High voltage frequency converter refers to the series superposition high voltage frequency converter, that is, multiple single-phase three-level inverters are connected in series to output high-voltage AC with variable frequency and voltage. This paper focuses on the composition of the down-voltage converter and the working principle of the high-voltage converter.

High voltage frequency converter consists of several parts

The main circuit is mainly composed of three-phase or single-phase rectifier bridge, smoothing capacitor, filter capacitor, IPM inverter bridge, current limiting resistor, contactor and other components. Many common faults are caused by electrolytic capacitors. The service life of electrolytic capacitor is mainly determined by the DC voltage applied at both ends and the internal temperature. The model of capacitor has been selected in the circuit design, so the internal temperature plays a decisive role in the service life of electrolytic capacitor. Electrolytic capacitor will directly affect the service life of frequency converter. Generally, the service life will be halved every 10 ℃ rise in temperature. Therefore, on the one hand, appropriate ambient temperature should be considered during installation, on the other hand, measures can be taken to reduce pulsating current. The use of AC or DC reactors with improved power factor can reduce the pulsating current and prolong the service life of electrolytic capacitors.

During capacitor maintenance, the deterioration of electrolytic capacitor is usually judged by the easily measured electrostatic capacity. When the electrostatic capacity is lower than 80% of the rated value and the insulation impedance is below 5 m Ω, the electrolytic capacitor shall be considered to be replaced.

Working principle of high voltage frequency converter

According to the basic principle of electromotology, the speed of the motor meets the following relationship: n = (1-s) 60F / P = n. × (1-s) (P: pole pairs of motor; F: operating frequency of motor; s: slip) it can be seen from the formula that the synchronous speed of motor n. It is directly proportional to the operating frequency of the motor (N. = 60fp). Since the slip s is generally small (0-0.05), the actual speed n of the motor is about equal to the synchronous speed n of the motor., Therefore, by adjusting the power supply frequency f of the motor, the actual speed of the motor can be changed. The slip s of the motor is related to the load. The greater the load, the greater the slip. Therefore, the actual speed of the motor will decrease slightly with the increase of the load.

The single-phase or three-phase alternating current is first transformed into direct current through the rectifier circuit, and the rectified direct current is added to the high-frequency switch circuit. The on-off time of the switch circuit is controlled through the control circuit. With the on-off change of the switch circuit, alternating current with variable frequency will be generated at the output end.

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