Development and Prospect of the hottest general in

Development and Prospect of general frequency converter technology

[Abstract] understanding the technical development of foreign general frequency converters in recent ten years is of great significance for in-depth understanding of the development trend of AC drive and control technology and how to stand at a high starting point and develop our own products in combination with China's national conditions.

[key words] general frequency conversion power electronics IGBT IPM PWM dtc

1 Preface

AC drive and control technology is one of the most rapidly developing technologies at present, which is closely related to the rapid development of ranli electronic device manufacturing technology, converter technology control technology, microcomputer and large-scale integrated circuit

as an early commodity, general frequency converter has been listed in China for nearly a decade. Its sales have increased year by year, and there is a market of more than billions of yuan (RMB) this year. Among them, most of them are imported brands, with power ranging from 100 watts to thousands of kilowatts; Simple or complex functions; Low or high accuracy; Slow or fast response: PG (tachometer) or no PG; With or without noise, etc

for many users, we have experienced many updates in the past decade. Most of the frequency converters used now belong to the most advanced models at present. From the perspective of application, our level is no different from that of developed countries. As a domestic manufacturer, through the marketing of foreign advanced technology in the past decade, it is also actively developing domestic frequency converters and trying to catch up with the level of developed countries in the world

reviewing the development of foreign general-purpose inverter technology in the past decade is of great significance for deeply understanding the trend of AC drive and control technology and how to develop our own products from a high starting point in combination with China's national conditions

2. About power devices, frequency conversion technology is based on power electronics technology. In the drive control of low-voltage AC motors, GTO, GTR, IGBT and intelligent power module (IPM) are the most widely used power devices. The latter two integrate the low saturation voltage characteristics of GTR and the high-frequency switching characteristics of MOSFET. They are the most widely used mainstream power devices in general-purpose converters at present. IGBT Collector Emitter Voltage VCE can be less than 3V, the frequency can reach 20kHz, and the embedded Collector Emitter ultra-high speed diode TRR can reach 150ns. It has been widely used in general-purpose converters since around 1992. Its development direction is lower loss, faster switching speed, higher voltage, and larger capacity (3.3kV, 1200A). At present, the fourth generation IGBT, which uses channel grid technology, non through technology and other methods to significantly reduce the saturation voltage between collector and emitter [vce (SAT)], has also come out

the application of the fourth generation IGBT has greatly improved the performance of the frequency converter. First, the heating of ICBT switching devices is reduced, reducing the heating of devices that once accounted for 50-70% of the heating of the main circuit by 30%. The second is high carrier control, which can significantly improve the output current waveform; Thirdly, the switching frequency is increased, which exceeds the range of human ears, that is, the static cyanization of motor operation is realized; Fourth, the driving power decreases and the volume tends to be smaller

and IPM is put into use about two years later than IGBT. Because IPM includes 1GBT chip and peripheral drive and protection circuits, and even some integrate optocoupler, it is a more useful integrated power device. At present, in the range of 10-600a rated current of the module, general-purpose converters have the tendency to adopt IPM, which has the following advantages:

(1) fast switching speed and small drive current, Control drive is simpler

(2) the embedded current sensor can detect the over-current and short-circuit current efficiently and quickly, provide sufficient protection for the power chip, and greatly reduce the failure rate

(3) due to the optimization of the wiring design of the internal power supply circuit and drive circuit of the device, the problems such as surge voltage, gate oscillation, interference caused by noise and so on can be effectively controlled

(4) the protection function is relatively rich, such as current protection, voltage protection and temperature protection. With the progress of technology, the protection function will be further improved

(5) the selling price of IPM has gradually approached that of igbt. after taking into account the factors such as the reduction of switching power supply capacity and driving power capacity, the saving of devices and the improvement of comprehensive performance after the adoption of IPM, its cost performance has been higher than that of IGBT on many occasions, and it has good economy

for this reason, in addition to being widely used in industrial frequency converters, economical IPM has also begun to be applied in some civil products in recent years, such as domestic air conditioning frequency converters, refrigerator frequency converters, and washing machine frequency converters. IPM is also developing to a higher level. ASIPM, a special intelligent module recently developed by Mitsubishi Electric in Japan, will not need an external optocoupler. It can be powered by a single power supply through an internal bootstrap circuit and adopts a low inductance packaging technology, which further advances the miniaturization, specialization, high performance and low cost of the system

3. About the control mode

most of the early general-purpose inverters, such as Toshiba tosfert-130 series, Fuji fvrg5/P5 series, Sanken SVF series, are open-loop constant voltage ratio (V/F = constant) control modes. Its advantages are simple control structure and low cost, but its disadvantages are low system performance, which is more suitable for fan and water pump regulation. Specifically, its control curve will change with the change of load; The torque response is slow, the utilization rate of TV torque is not high, and the performance decreases and the stability becomes worse due to the stator resistance and the dead time effect of inverter at low speed. The transformation of U/F control system of frequency converter has gone through three stages

first stage:

(1) In the early 1980s, Japanese scholars proposed the voltage space vector (or flux trajectory method) of the basic flux trajectory. This method takes the overall generation effect of the three-phase waveform as the premise, and aims to approximate the ideal circular rotating magnetic field track with the sales volume of the motor air gap of 40% (US200 million yuan), so as to generate the two-phase modulation waveform at one time. This method is called voltage space vector control. Typical models, such as Fuji frn5ooog5/p5 and Sanken MF Series, entered the Chinese market around 1989

(2) introduce frequency compensation control to eliminate the steady-state error of speed control

(3) based on the steady-state model of the motor, reconstruct the phase current with DC current signal, such as Siemens Micromaster series, so as to estimate the amplitude of the flux linkage, and eliminate the influence of stator resistance on Performance at low speed through feedback control

(4) the output voltage and current are closed-loop controlled to improve the voltage control accuracy and stability under dynamic load, and at the same time, the current waveform is improved to a certain extent. Another advantage of this control method is that it can significantly suppress the overvoltage and overcurrent caused by regeneration, so as to realize rapid acceleration and deceleration

after that, Fuji Electric launched the well-known FVR and frng7/P7 Series in 1991, which incorporated (2) (3) (4) technologies to varying degrees, so it is very representative. Mitsubishi Hitachi and Toshiba also have similar products. However, in the above four methods, the system performance has not been fundamentally improved because torque regulation is not introduced

second stage:

vector control. Also known as field oriented control. It was first put forward by asschke and others in West Germany in the early 1970s. This principle was analyzed and expounded by comparing DC motor with AC motor, thus creating a precedent for the equivalent DC motor control of AC motor. It makes people see that although the control of AC motor is complex, it can also realize the internal essence of independent control of torque and magnetic field

the basic point of vector control is to control the rotor flux linkage, orient the rotor flux, and then decompose the stator current into two components of torque and magnetic field, and realize orthogonal or decoupling control through coordinate transformation. However, due to the difficulty of accurate observation of rotor flux linkage and the complexity of vector transformation, the actual control effect is often difficult to achieve the effect of theoretical analysis, which is the deficiency of vector control technology in practice. In addition, it must directly or indirectly obtain the position of the rotor flux linkage in space to realize the stator current decoupling control. In this vector control system, it needs to be equipped with rotor position or speed sensors, which obviously brings inconvenience to many applications. Despite this, vector control technology is still trying to integrate into general-purpose converters. Since 1992, Siemens has developed 6SE70 general-purpose series, which can realize frequency control, vector control and servo control through FC, VC and SC boards respectively. In 1994, the series was expanded to more than 315KW. At present, the 6SE70 Series has a high cost performance above 200kW, except that the price below 200kW is higher

the third stage:

in 1985, Professor depenbrock of Ruhr University in Germany first proposed direct torque control (DTC). Direct torque control is different from vector control. Instead of indirectly controlling torque by controlling current, flux linkage and other quantities, it directly controls torque as a controlled quantity

the advantage of torque control is that torque control is to control the stator flux, which does not need speed information in essence; The control has good robustness to all motor parameter changes except stator resistance; The stator magnetic key observer can easily estimate the synchronous speed information. Therefore, speed sensorless can be easily realized. It is natural that this control method is applied to the design of general frequency converter. This control is called speed sensorless direct torque control. However, this control depends on the accurate mathematical model of the motor and the automatic identification of motor parameters (ID). Through ID operation, the actual stator impedance mutual inductance, saturation factor, motor inertia and other important parameters of the motor are automatically established, and then the actual torque, stator chain collision and rotor speed of the motor are estimated according to the accurate motor model, PWM signals are generated by the band band control of flux and torque to control the switching state of the inverter. This system can achieve fast torque response speed and high speed and torque control accuracy

the ACS600 direct torque control series first launched by ABB in 1995 has reached a torque response speed of 2ms, and the static speed accuracy with PG is up to 0.01%. Even without PG, the speed control accuracy of plus or minus 0.1% can be achieved even if it is affected by the change of input voltage or the sudden change of load. Other companies also aim at direct torque control, such as Yaskawa vs-676h5 high-performance speed sensorless vector control series. Although it is different from direct torque control, it has also achieved a torque response of 100ms and a speed control accuracy of plus or minus 0.2% (without PG) and plus or minus 0.01% (with PG). The torque control accuracy is about plus or minus 3%. Other companies, such as the FRN 5000g9/P9 and the latest frn5000gll/P11 series launched by Fuji Electric in Japan, have adopted a design similar to speed sensorless control, and their performance has been further improved. However, the price of frequency converters is not much more expensive than previous models

the development of control technology is entirely due to the development of microprocessor technology. Since Intel launched 8x196mc Series in 1991, chips specially used for motor control have made great progress in variety, speed, function, cost performance and so on. For example, the m3770 developed by Mitsubishi Electric in Japan for motor control