Fernando J. T. E. Ferreira

An Energy-Stored Quasi-Z-Source Inverter for Application to Photovoltaic Power System

The quasi-Z-source inverter (qZSI) with battery operation can balance the stochastic fluctuations of photovoltaic (PV) power injected to the grid/load, but its existing topology has a power limitation due to the wide range of discontinuous conduction mode during battery discharge. This paper proposes a new topology of the energy-stored qZSI to overcome this disadvantage. The operating characteristic of the proposed solution is analyzed in detail and compared to that of the existing topology. Two strategies are proposed with the related design principles to control the new energy-stored qZSI when applied to the PV power system. They can control the inverter output power, track the PV panels maximum power point, and manage the battery power, simultaneously. The voltage boost and inversion, and energy storage are integrated in a single-stage inverter. An experimental prototype is built to test the proposed circuit and the two discussed control methods. The obtained results verify the theoretical analysis and prove the effectiveness of the proposed control of the inverters input and output powers and battery power regardless of the charging or discharging situation. A real PV panel is used in the grid-tie test of the proposed energy-stored qZSI, which demonstrates three operational modes suitable for application in the PV power system.

Standards for Efficiency of Electric Motors

In this article, feasible minimum limits for IE4 class are analyzed, taking into account the estimated efficiency limits and rated efficiency for emergent or commercially best available line-start PMSM technologies. The presented results can be useful to set up future international standard super-premium or IE4-class levels/limits. The practicability and technical limits associated with the IE4-class efficiency levels are addressed, taking into account technical and economical limitations. It is expected that advanced technologies will enable manufacturers to design motors for the IE4-class efficiency levels, with mechanical dimensions compatible with the existing IMs of lower efficiency classes (e.g., flanges, shaft heights, or frame sizes as defined in standards EN 50347 and NEMA MG1). NEMA frames sizes are larger than the IEC frame sizes, allowing the use of more active materials. In addition, 60-Hz operation enables higher power density and higher efficiency levels with the same frame sizes.

Three phase tooth-concentrated multiple-layer fractional windings with low space harmonic content

The possibility of obtaining lower space harmonic content for the fractional-slot, tooth-concentrated windings by multiplying the number of coils, leading to bigger number of layers in the armature slots is presented. By using multiple layer windings it is possible to reduce or even to cancel some space sub-harmonics having as final result lower eddy current losses induced by the armature reaction in the iron structure of the rotor. The general method of doubling (or trebling) the winding and relatively shifting by one (or more) slots is presented in the paper. Two examples of three phase concentrated windings are presented and analyzed as primitive windings, 12 slots/10poles and 9 slots/8 poles. In the first case the only existing space sub-harmonic is reduced firstly from 35.9% (from fundamental wave) to 9.6% and finally it is canceled by using different number of turns per coils. In the second case the two existing sub-harmonics are reduced and balanced by using the developed method.

Beyond induction motors — Technology trends to move up efficiency

Premium/IE3 efficiency class motors are now mandatory in North America. Super-Premium/IE4 and Ultra-Premium/IE5 efficiency classes are to be defined in the 2nd Edition of the IEC 60034-30 standard. For line-start fixed-speed applications, Super-Premium/IE4-class line-start permanentmagnet (PM) motors and squirrel-cage induction motors are recent entrances in the industrial motor market. For variable-speed applications, IE4-class synchronous reluctance motors are also a recent entrance in the market. For the low-power range, moving from the IE4 to the IE5 class may require moving away from radial-flux induction motor technology and into PM and reluctance technology, either using rare-earth or ferrite magnets. In this paper, efficiency analysis on the best available emerging electric motor technologies, such as axial-flux PM synchronous motors, is presented. The potential efficiency gain associated with several design options as well as some considerations on the theoretical maximum efficiency achievable taking into account those existing design options.

Ecoanalysis of Variable-Speed Drives for Flow Regulation in Pumping Systems

Electronic variable-speed drives (VSDs) can produce large energy and environmental savings in variable-load variable-speed applications when compared with other conventional technologies. There are a very large number of applications (e.g., fluid motion, materials handling, and materials processing) which would benefit, both in terms of process improvement and energy savings, through the use of speed control. Previous studies have shown that the variable-flow fluid motion applications (pumps, fans, and compressors) have the largest savings potential. In this paper, an ecodesign analysis of two different technologies for the flow regulation in pumping systems - constant-speed pump with an output throttle valve (motor directly fed from the line) and variable-speed pump without an output throttle valve (motor fed by a variable-speed drive) - is presented. A European standard methodology for the ecodesign analysis of energy-using products is used in this paper. This paper includes a comparative analysis of the environmental impacts and life-cycle costs. The identified large environmental benefits on top of the energy savings provide a strong argument for an increased use of VSDs. The presented analysis is of major importance for industrial electronics manufacturers, designers, and users, widening their awareness to the importance of taking into account the energy and environmental issues when evaluating different system design options.

Design of Transverse Flux Linear Switched Reluctance Motor

We have developed a transverse flux linear switched reluctance motor (TFLSRM) in which 12 primary-side poles, each composed of four sectors, boost efficiency over the whole operating range. Each sector can operate independently, enhancing the robustness of the system by providing fault tolerance. We present a standard design method for the machine based on a power output equation related to the machine dimensions, air gap, force, efficiency, operating velocity, and required voltage. We illustrate the procedure by designing a sector for a three-phase, 250 W prototype TFLSRM. The method uses finite-element analysis to calculate the flux density, thrust force, and normal force versus position and current prior to construction of the prototype. Calculated and experimental results match within 5%, validating the design procedure.

Comparative analysis of IEEE 112-B and IEC 34-2 efficiency testing standards using stray load losses in low voltage three-phase, cage induction motors

Since 1998, 817 motor efficiency test data sets, in the 1-500 hp (0.75-375 kW) range have been collected from several sources in different countries around the world, for 50 Hz and 60 Hz. The test data includes the motor efficiency using the IEC 34-2 and IEEE 112-B test standards, and the quantification of the stray load losses (SLL), using the IEEE 112B method. In 125 cases the test data includes the efficiency computed with both methods. The aim of this study, was the quantification of the discrepancy between the two mentioned standards, and to establish an approximate method to allow the comparison between motors tested with the two standards. Based on the significant number of collected motor data sets, it was concluded that when compared to IEEE 112-B standard, the IEC 34-2 standard is less accurate, and gives typically higher efficiency values, mainly because of the SLL estimation procedure. The IEEE 112-B SLL computed values are typically higher then the IEC SLL predefined values, on average by 1.0 and 0.8 percentage point for 50 Hz and 60 Hz motors respectively. Also, the average SLL values vary slightly with the motor power. Using efficiency measurements made with both standards on the same motors, it was possible to confirm that the SLL are the main reason for the observed discrepancy in the efficiency values. The results of the analysis carried out in this paper offers the possibility to compare the efficiency of motors tested under the IEC 34-2 standard with motors tested with IEEE 112-B, leading to a choice of the most cost-effective option. This paper also provides a key contribution to the ongoing revision of IEC 34-2 standard.

Novel Multiflux Level, Three-Phase, Squirrel-Cage Induction Motor for Efficiency and Power Factor Maximization

In the European Union, the average load factor of electric motors in both industrial and tertiary sectors is estimated to be less than 60%. However, in some industrial sectors, the average load factor for some motor power ranges can be as low as 25%. Most oversized three-phase induction motors operate with low efficiency and power factor, which is, by far, the most important cause for poor power factor in industrial installations. In the low-load operating periods, motor performance can be improved both in terms of efficiency and power factor if the magnetizing flux is properly regulated. In this paper, a multiflux level, three-phase, squirrel-cage induction motor is proposed, in which the efficiency and power factor can be both maximized as a function of load. This novel motor can be a surplus value in industry due to its flexibility, particularly, for variable load applications in which significant energy savings can be obtained, and can also be used as new or rewound general purpose spare motor (with several levels of voltage, magnetizing flux and/or power). The proposed motor has a stator winding with two sets of turns, sharing the same positions in the stator slots (which can be connected either in series or in parallel). Among all the possible stator winding connections, six modes were selected and analyzed (two of which are new). The basic principles for proper connection mode change are discussed. An electronic device and a contactor concept for automatic connection mode change are proposed. As far as the authors know, this concept is described and analyzed for the first time.

Method for in-field evaluation of the stator winding connection of three-phase induction motors to maximize efficiency and power factor

The performance of the oversized three-phase induction motors can be improved, both in terms of efficiency and power factor, with the proper change of the stator winding connection, which can be delta or star, as a function of their load. A practical method is proposed to quickly and easily evaluate which stator winding connection is more appropriate for the actual motor load profile, in order to increase the motor efficiency and power factor. This new method is suitable for in-field evaluation, because it requires only the use of inexpensive equipment and has enough accuracy to allow a proper decision to be made. The automatic change of the stator winding connection, as a function of the motor line current, is also analyzed. When properly applied, these methods can lead to the improvement of the efficiency and power factor of permanently oversized motors, motors with a load variation between low load and near full load during their duty cycle, and/or motors driving high-inertia, low duty cycle loads. The proposed methods are particularly suitable to industrial plants where typically many electric motor systems are oversized and/or can have a wide load variation. In these conditions, the active and reactive electrical energy bill can be significantly reduced.

Adjustable Flux Three-Phase AC Machines With Combined Multiple-Step Star-Delta Winding Connections

In this paper, a complete theory and analysis method for combined star-delta three-phase windings from the point of view of magnetomotive force spatial harmonics content and equivalent winding factors calculation is developed. The method allows for any spatial angle and any coils percentage distribution between star and delta balanced winding systems. It is proven that connecting the inner delta in the clockwise or counterclockwise direction leads to two different space angles between the star and delta systems and two different steps in the flux level control. It means that, compared to what is known and applied up to now, using n winding sections in each phase, it is possible to obtain 2 n steps (instead of n + 1) in the flux level. To check the analysis validity, two experimental tests are reported on both squirrel-cage, integer-winding, induction motor, and permanent magnet, low-speed, fractional-winding, synchronous generator with a specially designed stator winding, allowing multiple-step air-gap flux regulation.