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Principle of frequency converter and matters needing attention in selection

  Release time: 2021-01-25   

Frequency converter is a kind of power control device which uses the on-off function of power semiconductor devices to convert the power frequency power supply to another frequency. The frequency converter we use now mainly adopts AC-DC-AC mode (VVVF frequency conversion or vector control frequency conversion). First, the power frequency AC power supply is converted into DC power supply through rectifier, and then the DC power supply is converted into AC power supply which can be controlled by frequency and voltage to supply the motor. The circuit of frequency converter is generally composed of rectifier, intermediate DC link, inverter and control. The rectifier is a three-phase bridge uncontrollable rectifier, the inverter is an IGBT three-phase bridge inverter, and the output is PWM waveform. The middle DC link is filtering, DC energy storage and buffering reactive power.
 
   frequency converter selection: the following points should be determined when selecting the frequency converter:
 
   1) the purpose of frequency conversion; constant voltage control or constant current control, etc.
 
   2) load type of frequency converter, such as vane pump or positive displacement pump, pay special attention to the performance curve of load, which determines the application method.
 
   3) matching problem between inverter and load;
 
   I. voltage matching; the rated voltage of frequency converter is consistent with the rated voltage of load.
 
II. Current matching; for ordinary centrifugal pump, the rated current of frequency converter is consistent with that of motor. For special loads such as deep-water pumps, it is necessary to refer to the motor performance parameters to determine the inverter current and overload capacity with the maximum current. 3. Torque matching; this situation may occur under constant torque load or with deceleration device.
 
   4) when the inverter is used to drive the high-speed motor, because the reactance of the high-speed motor is small, the output current value increases with the increase of high-order harmonics. Therefore, the capacity of frequency converter used in high-speed motor is slightly larger than that of ordinary motor.
 
   5) in case of long cable operation of frequency converter, measures should be taken to restrain the influence of long cable on ground coupling capacitance to avoid insufficient output of frequency converter. Therefore, in this case, the capacity of frequency converter should be enlarged by one gear or output reactor should be installed at the output end of frequency converter.
 
   6) for some special applications, such as high temperature and high altitude, the capacity of the frequency converter will be reduced at this time, and the capacity of the frequency converter should be increased by one gear.