Linear alternators coupled to Stirling power converters are promising candidates for high-efficiency heat-to-electricity power conversion in space. Presently, the external magnetic field emissions of such converters may exceed the allowed emission limits for use with certain sensitive scientific instrumentation. This invention, based on concepts of magnetic moment balancing, can reduce such field emissions sufficiently to enable use of the space power Stirling converters in sensitive instrumentation missions.
The most innovative part of this invention may be the use of the alternator’s own output current to reduce its own external magnetic field, everywhere in its far field space, without any significant power loss or extra mass or bulk. This enables the use of 100-watt-level Stirling converters with sensitive instrumentation in space, such as precision magnetometers. The field reduction technique could potentially replace existing methods that depend on heavy and bulky magnetic shields. It is expected that the same technique could be applied to higher power alternators and yield the same fractional reduction of external fields.
A dual-opposed power converter configuration equipped with the external magnetic field reduction technique was experimentally evaluated at NASA GRC. The innovation was tested on a pair of ASC-1 Stirling converters in the GRC EMI Lab. The results of this test indicated a 90% reduction in the peak AC magnetic field (from 88 dBpT, or 25.1 nT, at 1 m down to 70 dBpT, or 3.2 nT), and a 2.5% reduction in the DC component of the magnetic field. Finite element computations for a single alternator show that the AC external magnetic field at a distance of 1 m can be reduced to about 1.2 nT peak, or less, everywhere.