Motor losses in linear cryocooler compressors

J. Reed (1), P.B. Bailey (1), M.W. Dadd (1), T. Davis (2)

  1. Department of Engineering Science, University of Oxford, Parks Road, Oxford, OX1 3PJ, UK.
  2. AFRL, Albuquerque, NM, USA

Characterization of several Stirling cycle and pulse tube cryocoolers of varying size and duty indicated a systematic underestimate of the power consumed during operation. This was attributed to an extra loss which, since it appeared to be proportional to the work done on the gas in the compression space, was assumed to be thermodynamic in nature and related to cyclic heat transfer. As this loss can account for up to 45% of the total power delivered to the gas by the motor, even a modest reduction could significantly improve overall cryocooler efficiency. In this paper existing results are described outlining the evidence for the loss, along with an analysis of the problem. Experiments have been performed on an existing moving coil compressor, of a well established design, with the simplest compression space geometry, a flat cylinder head. Measurements were made of the intrinsic motor losses, including windage with different filling pressures and gases, and the power delivered to the gas. These are presented along with an analysis which appears to show that, for this particular machine, the motor losses are greater than previously thought, accounting for up to 30% of the total unidentified loss in the original Stirling cycle cooler configuration.

Sponsored by the Air Force Office of Scientific Research, USAF.