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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 de-link capacitor lifetime will be the same when either line inductors or a de-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. Index Terms--Adjustable-speed drives (ASDs), tic-link capacitor stress, de-link inductor stress, input voltage sag and unbalance, power quality.

This paper presents a modified vector control algorithm for a fractional-slot concentrated-winding surface permanent magnet (SPM) machine that has been developed to maximize the machine's partial-load efficiency over a wide range of operating conditions. By increasing the amplitude of the negative d-axis current, the resulting increase in the stator copper losses can be more than offset by the reduction in the iron core losses achieved by lowering the stator d-axis flux amplitude. The effectiveness of this technique has been demonstrated using both analytical models and finite element analysis for a 55-kW (peak) SPM machine design developed for a demanding set of traction drive performance requirements. For this example, the modified control strategy increases the partial-load efficiency at 20% of rated torque by > 6% at 2000 r/min compared to the maximum torque/ampere algorithm, making the machine much more attractive for its intended application. Index Terms--Concentrated windings, flux weakening, fractional-slot windings, partial-load efficiency, permanent-magnet (PM) synchronous machine, surface PM (SPM) motor, vector control.

AC adjustable-speed drives (ASDs) have become the primary choice for most new and retrofit precision motor-control applications with increased system efficiencies and reduced system losses. The significant increase in the use of ASDs brings the need for understanding compatibility issues between them and their electrical environment in order to design the electrical systems appropriately. Even though there have been numerous studies on ASD effects on power systems, models for predicting the effect of the source voltage sags or unbalance have been limited. This paper is devoted to analyze the characteristics of power quality indices such as input power factor, input current crest factor (CF), and the distribution transformer K-factor for practical ASD systems under input voltage unbalance and sag conditions. The relationships between the unbalanced input voltages and input currents, the input inductance and the input current unbalance, the input inductance and the input power factor, CF, and transformer K-factor are explored for 5-, 100-, and 600-hp power distribution systems with ASDs. The results can be used as a design guide for power distribution system design with ASDs. Theoretical derivations and simulation and experimental results are presented in this paper. Index Terms--Adjustable-speed drives (ASDs), crest factor (CF), input voltage sag and unbalance, power factor, power quality, total harmonics distortion, transformer K-factor.

This paper thoroughly investigates the impact of the winding layer number and the choice of magnet type on the performance characteristics of surface permanent magnet (SPM) machines with fractional slot concentrated windings designed for wide speed ranges of constant-power operation. This is accomplished by carefully examining the performance characteristics of three different SPM machines designed for the same set of performance requirements drawn from an automotive direct-drive starter/alternator application. These results show that double-layer stator windings yield lower torque ripple and magnet eddy current losses than do single-layer windings, but can contribute to a lower overload torque capability. Although the adoption of sintered magnets leads to the highest machine torque density, bonded magnets result in a significant reduction of the magnet losses because of their much higher value of resistivity. Index Terms---Concentrated, flux weakening, fractional slot, interior, permanent magnet, surface, synchronous machines, windings.