High-speed Electrical Machines Research Articles

High-speed electrical machines for power engineering: Current state and development trends

The current state of high-speed electrical machines with shaft rotation velocity within the range of 10 000 to 50 000 rpm is considered and the development trends are analyzed. The analysis includes recent publications and the results of the authors’ studies. Data of the developed motors and generators are presented. Forecasts regarding directions in the development of high-speed electrical machines are made.

Fabrication of Deep, Narrow Holes in Steel Using High-speed Electrical Discharge Machine with Conductive Aqueous Working Fluid

Conductive aqueous working fluid is increasingly being used instead of dielectric oils or deionized water for hole-fabrication in steel by means of electrical discharge machining (EDM). In the present study, the drilling speed, rate of wear on a tool electrode, and the precision of a hole-fabrication EDM system using aqueous working fluid (conductivity <1000 μS/cm) were examined. The working fluid was deionized water containing an additive which was composed of a surfactant (dispersant for other contents), a cleaner, an extreme pressure agent, an antirust agent, an antiseptic, and an antifoaming agent. For holes of diameter 0.40–1.20 mm and depth of 20 mm fabricated in S50C steel, the drilling speed and the wear on the tool electrode were improved by increasing the amount of additive. However, the precision of the fabricated hole decreased when the additive was used.

Design and Development of a Two-Megawatt, High Speed Permanent Magnet Alternator for Shipboard Application

Conventional gas turbine generator sets consist of a high speed turbine coupled to a low speed alternator through a speed reduction gearbox. This is required to maintain the alternator output frequency at 50 Hz or 60Hz, as output frequency is directly proportional to speed. Since power is also directly proportional to speed, the conventional system is bulky and possesses a very large footprint. The advent of solid-state inverters with their unique ability to efficiently and cost effectively change the alternator output frequency has made it possible to eliminate the need to link the alternator speed to the required 50/60 Hz output frequency. This output can be produced with a high-speed alternator, eliminating the need for a gearbox and greatly reducing the size, complexity, and weight of the machine by trading speed for torque. A direct drive system in which an alternator is coupled directly to a gas turbine is much more compact and highly efficient and requires much less maintenance. In this paper we will review the design and development of a high-speed permanent magnet alternator in an advanced cycle gas turbine system for shipboard applications. In addition to the alternator's design features, we will discuss design considerations including electromagnetic design, thermal design and structural design of high speed electrical machines, and review the alternator development including risk mitigation.

Numerical and experimental modelling of gas flow and heat transfer in the air gap of an electric machine. Part II: Grooved surfaces

The study deals with the cooling of a high-speed electric machine through an air gap with numerical and experimental methods. The rotation speed of the test machine is between 5000–40000 r/min and the machine is cooled by a forced gas flow through the air gap. In the previous part of the research the friction coefficient was measured for smooth and grooved stator cases with a smooth rotor. The heat transfer coefficient was recently calculated by a numerical method and measured for a smooth stator-rotor combination. In this report the cases with axial groove slots at the stator and/or rotor surfaces are studied. Numerical flow simulations and measurements have been done for the test machine dimensions at a large velocity range. At constant mass flow rate the heat transfer coefficients by the numerical method attain bigger values with groove slots on the stator or rotor surfaces. The results by the numerical method have been confirmed with measurements. The RdF-sensor was glued to the stator and rotor surfaces to measure the heat flux through the surface, as well as the temperature.

Investigation of windage splits in an enclosed test fixture having a high-speed composite rotor in low air pressure environments

The University of Texas at Austin Center for Electromechanics (UT-CEM) has designed and conducted a series of composite rotor spin tests to measure the windage losses and temperature distributions of a test setup at high rotor speeds and low air pressures. The intent of the windage tests is to validate the windage loss predictions and investigate how the air-gap windage is distributed between the rotor and stator. The findings of the spin tests will then be used to perform windage-related thermal design and analysis of a high-speed electrical machine. The radial air-gap flows under the test conditions, a low rotor cavity air pressure of 1 torr and high rotor surface velocities of 333 and 614 m/s, were in a laminar flow regime. Transient rotor and stator finite-element thermal analyses, using the measured windage losses and predicted laminar-flow windage splits, have been carried out to analyze the rotor and stator temperature distributions. This paper shows the detailed thermal analysis and compares the predictions with the measurements. The predicted and measured transient rotor and stator temperatures are in good agreement.

Numerical and experimental modelling of gas flow and heat transfer in the air gap of an electric machine

This work deals with the cooling of high-speed electric machines, such as motors and generators, through an air gap. It consists of numerical and experimental modeling of gas flow and heat transfer in an annular channel. Velocity and temperature profiles are modeled in the air gap of a high-speed test machine. Friction and heat transfer coefficients are presented in a large velocity range. The goals are reached acceptably using numerical and experimental research. The velocity field by the numerical method does not match in every respect the estimated flow mode. The absence of secondary Taylor vortices is evident when using time averaged numerical simulation.

SMALL AMPLITUDE OSCILLATIONS OF A CENTERED GYROSCOPE

The small amplitude oscillations of a centered gyroscope are studied for the cases where the axis of the gyroscope is connected by elastic links to a fixed reference. The proposed approach is based on an analysis with respect to a non-rotating reference system with fixed axes and may be used in practical applications such as rotors attached to elastic structures and high-speed electric machines supported by elastic foundations. The differential equations obtained for these categories of gyroscopic systems are distinct from and consistent with the equations governing the dynamics of other gyroscopic systems

Loss Analysis and Efficiency Improvement of the Axial Flux Type Generator with Coreless Windings

Lately, the application of axial flux type permanent magnet machine with coreless windings for high speed electric machines and electric vehicles have been discussed, from the standpoint of efficiency and size.In this paper, loss analysis and efficiency improvement of the axial flux type with coreless windings generator are given. The loss is examined based on the experimental data. In order to separate each loss, the prototype was fabricated of which component materials are interchangeable. The eddy current loss generated in the frame structure and the inside of the copper wire are examined. To decrease the eddy current loss, the thin Litz wire and the non -magnetic frame structure are recommended. Finally, the prototype having the output of 750W and the rated speed of 3, 600rpm, has been constructed, which has shown high efficiency of 96% (including machine loss) and 98.9% (excluding machine loss).

Using basic research as a catalyst to exploit new technology-based innovations — a case study

The difficulties related to the commercialization, or even to the introduction, of new technologies and solutions to common problems are well documented. The paper presents a case on how to introduce new technology to a complex research environment, in order to catalyse its commercialization. The central theme of the approach is on understanding all the possible applications and implications that the new technology shares when implemented to replace conventional solutions. The potential customer of the new technology and the related technological domains must be mapped systematically to establish technological trajectories which overlap with the possible applications. This mapping provides the framework for detailed application analysis with pointers to the right people responsible for the potential application area. The results produced indicate that three characteristic application classes for a new technology can be mapped. Special solutions pinpoint a non-standard application area where the new design is being implemented as a prototype. Research and development based applications require collaboration with the customer as the new technology may assist in achieving the goals set for the future. System applications emphasize the use of the new technology as part of a complete and widely used conventional system. A thorough case study is documented to demonstrate the classification of alternative application areas of a new technology. Also, the evolution of a new technology through different application classes is discussed. The case technology is related to high-speed electrical machines and the potential user is CERN, the European Laboratory for Particle Physics.

Design and analysis of permanent magnet-type bearingless motors

Magnetic bearings have been applied to high speed and high power electric machines for machine tools, turbomolecular pumps, etc. Bearingless motors can be expected to realize high speed and high power ratings because magnetic bearing functions are integrated into high-speed motors, which results in a simplified structure with short shaft length. In this paper, permanent magnet type bearingless motors, having built-in capability to achieve high power factor and high efficiency, are proposed. The shaft is suspended and centered by electromagnetic forces produced by currents in the additional radial force windings of the stator slots. At first the relationships of these radial forces, currents and voltages are derived analytically. Moreover, the relationships between radial forces and permanent magnet thickness are found. The optimal permanent magnet thickness is determined. The ratio of radial force over current as well as the peak air-gap flux density are discussed. These relationships are confirmed by prototype machines.

Structure and Characteristics of a High Speed Axial Gap Motor with Two Rotor Discs.

Super high speed electrical machines for electrical vehicles, compressors and co-generation systems have been studied. The speed range of the machines is 10, 000_??_30, 000 rpm. This paper describes a basic structure, analysis and characteristics of a newly developed super high speed machine, which rotates at 20, 000 rpm. The new machine is an axial-field type with a high energy permanent magnet (Nd-Fe-B). The stator has mold coils of air gap winding which can be divided in half. The rotor has two discs sandwiched by the mold coils. Many columnar magnets are embeded in the duralumin discs (non magnetic material) of the rotor. As the rotor is yokeless, the magnetic circuit of field is composed of back yokes of the stator and the permanent magnets of the rotor. This construction allows us to make the considerably light rotor which can withstand large centrifugal force at high speed. Then a little amature reaction, little cogging torque and small noise are realized. The analysis and test results show that the machine has the good stability at high speed and the capacity of about 7kW at 20, 000 rpm is achieved.

Temperature field in the rotor of a thermally stressed electrical machine

The temperature field in the rotor of a thermally stressed high-speed electrical machine is investigated. The problem is solved on a computer by the grid method. Results of computation of the temperature field in the rotor for different conditions of cooling are presented.

Ball and roller bearings for small high-speed electrical machines

Ball and roller bearings for small high-speed electrical machines

Tooth pulsations in rotating machines surface iron losses with reference to laminated materials

G. E. Luke: These two papers on iron losses are very worthy papers in that their purpose is to give the designing engineers a knowledge of the stray losses which occur in a machine. These stray losses are very large in the case of some induction motors and in the case of turbo-generator and other high speed electric machines. The old d-c. machines had solid pole faces; the losses on those were large. The only thing that limited them and kept them down within reasonable value was the fact that they used large air gaps. But as the designs were improved, the air-gap was brought down, particularly with the use of t he commutating pole, and it was necessary to reduce these pole-face losses; this necessitated a laminated structure.