Several Speed Regulation Methods of Asynchronous Motor(2)

Update:25-01-2019
Summary:

Frequency Control When the frequency f1 of the power su […]

Frequency Control

When the frequency f1 of the power supply changes, the synchronous speed n1 = 60f1/P is proportional to the frequency, so the speed n of the motor also changes, so changing the frequency of the power supply can smoothly adjust the speed of the asynchronous motor.

Variable frequency speed regulation can be divided into U/f control, slip frequency control, vector control and direct torque control.

1. U/f control. U/f control is proposed to obtain ideal torque-speed characteristics. It is based on the idea of changing the frequency of the power supply to adjust the speed while ensuring that the flux of the motor remains unchanged. Generally speaking, this control mode is used in all kinds of converters. The structure of U/f control frequency converter is very simple. The disadvantage is that the open-loop control mode of the converter can not achieve high control performance. Torque compensation must be carried out at low frequency to improve the low frequency torque characteristics.

2. Slip frequency control. Slip frequency control is a direct control method of torque. It is based on U/f control, according to the power frequency corresponding to the actual speed of the asynchronous motor, and according to the desired torque to adjust the output frequency of the converter, the motor can have the corresponding output torque. In order to control frequency and current, this control method needs to install speed sensor in the control system, and sometimes has current feedback, so it is a closed-loop control method. This method can make the frequency converter have good stability, and has good response characteristics to rapid acceleration and deceleration and load changes.

3. Vector control. Vector control is to control the magnitude and phase of stator current of motor through vector coordinate circuit, so as to control the excitation current and torque current of motor in D, Q and O coordinate axes respectively, and then to control the motor torque. By controlling the action order, time and zero vector action time of each vector, various PWM waves can be formed to achieve various control purposes, such as forming PWM waves with the least switching times to reduce switching losses. At present, there are two kinds of vector control methods in practical application of frequency converter: vector control based on dedicated frequency control and vector control without speed sensor.

Vector control based on slip frequency has the same steady characteristics as slip frequency control, but vector control based on slip frequency also needs coordinate transformation to control the phase of motor stator current. It satisfies certain conditions to eliminate the fluctuation in the process of torque and current transition. Therefore, the vector control method based on slip frequency can greatly improve the output characteristics compared with the slip frequency control method. However, this control mode belongs to the closed-loop mode, which requires the installation of speed sensors on the motor, so its application is limited.

Velocity sensorless vector control is to control excitation current and torque current by coordinate transformation, and then to identify speed by controlling voltage and current on motor stator winding, so as to achieve the purpose of controlling excitation current and torque current. This control mode has wide speed range, large starting torque, reliable operation and convenient operation, but its calculation is complex, and it usually needs a special processor to calculate. Therefore, the real-time performance of this method is not ideal, and the control accuracy is affected by the calculation accuracy.

4. Direct Torque Control. Direct Torque Control (DTC) uses the concept of space vector coordinates to analyze the mathematical model of AC motor in the stator coordinate system, to control the flux and torque of the motor, and to observe the stator flux by detecting the stator resistance. Therefore, the complex transformation calculation such as vector control is omitted. The system is intuitive and concise, and the calculation speed and accuracy are higher than those of vector control. High. Even in the open-loop state, it can output 100% rated torque, which has the function of load balance for multi-drive of a frequency converter supplying power to multiple motors.

Chapter Summary

The energy-saving effect of frequency conversion speed regulation for fan and pump machinery is obvious.

The above kinds of control are suitable for special motor with frequency conversion and speed regulation. Frequency conversion motor is developed from the traditional squirrel cage motor. The traditional motor fan is replaced by an independent fan, and the insulation performance of the motor winding is improved.