Homopolar Motor Research Articles

Electrical fiber brushes-theory and observations

The performance limits of electrical fiber brushes are assessed by means of an expanded, refined theory. Measurements back to 1979 are in accord with the theory, including 1993 measurements on a set of 64 fiber brushes in a homopolar motor at the U.S. Navy Annapolis Laboratory, At /spl nu/=7.6 m/s velocity and j=1 MA/m/sup 2/ the electric loss was 0.026 W/A, and after a 20 h test the brushes showed no detectable wear. On the other end of the scale, electrical brushes are being developed for a maglev train system (American Maglev Tech of Florida, Inc.) to operate up to 150 m/s. According to the theory, this will be possible in the open atmosphere with dimensionless wear rates smaller than 10/sup -8/ at current densities up to a few MA/m/sup 2/. At speeds to 300 m/s, projected losses may range up to 4.3 W/A at flash temperatures at and below 100/spl deg/C. The newest fiber brushes preserve the high contact spot density at the ends of individually flexible fibers, independent of amount of wear. Correspondingly, with 10 cm long brushes, for example, the projected maglev brushes should last at least for a New York-Los Angeles return trip. Also, on account of the changed method of construction, manufacturing costs have decreased by more than a factor of ten in the past two years. Prospects are that with industrial manufacturing techniques the cost of fiber brushes, based on ampere hours conducted, will become easily competitive with monolithic graphite brushes. Pending further practical experience, it is therefore expected that: (1) metal fiber brushes will expand the range of sliding brush applicability and (2) ultimately displace graphite brushes for most of their present applications.

Superconducting coil development and motor demonstration: Overview

Superconducting bismuth-cuprate wires, coils, and magnets are being produced by industry as part of a program to test the viability of using such magnets in Naval systems. Tests of prototype magnets, coils, and wires reveal progress in commercially produced products. The larger magnets will be installed in an existing superconducting homopolar motor and operated initially at 4.2K to test the performance. It is anticipated that approximately 400 Hp will be achieved by the motor. This article reports on the initial tests of the magnets, coils, and wires as well as the development program to improve their performance.

High-temperature superconducting magnet motor demonstration

A superconducting field winding, assembled with coils constructed with Bismuth 2223 HTS wire, has been successfully operated in an electric homopolar motor. At a field winding temperature of 4.2 K, the motor produced 125 kW of output power. At a temperature of 28 K, the motor power developed was 91 kW. These measured power levels are the highest that have been produced by an electric motor containing a HTS field winding.

Transition from steady to periodic liquid‐metal magnetohydrodynamic flow in a sliding electrical contact

In homopolar motors and generators, large dc electric currents pass through the sliding electrical contacts between rotating copper disks (rotors) and static copper surfaces shrouding the rotor tips (stators). A liquid metal in the small radial gap between the rotor tip and concentric stator surface can provide a low‐resistance, low‐drag electrical contact. Since there is a strong magnetic field in the region of the electrical contacts, there are large electromagnetic body forces on the liquid metal. The primary, azimuthal motion consists of simple Couette flow, plus an electromagnetically driven flow with large extremes of the azimuthal velocity near the rotor corners. The secondary flow involves the radial and axial velocity components, is driven by the centrifugal force associated with the primary flow, and is opposed by the electromagnetic body force, so that the circulation varies inversely as the square of the magnetic‐field strength. Three flow regimes are identified as the angular velocity Ω of the rotor is increased. For small Ω, the primary flow is decoupled from the secondary flow. As Ω increases, the secondary flow begins to convect the azimuthal‐velocity peaks radially outward, which in turn changes the centrifugal force driving the secondary flow. At some critical value of Ω, the flow becomes periodic through the coupling of the primary and secondary flows. The azimuthal‐velocity peaks begin to move radially in and out with an accompanying oscillation in the secondary‐flow strength.

Superconducting Marine Propulsion Power

Over the last 25 years a large amount of research and development has been undertaken on the application of superconductors to marine propulsion systems and a number of superconducting homopolar motors and generators were constructed between the mid 1960s and the early 1980s. The paper reviews this work and shows that the technology had almost reached the point where industrial exploitation could have commenced. The reason why these machines did not reach the market place is discussed together with the impact which the recently discovered higher temperature superconductors may have upon future developments. Reference is made to a new ship which has been constructed in Japan and which derives its thrust directly from electrical energy using superconducting magnets in an engine based upon magnetohydrodynamics (MHD). With the exception of the MHD ship and the programme in the United States all of the work on d.c. machines described in this paper has been carried out by or under the direction of the author.

４８０ｋＷ舶用超電導直流単極電動機の試作特性

The superconducting electric propulsion system using superconducting motor seems to be an ideal propulsion system for the future. We have constructed a 480kW superconducting DC homopolar motor for the purpose of developing this new system. This motor is composed of superconducting field winding, segmented armature drum and copper-graphite current brushes. The superconducting field winding is cooled by the condensation heat exchangers and helium refrigerating system being built into the cryostat. We have obtained good test results that operating parameters of this DC homopolar motor accorded with the design parameters.

Homopolar DC motor and trapped flux brushless DC motor using high temperature superconductor materials

Two motors have been designed and built for use with high-temperature-superconductor (HTSC) materials. They are a homopolar DC motor that uses HTSC field windings and a brushless DC motor that uses bulk HTSC materials to trap flux in steel rotor poles. The HTSC field windings of the homopolar DC motor are designed to operate at 1000 A/cm/sup 2/ in a 0.010-T (100-G) field. In order to maximize torque in the homopolar DC motor, an iron magnetic circuit with small air gaps gives maximum flux for minimum Ampere turns in the field. A copper field winding version of the homopolar DC motor has been tested while waiting for 575 Ampere turn HTSC coils. The trapped flux brushless DC motor has been built and is ready to test melt textured bulk HTSC rings that are currently being prepared. The stator of the trapped flux motor will impress a magnetic field in the steel rotor poles with warm HTSC bulk rings. The rings are then cooled to 77 K to trap the flux in the rotor. The motor can then operate as a brushless DC motor.

Theoretical model of gravitational perturbation of current collector axisymmetric flow field

Some designs of liquid-metal current collectors in homopolar motors and generators are essentially rotating liquid-metal fluids in cylindrical channels with free surfaces and will, at critical rotational speeds, become unstable. An investigation at David Taylor Research Center is being performed to understand the role of gravity in modifying this ejection instability. Some gravitational effects can be theoretically treated by perturbation techniques on the axisymmetric base flow of the liquid metal. This leads to a modification of previously calculated critical-current-collector ejection values neglecting gravity effects. The purpose of this paper is to document the derivation of the mathematical model which determines the perturbation of the liquid-metal base flow due to gravitational effects. Since gravity is a small force compared with the centrifugal effects, the base flow solutions can be expanded in inverse powers of the Froude number and modified liquid-flow profiles can be determined as a function of the azimuthal angle. This model will be used in later work to theoretically study the effects of gravity on the ejection point of the current collector.

Further Studies of a Low-Melting Point Alloy Used in a Liquid Metal Current Collector

Low-melting point bismuth-based alloys are potential replacements for NaK <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">78</inf> as liquid metal slip ring material because of their lower reactivity and potentially greater hydrodynamic stability. This paper describes experiments of one such alloy in a model of a 300-kW superconducting homopolar motor using close clearance braid-type collectors. Slip ring tip velocities varied from 5 to 20 m/s and currents ranging from 500 to 2500 A. Viscous power losses tend to follow a simple turbulent model. In all, the data supports the use of low-melting point alloys (LMPA) us an alternative to NaK <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">78</inf> .

Efficiency of Small Homopolar Synchronous Motors at High Speeds

Summary In this paper, some efforts have been made to relate the efficiency of homopolar synchronous machines of the ‘reluctance rotor’ type to their design parameters. Particularly emphasized are the influences of the rotor pole shape, supply frequency and stator slotting. The indications given are based on results of numerous experiments on to prototype motors of a special design with a limited active length. From these results it is evident that only under exceptional circumstances, when despite high-speed operation excessive windage losses can be avoided, this kind of motor may be interesting for practical application.

Design and manufacture of a large superconducting homopolar motor (and status of superconducting A.C. generator)

This paper describes the design and manufacture of a large superconducting motor which, in the present time of financial restraints, is continuing at least to the point of having a completed cryostat with its superconducting winding operating with a dedicated helium refrigeration plant. Comments are also made on the superconducting a.c. generator project, although a final decision on the rating of a prototype and the approval of the funding has not yet been made, the selected rating is expected to be between 200 MW and 6OO MW.

Desgin and control studies of a linear synchronous motor for urban transit application

Magnetic circuit analysis is used for a design study of an iron-cored homopolar linear synchronous motor (LSM) for urban transit application. Using a suitable field or current angle, efficiency, power factor, and ratio of output power to total voltamps input are all calculated to be better than 85 percent. Other parameters including field or armature current, rail bar size, tooth or slot length, frequency, etc., are examined and found to be relatively insensitive. Control options are examined and field weakening, field or current angle control, or armature current control all appear to be feasible alternatives.

A homopolar motor for the demonstration of new high current brushes

Recent research has demonstrated that very high currents can be transferred efficiently across sliding electrical contacts under laboratory conditions. Further effort is required to apply these research concepts to practical machine designs and to determine their impact on such important machine parameters as power density, efficiency and operating life. Toward this objective, a high technology 50 h.p. homopolar motor was designed and built. It incorporates novel high current metal fiber brushes and water-cooled components in a configuration that has been optimized for maximum power density. The machine provides a wide range of possible speeds and current levels for brush testing in a controlled environment and may operate as either a motor or a generator. Initial testing will be performed with copper fiber brushes, although other brush materials and configurations may also be evaluated.

U.K. ministry of defence current collection programme for superconducting homopolar motors

A programme which at the time of writing was beginning to yield preliminary results is described in this paper. The objectives of the programme and the test plan are outlined. The experimental facilities which have been purpose designed and constructed are described. A summary of the results obtained up to the time of writing, albeit at an early stage in the test schedule, is presented.

Linear homopolar synchronous motor for urban transit application

A linear homopolar synchronous motor suitable for application in tracked urban transit vehicles is described, analyzed and tested as a small machine in a rotating test wheel. In addition to providing propulsion which is independent of adhesion limitations, this motor promises to provide better efficiency and power factor than other types of linear motor and thus to require a smaller on-board inverter. Potential economics in track construction are also envisaged. The paper presents some design information and an equivalent circuit model of the motor.

A magnetic circuit analysis of the iron-cored homopolar linear synchronous motor

An analysis of the iron-cored homopolar linear synchronous motor (LSM) is presented using tooth-by-tooth magnetic circuit theory and field dependent relative permeabilities. Sophisticated iterative procedures are used to obtain numerical solutions to the resulting set of simultaneous nonlinear equations. The analysis is validated by comparison between calculated and reported experimental tests results.

A 50 kW homopolar motor with superconducting field windings

A short description of the first superconducting motor in Finland is given. The project was completed in two years giving information of the feasibility of superconductivity and liquid metal current collection in motors with application in industry and ship propulsion.

HOMOPOLAR LINEAR SYNCHRONOUS MOTOR: STEADY STATE ANALYSIS AND EXPERIMENTAL RESULTS

ABSTRACT The main electrical and mechanical performances of a homopolar linear synchronous motor (HLSM) are calculated using classical theories which are usually applied to rotating synchronous machines. An experimental study on a low-power test motor permits the verification of theoretical results. It cranes to evidence that simple theories are applicable under certain conditions to such a motor of original conception. A full scale operational motor proposed for a combined propulsion and magnetic suspension system for high speed ground transportation is designed through extrapolation.

The circular form of the linear superconducting machine for marine propulsion

The superconducting linear synchronous machine (LSM) has been shown to be a highly efficient means of propulsion for application to advanced ground transport systems. We have shown that it would also appear to have significant advantages in marine engineering, e.g. for the propulsion of large commercial ships, tankers and military vessels, where the basic requirements of high torque at relatively low shaft speeds, together with the ease of reversibility, are readily met. A design utilising the circular form of the LSM as a ship's drive motor has been proposed and the details of a specific form of the LSM, namely the linear commutator machine (LCM) are discussed. The many advantages of this type of heteropolar machine are listed and compared with the superconducting homopolar motors which have been developed for ship propulsion. In particular, its flexibility in design, the built-in redundancy factors, hence reliability, together with other design details of the cryogenic magnets and the armature winding are described.

AN EXPERIMENTAL STUDY OF A HOMOPOLAR LINEAR SYNCHRONOUS MOTOR

Abstract This paper presents some experimental results obtained from a small homo-polar linear synchronous motor which has been tested on the two-metre test wheel facility at the University of Toronto. The primary of the motor has both armature and field windings while the secondary consists of alternately-placed solid iron poles on the wheel rim. A short three-pole primary has been used so that end effects are accentuated. Test results are presented over a wide range of operating speed, load angle and winding currents. These results indicate that the motor is a promising contender for urban transit propulsion.