AC Motor control - Frequency converter
A complete installation with a frequency converter controlled motor consists of a series of different components which should all be selected carefully for a given application.
The components in an installation are selected according to the actual application, starting with selecting the right pump for the application. A suitable motor for the actual pump is chosen. The output filter of the frequency converter has to be able to handle the full load of the pump, and at the same time fit the frequency converter. The frequency converter should have the right power rating for the pump, and the fuses and the protective circuit breaker should fit the frequency converter. What follows is some information about how to choose the right components.
A frequency converter makes it possible to control the speed (rpm) of an asynchronous motor. This is done by controlling the output frequency to the motor.
Components in a typical installation
A standard frequency converter is shown to the right, the power-handling sub- circuits are:
- Input filter
- Energy storage circuit or Intermediate circuit
The function of the different components in the sub circuit is:
The input filter prevents that noise generated inside the frequency converter reaches other components connected to the mains. It also prevents that noise from the mains reaches and disturbs the frequency converter.
The rectifier rectifies the mains voltage into a pulsating DC voltage.
Energy storage circuit or intermediate circuit
The pulsating DC voltage from the rectifier is sent to the energy storage circuit, where it is transformed into a DC voltage with an additional AC ripple. The magnitude of the AC ripple depends on the load on the inverter. No load means no ripple.
The inverter converts the DC voltage into an output voltage of variable frequency and amplitude. The inverter has six switches that can either be switched on or off. In the next section we will concentrate on the performance of the inverter.
The inverter has six switches that can either be switched on or off