Instrumentation Technology: Frequency Converter Definition and Working Principle Overview

The frequency converter is a device that converts the industrial frequency power supply (50Hz or 60Hz) into AC power supply of various frequencies to achieve variable speed operation of the motor. The control circuit completes the control of the main circuit, the rectifier circuit converts AC power into DC power, the DC intermediate circuit smoothes and filters the output of the rectifier circuit, and the inverter circuit converts DC power into AC power. For frequency converters that require a lot of calculations, such as vector control frequency converters, a CPU for torque calculation and some corresponding circuits are sometimes required. Variable frequency speed regulation achieves the purpose of speed regulation by changing the frequency of the power supply to the motor stator winding.

Frequency conversion technology was born to meet the needs of stepless speed regulation of AC motors. After the 1960s, power electronic devices have gone through the development process of SCR (thyristor), GTO (gate turn-off thyristor), BJT (bipolar power transistor), MOSFET (metal oxide field effect transistor), SIT (static induction transistor), SITH (static induction thyristor), MGT (MOS control transistor), MCT (MOS control thyristor), IGBT (insulated gate bipolar transistor), HVIGBT (high voltage insulated gate bipolar thyristor), and the update of devices has promoted the continuous development of power electronic conversion technology. Since the 1970s, the research on pulse width modulation variable voltage and frequency (PWM-VVVF) speed regulation has attracted great attention. In the 1980s, the PWM mode optimization problem, as the core of frequency conversion technology, attracted people's strong interest, and many optimization modes were obtained, among which the saddle wave PWM mode had the best effect. Since the second half of the 1980s, VVVF inverters from developed countries such as the United States, Japan, Germany, and the United Kingdom have been put on the market and have been widely used.

There are many ways to classify inverters. According to the working mode of the main circuit, they can be divided into voltage type inverters and current type inverters; according to the switching mode, they can be divided into PAM control inverters, PWM control inverters and high carrier frequency PWM control inverters; according to the working principle, they can be divided into V/f control inverters, slip frequency control inverters and vector control inverters, etc.; according to the purpose, they can be divided into general inverters, high-performance special inverters, high-frequency inverters, single-phase inverters and three-phase inverters, etc.

VVVF: variable voltage, variable frequency CVCF: constant voltage, constant frequency. The voltage and frequency of the AC power supply used in various countries, whether for home or factory, are 400V/50Hz or 200V/60Hz (50Hz), etc. Usually, the device that converts the AC power with fixed voltage and frequency into the AC power with variable voltage or frequency is called "inverter". In order to generate variable voltage and frequency, the device must first convert the AC power of the power supply into direct current (DC).

The frequency converter used for motor control can change both voltage and frequency.

Working principle of frequency converter

We know that the synchronous speed expression of AC motor is:

n=60 f(1-s)/p (1)

In the formula

n——the speed of the asynchronous motor;

f——the frequency of the asynchronous motor;

s——motor slip rate;

p——The number of motor pole pairs.

From formula (1), we can see that the speed n is proportional to the frequency f. The speed of the motor can be changed by changing the frequency f. When the frequency f varies within the range of 0 to 50 Hz, the speed adjustment range of the motor is very wide. The frequency converter achieves speed regulation by changing the power supply frequency of the motor. It is an ideal high-efficiency and high-performance speed regulation method.

Inverter control mode

The low voltage general frequency conversion output voltage is 380~650V, the output power is 0.75~400kW, the operating frequency is 0~400Hz, and its main circuit adopts AC-DC-AC circuit. Its control method has gone through the following four generations.

1U/f=C Sinusoidal Pulse Width Modulation (SPWM) control method

其特点是控制电路结构简单、成本较低，机械特性硬度也较好，能够满足一般传动的平滑调速要求，已在产业的各个领域得到广泛应用。但是，这种控制方式在低频时，由于输出电压较低，转矩受定子电阻压降的影响比较显著，使输出最大转矩减小。另外，其机械特性终究没有直流电动机硬，动态转矩能力和静态调速性能都还不尽如人意，且系统性能不高、控制曲线会随负载的变化而变化，转矩响应慢、电机转矩利用率不高，低速时因定子电阻和逆变器死区效应的存在而性能下降，稳定性变差等。因此人们又研究出矢量控制变频调速。
2电压空间矢量(SVPWM)控制方式

It is based on the overall generation effect of the three-phase waveform, with the purpose of approaching the ideal circular rotating magnetic field trajectory of the motor air gap, generating a three-phase modulated waveform at one time, and controlling it in the way of an inscribed polygon approaching a circle. After practical use, it has been improved, that is, the introduction of frequency compensation can eliminate the error of speed control; the feedback estimation of the flux amplitude can eliminate the influence of the stator resistance at low speed; the output voltage and current are closed-loop to improve the dynamic accuracy and stability. However, there are many control circuit links, and no torque adjustment is introduced, so the system performance has not been fundamentally improved.

Vector control (VC) mode

The method of vector control variable frequency speed regulation is to convert the stator current Ia, Ib, Ic of the asynchronous motor in the three-phase coordinate system into the AC current Ia1Ib1 in the two-phase stationary coordinate system through three-phase-two-phase transformation, and then convert it into the DC current Im1 and It1 in the synchronous rotating coordinate system through the directional rotation transformation according to the rotor magnetic field (Im1 is equivalent to the excitation current of the DC motor; It1 is equivalent to the armature current proportional to the torque). Then imitate the control method of the DC motor to obtain the control quantity of the DC motor, and realize the control of the asynchronous motor through the corresponding coordinate inverse transformation. Its essence is to convert the AC motor into a DC motor and control the speed and magnetic field components independently. By controlling the rotor flux, the stator current is decomposed to obtain the torque and magnetic field components, and the coordinate transformation is used to realize orthogonal or decoupling control. The introduction of the vector control method is of epoch-making significance. However, in practical applications, since the rotor flux is difficult to observe accurately, the system characteristics are greatly affected by the motor parameters, and the vector rotation transformation used in the equivalent DC motor control process is relatively complex, making it difficult for the actual control effect to achieve the ideal analysis result.

Direct Torque Control (DTC)

In 1985, Professor DePenbrock of Ruhr University in Germany first proposed direct torque control frequency conversion technology. This technology has largely solved the shortcomings of the above-mentioned vector control, and has developed rapidly with its novel control ideas, concise and clear system structure, and excellent dynamic and static performance. At present, this technology has been successfully applied to high-power AC transmission traction of electric locomotives. Direct torque control directly analyzes the mathematical model of the AC motor in the stator coordinate system and controls the magnetic flux and torque of the motor. It does not need to equate the AC motor to a DC motor, thus eliminating many complex calculations in the vector rotation transformation; it does not need to imitate the control of the DC motor, nor does it need to simplify the mathematical model of the AC motor for decoupling.

Matrix cross-cross control method

VVVF frequency conversion, vector control frequency conversion, and direct torque control frequency conversion are all types of AC-DC-AC frequency conversion. Their common disadvantages are low input power factor, large harmonic current, large energy storage capacitors required for DC circuits, and the regenerated energy cannot be fed back to the power grid, that is, four-quadrant operation is not possible. For this reason, matrix AC-AC frequency conversion came into being. Since the matrix AC-AC frequency conversion eliminates the intermediate DC link, it eliminates the large and expensive electrolytic capacitors. It can achieve a power factor of 1, a sinusoidal input current and can operate in four quadrants, and the system has a high power density. Although this technology is not yet mature, it still attracts many scholars to conduct in-depth research. Its essence is not to indirectly control current, magnetic flux and other quantities, but to directly use torque as the controlled quantity. The specific method is:

——Control the stator flux linkage and introduce the stator flux linkage observer to realize the speed sensorless method;

——Automatic identification (ID) relies on accurate motor mathematical models to automatically identify motor parameters;

——Calculate the actual values ​​corresponding to stator impedance, mutual inductance, magnetic saturation factor, inertia, etc. to calculate the actual torque, stator flux, and rotor speed for real-time control;

——Realize Band-Band control. Generate PWM signal according to the Band-Band control of flux and torque to control the switching state of the inverter.

Matrix AC-AC frequency conversion has fast torque response (<2ms), very high speed accuracy (±2%, no PG feedback), and high torque accuracy (<+3%); it also has high starting torque and high torque accuracy, especially at low speed (including 0 speed), and can output 150%~200% torque.