Four kinds of control methods of the inverter to the motor

(1)U/f constant control U/f control is to change the motor power supply voltage while changing the motor power frequency, so that the motor magnetic flux is kept constant. Within a wide speed range, the motor efficiency, power factor does not drop . Because it is the ratio of voltage and frequency, it is called U/f control. The main problem of constant U/f control is poor low-speed performance. When the rotational speed is extremely low, the electromagnetic torque cannot overcome the large static friction force, and the torque compensation of the motor and the change of the load torque can not be properly adjusted; Can not accurately control the actual speed of the motor. Since the constant U/f converter is an open-loop control of the speed, the mechanical characteristic diagram of the asynchronous motor shows that the set value is the ideal no-load speed, and the actual speed of the motor is determined by the slip rate, so U/ f The constant error in the constant control mode cannot be controlled, so it is impossible to accurately control the actual speed of the motor.

(2) Slip frequency control The slip frequency is the frequency of the difference between the AC power frequency applied to the motor and the motor speed. According to the stable mathematical model of the asynchronous motor, when the frequency is constant, the electromagnetic torque of the asynchronous motor is proportional to the slip rate, and the mechanical characteristic is a straight line.

Slip frequency control is to control the torque and current by controlling the slip frequency. Slip frequency control needs to detect the speed of the motor, constitute a speed closed loop, the output of the speed regulator is the slip frequency, and then the sum of the motor speed and slip frequency as a given frequency of the inverter. Compared with U/f control, its acceleration and deceleration characteristics and ability to limit over-current are improved. In addition, it has a speed regulator that uses closed-loop control with speed feedback and a small static error in speed. However, to achieve the steady-state control of the automatic control system, good dynamic performance cannot be achieved.

(3) vector control vector control, also known as magnetic field orientation control. It was in the early 1970s by West Germany F. Blasschke et al. first proposed this principle by comparing the DC motor with the AC motor. This created a precedent for AC motors and equivalent DC motors. The vector control frequency conversion velocity modulation method is the asynchronous motor stator current in the three-phase coordinate system AC current Ia, Ib, Ic. Through the three-phase-two-phase transformation, it is equivalent to the alternating current Ia1, Ib1 in the two-phase stationary coordinate system, and then through the rotation and transformation of the magnetic field orientation of the rotor, equivalent to the direct current Im1, It1 in the synchronous rotating coordinate system (Im1 quite In the DC motor excitation current; It1 equivalent to the DC motor armature current), and then imitate the DC motor control method, to obtain the DC motor control, after the corresponding coordinate inverse transformation to achieve asynchronous motor control. The emergence of the vector control method, so that the asynchronous motor frequency control in the motor speed control in a full range of advantages. However, the vector control technology needs to correctly estimate the motor parameters. How to improve the accuracy of the parameters is a topic that has been researched.

(4) Direct Torque Control In 1985, Prof. De Penbrock of the University of Luhr in Germany presented the theory of direct torque control for the first time. This technology solves the problem of vector control to a large extent. It does not control the current and the flux linkage is equivalent. Instead of controlling the torque indirectly, the torque is directly controlled as a controlled variable. The superiority of torque control lies in that torque control is to control the stator flux linkage, which does not need the speed information in essence, and the control is robust to changes of all motor parameters except the stator resistance; the introduced stator flux observation The synchronizing speed information can be easily estimated by the device, so that a speed-free sensor can be conveniently implemented. Such a control is called speed sensorless direct torque control.