A three stage stator switching scheme for MWh and MVARh saving in induction motors

The delta-star three-phase winding developed earlier for harmonic reduction in three-phase induction motors is now applied for an energy saving proposition for motors which operate at less than rated load for a considerable duration in the daily load cycle. A case study on a three-phase 250 kW, 400 V induction motor working with different duty cycles has been presented, to emphasise the MWh and MVARh saving that could be obtained by reconnecting the delta-star stator winding in delta or star at suitable reduced loads, the motor being operated across the same supply. A clear procedure has been formulated for arriving at the operating points at which such switchings should be made to obtain the maximum savings. A computer program has been developed for the performance predetermination of the motor operated with these different connections. Experimental results on a 3.75 kW three-phase induction motor, designed and operated with a suitable delta-star winding, validate the usefulness of the proposed three stage switching scheme.