William E. Berkopec

DC bus electrolytic capacitor stress in adjustable- speed drives under input voltage unbalance and sag conditions

This work analyzes the effects of the input voltage unbalance and sags on the DC bus electrolytic capacitors in adjustable-speed drives (ASDs) in order to predict their impact on expected capacitor lifetime. The key phenomenon that causes these problems is the transition of the rectifier stage from three-phase to single-phase operation. Since the ESR (equivalent series resistance) increases at low frequencies, the low-order harmonic current components (120 Hz, 240 Hz, etc.) contribute disproportionately to the capacitor power losses and temperature rise, resulting in reduced lifetime. Closed-form expressions are developed for predicting these effects including the impact of finite line impedance, finite bus capacitance, and varying load. The impact of inverter SVPWM (space vector pulse width modulation) switching on the capacitor loss is also included. Simulations and experimental tests are used to verify the accuracy and effectiveness of the closed-form analysis using a 5 hp ASD system.

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 rectifiers 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 will be 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 or 3730-W ASD system.

Closed-form analysis of adjustable speed drive performance 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 single-phase rectifier operation. This degradation of the input power quality can have a significant negative impact on the induction machine performance characteristics. This paper provides an approximate closed-form analysis of the impact of line voltage sags and unbalance on the induction machine phase voltages, currents, and torque pulsations for a general-purpose ASD consisting of a three-phase diode bridge rectifier, dc link, and PWM inverter delivering constant volts-per-Hertz excitation. Attention is focused on the impact of the dominant 2nd harmonic of the line frequency that appears in the DC link voltage during the sag/unbalance conditions, neglecting the impact of the other higher-order harmonics. In addition to the closed-form analytical results that assume constant rotor speed, both simulation and experimental results are presented that confirm the key analytical results, including the dominance of the 2nd harmonic in the resulting torque pulsations. The analytical results can be used as a valuable design tool to rapidly evaluate the approximate impact of unbalance/sag conditions on ASD machine performance.

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 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 drives 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 ASB topology on the drives 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

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 rectifiers 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 will be 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 or 3730-W ASD system.

Influence of deep bar effect on induction machine modeling with gamma-controlled soft starters

Direct line starting of induction machines causes high torque transients and inrush currents that can be attenuated using soft starters that adjust the applied stator voltage amplitude. Startup performance of the induction machine with a soft starter and gamma control strategy can be predicted much more accurately when deep bar effect is incorporated into the induction machine model. In this paper, a simplified, yet accurate deep bar effect model is presented in starting induction machine applications. The evaluations of the induction machine starting characteristics with and without considering the deep bar effects are conducted for both line start and soft start conditions. The gamma-controlled soft starter is described in controlling a 5 hp induction machine with the deep bar effects. Full time domain simulations for the line starting and the soft starting an induction machine have been implemented. Experimental results are presented to verify the improved simulation model, confirming the value of this analytical tool for practicing engineers