High Frequency Conducted Emission in AC Motor Drives Fed By Frequency Converters by jaroslaw luszcz

Author:jaroslaw luszcz
Language: eng
Format: epub
Published: 2018-08-22T16:00:00+00:00

Furthermore, LF components of output currents of FC correlated with the motor operating current are predominantly DM, which means that related current path for these components are well defined by power wires and low DM impedances of motor windings. Areas of loops formed by DM output currents are usually of much smaller area than for CM currents, because of the use of three wires cabling, where wires of each phase are very tightly arranged with wires of other phases. Apart from the unique cases, contribution of motor operational current ripples to overall intensities of interference originated by means of magnetic field couplings is usually not significant, because of relatively low frequency of the dominating spectral content. Therefore, this type of magnetic field emission is usually not emphasized in typical ASD applications.

Capacitive parasitic couplings occurring in ASDs result in more significant effects in relation to magnetic fields generated by drive operating currents. First, output voltage changes at FC terminals are significantly higher in magnitudes, more than 100% of nominal voltage, in relation to the ripples of the motor's operational currents that usually do not exceed 10 Second, output voltage changes are much faster ( Figure 5.1), because they are limited only by small parasitic capacitances. Thus, voltage steepness easily follow transistor switching characteristics, changes of which are often much faster than .

Rapid changes of output voltages as a primary reason, together with omnipresent parasitic capacitances as necessary second condition, are the dominant cause of HF parasitic leakage currents. These parasitic transient leakage currents add themselves at the FC output to motor operating currents with much slower ripples, and then return back to the converter via parasitic capacitances, spreading out into many possible return paths. Simplified example of parasitic HF capacitive current transients distribution at the motor side of ASD is presented in Figure 5.2.

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