Specifically, its control curve will change with the change of load; The torque response is slow, and the torque utilization rate of TV is not high. At low speed, due to stator resistance and inverter dead-time effect, the performance is degraded and the stability is poor.
The transformation of U/F control system of frequency converter has mainly gone through three stages;
The first stage:
1In the early 1980s, Japanese scholars proposed the voltage space vector (or flux trajectory method) of the basic flux trajectory.
This method is based on the overall generation effect of three-phase waveform, and takes the ideal circular rotating magnetic field trajectory approaching the air gap of the motor as the goal to generate two-phase modulation waveform at one time.
This method is called voltage space vector control.
Typical models include Fuji FRN5OOOG5/P5 and SANKEN MF series, which entered the China 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, the phase current is reconstructed by using DC current signals such as Siemens MicroMaster series, so as to estimate the magnitude of flux linkage, and the influence of stator resistance on performance at low speed is eliminated through feedback control.
(4) Closed-loop control of output voltage and current can improve the accuracy and stability of voltage control under dynamic load, and at the same time improve the current waveform to some extent.
Another advantage of this control method is that the overvoltage and overcurrent caused by regeneration are obviously suppressed, so that rapid acceleration and deceleration can be realized.
Later, in 199 1, Fuji Electric introduced the famous designs of FVR and FRNG7/P7 series, which included 23. 4 different levels of technology, 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 adjustment is not introduced.
The second stage:
Vector control.
Also known as field-oriented control.
It was first put forward by F.Blasschke of West Germany in the early 1970s, and this principle was analyzed and expounded by comparing DC motor with AC motor, thus creating a precedent for equivalent DC motor control of AC motor.
It makes people see that although the control of AC motor is complicated, it can also realize the inherent essence of independent control of torque and magnetic field.
The basic point of vector control is to control and orient the rotor flux linkage, and then decompose the stator current into two components: torque and magnetic field, and realize orthogonal or decoupling control through coordinate transformation.
However, because the rotor flux linkage is difficult to observe accurately and the vector transformation is complex, the actual control effect is often difficult to achieve the effect of theoretical analysis, which is the deficiency of vector control technology in practical application.
In addition, the position of rotor flux linkage in space must be obtained directly or indirectly in order to realize stator current decoupling control. In this vector control system, it is necessary to keep the rotor position or speed sensor, which obviously brings inconvenience to many applications.
Even so, vector control technology is still trying to be integrated into the general frequency converter. Starting from 1992, Siemens Germany developed 6SE70 universal series, which can realize frequency control, vector control and servo control through FC, VC and SC boards respectively.
1994, the series expanded to more than 3 15KW.
At present, 6SE70 series is high in price except below 200KW, and high in cost performance above 200KW.
The third stage:
1985, Professor Depenbrock of Ruhr University in Germany first put forward the theory of direct torque control (DTC).
Different from vector control, direct torque control does not indirectly control torque by controlling current and flux linkage, but directly controls torque as a controlled quantity.
The advantages of torque control are as follows: torque control is to control stator flux linkage, and it does not need speed information in essence; The control is robust to all motor parameters except stator resistance. The stator magnetic key observer can estimate the synchronous speed information conveniently.
Therefore, speed sensorless can be realized conveniently.
This control method is naturally applied to the design of general frequency converter, which is called speed sensorless direct torque control.
But this control depends on the precise mathematical model of the motor and the automatic identification of motor parameters (identification gives you ID). The actual stator impedance mutual inductance, saturation factor, motor inertia and other important parameters of the motor are automatically established by ID operation, and then the actual torque, stator chain collision and rotor speed of the motor are estimated according to the accurate motor model, and PWM signals are generated by band-to-band control of flux linkage and torque to control the switching state of the inverter.
The system can achieve fast torque response speed and high speed and torque control accuracy.
1995 ABB's first ACS600 direct torque control series has been achieved.
Other companies also aim at direct torque control, such as An Chuan VS-676H5 high-performance speed sensorless vector control series. Although it is still different from direct torque control, it has achieved the torque response of 100ms and the speed control accuracy of plus or minus 0.2% (without PG) and plus or minus 0.0 1% (with PG), and the torque control accuracy is about plus or minus 3%.
Other companies, such as FRN5000G9/P9 manufactured by Fuji Electric and the latest FRN5000GLL/P1series, have adopted similar speed sensorless control designs, and their performance has been further improved. However, the price of inverter is not much more expensive than the previous model.
The development of control technology is entirely due to the development of microprocessor technology. Since 1 INTEL introduced the 8x 19965438 series, the chips specially used for motor control have made great progress in variety, speed, function and cost performance.
For example, M37705 and M7906 single-chip computers developed by mitsubishi electric for motor control, and TMS320C240DSP developed by Texas Instruments are all representative products.