Showing papers by "Kevin Lee published in 2005"

System and method of controlling power to a non-motor load

Abstract: A motor drive for conditioning power to be delivered to a non-motor load includes a voltage feedback circuit that monitors a DC bus voltage and, based on changes in the DC bus voltage, adjusts a power conditioning scheme such that near steady-state load conditions are maintained in response to a transient load condition. The voltage feedback circuit has a voltage sensor that provides voltage feedback to a controller that determines what changes in power conditioning are needed in response to a transient load condition that manifests itself in a change in DC bus voltage.

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

Abstract: 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 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.

System and method of controlling the start-up of an adjustable speed motor drive based sinusoidal output power conditioner

Abstract: A motor drive for conditioning power to be delivered to a load includes a current feedback circuit that monitors current being fed to a transformer connected between the motor drive and the load. Based on the total current input to the transformer, a controller adjusts the V/Hz profile along which the motor drive operates. In this regard, the motor drive operates according to a V/Hz profile during start-up that prevents transformer saturation.

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 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 ASB 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

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

Abstract: 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

A high performance and cost effective drive based power conditioner for critical applications

Abstract: In this paper, a high performance and cost effective power conditioner based on off-the-shelf adjustable-speed drives (ASDs) is presented for critical applications such as the CT, MRI type loads. The high performance resides in the load voltage regulation of /spl plusmn/1% in steady state with less than 3% total harmonic distortion, /spl plusmn/0.1 Hz with fixed output frequency (60 Hz), and fast dynamic step load response. The system frequency domain analysis, the PID controller design with a unique derivative compensating term based on the DC bus voltage, and active damping for improved load transient voltage characteristics are explored. The control is implemented in the ASD's user programmable application software layer and interfaces seamlessly with the existing volts-per-hertz algorithms. The overall system simulation has been developed to demonstrate the steady state and load transient characteristics. The distinguishing control implementation is accomplished by using programming languages to comply with the IEC61131-3 standard. A prototype system consisting of a 150 hp, 460 V, 60 Hz ASD, a three-phase output LC filter, and a 75 kVA step-up isolation transformer has been built and tested to verify the analytical and simulation results. The significant contribution of this paper validates the feasibilities of using low cost off-the-shelf ASDs and their existing volts-per-hertz controls on non-motor driven applications such as the critical CT, MRI type loads in medical patient care and diagnostic areas.

A coupled thermal and electrical time-domain soft starter system model

Abstract: A coupled thermal and electrical dynamic system model was developed to aid in determining the transient ratings of a medium voltage soft start motor starter for various motor and load combinations and cycles (start-run-stop). This model includes the medium voltage (4160 V) power circuit, the control algorithms (for a soft start scheme), the induction machine model, and a detailed transient thermal model. The formulation for the transient thermal model is based on the electrical analogy for thermal resistance, thermal capacitance and airflow. Soft starters can significantly attenuate the high torque transients and inrush currents that can occur with direct line starting of induction machines. The soft start scheme reduces the effective RMS voltage applied to the motor by controlling thyristors to switch "ON" at specific times during each half cycle. Average power thermal simulations and full time-domain simulations have been completed for the soft starter connected to a low voltage 200 hp induction machine. Experimental results provide verification for the thermal model and the dynamic electrical system. Additionally, a medium voltage 1000 hp induction machine was modeled.