How To Reduce Losses in Film-Coated Wires Through Material Selection

In today’s world, film-coated wires are one of the important components of various industries, including telecommunications, automotive, home circuits, electronics, and more. Different material is used to coat different wires, which provides them with the needed insulation for minimizing energy losses. This also improves performance while lowering utility bills by reducing energy losses.

But, to minimize the electrical losses, it is essential to choose the best material for the wire core and coating. Poor material can lead to reduced efficiency, overheating, and, ultimately, equipment failure.

Let’s understand what electrical loss is, how it occurs, and how it can be reduced with film-coated wires through material selection.

What are Electrical Losses?

Electrical loss is the loss of electrical energy during its transmission between conductors or conversion in a system. This lost energy is dissipated as heat.

Eddy Current Losses

Eddy current losses are caused by two effects – skin effect and proximity effect:

• Skin effect: When a high-frequency alternating current passes through a conductor due to electromagnetic induction, the magnetic field is generated by the current in the center of the conductor. This induces an electromotive force in the central region that generates eddy currents. The direction of these eddy currents is opposite to that of the original current, forcing the current to concentrate towards the surface of the conductor, thus forming the skin effect.
• Proximity effect: When two conductors in a balanced transmission line carry alternating currents in opposite directions, the alternating magnetic fields they generate induce eddy currents in each other. This phenomenon, where the current in one conductor induces eddy currents in an adjacent conductor, causing the actual current distribution in the conductor to concentrate towards the side closer to the adjacent conductor, is known as the proximity effect.

How to Reduce Current Loss

Here are ways to reduce current loss:

Focus on the number of strands of wire

To reduce eddy current loss, you need to consider the number of strands of wire. In high-frequency operating environments, increasing the number of strands in a conductor is not beneficial. While increasing the number of strands reduces the diameter of each individual strand, mitigating the skin effect, it also intensifies the internal proximity effect.

For instance, in the ultra-high frequency (e.g., GHz level) electronic devices such as small-cell base station inductors, excessive strands can lead to losses from the proximity effect that offset the benefits of reduced skin effect.

Therefore, a careful balance needs to be struck between operating frequency and the number of strands. For low-frequency applications, the impact of the number of strands on the skin effect and proximity effect is relatively minor.

However, other factors, such as cost and mechanical strengt,h should also be considered. In cost-sensitive low-frequency applications, such as ordinary low-frequency transformers, there is no need to choose conductors with an excessive number of strands, as this would unnecessarily increase costs.

Reduce skin effect and proximity effect in transformers and inductors

The skin effect causes non-uniform current density distribution, leading to increased losses. On the other side, the proximity effect cancels the magnetic flux entering the conductor which decreases the magnetic flux.

In conductors, the proximity effect is the primary source of eddy current loss. To overcome this loss, film-coated wires are developed based on the principle of reducing proximity effect, and their selection is also constrained by it.

Role Of Film-Coated Stranded Conductors In Reducing Loss

When two unstranded conductors are connected end-to-end, applying an external magnetic field induces an EMF, causing an additional current to flow in the same direction. This results in an uneven current distribution and increased losses.

Multi-strand conductors without stranding cannot mitigate this issue, even with a large number of strands. To overcome this limitation, film-coated stranded conductors have been introduced.

When strands are twisted together, the induced electromotive forces cancel each other out, resulting in no net-induced EMF in the closed loop and, therefore, no eddy currents, thus achieving a balanced current distribution. This is the fundamental principle of stranded conductors. By twisting the strands, the current is evenly distributed across each conductor.

Key Factors Affecting Loss In Film Coated Wires

For film-coated or Litz wires, the key factors affecting their performance are strand diameter, number of strands, pitch, and stranding method. In particular, strand diameter and number of strands are the two most critical parameters. So, is it always better to have more strands?

The answer is not necessarily!

Strand diameter

Strand diameter directly affects current-carrying capacity. A smaller strand diameter can effectively reduce skin effect at high frequencies, but if the diameter is too small, it may not meet the required current-carrying capacity.

For example, in the windings of high-power motors, if the selected strand diameter is too small, the conductor may overheat due to current overload, and even damage the insulation.

Number of strands

Strand diameter and number of strands are interrelated. When determining the strand diameter, the number of strands must also be considered. As the number of strands increases, the strand diameter decreases accordingly. It is necessary to evaluate the overall impact of this change on performance, including eddy current losses, mechanical strength, and other factors.

Pitch of stranded conductors

The pitch of stranded conductors has a significant impact on their performance. An appropriate pitch can better cancel out induced electromotive forces and achieve a balanced current distribution.

If the pitch is too large, it may not effectively cancel out induced electromotive forces, leading to increased eddy current losses. If the pitch is too small, the stranded conductor may become too tight, affecting its flexibility and heat dissipation.

For example, in applications that require frequent bending, film-coated stranded conductors with too small a pitch may be prone to breakage.

Stranding methods

Different stranding methods (such as regular stranding, composite stranding, etc.) can also affect the performance of stranded conductors.

Regular stranding has a simple structure and stable performance, suitable for most general applications. Composite stranding can meet special performance requirements, such as strong anti-interference ability, through different combination methods, but the cost may be relatively higher.

The appropriate stranding method should be selected based on specific application scenarios and performance requirements.

Final Words

At high frequencies, for each individual conductor, the skin effect is significantly reduced due to the smaller diameter of each strand. However, the magnetic fields from other conductors will induce electromotive force in this conductor.

For each strand in a multi-strand conductor, the current in other strands induces internal proximity effects, and the more strands there are, the stronger the internal proximity effects become.

Although the skin effect is reduced, the internal proximity effect is increased. Therefore, when selecting a conductor, it is necessary to comprehensively consider the relationship between the product’s frequency, the number of strands, and the strand diameter.