In this technical engineering guide, Clayton Elliott, Design Engineering Manager at New England Wire Technologies, addresses a common winding design question: Should you serve or not serve your Litz wire?
A “serve” is an outer protective jacket made of spirally wrapped textile fibers (such as nylon, polyester, Nomex, or silk) applied directly over the Litz wire bundle. To understand whether your application requires a serve, engineers must consider the manufacturing realities of magnet wire:
- Allowable Pinholes in Magnet Wire: Individual fine magnet wire strands rely on a micro-thin enamel coating. Industry specifications explicitly permit a small, statistically predictable incidence of microscopic pinholes in virgin magnet wire. In a single-strand winding, the chance of a pinhole contacting ground or another bare spot is virtually negligible.
- The Statistical Multiplier in Litz Bundles: Litz wire combines dozens, hundreds, or even thousands of individually insulated strands bundled in parallel. When wound around a core or bobbin, the statistical probability that exposed enamel pinholes will align or touch a conductive metallic core increases dramatically, raising the risk of catastrophic turn-to-ground or turn-to-core shorts.
- Bundle Integrity and Splaying Prevention: Without an external wrap, unserved Litz strands can splay or flatten across the sharp corners of bobbins or toroidal cores. A textile serve binds the strands into a stable, compact, uniform profile, maintaining dimensional stability during high-tension winding.
- Protecting High-Value Assemblies: Litz coils represent significant engineering labor, material investment, and manufacturing value compared to basic single-conductor windings. Scraping an entire transformer or motor stator due to an internal short during potting or assembly is costly. A textile serve provides critical mechanical cut-through defense and added electrical voltage standoff.
Serving materials are selected based on the coil’s required UL/NEMA thermal temperature class, overall build thickness constraints, and chemical compatibility with impregnation varnishes or potting resins.
Key Technical Takeaways
- Pinhole Isolation: Mitigates the risk of allowed manufacturing pinholes in individual enamel strands aligning to cause shorts to the bobbin or ground.
- Bundle Shape Retention: Keeps fine strands tightly bundled and uniform, preventing splaying across sharp toroid edges during winding.
- Voltage Standoff: Adds a continuous physical barrier that boosts dielectric strength and electrical isolation between coil turns and magnetic cores.
- High-Value Assembly Insurance: Protects high-cost, multi-strand winding assemblies against scrap and rework caused by handling or mechanical abrasion.
- Material Selection: Textile wraps (nylon, polyester, Nomex, silk) are matched to operating temperature classes, wall thickness budgets, and varnish compatibility.
Episode Transcript – Litz Wire: Do You Need a Serve?
Introduction to Litz Wire Serves
Good day. I’m Clayton Elliott from New England Wire Technologies, long-time Design Engineer and now Design Engineering Manager.
So you have been trying to decide: serve or don’t serve your Litz wire? Well, in order to answer that, first let’s talk about magnet wire.
Magnet Wire Pinholes and Statistical Alignment
Magnet wire is usually copper with a very thin insulation. It’s used for winding coils and making Litz wire. This thin insulation very occasionally has pinholes. They are allowed by specification. Magnet wire is intended to be used in windings of many types, and the incidence of pinholes in the single end of magnet wire does not align in the winding, does not align to ground, or does not occur often enough to impact the devices or cause any problems.
Still, many coils are protected by the bobbin or by a tape wound overall. A Litz wire is similar to a coil in that many ends are wound together, though in a special manner, and they are all in common. This makes the chance of pinholes aligning to ground much higher. I’ve kind of accentuated that these ends are all splayed over this toroid, so that you can see they may short to the bobbin.
Bundle Shape Retention and High-Value Assembly Protection
And as you can also see, served versus unserved keeps the wire more in shape.
Plus, when you make a Litz coil, you are making something from a higher-level assembly. So loss of a winding from a short is more loss of time and material than a basic magnet wire winding. Adding a textile serve protects this higher-level assembly and provides greater isolation, both mechanically and electrically, to ground.
Textile Serve Materials and Temperature Classes
There are many types of serve insulation materials. These different materials are for different temperature classes, thicknesses, and insulation system compatibility.
In summary, a serve can protect your higher-level assembly and give you more voltage standoff.
Please get in touch with New England Wire Technologies for more questions about what serve would work well on your Litz wire, or any other questions on Litz wire. Thanks for your attention today.
