Impact of Motor Feeding Cable on CM Currents Generated in ASD

The most considerable part of CM currents generated in ASDs is initiated at the output side of FC during voltage commutations. The motor feeding cable is usually connected directly to FC output terminals or sometimes via passive filters with simple topologies, L or LC. Therefore parasitic parameters of the motor feeding cable play a key role in CM currents generation and propagation towards the AC motor and the power grid.Broadband characteristics of a motor cable, as the most broadly excited component of an ASD, powerfully influence magnitudes and spectral distribution of CM currents between the motor, FC, and the power grid. CM currents injected into AC motor windings, transmitted via DC bus of FC to the grid side of FC and also collected in grounding connection of FC, can be significantly influenced due to broadband properties of the motor feeding cable.Broadband characteristics of three wire power cables commonly used for feeding AC motors in ASD, have a relatively regular and balanced structure in comparison to other ASD components, eg., FC or motor windings. Nevertheless, the length of motor feeding cable, which in each case is individually matched for a particular application, can significantly change the overall performance of an ASD in terms of conducted emission generation. Moreover, the motor feeding cable should meet requirements related to transfer of high power at low frequencies, therefore optimization of its performance in HF range is additionally limited.Various recommendations for motor feeding cables used in ASDs, have been developed by manufactures. Those recommendations help to avoid excessive EMC problems, nevertheless the influence of the motor feeding cable on overall EMC performance of an ASD can still be crucial in many of contemporary applications. Accurate modeling of HF phenomena occurring at the output side of FC is very difficult because of omnipresent parasitic couplings which become meaningful in the frequency range of conducted emission. The method of simplified analysis of broadband behavior of FC's load in the frequency range of conducted emission, presented in this chapter, allows to recognize fundamental effects of influence of motor feeding cable on CM currents generation and distribution in ASD.