Impact of input voltage sag and unbalance on dc link inductor and capacitor stress in adjustable speed drives

This paper investigates the impact of input voltage unbalance and sags on stresses in the dc bus choke inductor and dc bus electrolytic capacitors of adjustable-speed drives (ASDs). These stresses are primarily attributable to the rectifier´s transition into single-phase operation, giving rise to low-order harmonic voltages (120 Hz, 240 Hz, etc.) that are applied to the dc link filter components. These harmonics elevate the ac flux densities in the dc choke core material significantly above values experienced during normal balanced excitation conditions, causing additional core losses and potential magnetic saturation of the core. It is shown that the effects of voltage unbalances and sags on the dc link capacitor lifetime is the same when either line inductors or a dc link choke inductor are used if the dc choke inductance value is twice the value of the line inductance. Simulations and experimental tests are used to verify the accuracy of predictions provided by closed-form analysis and simulation for a 5 hp ASD system.