The change of frequency conversion control mode

- Jan 21, 2019-

In the actual speed regulation process, an ordinary frequency-adjustable AC power supply can not meet the requirements of speed control of the asynchronous motor. It must also consider the effective use of the motor magnetic field, suppress the starting current and obtain the ideal torque characteristics, such as Low frequency torque characteristics and other issues.

    In order to obtain the ideal frequency conversion speed regulation effect, the V/f control mode, the slip frequency control mode, the vector control mode and the direct torque control mode are adopted in the development process of the frequency conversion control technology.

1. V/f control of frequency conversion control mode

The V/f control mode means that in the process of frequency conversion speed regulation, in order to keep the main magnetic flux constant, the output of the motor is guaranteed, and the ratio of voltage and frequency is kept constant, that is, the control mode of V/f=constant. For a 380V, 50Hz motor, when the operating frequency is 40Hz, to maintain a constant V/f, the supply voltage of the motor at 40Hz is 380 × (40 / 50) = 304V.

This kind of frequency converter is cost-effective and is widely used in various occasions where energy saving is the purpose and speed accuracy is not high. It is the basic control method of the inverter. However, satisfactory control performance cannot be given in terms of speed control, and when the output frequency is low, the output voltage is reduced, the stator winding current is reduced, and the motor torque is insufficient. The output voltage needs to be appropriately increased to improve the motor torque. , for torque compensation.

2. Slip frequency control of frequency conversion control mode

This is a closed-loop control method for speed feedback control. Its dynamic and static performance are superior to the V/f control method. Therefore, it can be applied to various speed control systems with high requirements on speed and accuracy. However, because the inverter with this control method is worse than the vector control inverter in terms of control performance, and the complexity of the hardware circuits is quite similar, the inverter with the slip frequency control method is basically vector controlled. Taken by the inverter.

3. Vector control of frequency conversion control mode

Vector control, also known as field oriented control. It was first proposed by West German F. Blasschke et al. in the early 1970s, and this principle was elaborated by comparing DC motors with AC motors.

This method mimics the control method of the DC motor, and uses the vector coordinate transformation to divide the stator current of the asynchronous motor into a current component (excitation current) that generates a magnetic field and a current component (torque current) that generates a vertical torque, and simultaneously controls the asynchronous motor. The magnitude and phase of the stator current, that is, the stator current vector, is called the vector control method. The control method

The constant magnetic flux of the motor is maintained, thereby achieving good torque control performance and achieving high performance control.

The emergence of vector control methods makes the asynchronous motor frequency conversion speed control in an all-round position in the field of motor speed regulation. It has many advantages, it can be controlled from zero speed, wide speed range, precise control of torque, fast system response and good acceleration/deceleration performance. Therefore, this control method is widely used in speed control systems with high speed performance requirements. However, vector control technology requires accurate estimation of motor parameters, and how to improve the accuracy of parameters is a topic that has been studied.

4. Direct torque control of frequency conversion control mode

     The direct torque control technology uses the analysis method of space vector and stator field orientation to analyze the mathematical model of the asynchronous motor directly in the stator coordinate system, calculate and control the flux linkage and torque of the asynchronous motor, and adopt discrete two-point adjustment. The device (Band-Band control) compares the torque detection value with the torque reference value to limit the torque fluctuation within a certain tolerance range. The tolerance is controlled by the frequency regulator and generates the PWM pulse. The wide modulation signal directly controls the switching state of the inverter to obtain a high dynamic performance torque output. Its control effect does not depend on whether the mathematical model of the asynchronous motor can be simplified, but depends on the actual condition of the torque. It does not need to compare, convert, and convert the AC motor with the DC motor, that is, it does not need to imitate the control of the DC motor. Because it eliminates the coordinate transformation and calculation of vector transformation mode and simplifies the asynchronous motor mathematical model for decoupling, there is no common PWM pulse width modulation signal generator, so its control structure is simple, the physical concept of control signal processing is clear, The torque response of the system is fast and there is no overshoot. It is an AC speed regulation mode with high static and dynamic performance. Compared to the vector control method, the direct torque control field orientation uses a custom flux linkage that uses discrete voltage states and hexagonal flux linkage trajectories or an approximate circular flux linkage trajectory concept. As long as you know the nail resistance, you can observe it. The vector control magnetic field orientation uses the rotor flux linkage. Observing the rotor flux linkage requires knowing the rotor resistance and inductance of the motor. Therefore, direct torque control greatly reduces the problem that the control performance in vector control technology is susceptible to parameter changes.

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