1. U/f constant control U/f control changes the voltage of the motor power supply while changing the frequency of the motor power supply, so that the magnetic flux of the motor is kept constant. In the wide speed regulation range, the efficiency of the motor and the power factor do not decrease. Because it is the ratio of the control voltage (Voltage) to the frequency (Frequency), it is called U/f control. The main problem of constant U/f control is that the low speed performance is poor. When the speed is extremely low, the electromagnetic torque cannot overcome the large static friction force, and the torque compensation of the motor and the adaptation of the load torque cannot be properly adjusted. The actual speed of the motor cannot be accurately controlled. Since the constant U/f inverter is the open-loop control of the speed, the mechanical characteristic diagram of the asynchronous motor shows that the set value is the ideal idle speed of the stator frequency, and the actual speed of the motor is determined by the slip rate, so U/ The stability error existing in the f constant control mode cannot be controlled, so the actual rotational speed of the motor cannot be accurately controlled. Second, the slip frequency control slip frequency is the difference frequency between the AC power frequency applied to the motor and the motor speed. According to the asynchronous motor stability mathematical model, when the frequency is constant, the electromagnetic torque of the asynchronous motor is proportional to the slip ratio, and the mechanical characteristic is a straight line.
Slip frequency control is to control the torque and current by controlling the slip frequency. The slip frequency control needs to detect the speed of the motor to form a speed closed loop. The output of the speed regulator is the slip frequency, and then the sum of the motor speed and the slip frequency is used as the given frequency of the inverter. Compared with U/f control, its acceleration and deceleration characteristics and ability to limit overcurrent are improved. In addition, it has a speed regulator that uses speed feedback to form a closed loop control with a small static error. However, to achieve steady state control of the automatic control system, good dynamic performance is not achieved. Third, vector control vector control, also known as field oriented control. It was first proposed by West German F. Blasschke et al. in the early 1970s to illustrate this principle by comparing DC motors with AC motors. This led to the creation of AC motors and equivalent DC motors. The vector control frequency conversion speed regulation method is to stator asynchronous current Ia, Ib, Ic of the asynchronous motor in the three-phase coordinate system. Through the three-phase-two-phase transformation, the alternating currents Ia1 and Ib1 in the two-phase stationary coordinate system are equivalent to the direct current rotation according to the rotor field, which is equivalent to the direct current Im1 and It1 in the synchronous rotating coordinate system (Im1 is equivalent). The excitation current of the DC motor; It1 is equivalent to the armature current of the DC motor), and then the control method of the DC motor is simulated, and the control amount of the DC motor is obtained, and the control of the asynchronous motor is realized through the inverse transformation of the corresponding coordinates. The emergence of the vector control method makes the asynchronous motor frequency conversion speed regulation in an all-round position in the field of motor speed regulation. However, vector control technology needs to correctly estimate the motor parameters. How to improve the accuracy of the parameters is a topic that has been studied. Direct Torque Control In 1985, Professor DePenbrock of Ruhr University in Germany first proposed the theory of direct torque control. This technology largely solved the problem of vector control. It is not controlled by current, flux linkage, etc. The torque is controlled, but the torque is directly controlled as the controlled amount. The advantage of torque control is that the torque control is to control the stator flux linkage. In essence, the speed information is not needed. The control is robust to all motor parameters except the stator resistance. The stator flux linkage observation is introduced. The device can easily estimate the synchronous speed information, so that it can easily realize the speed sensorless. This control is called speed sensorless direct torque control.
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