In the process of practical speed regulation, an ordinary AC power supply with adjustable frequency can not meet the requirements of speed regulation and control of the asynchronous motor, and it is necessary to take into account the effective use of the motor's magnetic field, the pressing starting current and the desired torque characteristics, such as low-frequency torque characteristics.
In order to obtain the more ambitious role of frequency conversion speed regulation, V/f control method, slip frequency control method, vector control method and direct torque control method were selected in the development process of frequency conversion control technology.
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 basic control method of inverter. However, in terms of speed control, the satisfactory control function can not be given, and when the output frequency is low, due to the drop of output voltage, the stator winding current decreases, and the motor torque is insufficient, the output voltage needs to be appropriately improved to improve the motor torque, and torque compensation is carried out.
2. Differential frequency control of frequency conversion control method
This is a closed-loop control method for speed response control, and its dynamic and static functions are better than V/f control methods, so it can be applied to various speed control 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 appropriate, so the inverter that selects the slip frequency control method has been basically 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 discussed this principle by comparing DC motors and AC motors.
The 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 perpendicular current weight of the 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 a good torque control function and achieve high-function control.
The presentation of the 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, the speed regulation scale is wide, the torque can be accurately controlled, 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 technology requires the correct estimation of motor parameters, and how to improve the accuracy of parameters has been a topic of discussion.
4. Direct torque control of frequency conversion control method
Direct torque control technology 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 scale, the size of the tolerance is controlled by the frequency regulator, and the PWM pulse width modulation signal occurs. The switching situation of the inverter can be directly controlled to obtain a torque output with high dynamic functions. Its handling role does not depend on whether the mathematical model of the asynchronous motor can be simplified, but on the practical situation of the torque. It does not need to compare, equivalence, and conversion of AC motors with DC motors, that is, it does not need to imitate the control of DC motors. Because it omits the coordinate change and accounting of the vector switching method and simplifies the mathematical model of the asynchronous motor for decoupling, there is no usual PWM pulse width modulation signal generator, so 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 an AC speed regulation method with high static and dynamic functions. In contrast to the vector manipulation method, the direct torque manipulation of the magnetic field direction uses a custom flux, which uses a discrete voltage situation and a hexagonal flux track or an approximate circular flux track concept. As long as you know the resistance of the nail, you can 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 control greatly reduces the problem that the control function in the vector control technology is easily affected by parameter changes.
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