to slip frequency control. However, this control method is a closed-loop control, requiring a speed sensor on the motor, thus limiting its application.
Sensor-based vector control controls the excitation current and torque current separately through coordinate transformation. It then identifies the speed by controlling the voltage and current on the motor stator windings to control both the excitation and torque currents. This control method offers a wide speed range, high starting torque, reliable operation, and ease of use. However, the calculations are complex and generally require a dedicated processor. Therefore, its real-time performance is not ideal, and the control accuracy is affected by the accuracy of the calculations.
How does vector control enable a motor to have a large torque?
(1) Torque Boost
This function increases the inverter's output voltage so that the motor's output torque increases proportionally to the square of the voltage, thereby improving the motor's output torque.
(2) Techniques to improve insufficient low-speed output torque of motors
Using "vector control", the output torque of the motor at low speeds, such as 1Hz (for a 4-pole motor, the speed is about 30r/min) can be the same as the output torque of the motor at 50Hz power supply (up to about 150% of the rated torque).
In conventional V/F control, the voltage drop across the motor increases relatively as the motor speed decreases. This leads to insufficient excitation, preventing the motor from obtaining enough rotational force. To compensate for this deficiency, the frequency converter needs to increase the voltage to offset the voltage drop caused by the decrease in motor speed. This function of the frequency converter is called "torque boost."
The torque boost function increases the inverter's output voltage. However, even with a significant increase in output voltage, the motor torque does not increase proportionally to the increase in current. This is because the motor current includes both the torque component generated by the motor and other components (such as the excitation component).
"Vector control" distributes the motor current value to determine the values of the motor current component that generates torque and other current components (such as the excitation component).
Vector control optimizes the response to voltage drops at the motor terminals, allowing the motor to produce high torque without increasing current. This feature is also effective in reducing motor temperature rise at low speeds.
(3) Direct Torque Control (DTC)
In 1985, Professor DePenbrock of Ruhr University in Germany first proposed direct torque control frequency conversion technology. This technology largely solved the shortcomings of the aforementioned vector control and has rapidly developed due to its novel control concept, simple and clear system structure, and excellent dynamic and static performance. Currently, this technology has been successfully applied to high-power AC drives for electric locomotive traction.
Direct torque control (DTC) utilizes the concept of spatial vector coordinates to analyze the mathematical model of an AC motor in the stator coordinate system, controlling the motor's flux linkage and torque. It achieves this by detecting the stator resistance, thus eliminating the complex transformation calculations required by vector control. The system is intuitive and simple, with improved calculation speed and accuracy compared to vector control. Even in open-loop operation, it can output 100% of the rated torque and provides load balancing for multi-drive applications.
(4) Optimal control
The practical applications of optimal control vary depending on the requirements. Based on optimal control theory, individual parameters can be optimized for a specific control requirement. For example, in the control applications of high-voltage frequency converters, time-segmented control and phase-shift control strategies have been successfully employed to achieve the optimal voltage waveform under certain conditions.
(5) Matrix-based cross-cross control mode
VVVF frequency converters, vector control frequency converters, and direct torque control frequency converters are all types of AC-DC-AC frequency converters. Their common drawbacks are low input power factor, high harmonic current, the need for large energy storage capacitors in the DC circuit, and the inability to feed regenerated energy back to the grid, meaning they cannot operate in four quadrants. To address these issues, matrix AC-AC frequency converters were developed. Because matrix AC-AC frequency converters eliminate the intermediate DC link, they also eliminate the need for large and expensive electrolytic capacitors. They can achieve a power factor of 1, sinusoidal input current, and four-quadrant operation, resulting in a high system power density. Although this technology is not yet mature, it continues to attract in-depth research from many scholars. Its essence is not to indirectly control quantities such as current and flux linkage, but rather to directly use torque as the controlled variable. The specific method is:
—Control the stator flux linkage by introducing a stator flux linkage observer to achieve a sensorless speed operation;
—Automatic identification (ID) relies on a precise mathematical model of the motor to automatically identify motor parameters;
—Calculate the actual values corresponding to stator impedance, mutual inductance, magnetic saturation factor, inertia, etc., and then calculate the actual torque, stator flux linkage, and rotor speed for real-time control;
— To achieve Band-Band control, PWM signals are generated based on the flux linkage and torque to control the inverter switching state.
Matrix AC-AC converters have fast torque response (<2ms), high speed accuracy (±2%, no PG feedback), and high torque accuracy (<+3%). They also have high starting torque and high torque accuracy, especially at low speeds (including 0 speed), where they can output 150% to 200% torque.
(6) Other non-intelligent control methods
In practical applications, some non-intelligent control methods can also be implemented in the control of frequency converters, such as adaptive control, sliding mode variable structure control, differential frequency control, circulating current control, and frequency control.
2. Intelligent control method
Intelligent control methods mainly include neural network control, fuzzy control, expert systems, and learning control. There are some successful examples of using intelligent control methods in the control of frequency converters.
(1) Neural network control
Neural network control is typically used in inverter control for complex systems where little is known about the system model. Therefore, the neural network must perform both system identification and control. Furthermore, neural network control can control multiple inverters simultaneously, making it suitable for cascaded inverter configurations. However, an excessively large number of neural network layers or overly complex algorithms can introduce practical difficulties in real-world applications.
(2) Fuzzy control
Fuzzy control algorithms are used to control the voltage and frequency of frequency converters, thereby controlling the acceleration time of the motor to avoid the impact of excessive acceleration on motor lifespan and excessive acceleration on work efficiency. The key to fuzzy control lies in the universe of discourse, membership degree, and fuzzy level division. This control method is particularly suitable for multi-input single-output control systems.
(3) Expert System
Expert systems are a control method that utilizes the experience of so-called "experts." Therefore, an expert system typically requires an expert database to store expert information, as well as an inference mechanism to seek the ideal control result based on known information. The design of the expert database and the inference mechanism is particularly important, as they determine the quality of the expert system's control. Expert systems can control both the voltage and current of frequency converters.
(4) Learning control
Learning control is primarily used for repetitive inputs, and regular PWM signals (such as center-modulated PWM) perfectly meet this condition. Therefore, learning control can also be used in inverter control. Learning control doesn't require much system information, but it does need 1-2 learning cycles, resulting in relatively poor speed. Furthermore, the algorithm sometimes requires a lead element, which is impossible with analog devices. Additionally, learning control involves stability issues, requiring special attention during application.
3. Prospects for Variable Frequency Drive Control
With the development of high-tech technologies such as power electronics, microelectronics, and computer networks, the control methods of frequency converters will develop in the following aspects in the future.
(1) Implementation of digital control frequency converter
Currently, the control method of frequency converters can achieve relatively complex calculations using digital processors. Frequency converter digitization will be an important development direction. At present, frequency converter digitization mainly uses single-chip microcomputers such as MCS51 or 80C196MC, supplemented by SLE4520 or EPLD LCD displays to achieve more complete control performance.
(2) Combination of multiple control methods
Different control methods have their own advantages and disadvantages. There is no "one-size-fits-all" control method. In some control situations, it is necessary to combine some control methods, such as combining learning control with neural network control, adaptive control with fuzzy control, and direct torque control with neural network control, or what is called "hybrid control". This way, the strengths of each method can be combined to make up for their weaknesses, and the control effect will be better.
(3) Implementation of remote control
The development of computer networks has made "the ends of the earth seem near," and remote control of frequency converters via computer networks is also a future trend. Remote control of frequency converters through RS485 interfaces and other network protocols allows for easy achievement of control objectives even in situations where on-site human operation is not feasible.
(4) Green frequency converter
With the introduction of sustainable development strategies, environmental protection has received increasing attention. The high-order harmonics generated by frequency converters can pollute the power grid. Addressing issues such as reducing noise during inverter operation and enhancing its reliability and safety are all attempts to solve through appropriate control methods, leading to the design of green frequency converters.
4. Conclusion
The control method of frequency converters is a problem worthy of study.
