Effect of Inductor Placement on DC Bus Capacitor of Adjustable Speed Drives

An evaluation of the DC-link capacitor heating in adjustable speed drive systems with different utility interface options

Abstract: In this paper, an evaluation of DC-link capacitor heating in adjustable speed drive systems with different utility interface options is presented. The evaluation is based on the level of ripple currents DC-link capacitors can endure that lead to self-heating and reduction of capacitors' operating life. Three popular utility interface options for ASD systems are considered for evaluation. First, a standard six-pulse diode rectifier with and without DC-link inductor is examined. Second, an active PWM rectifier, which draws nearly sinusoidal current from the utility, is studied. Third, an auto-connected 12-pulse rectifier system for utility interface is analyzed. A new term capacitor heating factor (CHF) based on the dependence of ESR on frequency is introduced and is computed for various utility interface options and the results are compared. A mathematical procedure is outlined to analytically compute the capacitor ripple current and hence heating. Simulation and experimental results are presented to verify analytical calculation. Finally, a relative comparison of capacitor heating as a function of utility interface options is presented.

Influence of reactors on input harmonics of variable frequency drives

Abstract: AC line reactors and dc link reactors are widely used in variable frequency drives (VFDs) to improve the drive performance such as reducing input harmonics, improving the input power factor, and protecting the drives from surges, etc. The effectiveness of both types of reactors in reducing input harmonics is affected by many factors, including the loading of the drives and the system source impedance. In this paper, a simulation is conducted to investigate the influence of such factors. Two power distribution systems with VFDs as the dominant loads in the oil field are used as the case studies. The rules to evaluate the needs and effectiveness using ac line or dc link reactors are proposed for practical applications.

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

Abstract: 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

The impact of grid unbalances on the reliability of DC-link capacitors in a motor drive

Abstract: DC-link capacitor is one of the reliability-critical components in motor drive applications, which contributes to a considerable cost, size and failure. Its reliability performance depends on both inherent physical strength and external loading. The grid unbalances could alter the electro-thermal stresses of key components in a motor drive. Therefore, this paper investigates the impact of grid voltage amplitude and phase unbalances on the lifetime of DC-link capacitors used in a standard three-phase motor drive. The theoretical stress models and experimental measurements of the capacitor voltages and ripple currents are presented. The relationship between the DC-link capacitor lifetime and the level of unbalances and loads are discussed based on a 7.5 kW motor drive system. The results serve as a guideline to size the DC-link capacitors to be robust enough at the presence of grid unbalance conditions.