Gene E. Schwarze

A 3-D magnetic analysis of a linear alternator for a Stirling power system

The NASA Glen Research Center and the Department of Energy (DOE) are developing advanced radioisotope Stirling convertors, under contract with Stirling Technology Company (STC), for space applications. Of critical importance to the successful development of the Stirling convertor for space power applications is the development of a lightweight and highly efficient linear alternator. This paper presents a 3-D finite element method (FEM) approach for evaluating Stirling convertor linear alternators. Preliminary correlations with open-circuit voltage measurements provide an encouraging level of confidence in the model. Spatial plots of magnetic field strength (H) are presented in the region of the exciting permanent magnets. These plots identify regions of high H, where at elevated temperature and under electrical load, the potential to alter the magnetic moment of the magnets exists. This implies the need for further testing and analysis.

Overview of NASA Magnet and Linear Alternator Research Efforts

The Department of Energy, Lockheed Martin, Stirling Technology Company, and NASA Glenn Research Center are developing a high‐efficiency, 110 watt Stirling Radioisotope Generator (SRG110) for NASA Space Science missions. NASA Glenn is conducting in‐house research on rare earth permanent magnets and on linear alternators to assist in developing a free‐piston Stirling convertor for the SRG110 and for developing advanced technology. The permanent magnet research efforts include magnet characterization, short‐term magnet aging tests, and long‐term magnet aging tests. Linear alternator research efforts have begun just recently at GRC with the characterization of a moving iron type linear alternator using GRC’s alternator test rig. This paper reports on the progress and future plans of GRC’s magnet and linear alternator research efforts.

High Frequency, High Temperature Specific Core Loss and Dynamic B-H Hysteresis Loop Characteristics of Soft Magnetic Alloys

Limited experimental data exists for the specific core loss and dynamic B-H loops for soft magnetic materials for the combined conditions of high frequency and high temperature. This experimental study investigates the specific core loss and dynamic B-H loop characteristics of Supermalloy and Metglas 2605SC over the frequency range of 1-50 kHz and temperature range of 23-300 C under sinusoidal voltage excitation. The experimental setup used to conduct the investigation is described. The effects of the maximum magnetic flux density, frequency, and temperature on the specific core loss and on the size and shape of the B-H loops are examined.

Design and technology of compact high-power converters

New material technologies such as silicon carbide (SiC) are promising in the development of compact high-power converters for next-generation power electronics applications. This paper presents an optimized converter design approach that takes into consideration nonlinear interactions among various converter components, source and load. It is shown that with the development of high-temperature, high-power SiC power module technology, magnetic components and capacitors become important technology challenges, and cannot be ignored. A 50% improvement in power density is calculated for a 100 V-2 kV, 7k W SiC DC-DC power converter operating at 150/spl deg/C compared to a silicon power converter. The SiC power converter can be operated at junction temperatures in excess of 300/spl deg/C (as compared to 150/spl deg/C for a silicon power converter) with reasonable efficiency that potentially leads to a significant reduction in thermal management.

Comparison of high temperature, high frequency core loss and dynamic B-H loops of a 2V-49Fe-49Co and a grain oriented 3Si-Fe alloy

The design of power magnetic components such as transformers, inductors, motors, and generators, requires specific knowledge about the magnetic and electrical characteristics of the magnetic materials used in these components. Limited experimental data exists that characterizes the performance of soft magnetic materials for the combined conditions of high temperature and high frequency over a wide flux density range. An experimental investigation of a 2V-49-Fe-49Co (Supermendur) and a grain oriented 3 Si-Fe (Magnesil) alloy was conducted over the temperature range of 23 to 300 C and frequency range of 0.1 to 10 kHz. The effects of temperature, frequency, and maximum flux density on the core loss and dynamic B-H loops for sinusoidal voltage excitation conditions are examined for each of these materials. A comparison of the core loss of these two materials is also made over the temperature and frequency range investigated.

Comparison of high frequency, high temperature core loss and B-H loop characteristics of an 80 Ni-Fe crystalline alloy and two iron-based amorphous alloys

Limited experimental data exists for the specific core loss and dynamic B‐H loops for soft magnetic materials for the combined conditions of high frequency and high temperature. This experimental study investigates the specific core loss and dynamic B‐H characteristics of a nickel‐iron crystalline magnetic alloy (Supermalloy) and two‐iron‐based amorphous magnetic materials (Metglas 2605S‐3A and Metglas 2605SC) over the frequency range of 1–50 kHz and temperature range of 23–300 C under sinusoidal voltage excitation. The effects of maximum magnetic flux density, frequency, and temperature on the specific core loss and on the size and shape of the B‐H loops are examined. The Supermalloy and Metglas 2605S‐3A and 2605SC data are used to compare the core loss of transformers with identical kVA and voltage ratings.

Magnetic and Electrical Characteristics of Cobalt-Based Amorphous Materials and Comparison to a Permalloy Type Polycrystalline Material

Magnetic component designers are always looking for improved soft magnetic core mater ials to increase the efficiency, temperature rating and power density of transformers, motors, generators and alternators, and energy density of inductors. In this paper, we report on the experimental investigation of commercially available cobalt -based am orphous alloys which, in their processing , w ere subjected to two different types of magnetic field anneals: A longitudinal magnetic field anneal or a transverse magnetic field anneal . The longitudinal field annealed material investigated was Metglas® 2714A. The electrical and magnetic characteristics of this material were investigated over the frequency range of 1 - 200 kHz and temperature range of 23 -150 C for both sine and square wave voltage excitation. The specific core loss was lower for the square th an the sine wave voltage excitation for the same maximum flux density, frequency and temperature. The transverse magnetic field annealed core materials include Metglas® 2714AF and Vacuumschmelze 6025F. These two materials were experimentally characterized over the frequency range of 10 - 200 kHz for sine wave voltage excitation and 23 C only. A comparison of the 2174A to 2714AF found that 2714AF always had lower specific core loss than 2714A for any given magnetic flux density and frequency and the ratio of specific core loss of 2714A to 2714AF was dependent on both magnetic flux density and frequency. A comparison was also made of the 2714A, 2714AF, and 6025F materials to two different tape thicknesses of the polycrystalline Supermalloy material and the res ults show that 2714AF and 6025F have the lowest specific core loss at 100 kHz over the magnetic flux density range of 0.1 - 0.4 Tesla.

Magnetic and Electrical Characteristics of Permalloy Thin Tape Bobbin Cores

The core loss, that is, the power loss, of a soft ferromagnetic material is a function of the flux density, frequency, temperature, excitation type (voltage or current), excitation waveform (sine, square, etc.) and lamination or tape thickness. In previously published papers we have reported on the specific core loss and dynamic B-H loop results for several polycrystalline, nanocrystalline, and amorphous soft magnetic materials. In this previous research we investigated the effect of flux density, frequency, temperature, and excitation waveform for voltage excitation on the specific core loss and dynamic B-H loop. In this paper, we will report on an experimental study to investigate the effect of tape thicknesses of 1, 1/2, 1/4, and 1/8-mil Permalloy type magnetic materials on the specific core loss. The test cores were fabricated by winding the thin tapes on ceramic bobbin cores. The specific core loss tests were conducted at room temperature and over the frequency range of 10 kHz to 750 kHz using sine wave voltage excitation. The results of this experimental investigation will be presented primarily in graphical form to show the effect of tape thickness, frequency, and magnetic flux density on the specific core loss. Also, the experimental results when applied to power transformer design will be briefly discussed.

Static and Turn-on Switching Characteristics of 4H-Silicon Carbide SITs to 200 °C

†Test results are presented for normally -off 4H -SiC Static Induction Transistors (SITs) intended for power switching and are among the first normally -off such devices realized in SiC. At zero gate bias, the gate p -n junction depletion layers extend far enough into the condu ction channel to cut off the channel. Application of forward gate bias narrows the depletion regions, opening up the channel to conduction by majority carriers. In the present devices, narrow vertical channels get pinched by depletion regions from opposite sides. Since the material is SiC, the devices are usable at temperatures above 150 C. Static curve and pulse mode switching observations were done at selected temperatures up to 200 C on a device with average static characteristics from a batch of similar devices. Gate and drain currents were limited to about 400 mA and 3.5 A, respectively. The drain voltage was limited to roughly 300 V, which is conservative for this 600 V rated device. At 23 C, 1 kW, or even more, could be pulse mode switched in 65 ns (1 0% to 90%) into a 100 � load. But at 200 C, the switching capability is greatly reduced in large part by the excessive gate current required. Severe collapse of the saturated drain -to -source current was observed at 200 C. The relation of this property to c hannel mobility is reviewed.

Performance of the NASA Digitizing Core-Loss Instrumentation

The ‘standard method’ of magnetic core loss measurement was implemented on a high frequency digitizi ng oscilloscope in order to explore the limits to accuracy when characterizing high Q cores at frequencies up to 1 MHz. This method computes core loss from the cycle mean of the product of the exciting current in a primary winding and induced voltage in a separate flux sensing winding. It is pointed out that just 20% accuracy for a Q of 100 core material requires a phase angle accuracy of 0.1 ° between the voltage and current measurements. Experiment shows that at 1 MHz, even high quality, high frequency cur rent sensing transformers can introduce phase errors of a degree or more. Due to the fact that the Q of some quasilinear core materials can exceed 300 at frequencies below 100 kHz, phase angle errors can be a problem even at 50 kHz. Hence great care is nec essary with current sensing and ground loops when measuring high Q cores. Best high frequency current sensing accuracy was obtained from a fabricated 0.1 -ohm coaxial resistor, differentially sensed. Sample high frequency core loss data taken with the setup for a permeability -14 MPP core is presented.