Material Effects on the 4 Kelvin AC Efficiency of Moving Coil Linear Actuators
We describe our measurements of the power dissipation of moving coil linear actuators operating at 4 K. The “voice coil” assembly is approximately 25 mm in diameter and 25 mm long and is fabricated from 28 MGOe energy product samarium cobalt and 48 percent nickel steel. The effects of using different wire materials for the coil are described. A set of six nominally identical coils were wound with either copper wire formed from raw material varying in purity from 0.9995 to 0.999999 or with solid core NbTi (copper clad) wire or with twisted multifilamentary NbTi (copper-nickel alloy matrix) wire. For the copper wire we have correlated the Residual Resistance Ratio (RRR) with purity and annealed state. The coils are operated in the magnet assembly’s 0.5 T DC field yielding a force constant of about 6 N/A. At room temperature the resistive losses in the coil are the major source of actuator power dissipation, but at 4 K non-resistive losses predominate. For 50 Hz operation both superconducting wire voice coils exhibited higher power dissipation than the high purity copper voice coil.
KeywordsPower Dissipation Liquid Helium Copper Wire Resistive Loss Electromagnetic Actuator
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