Effect of Choke Placement on Common-Mode Noise in Three-Phase Variable Speed Drives

Three-phase variable speed drives play a key role in energy conversion systems. To comply with the electromagnetic compatibility standards for the 0-2 kHz frequency range, magnetic chokes are typically placed at the dc or ac sides of the drive systems. Recently, compatibility levels have been defined at 9-150 kHz for the public network in accordance with IEC 61000-2-2. At this frequency range, common-mode noise is an important factor affecting electromagnetic compatibility. This article analyzes the effect of choke placement on common-mode noise at the 9-150 kHz frequency range. In particular, a comparative study on choosing either two dc-link choke inductors or three ac-line inductors is carried out via mathematical calculations. To this end, single-phase common-mode equivalent models are extracted for each placement of dc and ac chokes. Subsequently, by calculating the transfer functions of the currents in the single-phase equivalent circuits, the attenuation rate of common-mode currents from the motor to the grid side is analytically modeled for each choke configuration. The developed theories prove that ac-line inductors are more effective at suppressing low-frequency common-mode contents, while dc chokes are useful in attenuating high-order noise contents. Laboratory test results are used to prove the validity of the presented theoretical modeling and analysis.