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The variable-frequency motor speed control system mainly consists of 4 components: the rectifier circuit, the filter circuit, the braking circuit, and the inverter circuit. The output waveform of this circuit is a pulse square wave, with the main components of this waveform being higher harmonics. When the voltage in the circuit changes, the frequency also changes in a proportional manner; neither of these parameters can be adjusted independently. It is evident that this circuit does not meet the requirements of power supply applications in practical use. I. Rectifier circuit. The rectifier circuit operates primarily through a three-phase uncontrolled bridge rectifier circuit; this system converts three-phase alternating current into direct current for subsequent power supply. II. Filtering circuit. In the rectifier circuit of variable-frequency motors, the current resulting from the conversion process contains a high level of higher harmonics. The filtering circuit is used to further process this current; in addition, it helps to eliminate the coupling between the inverter circuit and the rectifier circuit, preventing interference in the circuit and thus contributing to an improved power factor. When the circuit is connected to the power supply, the voltage across the capacitor is 0; as a result, when power is applied, the charging of the filter capacitor occurs at a high current level that can easily damage the diodes in the rectifier bridge. A resistor is needed to prevent this from happening, and once the current stabilizes, the resistor can be short-circuited. https://www.zuolidianji.com/uploads/allimg/20191125/1-1911251HT0Y9.png III. Braking circuit. The function of the braking circuit is primarily to eliminate the feedback energy in the circuit. This energy arises when, during the deceleration of the variable-frequency motor, the rotor speed becomes too high, even exceeding the motor’s speed, which results in an excess of kinetic energy that increases the voltage across the DC bus filter capacitors. If this energy is not eliminated in a timely manner, it can cause severe damage to the inverter. The braking circuit is designed to handle and counteract this energy effectively. IV. Inverter circuit. The main component of the inverter circuit for variable-frequency motors is the inverter transistor, which is capable of inverting the direct current generated by the conventional three-phase bridge rectifier circuit; as a result, both the amplitude and frequency in the circuit can be adjusted. In practical applications, inverter circuit systems are primarily controlled in two ways: closed-loop feedback control and open-loop feedback control. In practical applications, open-loop feedback control is primarily used to provide a frequency signal to the inverter, thereby controlling its cyclic operation and enabling the control of the motor. In actual operation, the motor controls its speed through power regulation; there is a certain difference between this speed and the desired speed, and this error is unavoidable. Therefore, this control method is not suitable for applications where high precision is required. Closed-loop feedback control represents a further refinement of traditional open-loop feedback control; the main approach involved is the addition of feedback mechanisms, which allow for adjustments to be made based on the actual operating conditions in order to achieve the desired output results. In most closed-loop feedback control systems, a speed sensor is installed in practical applications. The installation of this sensor enables the prediction of the speed signal to be fed back and compared with the desired reference value; adjustments are then made based on the results of this comparison in order to reduce the discrepancy in speed and ensure the proper operation of the motor.