Variable frequency speed control technology is an important development direction of modern electric drive technology. As the core of the variable frequency speed control system, the performance of the frequency converter is increasingly becoming the decisive factor in the quality of speed control. Besides the inherent conditions of the frequency converter's manufacturing process, the control method adopted is also crucial. This article, based on industrial practice, reviews the characteristics of various frequency converter control methods in recent years and looks forward to future development directions.
Inverter Introduction
1) Basic structure of frequency converter
A frequency converter is a device that transforms mains frequency power (50Hz or 60Hz) into AC power of various frequencies to enable variable speed operation of a motor. The control circuit controls the main circuit, the rectifier circuit converts AC to DC, the DC intermediate circuit smooths and filters the output of the rectifier circuit, and the inverter circuit converts the DC back into AC. For frequency converters like vector control frequency converters that require extensive computation, a CPU for torque calculation and other corresponding circuits are sometimes also needed.
2) 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.
Selection Criteria for Variable Frequency Drive Control Method
The control method should be selected based on the specific requirements of the production machinery.
1. For quadratic law loads such as centrifugal fans, water pumps, and air compressors, V/F control is generally preferred. This is because V/F control has a low excitation U/f line, which can better save energy during low-frequency operation. Vector control essentially maintains the motor's rated magnetic flux at all times, making low excitation impossible. 2. Constant torque loads
(1) For loads with frequently changing load rates and small speed ranges, it is generally better to choose non-feedback vector control, because the "torque boost" of V/F control is not easy to preset perfectly. However, when using non-feedback vector control, the following should be noted:
1) The motor parameters must be measured by the user.
2. If the minimum operating frequency is below 5Hz, it is necessary to understand the low-frequency operating characteristics of the selected inverter brand. Some inverters are often not stable enough when operating at low frequencies in a non-feedback vector control mode.
(2) For loads with a stable load rate, V/F control can be used because a general-purpose frequency converter without vector control function can be selected at a lower price.
(3) The lifting machinery adopts the "feedback vector control" method, while some frequency converters can adopt the "no feedback vector control" method.
3. Constant power load
(1) The winding machine can adopt V/F control mode or "no feedback vector control mode".
(2) Metal cutting machine tools have high requirements for dynamic response, so it is best to use the "feed vector control" method.
Constant torque load: P=ML*n/9550, where ML is the constant torque and power is directly proportional to speed. Constant power load: ML=9550P/n, where power P is constant and torque is inversely proportional to speed.
Detailed Explanation of Variable Frequency Drive Control Methods
1. Inverter control method: Sinusoidal pulse width modulation (SPWM) with U/f=C
The SPWM control method of frequency converters is characterized by its simple control circuit structure, low cost, and good mechanical stiffness, which can meet the smooth speed regulation requirements of general drives and has been widely used in various industrial fields. However, at low frequencies, due to the low output voltage, the torque is significantly affected by the stator resistance voltage drop, resulting in a reduction in the maximum output torque. In addition, its mechanical characteristics are ultimately not as stiff as those of a DC motor, and its dynamic torque capability and static speed regulation performance are not satisfactory. Furthermore, the system performance is not high, the control curve changes with the load, the torque response is slow, the motor torque utilization rate is low, and the performance degrades and the stability deteriorates at low speeds due to the stator resistance and inverter dead-zone effect. Therefore, vector control frequency converter speed regulation has been developed.
2. Voltage Space Vector Pulse Width Modulation (SVPWM) as a Frequency Converter Control Method
The SVPWM control method of the frequency converter is based on the overall generation effect of the three-phase waveform, 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 application, 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.
3. Vector Control (VC) Method of Variable Frequency Drive (VFD)
The VC control method of a frequency converter involves transforming the stator currents Ia, Ib, and Ic of an asynchronous motor in a three-phase coordinate system into equivalent AC currents Ia1 and Ib1 in a two-phase stationary coordinate system through a three-phase to two-phase transformation. Then, through a rotor field-oriented rotational transformation, these are equivalent to DC currents Im1 and It1 in a synchronous rotating coordinate system (Im1 is equivalent to the excitation current of a DC motor; It1 is equivalent to the armature current proportional to torque). Then, mimicking the control method of a DC motor, the control quantities for the DC motor are obtained. After a corresponding inverse coordinate transformation, the asynchronous motor is controlled. Essentially, it equates an AC motor to a DC motor, independently controlling the speed and magnetic field components.
By controlling the rotor flux linkage and then decomposing the stator current to obtain the torque and magnetic field components, orthogonal or decoupled control can be achieved through coordinate transformation. The proposal of the vector control method is of epoch-making significance. However, in practical applications, due to the difficulty in accurately observing the rotor flux linkage, the significant influence of motor parameters on system characteristics, and the complexity of the vector rotation transformation used in the equivalent DC motor control process, the actual control effect is difficult to achieve the ideal analysis results.
4. Direct Torque Control (DTC) Method of Variable Frequency Drive (VFD)
The DTC control method of frequency converters originated in 1985 when Professor DePenbrock of Ruhr University in Germany first proposed the direct torque control frequency converter technology. This technology largely solves the shortcomings of the aforementioned vector control and has been 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 analyzes the mathematical model of the AC motor directly in the stator coordinate system, controlling the motor's flux linkage and torque. It does not require equating the AC motor to a DC motor, thus eliminating many complex calculations in vector rotation transformation; it does not require mimicking the control of a DC motor, nor does it require simplifying the mathematical model of the AC motor for decoupling.
5. Matrix AC-AC control method for frequency converters
The matrix AC-AC mode of the frequency converter eliminates the intermediate DC link, thus eliminating the need for bulky and expensive electrolytic capacitors. It 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 numerous scholars for in-depth research. Its essence is not to indirectly control quantities such as current and flux linkage, but rather to directly control torque as the controlled variable.
The specific method is as follows:
1) Control the stator flux linkage by introducing a stator flux linkage observer to achieve a sensorless speed mode;
2) Automatic identification (ID) relies on a precise mathematical model of the motor to automatically identify motor parameters;
3) Calculate the actual values corresponding to stator impedance, mutual inductance, magnetic saturation factor, inertia, etc., and calculate the actual torque, stator flux linkage, and rotor speed for real-time control;
4) Implement Band-Band control: Generate PWM signals based on 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.
Of course, reading this far doesn't mean you've fully understood frequency converters. To better understand how frequency converters work, you need to break down the above knowledge, apply it to real-world situations, and verify it before you can say you've truly grasped their working principles (the old adage: practice makes perfect).
