Common-mode choke and transformer solutions for induction motor drives

Common-Mode (CM) currents, shaft voltages and bearing currents become more noticeable in all two-level three-phase voltage source inverter-fed Induction Motor (IM) drive systems when particularly incorporating fast-switching IGBT devices. This dissertation presents selected solutions to these problems using CM chokes and CM transformers (passive- and active-based methods).; An enhanced approach is presented to specify CM chokes whose inductance should be the minimum practical value achieving the specified reduction of the CM currents, and not simply a large CM choke inductance that require a larger CM choke volume and possible cause CM voltage ringing. The procedure calculating the CM choke inductance considers the change in the CM system response when adding the CM choke. Experimental results of a prototype design verify this approach. CM chokes cannot reduce shaft voltages and bearing currents, the shaft grounding method combined with a CM choke is a cheap and simple solution to reduce CM currents, cancel shaft voltages and bearing currents. This solution is included for comparison purposes.; The passive method of the four-windings CM transformer is not a practical solution because it can never completely eliminate the IM CM voltages because it cannot solve the conflict of the required magnetizing current and the negligible voltage drops along the path of the fourth winding. Theoretical analysis, simulation and experimental results are presented to verify this conclusion.; The active method of the CM transformer cancels CM voltages and thus CM currents, shaft voltages and bearing currents. However, separate power supplies are needed to make the active method work properly for IM drives rated 460-V and above. Theoretical and power analysis, practical implementation issues, active circuit noise problems and solutions are presented together with experimental results verifying the effectiveness of the proposed ideas.