Litz wire loss performance and optimization for cryogenic windings

Abstract

Litz wires operating in a cryogenic environment can potentially improve both the efficiency and power density of electrical machines and passive components. However, due to the low resistivity and high magnetic fields, eddy-current losses may become significant in cryogenically cooled windings, especially in airgap winding arrangements or in the case of significant slot leakage fields, unless the litz wire parameters are carefully chosen. A framework for litz wire loss performance optimization and experimental characterisation at cryogenic temperatures is provided. An optimum operating temperature for minimum loss is derived based on analytical expressions, which highlights the role of litz wire parameters, current density and external field. The proximity loss model, used to calculate the optimum operating temperature, is validated experimentally. Two test rigs with different magnetic cores were designed and built. Copper and aluminium litz wires with a strand diameter down to 0.1 mm were tested in a liquid nitrogen bath with a uniform harmonic external magnetic field up to 0.5 T peak and a frequency up to 1 kHz. Measurements show good agreement with the theoretical results and confirm that the proposed model can be confidently used during the preliminary design of cryogenic windings.

Electromagnetic Characterization of Hyperconducting Aluminum Litz Wires at Cryogenic Temperatures

“…A correctly chosen Litz wire should be able to minimize the skin effect, i.e., the strand diameter ought to be no higher than the skin depth, δ. Under the condition ds ≤ δ, according to [26], the eddy current loss caused by the skin effect and proximity effect can be written as:…”

“…However, the above-mentioned test rigs are all focused on room temperature measurements for Cu Litz wires. A cryogenic test rig has been recently proposed in [26] to study the proximity loss of different Cu and Al Litz wires; however the temperature was confined at 77 K and only the proximity effect was investigated.…”

Perspectives on Electric Machines with Cryogenic Cooling

“…Copper: copper (Cu) is commonly used as the conductor material in the resistive windings of electric motors for cryogenic cooling systems [36,37]. It has good electrical conductivity and is relatively inexpensive.…”