Filters

In currently used frequency converters the switching frequency of transistors reaches the value 15 kHz. In these converters most commonly used is modern IGBT transistors with is characterized by very short switching time. This results in high steepness of voltage du / dt at the inverter output. This may cause the appearance of waves effects in the cables supplying the motor, causing dangerous voltage rise on motor terminals, the occurrence of surges and the associated aging of insulation. Application at the inverter output of the appropriate filter enables obtaining the relevant sinusoidal currents and voltages, while significantly reducing the current flowing into the motor housing and protective conductor PE. When selecting the filter parameters it is important to take into consideration that the phase shift of currents and voltages before and after the filter should be as small as possible. If this requirement is not met than impossible will be the proper operation of inverter drive system for example in systems with flux-oriented control systems and with direct control of motor torque (DTC inverters).