Ansys Maxwell – Twisted Litz Wire

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Introduction

In the 2024R1 release of Ansys Maxwell, a new feature was introduced to allow for a twisting factor to be included in Litz Wire models. This allows for a more robust solution that includes the increase in total wire length that occurs when twisting the bundles together. This article will detail how to set up the material properties and calculate the twisting length factor, and then show a simple Helmholtz coil model and the difference in results between a solid wire, stranded wire, and litz wire model.

Defining the Material

In the toolbar at the top, navigate to “Tools -> Edit Libraries -> Materials… “ Search for ‘copper’ (do NOT press Enter or it will close the window) and then select “Clone Material(s)” with copper selected.

Ansys maxwell materials library for defining the materials

This will bring up the View/Edit Material window for our copy of copper, which will be the basis for our litz wire. The first step is to change the Composition of the material from Solid to Litz Wire, which will allow us to put information such as the number of strands in the litz bundle, the individual wire diameter, and the twisting length factor.

Ansys Maxwell material properties window

For this example, our litz wire is 64 strands of Round 38 AWG wire, which is .1007 mm in diameter for each strand; this comes to a total wire size of 20 AWG. Twisted Length Factor is the new feature introduced into Ansys 2024R1 that accounts for the increase in total strand length due to the wires being twisted together to calculate the DC resistance more accurately. Twisted Length Factor (TLF) is the ratio between the twisted-strand length and the overall bundle length and is a value greater than or equal to 1. To calculate the twisted length factor, the following equation can be used:

Equation for twisting length factor, being 1 plus the quantity of pi squared times n times d sub s squared divided by the quantity 4 times K sub a times p squared

Where n is the number of strands, ds is the strand diameter, p is the pitch length, and Ka is the packing factor. The packing factor is found by dividing the sum of the cross sectional areas of all strands by the overall bundle area; this comes to approximately 3.93 for the chosen wire. The pitch for this wire is given as 12 twists-per-foot, which translates to a pitch of 1 inch, or 25.4 mm. For this example, the TLF is calculated to be 1.000632.

Example Helmholtz Case Study

Now we want to see the effects of setting up this litz wire material. To do that, a simple Helmholtz coil model was created, with two 10 mm radius loops separated by 10 mm, with the current flowing in the same direction. We’re going to set up three different versions of this model, varying the wire composition. The three models will be the default solid and stranded models using copper as the material, and the final will be using our new copper_litz material with a stranded model. We’ll run each of these models, using the default solution setup, and compare the losses in the wires.

Helmholtz coil modeled in Ansys Maxwell

Loss Comparison

After the simulations complete, we can examine the coil impedance and the wire losses that Maxwell reports and see if they correspond to our earlier hypothesis. First, we’ll look at the wire losses for the litz wire case (both DC and Eddy current), the ideal stranded wire case, and the solid wire case.

Ansys Maxwell results window showing the wire losses between the three different wire types simulated

We can see that the solid wire has the greatest losses and the ideal stranded wire has the lowest losses, both of which are in agreement with our hypothesis. The litz wire has two values associated with it – we’ve computed both the DC loss and the total loss. The DC loss is close to that of the ideal stranded wire, just slightly higher due to the twisted length factor we added in earlier. Since the DC loss report doesn’t include eddy effects, and neither does the ideal stranded wire, this result also agrees with our ideas. The total litz wire loss sits in-between the DC loss and Solid Loss, which is our expected outcome. Solid wires have the highest impedance and therefore the highest loss, especially when excited with higher frequency currents, like the 500 kHz we used in this example.

Ansys Maxwell results window showing the coil impedances between the three different wire types simulated

Conclusion

We have successfully demonstrated how to set up the new Litz Wire model that includes the Twisting Length Factor for accurate determination of DC resistance and shown how this impacts the ohmic losses and resistance of the wire. The litz wire feature, combined with the twisting length factor addition in Ansys Maxwell 24R1, allows for more robust simulations that can include the eddy current effects on stranded windings without having to model every wire individually.

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