A Reconsideration of the Design of the Modified McMurray Inverter Circuit

When a ``modified´´ McMurray inverter is used to drive 400-V cage induction machines from a nominal 560-V dc supply using pulsewidth modulation techniques, the voltage stresses the thyristor elements must withstand become severe. With a view to predicting these stresses accurately, the analysis of the inverter commutation interval¿and the implementation in practice of commutation circuitry¿is reviewed. Some consideration is also given to the energy losses in this circuitry. Generally, ``snubber´´ networks must be incorporated in the inverter to limit the rate of reapplication of voltage stress to the thyristors. It is demonstrated that conventional theory is not applicable directly to the design of networks for these inverters; the constraints on network design are discussed and illustrated. Consideration is also given to the selection of the thyristor devices, and the effect on the inverter as a whole of the timing of the ``main´´ thyristor switching instants is briefly investigated. The discussion is supported by computer simulation and clarified with reference to specific numerical examples. Some experimental measurements are also provided.