Impact of Inductor Placement on the Performance of Adjustable-Speed Drives Under Input Voltage Unbalance and Sag Conditions

Voltage unbalance or sag conditions generated by the line excitation can cause the input rectifier stage of an adjustable-speed drive (ASD) to enter a single-phase rectifier operation. This degradation of the input power quality can have a significant negative impact on the induction machine performance characteristics, but the presence of an LC filter in the drive's input rectifier stage can be used to attenuate these undesired effects. The purpose of this paper is to investigate the impact of the inductor placement in the ASD topology on the drive's performance under voltage unbalance or sag conditions. More specifically, the relative advantages of choosing either a dc-link choke inductor or three ac line inductors are discussed using a combination of closed-form analysis and simulations. The results of first-order sizing calculations show that a dc-link choke inductor may offer some volume and mass advantages over three separate ac line inductors for the same ASD performance under unbalanced voltage conditions. Experimental results using a 5-hp ASD confirm the key analytical performance predictions