Inverter is the use of power semiconductor devices the on-off function of the power frequency power conversion to another frequency control device of electric power.We now use the frequency converter mainly adopts cross - straight - to pay way (VVVF inverter or vector control frequency conversion), the power frequency ac power first through the rectifier converts dc power supply, and then to convert the dc power supply frequency, voltage control of the ac power supply electric motor.General by rectifying circuit of frequency converter, intermediate dc link inverter and the control of four parts.Rectifying section for the three-phase bridge controlled rectifier, inverter part for IGBT three-phase bridge type inverter, and the output of PWM waveform, intermediate dc link for filtering, dc energy storage and buffer reactive power.Frequency converter selection: model selection of the frequency converter to determine the following:1) the purpose of using frequency;Constant pressure control or constant flow control, etc.2) inverter load type;Such as vane pump or pump volume, special attention to the performance of the load curve, the performance curve determines the methods of the application.3) the inverter and the load matching problem;I. voltage matching;Rated voltage of the inverter consistent with the rated voltage of the load.II. The current matching;Ordinary centrifugal pump, the rated current of the inverter and motor rated current.For a special load such as deep water pump requires the reference performance parameters of the motor, with maximum current inverter current and overload ability.III. The torque matching;In the constant torque load or speed reducer that may happen.4) in the use of frequency converter drive high-speed motor, because of the high speed motor reactance is small, high order harmonics in output current value increases.So the high-speed motor used for the selection of frequency converter, its capacity is slightly larger than ordinary motor selection.5) inverter for long cable runs, at this time to take measures to restrain long cable to the influence of the coupling capacitance, avoid the inverter output is insufficient, so in this case, the frequency converter capacity to zoom in a reactor or installed in the output of the inverter output.6) for some special applications, such as high temperature, high altitude, at this point would cause reduced capacity of the inverter, the inverter capacity to enlarge a block.