. U/f constant control U/f control is to change 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 a wide speed regulation range, the efficiency and power factor of the motor 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 cannot be properly adjusted and adapt to the change of load torque; followed by The actual speed of the motor cannot be accurately controlled. Since the constant U/f frequency converter is a speed open-loop control, it can be seen from the mechanical characteristic diagram of the asynchronous motor that the set value is the stator frequency, which is the ideal no-load speed, and the actual speed of the motor is determined by the slip rate, so U/ The stable error existing in the f constant control method cannot be controlled, so the actual speed of the motor cannot be accurately controlled.

2. Slip frequency control Slip frequency is the difference frequency between the frequency of the AC power applied to the motor and the speed of the motor. 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, and the mechanical characteristics are straight lines.
Slip frequency control is to control torque and current by controlling 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 the ability to limit overcurrent have been improved. In addition, it has a speed regulator, which uses the speed feedback to form a closed-loop control, and the static error of the speed is small. However, to achieve the steady state control of the automatic control system, good dynamic performance cannot be achieved.
Third, vector control, also known as field-oriented control. It was first proposed by West German F.

3.Blasschke and others in the early 1970s, and explained this principle by comparing the DC motor and the AC motor. This created a precedent for AC motors and equivalent DC motors. The method of vector control frequency conversion speed regulation is to convert the stator AC currents Ia, Ib and Ic of the asynchronous motor in the three-phase coordinate system. Through three-phase-two-phase transformation, it is equivalent to AC currents Ia1 and Ib1 in a two-phase stationary coordinate system, and then through rotational transformation according to the rotor magnetic field orientation, it is equivalent to DC currents Im1 and It1 in a synchronous rotating coordinate system (Im1 is equivalent to It is equivalent to the excitation current of the DC motor; It1 is equivalent to the armature current of the DC motor), and then imitates the control method of the DC motor to obtain the control amount of the DC motor, and realizes the control of the asynchronous motor through the corresponding coordinate inverse transformation. The emergence of the vector control method makes the asynchronous motor variable frequency speed regulation in an all-round dominant position in the field of motor speed regulation. However, the vector control technology needs to estimate the motor parameters correctly, and how to improve the accuracy of the parameters is a topic of constant research.

4. Direct torque control In 1985, Professor DePenbrock of Ruhr University in Germany first proposed the theory of direct torque control. This technology solves the shortcomings of vector control to a large extent. It is not indirect by controlling current and flux linkage. To control the torque, the torque is directly controlled as the controlled variable. The advantages of torque control are: torque control is to control the stator flux linkage, it does not need speed information in essence, and the control is robust to all motor parameter changes except the stator resistance; the introduced stator flux linkage observation The controller can easily estimate the synchronous speed information, so it can easily realize the speed sensorless, this kind of control is called the speed sensorless direct torque control.