Steven M. Geng

Glenn Research Center, Cleveland, Ohio

Janis M. Niedra

ASRC Aerospace Corporation, Cleveland, Ohio

Gene E. Schwarze

Glenn Research Center, Cleveland, Ohio

Demagnetization Tests Performed on a Linear

Alternator for a Stirling Power Convertor

NASA/TM—2012-217288

May 2012

AIAA–2011–5728
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Steven M. Geng

Glenn Research Center, Cleveland, Ohio

Janis M. Niedra

ASRC Aerospace Corporation, Cleveland, Ohio

Gene E. Schwarze

Glenn Research Center, Cleveland, Ohio

Demagnetization Tests Performed on a Linear

Alternator for a Stirling Power Convertor

NASA/TM—2012-217288

May 2012

AIAA–2011–5728

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for the

9th International Energy Conversion Engineering Conference (IECEC)

sponsored by the American Institute of Aeronautics and Astronautics

San Diego, California, July 31–August 3, 2011
Acknowledgments

The authors acknowledge the technical contributions of Dr. Reinhold M.W. Strnat of KJS Associates, Magnetic Instrumentation 

Inc. Dr. Strnat built the arc-magnet characterization fi xture and the electromagnet pole extensions used to measure the magnetic 

properties of the various magnet samples discussed in this paper. The authors also wish to acknowledge the efforts of Stacey Bagg 

in preparing this paper for publication and presenting the work at the 9th annual International Energy Conversion Engineering 

Conference. The work described in this paper was performed for the NASA Headquarters Science Mission Directorate under the 

auspices of the Radioisotope Power System Program.

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Trade names and trademarks are used in this report for identifi cation 

only. Their usage does not constitute an offi cial endorsement, 

either expressed or implied, by the National Aeronautics and 

Space Administration.

Level of Review: This material has been technically reviewed by technical management. 
NASA/TM—2012-217288 1 

Demagnetization Tests Performed on a Linear Alternator 

for a Stirling Power Convertor 

Steven M. Geng 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Janis M. Niedra 

ASRC Aerospace Corporation 

Cleveland, Ohio 44135 

Gene E. Schwarze1

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Abstract 

The NASA Glenn Research Center (GRC) is conducting in-house research on rare-earth permanent 

magnets and linear alternators to assist in developing free-piston Stirling convertors for radioisotope 

space power systems and for developing advanced linear alternator technology. This research continues at 

GRC, but, with the exception of Advanced Stirling Radioisotope Generator references, the work 

presented in this paper was conducted in 2005. A special arc-magnet characterization fixture was 

designed and built to measure the M-H characteristics of the magnets used in Technology Demonstration 

Convertors developed under the 110-W Stirling Radioisotope Generator (SRG110) project. This fixture 

was used to measure these characteristics of the arc magnets and to predict alternator demagnetization 

temperatures in the SRG110 application. Demagnetization tests using the TDC alternator on the 

Alternator Test Rig were conducted for two different magnet grades: Sumitomo Neomax 44AH and 

42AH. The purpose of these tests was to determine the demagnetization temperatures of the magnets for 

the alternator under nominal loads. Measurements made during the tests included the linear alternator 

terminal voltage, current, average power, magnet temperatures, and stator temperatures. The results of 

these tests were found to be in good agreement with predictions. Alternator demagnetization temperatures 

in the Advanced Stirling Convertor (ASC—developed under the Advanced Stirling Radioisotope 

Generator project) were predicted as well because the prediction method had been validated through the 

SRG110 alternator tests. These predictions led to a specification for maximum temperatures of the ASC 

pressure vessel. 

Introduction 

As part of the NASA radioisotope power system development, the NASA Glenn Research Center 

(GRC) is conducting an in-house technology project that previously supported the Department of Energy 

(DOE), Lockheed-Martin (LM), and the Infinia Corporation by developing a high-efficiency 110-W 

Stirling Radioisotope Generator (SRG110) for possible use on future NASA space science missions 

(Refs. 1 to 3). The SRG110 development has been stopped, and recent program changes have been made 

to increase the specific power of the generator. This effort is supporting DOE, LM, and Sunpower Inc. in 

the development of the Advanced Stirling Radioisotope Generator (ASRG). As a part of this in-house 

 

1

 Gene E. Schwarze, retired. 
NASA/TM—2012-217288 2 

effort, GRC is conducting research on permanent magnets and linear alternators to assist in developing 

the Stirling power convertor for space qualification and mission implementation. The in-house effort 

includes both analytical (finite-element modeling) and experimental research on magnets and linear 

alternators (LAs). The majority of the work in this paper was conducted for the SRG110 project in 2005. 

In 2000, GRC developed a three-dimensional magnetostatic Maxwell model of the Technology 

Demonstration Convertor (TDC) linear alternator (LA used in SRG110) (Refs. 4 and 5). The model was 

used to predict the temperature that would cause demagnetization of the initially selected NdFeB magnets 

(Ugimag 40HC2) used in the alternator. The prediction for these magnets was later verified through a 

linear alternator demagnetization test conducted by Infinia. 

GRC identified several other grades of neodymium-iron-boron (NdFeB) magnets as potential 

replacement candidates that could offer improvements in temperature margin based on the M-H

characteristics provided by the vendor. All of the identified magnet grades offered superior intrinsic 

coercivity (resistance to demagnetization) in comparison with the TDC LA baseline magnets without 

sacrificing magnet remanence. The magnetostatic model of the TDC LA was used to predict the 

demagnetization temperatures for each of these grades in the TDC application. The predictions assumed 

nominal magnetic properties for each magnet grade. 

Of the four new magnet grades identified by GRC, LM selected the two magnet grades (Sumitomo 

Neomax 44AH and 42AH) with the highest remanence for further consideration. LM asked GRC to perform 

demagnetization tests on these two magnet grades in the TDC application to confirm the predictions. 

In 2004, an arc-magnet characterization paddle was fabricated to measure the intrinsic induction 

(intrinsic induction versus magnetizing force, M-H) curves of the TDC LA magnets (Ref. 6). Initially, 

only the magnetizing force H measurements seemed reasonable. The B measurements were well below 

expectations because of leakage flux at the curved ends of the magnets. In 2005, special electromagnet 

pole extensions were designed and fabricated to improve the accuracy of the M-H measurements by 

reducing the leakage flux at the curved ends of the magnets. The M-H curves measured using the arcmagnet characterization paddle and pole extensions are now shaped more like those of the cube magnets. 

Nomenclature 

ASRG Advanced Stirling Radioisotope Generator 

ATR Alternator Test Rig 

Br remanence (T) 

BOM beginning of mission 

DOE Department of Energy 

GRC Glenn Research Center 

H magnetizing force (kOe) 

Hc coercivity (kOe) 

Hci intrinsic coercivity (kOe) 

Hk knee location (as defined in this report) on magnetization curve (kOe) 

LA linear alternator 

LCR Induction, Capacitance, Resistance 

LM Lockheed-Martin 

M magnetization (T) 

NdFeB neodymium-iron-boron 

NIST National Institute of Standards and Technology 

RMS Root Mean Square 

RTV room temperature vulcanized silicone rubber 

SRG110 Stirling Radioisotope Generator, 110 We 

TDC Technology Demonstration Convertor 
NASA/TM—2012-217288 3 

Arc-Magnet Characterization 

The magnetic properties of the arc magnets were measured both before and after the demagnetization 

tests. The procedure used to measure the M-H characteristics of the arc magnets is basically the same as 

that discussed by Niedra (Ref. 7) for cube magnets. The only difference is in the characterization paddle 

used to make the measurements. An arc-magnet characterization paddle designed specifically for the TDC 

LA magnets was used to measure the M-H curve for all arc-shaped magnets at room temperature (23 °C). 

The arc-magnet characterization paddle, along with the electromagnet pole extensions and the arc-magnet 

nickel standard sandwiched in between, is shown in Figure 1. The pole extensions shown in Figure 1(b) 

were designed by Niedra and Geng in collaboration with Dr. Reinhold Strnat of KJS Associates, Inc., of 

Indianapolis, Indiana. These extensions are used to control the demagnetization field applied to the arc 

magnet. The flat, square-shaped surfaces of the pole extensions fit snugly between the flat and parallel 

pole extensions of a 10-in., variable-gap electromagnet. The electromagnet pole extensions are a spatially 

fixed part of a temperature control fixture. The electromagnet is used to apply an external demagnetizing 

field to the magnet. The arc-magnet characterization paddle is fitted with built-in magnetization and fieldstrength-sensing coils that are used to measure the M-H characteristics of the arc-shaped magnets. The 

arc-magnet nickel standard is used as the calibration reference, and its material is traceable to the National 

Institute of Standards and Technology (NIST) through KJS. 

For the 42AH magnets, the measurements indicated a range of intrinsic coercivity Hci between 22.8 

and 24.3 kOe (vendor minimum Hci for cube-shaped magnets = 24 kOe at 20 °C) and a range of 

remanence Br between 1.22 and 1.24 T (vendor minimum Br for cube-shaped magnets = 1.28 T at 20 °C) 

for the eight arc-shaped alternator magnets. For the 44AH magnets, the measurements indicated a range 

of Hci between 20.5 and 21.3 kOe (vendor minimum Hci for cube-shaped magnets = 21 kOe at 20 °C) and 

a range of Br between 1.25 and 1.27 T (vendor minimum Br for cube-shaped magnets = 1.30 T at 20 °C) 

for the eight arc-shaped alternator magnets. Both the Hci and Br measurements for the arc-shaped magnets 

were slightly below the vendor minimum values for cube-shaped magnets. It should be noted that 

although the arc-shaped magnet measurements were repeatable, the accuracy of the measurements is 

unknown at this time. The difference between the arc- and cube-shaped magnet measurements could be 

due to geometry or the effect of the machining process, but it also may be due to differences in the 

measurement techniques. 

Figure 1.—Arc-magnet characterization paddle, pole extensions, and nickel arc-magnet standard.

 (a) Arc-magnet characterization paddle (b) Pole extensions with nickel arc-magnet standard

Pole 

Magnet extensions
NASA/TM—2012-217288 4 

Figure 2.—Cube-shaped magnet characterization data compared with Arc-magnet characterization data at 23 C. 

Figure 2 shows a comparison between the M-H curves measured for cube-shaped magnet samples and 

for arc-shaped magnet samples. These plots show that the knee location of the 42AH arc-shaped magnet 

M-H curve is about 1 kOe less than that of a cube-shaped magnet sample, whereas the knee location of 

the 44AH arc-shaped magnet M-H curve is about 2 kOe less than that of a cube-shaped magnet. The 

points of onset of rapid demagnetization, or knee locations (Hk), as referred to in this report, were 

determined by visual inspection of the M-H curves. Recalculating the predicted demagnetization 

temperature based on the M-H curve for the magnet with the least demagnetization resistance (i.e., lowest 

absolute Hk) of the 42AH arc-shaped magnets resulted in a new prediction of 132 °C, whereas the initial 

prediction was 136 °C. The same procedure applied to the 44AH arc-shaped magnets resulted in a new 

prediction of 117 °C, whereas the previous prediction was 122 °C. The methodology for predicting the 

onset of demagnetization temperatures is described in Appendix A. 

Demagnetization Test 

Apparatus and Procedure 

The GRC alternator test rig (ATR) was used to drive the test alternator for this test. The ATR features a 

Sunpower linear motor (DTR0101) as the drive motor. The DTR0101 is a variable-stroke and variablefrequency linear motor that can be used to drive linear alternators at mover amplitudes up to 6.5 mm and 

frequencies up to 120 Hz. The DTR0101 can deliver a nominal output of 150 W at 60 Hz and a stroke of 

13 mm. It is powered using a Chroma model 6404 programmable alternating-current (AC) source. A 

Sorensen model DCR80–6B direct-current (DC) power supply is used to provide power to an electric 

resistance heater blanket that is wrapped around the test alternator to control temperature. The heater blanket 

is wired to minimize the magnetic field that it generates. The DTR0101, power supplies, data acquisition 

system, and all support equipment are collectively referred to as the ATR, which is shown in Figure 3. 

A high-temperature red room temperature vulcanized (RTV) silicone rubber was used to secure the 

arc magnets to the stator laminations. A bead of RTV was placed along the straight edges of each magnet. 

This method of securing the magnets allowed for easy removal after the test had been completed so that a 

posttest magnet characterization could be performed. 

Type T thermocouples were used to measure the temperatures of the magnets. Type T thermocouples 

were selected for this test since both conductors (copper and constantan) are nonmagnetic. One 

thermocouple was epoxied to each of the eight magnets. Thermocouples were also placed with Kapton 

tape on the outside cylindrical surface of the stator laminations adjacent to each of the four pole/coils. 
NASA/TM—2012-217288 5 

Figure 3.—TDC 55-We linear alternator covered 

with insulation boot and mounted on ATR. 

A set of baseline operating conditions was selected for the demagnetization tests. The baseline mover 

amplitude was 6.1±0.05 mm and the baseline magnet temperature was 800.5 °C. The alternator was 

maintained at the design frequency of 82 Hz. The inductance of the test alternator was measured with an 

HP 4284A Precision Induction–Capacitance-Resistance (LCR) meter to determine the tuning capacitance 

required to achieve full alternator power output. The tuning capacitor was wired in series with a resistive 

load such that the inductive reactance canceled the capacitive reactance. The load resistance was adjusted 

until the output power of the alternator was approximately 68.8 W (SRG beginning of mission (BOM) 

predicted power is about 68.6 W), while maintaining the baseline mover amplitude and magnet 

temperature. The load resistance needed to achieve this alternator output power was 75.3 Ω, and the 

resultant current was 0.97 A, root mean square (ARMS). The load resistance and tuning capacitance were 

held constant throughout the demagnetization test. 

The alternator magnets were heated using a 360-W fiberglass-reinforced silicone-rubber heat blanket 

wrapped around the alternator stator and end rings as shown in Figure 4. Heat was conducted from the 

blanket, through the stator, and to the magnets. The magnet temperatures were raised gradually (at a rate 

of about 2 °C/min) to prevent localized hot spots at the magnet/stator interface. An insulation boot was 

placed over the alternator while the magnets were heated as shown in Figure 3. 

For cooling the magnets to the baseline temperature of 80 °C, the insulation boot was slid up about 

1.5 in. to expose the ATR/LA interface ring as shown in Figure 5. A muffin fan positioned near this 

interface ring was then used to convectively cool the interface ring, which helped to conduct heat away 

from the magnets. The magnets could be cooled in this manner at a rate of about 2 °C/min. 

The highest magnet temperature of the eight magnets was initially increased in increments of 5 °C. 

When the magnet temperature reached within 10 °C of the predicted demagnetization temperature, the 

increment was reduced to 2 °C. Data were recorded after each incremental increase in temperature before 

the magnets were returned to baseline operating conditions. Then the data were recorded again. The 

maximum magnet temperatures reached during this test were 151 °C for the 42AH magnets and 134 °C 

for the 44AH magnets. 

Complete demagnetization of the alternator magnets was purposely avoided to reduce the chance of 

magnet separation from the stator laminations. The magnetic forces of the magnets along with beads of 

RTV were relied on to keep the magnets fixed to the stator poles. Complete demagnetization would have 

placed the entire burden of holding the magnets onto the RTV. 

The AC-source RMS voltage and current as well as the linear alternator terminal RMS voltage and 

current, average power, eight magnet temperatures, and four stator temperatures were recorded at each 

test operating point. 

Insulation 

boot 
NASA/TM—2012-217288 6 

Figure 4.—A 55-We linear alternator mounted on ATR with 360-W 

thermal blanket wrapped around stator and end rings. 

Figure 5.—Insulation boot position when cooling magnets. 

Demagnetization Test Results 

In Figure 6 the alternator terminal voltage measured at the baseline alternator operating conditions 

(frequency = 82 Hz, mover amplitude = 6.1 mm, and magnet temperature = 80 °C) is plotted as a function 

of the maximum magnet temperature reached before the magnets were returned to baseline temperature. 

These plots indicate that the onset of magnet demagnetization occurred between 131 and 135 °C for the 

42AH magnets and between 116 and 120 °C for the 44AH magnets. These results agree well with the 

GRC prediction of 132 and 117 °C for magnet grades 42AH and 44AH, respectively. 

It should be noted that Figure 6 indicates the degradation of magnet Br as a function of magnet 

temperature. It does not give any indication of degradation regarding Hci. Tables I and II list the magnetic 

properties of the arc-shaped magnets measured at room temperature (2 °C) both before and after the LA 

demagnetization test. The information shown in the tables includes the pretest and posttest magnet Br, Hk, 

Test alternator 

Heat blanket 

Muffin fan 

LA/ATR 

interface ring 

1.5” 

Safety shield 
NASA/TM—2012-217288 7 

and Hci. For the pretest data, the arc magnets were fully charged, characterized, and recharged prior to 

insertion into the alternator. For the posttest data, the magnets were removed from the alternator, 

characterized, and recharged in preparation for reuse. Comparing the pretest and posttest measurements, 

Table I shows that the average 42AH magnet Br dropped by 1.7 percent, whereas Hk dropped by 

0.7 percent during this test, and Table II shows that the average 44AH magnet Br dropped by 2.1 percent, 

whereas Hk dropped by 1.7 percent. 

Figure 6.—Alternator terminal voltage at baseline conditions (80 C) 

following operation at elevated temperatures. 

TABLE I.—42AH ARC-MAGNET CHARACTERIZATION DATA AT 23 °C 

Magnet 

identification 

Pretest Posttest Percent change 

Br (T) Hk (kOe) Hci (kOe) Br (T) Hk (kOe) Hci (kOe) Br Hk Hci

42AH–1 1.24 –21.0 –23.1 1.20 –20.7 –22.7 –3.2 –1.4 –1.7 

42AH–2 1.24 –20.8 –23.4 1.22 –20.6 –23.0 –1.6 –1.0 –1.7 

42AH–3 1.23 –20.9 –24.1 1.22 –20.7 –24.0 –0.8 –1.0 –1.7 

42AH–4 1.23 –20.6 –23.0 1.20 –20.6 –22.7 –2.4 0.0 –1.3 

42AH–5 1.23 –21.1 –24.3 1.22 –21.1 –22.9 –0.8 0.0 –5.8 

42AH–6 1.23 –20.6 –23.5 1.22 –20.6 –22.4 –0.8 0.0 –4.7 

42AH–7 1.23 –20.6 –23.6 1.21 –20.4 –23.1 –1.6 –1.0 –2.1 

42AH–8 1.22 –20.6 –22.8 1.19 –20.3 –22.6 –2.5 –1.5 –0.9 

TABLE II.—44AH ARC-MAGNET CHARACTERIZATION DATA AT 23 °C 

Magnet 

identification 

Pretest Posttest Percent change 

Br (T) Hk (kOe) Hci (kOe) Br (T) Hk (kOe) Hci (kOe) Br Hk Hci

44AH–1 1.25 –19.7 –21.0 1.24 –19.4 –20.7 –0.8 –1.5 –1.4 

44AH–2 1.27 –19.5 –20.8 1.24 –19.1 –20.4 –2.4 –2.1 –1.9 

44AH–3 1.27 –19.2 –20.5 1.22 –18.9 –20.2 –3.9 –1.6 –1.5 

44AH–4 1.26 –19.6 –20.9 1.24 –18.9 –20.2 –1.6 –3.6 –3.3 

44AH–5 1.25 –19.8 –21.1 1.24 –19.4 –20.7 –0.8 –2.0 –1.9 

44AH–6 1.25 –19.9 –21.3 1.22 –19.3 –20.6 –2.4 –3.0 –3.3 

44AH–7 1.26 –19.5 –20.8 1.24 –19.5 –20.8 –1.6 0.0 0.0 

44AH–8 1.27 –19.2 –20.5 1.23 –19.2 –20.5 –3.1 0.0 0.0
NASA/TM—2012-217288 8 

Advanced Stirling Radioisotope Generator Discussion 

The method for predicting demagnetization temperatures in an alternator was applied to the ASC as 

part of the ASRG project. A three-dimensional magnetostatic Maxwell model was created of the ASC 

alternator (used in the ASRG) to predict the temperature that would cause the LA magnets to 

demagnetize. The M-H characteristics for the ASC LA magnets also were measured at GRC and 

compared with vendor data to verify that appropriate values were being used in the model. This predicted 

value for demagnetization contributed to determining a specification for the ASC pressure vessel 

maximum temperature (the ASC LA is housed in the ASC pressure vessel). The final specification was 

determined to be 115 C for the maximum ASC pressure vessel temperature (Ref. 8), which included 

some margin. 

Conclusions 

The magnetic properties of the arc magnets used in this test were all slightly below the vendor 

minimum specifications for cubes. The predicted demagnetization temperature based on the arc-magnet 

M-H curve was 132 °C for 42AH magnets and 117 °C for 44AH magnets. In general, the predicted 

demagnetization temperatures were in good agreement with the test data. The demagnetization 

temperature of the Technology Demonstration Convertor (TDC) linear alternator can be accurately 

predicted provided that the magnetic properties of the actual alternator magnets are known. The shape of 

a magnet may affect its magnetic properties. Vendor data are typically given for cube-shaped magnets. 

However, variation of intrinsic properties with magnet shape can be expected. 

This method for predicting alternator demagnetization temperatures was validated using the TDC 

alternator tests on the Alternator Test Rig. The same method was then used to predict alternator 

demagnetization temperatures for the Advanced Stirling Radioisotope Generator (ASRG). This prediction 

was part of the method for determining the specification for the ASRG maximum convertor pressure 

vessel temperature. 
NASA/TM—2012-217288 9 

Appendix A—Methodology Used to Predict Demagnetization Temperatures 

A three-dimensional magnetostatic model of the TDC linear alternator was used to calculate the peak 

magnetic field strength at various temperatures for the arc magnets with the assumption of first 42AH 

magnets, then 44AH magnets, for the mover amplitude (6.1 mm) and alternator current (0.97 A RMS) as 

selected for the demagnetization tests. Demagnetization fields were calculated as documented by Geng 

(Ref. 5). Figure 7 shows the results of these predictions plotted along with curves that represent the 

estimated knees of the 42AH and 44AH arc-magnet intrinsic induction curves. The intersection of the 

lines represents the predicted demagnetization temperature for the two magnet grades. The predicted 

demagnetization temperature for the TDC linear alternator equipped with 42AH magnets and with the 

assumption of normal loads (no current spikes) was approximately 132 °C, whereas the predicted 

demagnetization temperature for the 44AH magnets was approximately 117 °C. 

Figure 7.—Sensitivity of resistance to demagnetization and maximum localized 

demagnetization field intensity. 
NASA/TM—2012-217288 10 

References 

1. Cockfield, R.D. and Chan, T.S., “Stirling Radioisotope Generator for Mars Surface and Deep Space 

Missions,” Proceedings of 37th Intersociety Energy Conversion Engineering Conference, 

Washington, D.C., IECEC 2002 Paper No. 20188, 2002. 

2. Schreiber, J.G. and Thieme, L.G., “Accomplishments of the NASA GRC Stirling Technology 

Development Project,” Proceedings of 2nd International Energy Conversion Engineering 

Conference, Providence, RI, Paper No. AIAA–2004–5517, 2004. 

3. Schreiber, J.G. and Thieme, L.G., “Final Results for the GRC Supporting Technology Development 

Project for the 110-Watt Stirling Radioisotope Generator (SRG-110),” in Proceedings of the Space 

Technology and Applications International Forum, (STAIF 2007), American Physical Society; 

NASA/TM—2007-214839. 

4. Geng, S.M., Niedra, J.M., and Schwarze, G.E., “A 3–D Magnetic Analysis of a Linear Alternator 

for a Stirling Power System,” Proceedings of the 35th Intersociety Energy Conversion Engineering 

Conference, Las Vegas, Nevada, July 24–28, 2000, AIAA–2000–2838, NASA/TM—2000-210249. 

5. Geng, S.M., Schwarze, G.E., Niedra, J.M., and Regan, T.F. “A 3–D Magnetic Analysis of a Stirling 

Convertor Linear Alternator Under Load,” Proceedings of the 36th Intersociety Energy Conversion 

Engineering Conference, Savannah, Georgia, July 29-August 2, 2001, IECEC2001–CT–34, 

NASA/TM—2001-211084, 2001. 

6. Geng, S.M., Niedra, J.M., and Schwarze, G.E., “Overview of NASA Magnet and Linear Alternator 

Research Efforts,” Proceedings of Space Technology and Applications International Forum 

(STAIF–2005), STAIF–2005–093, NASA/TM—2005-213411, 2005. 

7. Niedra, J.M., and Schwarze, G.E., “Makeup and Uses of a Basic Magnet Laboratory for 

Characterizing High-Temperature Permanent Magnets,” NASA TM–104508, 1991. 

8. “ASRG User ICD,” Section 6. Lockheed Martin, May 2010. NASA SMD Science Office for 

Mission Assessments Discovery Program Acquisitions Discovery 2010 AO Program Library. 

November 23, 2010. http://discovery.larc.nasa.gov/dpl.html. 
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NASA/TM-2012-217288 

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Unclassified-Unlimited 

Subject Category: 20 

Available electronically at http://www.sti.nasa.gov 

This publication is available from the NASA Center for AeroSpace Information, 443-757-5802

13. SUPPLEMENTARY NOTES

Gene E. Schwarze, NASA Glenn Research Center, retired. 

14. ABSTRACT

The NASA Glenn Research Center (GRC) is conducting in-house research on rare-earth permanent magnets and linear alternators to assist 

in developing free-piston Stirling convertors for radioisotope space power systems and for developing advanced linear alternator 

technology. This research continues at GRC, but, with the exception of Advanced Stirling Radioisotope Generator references, the work 

presented in this paper was conducted in 2005. A special arc-magnet characterization fixture was designed and built to measure the M-H 

characteristics of the magnets used in Technology Demonstration Convertors developed under the 110-W Stirling Radioisotope Generator 

(SRG110) project. This fixture was used to measure these characteristics of the arc magnets and to predict alternator demagnetization 

temperatures in the SRG110 application. Demagnetization tests using the TDC alternator on the Alternator Test Rig were conducted for two 

different magnet grades: Sumitomo Neomax 44AH and 42AH. The purpose of these tests was to determine the demagnetization 

temperatures of the magnets for the alternator under nominal loads. Measurements made during the tests included the linear alternator 

terminal voltage, current, average power, magnet temperatures, and stator temperatures. The results of these tests were found to be in good 

agreement with predictions. Alternator demagnetization temperatures in the Advanced Stirling Convertor (ASC-developed under the

Advanced Stirling Radioisotope Generator project) were predicted as well because the prediction method had been validated through the 

SRG110 alternator tests. These predictions led to a specification for maximum temperatures of the ASC pressure vessel. 

15. SUBJECT TERMS

Stirling engines; Stirling cycle; Linear alternators; Permanent magnets 

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