In the process of practical speed regulation, an ordinary frequency-adjustable communication power supply can not meet the requirements of speed regulation and control of asynchronous motors, and it is also necessary to take into account the effective use of the motor's magnetic field, the suppression of the starting current and the acquisition of the desired torque characteristics, such as low-frequency torque characteristics.
In order to obtain the more ambitious role of frequency conversion speed regulation, the V/f control method, the slip frequency control method, the vector control method and the direct torque control method were selected in the development process of the frequency conversion control skills.
1. V/f control of frequency conversion control method
The V/f control method refers to the control method in which the ratio of voltage and frequency insists on a certain amount in order to adhere to the stability of the main magnetic flux and ensure the output of the motor in the process of frequency conversion speed regulation, that is, V/f = constant. For example, for 380V and 50Hz motors, when the operating frequency is 40Hz, the V/f stability should be insisted on, and the power supply voltage of the motor at 40Hz is 380× (40/50)=304V.
This kind of inverter is cost-effective, and is widely used in various occasions for the purpose of energy saving and low requirements for speed accuracy, and is the fundamental control method of inverter. However, in terms of speed control, it can not give a satisfactory control function, and when the output frequency is low, due to the drop of output voltage, the stator winding current decreases, and the motor torque is lacking.
2. Differential frequency control of frequency conversion control method
This is a closed-loop control method for speed feedback control, and its dynamic and static functions are better than V/f control methods, so it can be applied to various speed regulation systems with high requirements for speed and accuracy. However, because the inverter that chooses this control method is worse than the vector control inverter in terms of control function, and the complexity of the hardware circuits of the two is comparable, so the inverter that selects the slip frequency control method has been fundamentally taken by the vector control inverter.
3. Vector control of frequency conversion control method
Vector manipulation, also known as field-oriented manipulation. In the early 70s of the 20th century, the West German F. · Blasschke et al. first proposed and illustrated this principle by comparing DC motors and communication motors.
This method imitates the control method of DC motor, selects vector coordinate replacement, divides the stator current weight of the asynchronous motor into the current weight of the magnetic field (excitation current) and the current weight of the straight torque (torque current), and controls the amplitude and phase of the stator current of the asynchronous motor together, that is, the control of the stator current vector, so it is called the vector control method. The manipulation
It insists on the stability of the magnetic flux of the motor, so as to achieve good torque control function and achieve high-function control.
The emergence of vector control method makes the frequency conversion speed regulation of asynchronous motor in an all-round advantageous position in the field of motor speed regulation. It has many advantages, it can be controlled from zero speed, has a wide range of speed regulation, can accurately control the torque, the system echoes quickly, and the acceleration/deceleration function is good. Therefore, the control method is widely used in the speed regulation system with high requirements for speed regulation function. However, vector control skills require the correct estimation of motor parameters, and how to improve the accuracy of parameters has always been a research topic.
4. Direct torque control of frequency conversion control method
The direct torque control skill is to use the analysis method of space vector and stator magnetic field orientation, directly analyze the mathematical model of the asynchronous motor under the stator coordinate system, calculate and control the flux and torque of the asynchronous motor, select a discrete two-point regulator (Band-Band control), compare the torque detection value with the given value of torque, so that the torque fluctuation is limited to a certain tolerance range, the size of the tolerance is controlled by the frequency regulator, and the PWM pulse width modulation signal occurs. The switching state of the inverter can be directly controlled to obtain a torque output with high dynamic functions. Its handling effect does not depend on whether the mathematical model of the asynchronous motor can be simplified, but on the practical state of torque. It does not need to compare, equivalence, and conversion between the AC motor and the DC motor, that is, it does not need to imitate the control of the DC motor. Because it omits the coordinate change and calculation of the vector switching method and simplifies the mathematical model of the asynchronous motor for decoupling, and does not have the usual PWM pulse width modulation signal generator, its control structure is simple, the physical concept of control signal processing is clear, the torque of the system echoes quickly and there is no overshoot, and it is a communication speed regulation method with high static and dynamic functions. In contrast to vector manipulation, direct torque manipulation of magnetic field orientation uses a custom flux, which uses a discrete voltage state and a hexagonal flux track or an approximate circular flux track concept. You only need to know the resistance of the nail to be able to observe it. The rotor flux is used to control the magnetic field direction in vector, and the rotor flux needs to know the rotor resistance and inductance of the motor to observe the rotor flux. Therefore, the direct torque manipulation greatly reduces the problem that the control function in the vector manipulation skill is easily affected by parameter changes.
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