- Detail

Abstract: combined with the popularization and application of domestic frequency conversion technology, this paper expounds the technical characteristics of several control modes of general frequency converter, and focuses on the selection and application of torque control frequency converter in view of the reasonable selection of frequency converter control mode

key words: control mode; Application selection; Precautions

1 introduction

frequency conversion technology is born in response to the need of stepless speed regulation of AC motor. Since the 1960s, power electronic devices have experienced SCR (thyristor), GTO (gate turn off thyristor), BJT (bipolar power transistor), MOSFET (metal oxide field effect transistor), sit (electrostatic induction transistor), Sith (electrostatic induction thyristor), Mgt (MOS control transistor), MCT (MOS control thyristor), IGBT (insulated gate bipolar transistor) The development of hvigbt (high voltage resistant insulated gate bipolar thyristor) and the renewal of devices have promoted the continuous development of power electronic conversion technology. Since the 1970s, the research on pulse width modulation variable voltage variable frequency (pwm-vvvf) speed regulation has attracted great attention. In the 1980s, if you directly extract the data from the database, the PWM mode optimization problem, as the core of frequency conversion technology, attracted people's great interest, and obtained many optimization modes, among which the saddle wave PWM mode was the best. Since the late 1980s, VVVF converters from developed countries such as the United States, Japan, Germany and Britain have been put into the market and widely used

2 frequency converter control mode

low voltage general frequency conversion output voltage is 380 ~ 650V, output power is 0.75 ~ 400KW, and working frequency is 0 ~ 400Hz. Its main circuit adopts AC DC AC circuit. Its control mode has experienced the following four generations

2.1 u/f=c sinusoidal pulse width modulation (SPWM) control mode

is characterized by simple control circuit structure, low cost, and good mechanical hardness. It can meet the smooth speed regulation requirements of general transmission, and has been widely used in various fields of industry. However, at low frequency, due to the low output voltage, the torque is significantly affected by the stator resistance voltage drop, which reduces the maximum output torque. In addition, its mechanical characteristics are not as hard as DC motor after all, and its dynamic torque capacity and static speed regulation performance are not satisfactory. In addition, the system performance is not high, the control curve will change with the change of load, the torque response is slow, the motor torque utilization is not high, and the performance will decline and the stability will deteriorate due to the existence of stator resistance and inverter dead time effect at low speed. Therefore, people have developed vector control variable frequency speed regulation

2.2 voltage space vector (SVPWM) control mode

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, generates the three-phase modulation waveform at one time, and controls in the way that the inscribed polygon approaches the circle. After practical use, it has been improved, that is, the introduction of frequency compensation can eliminate the error of speed control; The amplitude of flux linkage is estimated by feedback to eliminate the influence of 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 circuits and no torque regulation is introduced, so the system performance has not been fundamentally improved

2.3 vector control (VC) mode

the method of vector control variable frequency speed regulation is to equivalent the stator currents IA, IB, IC of asynchronous motor in the three-phase coordinate system to the AC current ia1ib1 in the two-phase static coordinate system through three-phase two-phase transformation, and then equivalent to the DC current IM1 in the synchronous rotating coordinate system through directional rotation transformation according to the rotor magnetic field It1 (IM1 is equivalent to the excitation current of the DC motor; it1 is equivalent to the armature current proportional to the torque), and 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 inverse transformation of coordinates. Its essence is that AC motor is equivalent to DC motor, and the two components of speed and magnetic field are controlled independently. By controlling the rotor flux linkage, and then decomposing the stator current, the torque and magnetic field are obtained. Through coordinate transformation, orthogonal or decoupling control is realized. The proposal of vector control method has epoch-making significance. However, in practical application, because it is difficult to accurately observe the rotor flux linkage, the system characteristics are greatly affected by the motor parameters, and the vector rotation transformation used in the process of equivalent DC motor control is complex, making the actual control effect difficult to achieve the ideal analysis results

2.4 direct torque control (DTC) this year

in 1985, Professor depenbrock of Ruhr University in Germany first proposed direct torque control frequency conversion technology. This technology solves the above shortcomings of vector control to a great extent, and has developed rapidly with novel control ideas, simple and clear system structure and excellent dynamic and static performance. At present, this technology has been successfully applied to high-power AC drive of electric locomotive traction

direct torque control directly analyzes the mathematical model of the AC motor in the stator coordinate system, and controls the flux linkage and torque of the motor. It does not need to equivalent AC motor to DC motor, so many complex calculations in vector rotation transformation are omitted; It does not need to imitate the control of DC motor, nor does it need to simplify the mathematical model of AC motor for decoupling

2.5 matrix AC control mode

vvvf frequency conversion, vector control frequency conversion and direct torque control frequency conversion are all one of AC DC AC frequency conversion. Its common disadvantages are low input power factor, large harmonic current, large energy storage capacitor required by DC circuit, and regenerative energy can not be fed back. Not only the structure and property requirements are different, that is, four quadrant operation cannot be carried out. Therefore, matrix AC-AC converter came into being. Because the matrix AC-AC frequency conversion eliminates the intermediate DC link, the electrolytic capacitor with large volume and expensive price is omitted. It can achieve power factor of L, input current of sine and four quadrant operation, and the power density of the system is large. 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, flux linkage and other quantities, but to directly take torque as the controlled quantity. The specific method is:

-- control the stator flux and introduce the stator flux observer to realize the speed sensorless mode

-- automatic identification (ID) relies on accurate motor mathematical model to automatically identify motor parameters

-- calculate the actual value corresponding to stator impedance, mutual inductance and magnetic saturation, and repeat 28; Calculate the actual torque, stator flux and rotor speed with factors and inertia for real-time control

-- realize band band control. According to the band band control of flux and torque, PWM signals are generated to control the switching state of the inverter

matrix AC AC converter has fast torque response（

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