Step-down Autotransformer Research Articles

Impact of Inter-Turn Short-Circuit Location on Induction Machines Parameters Through FE Computations

This paper studies the effect of the location of stator inter-turn short-circuit (ITSC) faults on the squirrel-cage induction machine (IM) model parameter's. The ITSC location can be affected in three different ways, however we studied two of them: the physical placement for a fixed number of turns fault and the variation of the number of turns short-circuited. Both different cases will influence the rotating magnetic field shape in the air gap and thus the stator and rotor parameters. Two investigations are conducted, the former one is a finite element analysis (FEA) of the four location cases of equal severity factor ITSC in the IM phase “a.” The latter is a theoretical and mathematical analysis, where the ITSC is modeled by a step-down auto-transformer in the faulty circuit. The results of the FEA corroborate that different locations impact the IM parameters. However, the effect is not substantial and the theoretical model indicates how the parameters vary with the fault. Moreover, a comparative study of two IM damaged models that present different approach to the stator leakage inductance is performed. The asymmetric mathematical model is presented, simulated, and experimentally compared.

A Family of Autoconnected Transformers for 12- and 18-Pulse Converters—Generalization for Delta and Wye Topologies

Multipulse rectifier topologies based on autoconnections are increasingly applied as interface stages between mains and power electronics converters. These topologies are attractive and cost-effective solutions for meeting the requirements of low total harmonic distortion of line current and high power factor. Furthermore, as only a small fraction of the total power required by the load is processed in the magnetic core, the overall resulting volume and weight are reduced. This paper proposes a mathematical analysis based on phasor diagrams that results in a single and general expression capable of unifying all delta and wye step-up or step-down autotransformer connections for 12- and 18-pulse ac-dc converters. The expression obtained allows the choice of a wide range of input/output voltage ratio for step-up or step-down autotransformer, and this general expression is also presented in a graphical form for each converter. Moreover, it simplifies the procedure for determining turn ratios and polarities for all windings of the autotransformer. A routine for easy and fast calculations is developed and validated by a design example. Finally, experimental results are presented along with comments on a 6-kW 220-V line voltage, 400-V rectified voltage, and 18-pulse delta-autoconnected prototype.

Development of kA‐class alternating current superconducting conductors and fabrication of a prototype coreless superconducting autotransformer

AbstractRecently, multifilamentary superconducting wires with very low ac losses have been produced and practical applications will now be considered. To realize actualsize power machines and apparatuses, it is necessary to develop 1 ‐ 10 kA ac conductors. However, the critical currents of multifilamentary wires at 1 T are several tens of A, and therefore it is necessary to use multistrand conductors consisting of several tens or several hundreds of strands. Such conductors sometimes show ac current degradation because of such factors as (1) nonuniform current distribution, (2) wire motion, (3) temperature increase, (4) longitudinal magnetic field effect, etc.Formerly, a coreless transformer was considered unpractical because of its large exciting current. However, Yamamoto and others proposed that a coreless superconducting transformer could be used as a stepdown autotransformer at the receiving side, utilizing its large exciting current as the reactive power source to cancel the charging current of an underground transmission line or UHV line, and therefore the shunt reactors could be eliminated.In this paper, development of ac‐superconducting conductors aimed at prevention of current degradation are discussed, as well as quench test results of two small coils made with these conductors. In these conductors, low ac low strands with ultrafine NbTi filaments are twisted around a central bundle of stainless steel wires. One of the coils has been designed as a model coreless autotransformer, and its test result is also described.

A simple preamplifier for use of 10-stage photomultipliers with commercial nanosecond electronics

An ac-coupled preamplifier of gain 8 is described which allows the use of 10-stage miniature hotomultipliers with commercial nanosecond electronics in high energy hodoscope applications. The preamplifier is located close to the photomultiplier base. Its essential components are a step-down autotransformer and a common base transistor stage.