Kiruba Sivasubramaniam

Development of a High Speed HTS Generator for Airborne Applications

General Electric, under contract with the Air Force Research Labs (AFRL), has successfully developed and tested a high speed, multimegawatt superconducting generator. The generator was built to demonstrate high temperature superconducting (HTS) generator technology for application in a high power density Multimegawatt Electric Power System (MEPS) for the Air Force. The demonstration tested the generator under load conditions up to 1.3 MW at over 10,000 rpm. The new MEPS generator achieved 97% efficiency including cryocooler losses. All test results indicate that the generator has a significant margin over the test points and that its performance is consistent with program specifications. This demonstration is the first successful full-load test of a superconducting generator for the Air Force. In this paper we describe the development of the generator and present some key test results used to validate the design. Extrapolation to a higher power density generator is also discussed.

Analysis of the far field of permanent-magnet motors and effects of geometric asymmetries and unbalance in magnet design

In the design of permanent-magnet synchronous machines for naval applications, exterior magnetic fields are of interest. These decay at a rate depending on the number of poles, with magnetic fields due to a higher number of poles decaying more rapidly. We have developed multipole expansion methods to study the effects of geometric asymmetries and unbalanced pole strength on the components of the far field. We have found that, if there is an imbalance in the set of poles, the lower order decay of the unbalanced poles dominates in the far field, and the advantage of using a higher number of poles is diminished. Multipole expansion in combination with the charge simulation method offers a quick and easy method of determining effects of imbalance on the far field of motors at the design stage. We present the results for a variety of imbalances and pole numbers, and discuss the unbalanced terms due to the demagnetization and space imbalance. We also explain the behavior of the far-field decay with the aid of analytical expressions.

Performance of an HTS Generator Field Coil Under System Fault Conditions

High-temperature superconducting (HTS) coils are generally stable against transient thermal disturbances. Protection against spontaneous quenches is not a main design issue for an HTS coil. However, HTS coils used in many electric devices such as motors, generators, transformers, and current limiters will operate under over-current fault conditions, which may result in a coil quench and thermal runaway. Those electric devices should be able to ride through some grid fault conditions and remain functional. This requires a certain over-current capability of the HTS coils. This paper discusses the over-current requirements from grid faults and the thermal transient responses of a BSCCO coil. It presents the analysis results of the coil subjected to over-current pulses at different operating conditions. The study also investigates the thermal runaway current and its relationship with the over-current pulse

High-Temperature Superconducting Homopolar Inductor Alternator for Marine Applications

This paper describes a high power density high-temperature superconducting (HTS) electric machine topology that is scalable for marine propulsion and power generation. The design, currently being pursued for airborne applications, is based on homopolar inductor alternator (HIA) technology, which is new within HTS applications. The basic machine design configuration of the HTS HIA is based on a stationary HTS field excitation coil, a solid rotor, and an advanced but conventional stator comprising liquid-cooled air-gap armature winding and an advanced iron core. High power density is obtained by the enhanced magneto-motive force capability of the HTS coil, the increased airgap flux density and armature current loading, and the high tip velocity of the rotor. Preliminary scaled up designs look attractive for three marine applications: propulsion drive, primary ship power generation, and power generation modules. The generators are driven directly by the turbines without the additional complexity of a clutch and gear system. A conceptual design study of a 36-MW 3600-r/min generator, a 4-MW 7000-r/min auxiliary generator, and a 36-MW 120-r/min and 4-MW 132-r/min propulsion motor are summarized.

Advanced permanent magnet machines for a wide range of industrial applications

This paper provides an overview of four distinct applications of advanced permanent magnet machine technologies. It describes the specific application benefits that are accomplished with permanent magnet solutions for Oil&Gas, aviation, automotive, and wind turbine applications. The technology development for these four first-of-a-kind machines is presented, each of which advancing the state of the art in power density, torque density, or speed.

The Thermal Performance of a 1.5 MVA HTS Generator

A 1.5‐MVA high temperature superconducting ( HTS ) generator of novel design has been designed, built and successfully tested by the General Electric Company. The 1.5‐ MVA generator has served as the engineering prototype for a much larger 100‐MVA beta unit now under design.The HTS coil in the 1.5 ‐ MVA demonstrator is designed to operate in the range of 20–40 K and is cooled with a closed‐cycle helium refrigeration system employing GM type cryocoolers. This paper will discuss the calculation of the thermal loads to the rotor from all anticipated sources. These sources include conduction losses through the coil suspension system, radiative heat loads to the cold‐system components, residual gas conduction losses, helium‐transfer coupling losses and lead losses. These predicted losses were compared to those measured during actual electrical testing of the rotor at 3600 RPM in order to validate the predictive calculations employed for the 100 MVA machine.

The effect of asymmetry on torque in permanent magnet motors

Electronically controlled permanent magnet motors are increasing in popularity due to high efficiency and power density. The noise and vibration issues in permanent magnet motors have not been studied extensively. This paper presents a general purpose method to understand the sources of torque pulsation in permanent magnet motors. This method is based on the analysis of the traveling wave components of the radial and tangential air gap flux. These flux densities are obtained from time domain finite element solutions which are then analyzed to find the time and frequency Fourier components. The harmonic components in the cogging torque can then be attributed to specific air gap flux harmonics. A surface mounted brushless DC motor is used as a case study to analyze the effect of rotor asymmetry on torque pulsation. Several guidelines relating to the finite element modeling of the machine are given concerning grid generation and the inclusion of external circuit connections. The effect of magnet imbalance, static rotor eccentricity, dynamic eccentricity, and combined static and dynamic eccentricity are investigated. Load cases are also studied. These results are then related to the expected frequency components in the torque ripple. The authors conclude the paper with overall comments on the modeling and a discussion of the suitability of these motors for low vibration applications.

High Power Density HTS Iron Core Machines for Marine Applications

We present a conceptual design study to develop high power density high temperature superconducting (HTS) machines for two navy applications: primary ship propulsion power generation and drive, and power generation modules (PGM). The objective of the study was to evaluate the design of a 36 MW, 3600 rpm generator, a 36 MW, 120 rpm propulsion motor, and a 4 MW, 7000 rpm power generation module. The generators are to be driven directly by the turbines without the benefit of clutch or gear. Several design concepts were evaluated, including iron core rotors and homopolar inductor alternator topologies. Engineering trade-offs make the re-introduction of ferromagnetic iron attractive for a number of reasons. High power density is obtained by the increased airgap flux density and armature loading, and, for high speed applications, the high tip velocity of the rotor.

Development of integral equation solution for 3D eddy current distribution in a conducting body

Eddy current analysis finds wide application in electrical machinery and devices, in power system analysis, non destructive testing, continuous casting, ship board applications and others. Finite Element methods such as T-Omega, A-phi and A-V methods do provide solutions of acceptable accuracy for small problems where the element size is comparable to skin-depth. Even for this, a large number of elements are required to model the entire space of the conducting medium and the surrounding air region. Integral equations require modeling of only the conducting parts and therefore offer an alternative approach to the problem. This paper presents an integral equation analysis and its application to a conducting slab with and without a crack excited by a transmission line source, to a slab excited by a dipole source, to phase segregated bus bars and others.

AC losses in a high temperature superconducting generator

As part of the DOE-SPI funded project a commercial HTS utility-size generator is being developed based on GEs iron core superconducting generator technology. The iron core concept has significant advantages over air core designs. The rotor consists of a cold superconducting field coil and coil supports and a warm iron core, which takes the torque and transmits to the shafts. Heat load due to AC losses in the cold mass of the rotor are a key design constraint. Analyses have been performed both at the component and system levels. AC loss tests have been conducted on an HTS generator. This paper presents and discusses the analytical and test results.