Initial AC and DC electromagnetic field characterization tests were performed on the Advanced Stirling Convertor ASC-1A-2 in March of 2006. The purpose of this initial test was to characterize the ASC electromagnetic fields and to evaluate the performance of three shielding configurations. The maximum field measured for both AC and DC measurements was measured at one meter with no shielding. With both Mu metal and iron shields positioned around the convertor, the maximum AC and DC fields were reduced. After this testing, it was concluded that more tests are needed to determine the characteristic of the linear alternator responsible for high magnetic field measurements (i.e. magnet movement, gap effect, current flow in coil, iron saturation, etc…)
An ASC research linear alternator (LA) was tested in the GRC EMI lab in August of 2006. The purpose of this test was to investigate the AC magnetic field emission sources of the ASC linear alternator and determine viable field reduction options to satisfy requirements for missions using low frequency magnetometer instruments. The ASC research LA was tested using the GRC Alternator Test Rig (ATR).
To determine the actual ASC convertor magnetic field emission level, further testing was completed. A pair of ASC convertors (ASC-1 #3 and #4) was tested in the GRC EMC lab in June of 2007. The purpose of this test was to determine the AC and DC magnetic field emission level of the ASC convertor pair. AC magnetic field measurements were made at various axial distances and radial positions using a Macintyre Electronic Design Associates, Inc. (MEDA) probe. A limited number of close-field DC magnetic field measurements were also made.
In December of 2007, GRC tested a bucking coil concept for the ASC-1 LAs that was predicted to reduce the AC magnetic field emissions. AC magnetic field measurements were made at various distances and radial positions and for various bucking coil configurations using a MEDA probe. Additionally, the electric field (E-field) was measured both with and without the bucking coil to determine if the E-field varies significantly with the addition of the external bucking coil. The bucking coil resulted in a decrease of the measured AC magnetic emissions in both the axial and radial direction.
During June of 2009, ASC-1 #3 and #4 underwent testing to characterize the AC magnetic field, DC magnetic field and electric field emitted by the convertors with Carpenter high permeability Perm-49 pressure vessels installed. The test plan combined the Perm-49 pressure vessels with bucking coils, showing a significant reduction of the DC magnetic field emissions.
ASC-E2 #1 and #2 underwent electromagnetic interference (EMI) characterization at GRC in August of 2010. The characterization included measurements of the AC magnetic field and electric field taken at varying distances in three directions: axial, radial, and 45°. This characterization provided a baseline for operation with a digital controller. Results showed that the emissions for the ASC-E2 convertors are comparable to those of the ASC-1 convertors with the 316 stainless steel housing in AC magnetic field, AC electric field, and DC magnetic field strength. The Carpenter high permeability Perm-49 pressure vessels, designed specifically to reduce EMI emissions, significantly reduced the DC magnetic field and the electric field emitted by the convertor pair, but only reduced the AC magnetic field emissions slightly.
Future testing may involve characterizing each successive generation of ASC convertors to determine if any of the design changes impacted the magnetic or electric field emissions of the convertors. Also, testing to determine the impact of a digital control system may occur at some point in the future.