Degree Name

Master of Engineering (Hons.)


School of Electrical, Computer and Telecommunications Engineering


In ac variable speed drives (VSDs) having an uncontrolled rectifier front-end, the effect of voltage sags are mainly observed in the dc bus characteristics. These VSDs are susceptible to nuisance tripping due to dc under-voltage or ac over-current faults which result in production loss, material wastage and require manual interventions in industrial processes. This thesis is aimed at improving the voltage sag ride-through performance of ac VSDs by improving the control algorithm. The proposed strategy recommends maintaining the dc link voltage constant at the nominal value utilising two control modes (a) by recovering the kinetic energy available in the rotating mass at high motor speeds and (b) by recovering the magnetising energy available in the motor winding inductances at low speeds. B y combining these two modes, the VSD can be configured to ride-through voltage sags at all speeds. Additional control loops are suggested for this dc link voltage control.

The proposed control strategy was applied on a synchronous reluctance motor (SRM) VSD and on an induction motor (IM) VSD. In the case of an SRM VSD, both modes of this control strategy are found to work satisfactorily and sag ride-through can be achieved at all motor speeds.

In an IM VSD, the first mode of the control strategy, viz. closed loop dc voltage control by recovering the kinetic energy available in the system inertia is found to work satisfactorily and sag ride-through can be achieved at high motor speeds. However, due to the inherent IM characteristics, it was found that the magnetising energy present in an IM is not recoverable. The reasons for this behaviour are analysed and an alternative ride-through scheme is suggested by accommodating the limitations encountered in the case of an IM so that the VSD is able to ride-through voltage sags at low speeds also.

This control strategy can provide a voltage sag ride-through performance at all motor speeds down to standstill. It is also shown that the transition between various control modes during a sag situation can be achieved relatively smoothly.