AC Resistance Factor of Litz-Wire Windings Used in Low-Voltage High-Power Generators

New types of heavy litz wires with noninsulated or insulated strands are offered for high-power low-voltage electrical machines, where skin and proximity effects can cause serious problems if traditional windings are used. This paper evaluates the ac resistance of a litz wire and its usability in megawatt-range low-voltage electrical machines. The evaluation is made by finite-element analysis and by resistance measurements with an experimental test setup. A simple amplifier configuration was used to minimize current and voltage phase shift errors, which are critical in this kind of low-impedance and low-frequency measurement. A simple measurement device based on the aforementioned configuration was prepared to ensure a precise measurement result. Tests were done in a frequency range of 50-200 Hz, to cover a wide range of practical high-power low-voltage electrical machines. It is found that the measured litz wire with originally noninsulated strands can be used in large electrical machines up to about 120 Hz if about 50% increase in ac resistance compared to the dc resistance is allowed. Impregnation of the originally noninsulated litz wire with vacuum pressure impregnation improved the ac resistance in one test case, and in the other case, it seemed to have no effect on the ac resistance.

[1]  Diego Puyal,et al.  Frequency-dependent resistance in Litz-wire planar windings for domestic induction heating appliances , 2006, IEEE Transactions on Power Electronics.

[2]  Marian K. Kazimierczuk,et al.  High-Frequency Magnetic Components , 2009 .

[3]  Di Wu,et al.  Stator Design for a 1000 kW HTSC Motor With Air-gap Winding , 2011, IEEE Transactions on Applied Superconductivity.

[4]  J. A. Ferreira.  Analytical computation of AC resistance of round and rectangular litz wire windings , 1992 .

[5]  Charles R. Sullivan.  Optimal choice for number of strands in a litz-wire transformer winding , 1997 .

[6]  Charles R. Sullivan,et al.  Computationally efficient winding loss calculation with multiple windings, arbitrary waveforms, and two-dimensional or three-dimensional field geometry , 2001 .

[7]  Fred C. Lee,et al.  A high frequency model for Litz wire for switch-mode magnetics , 1993, Conference Record of the 1993 IEEE Industry Applications Conference Twenty-Eighth IAS Annual Meeting.

[8]  W.. Nick,et al.  Operational Experience With the World's First 3600 rpm 4 MVA Generator at Siemens , 2007, IEEE Transactions on Applied Superconductivity.

[9]  Rafal Wrobel,et al.  Contribution of End-Winding Proximity Losses to Temperature Variation in Electromagnetic Devices , 2012, IEEE Transactions on Industrial Electronics.

[10]  C.R. Sullivan,et al.  Simplified high-accuracy calculation of eddy-current loss in round-wire windings , 2004, 2004 IEEE 35th Annual Power Electronics Specialists Conference (IEEE Cat. No.04CH37551).

[11]  Valter Mariani Primiani,et al.  Modelling of a Litz-wire planar winding , 2010 .

[12]  Charles R. Sullivan.  Cost-constrained selection of strand diameter and number in a Litz-wire transformer winding , 1998 .

[13]  Charles R. Sullivan,et al.  Stranded wire with uninsulated strands as a low-cost alternative to litz wire , 2003, IEEE 34th Annual Conference on Power Electronics Specialist, 2003. PESC '03..

[14]  C.R. Sullivan,et al.  Optimization of stranded-wire windings and comparison with litz wire on the basis of cost and loss , 2004, 2004 IEEE 35th Annual Power Electronics Specialists Conference (IEEE Cat. No.04CH37551).

[15]  P. Viarouge,et al.  Accurate analytical model of winding losses in round Litz wire windings , 2001 .

[16]  Johan Gyselinck,et al.  Homogenization of Form-Wound Windings in Frequency and Time Domain Finite-Element Modeling of Electrical Machines , 2010, IEEE Transactions on Magnetics.

[17]  Fabio Giulii Capponi,et al.  Axial-Flux Permanent-Magnet Generator for Induction Heating Gensets , 2010, IEEE Transactions on Industrial Electronics.

[18]  G.S. Dimitrakakis,et al.  High-Frequency Copper Losses in Magnetic Components With Layered Windings , 2009, IEEE Transactions on Magnetics.

[19]  T. Chevalier,et al.  Estimation of magnetic loss in an induction motor fed with sinusoidal supply using a finite element software and a new approach to dynamic hysteresis , 1999 .

[20]  P. L. Dowell,et al.  Effects of eddy currents in transformer windings , 1966 .

[21]  Marian K. Kazimierczuk,et al.  Analytical Optimization of Solid–Round-Wire Windings , 2013, IEEE Transactions on Industrial Electronics.

[22]  M. Bartoli,et al.  Modeling Litz-wire winding losses in high-frequency power inductors , 1996, PESC Record. 27th Annual IEEE Power Electronics Specialists Conference.

[23]  C.R. Sullivan,et al.  An Equivalent Complex Permeability Model for Litz-Wire Windings , 2009, IEEE Transactions on Industry Applications.

[24]  F. Lee,et al.  Proximity effects in coils for high frequency power applications , 1992 .

[25]  Marian K. Kazimierczuk,et al.  Winding resistance of litz-wire and multi-strand inductors , 2012 .