Analytical Formulation of Copper Loss of Litz Wire with Multiple Levels of Twisting Using Measurable Parameters

Kazuhiro Umetani, Shota Kawahara, Jesus Acero, Hector Sarnago, Oscar Lucia, Eiji Hiraki

Research output: Contribution to journalArticlepeer-review

13 Citations (Scopus)


Litz wire has been widely utilized in power transformers and inductors as a wire with low copper loss at high-frequency operation. The Litz wire is commonly made of many thin isolated strands twisted in multiple levels. Due to its complicated structure, the copper loss prediction of the Litz wire has been difficult, hindering the design optimization of the Litz wire structure. To overcome this difficulty, preceding studies have investigated the analytical copper loss models of the constituting elements of the Litz wire, i.e., the strands and the bundles of strands. The purpose of this article is to propose an analytical copper loss model of the Litz wire by utilizing these preceding knowledge. The proposed model is formulated only with parameters that can be measured by basic testing instruments. Besides, the proposed model considers the bundle structure of the Litz wire, which affects the local ac current distribution, and the twisting pitch, which causes the inclination of the Litz wire strands. The proposed model was tested by comparing the analytical prediction and experimental measurements of the ac resistance of commercially available Litz wires. As a result, the predicted ac resistance showed good agreement with the measured ac resistance, suggesting the appropriateness of the proposed model.

Original languageEnglish
Article number9369852
Pages (from-to)2407-2420
Number of pages14
JournalIEEE Transactions on Industry Applications
Issue number3
Publication statusPublished - May 1 2021


  • AC resistance
  • analytical model
  • copper loss
  • litz wire
  • proximity effect

ASJC Scopus subject areas

  • Control and Systems Engineering
  • Industrial and Manufacturing Engineering
  • Electrical and Electronic Engineering


Dive into the research topics of 'Analytical Formulation of Copper Loss of Litz Wire with Multiple Levels of Twisting Using Measurable Parameters'. Together they form a unique fingerprint.

Cite this