What Are the Differences Between Cotton Covered Wire, Silk Covered Wire, Film Insulated Wire, and Enameled Wire?

In the manufacturing of electrical equipment such as transformers, reactors, and motors, the insulation treatment of winding wires is a critical factor determining product performance and reliability. Cotton covered wire, silk covered wire, film insulated wire, and enameled wire are four common types of insulated conductors, each differing in insulation materials, manufacturing processes, performance characteristics, and application scenarios. This article provides a detailed comparison of these four types of insulated wires to help you make the best choice based on specific application requirements.

 

1. Enameled Wire

Enameled wire is the most widely used type of insulated wire in transformer and reactor manufacturing. It is formed by coating copper or aluminum conductors with multiple layers of high-polymer insulating varnish.

● Manufacturing Process and Material Properties

The production of enameled wire involves a "coating-baking" process, where the conductor undergoes multiple layers of varnish application and high-temperature curing. Common insulating varnishes include:

(1)Polyester:Offers excellent mechanical strength and heat resistance (130°C).

(2)Polyurethane:Easy to solder, often used in high-frequency applications.

(3)Polyimide:Withstands temperatures above 220°C.

(4)Polyamide-imide (AI Wire):Provides balanced performance with a heat resistance of 180°C.

The molecular structure of the insulating varnish forms a cross-linked network during high-temperature curing, giving enameled wire outstanding electrical and mechanical properties. The varnish thickness typically ranges from 0.02 to 0.1 mm and can be classified according to international standards such as IEC 60317 or the U.S. standard NEMA MW 1000.

● Performance Advantages and Applications

Enameled wire excels in the following areas:

(1)High space efficiency:Thin and uniform varnish allows for compact windings.

(2)Excellent dielectric strength:Typiquement 5-10 kV/mm.

(3)Good thermal stability:Resists aging at high temperatures.

(4)Mechanical flexibility:Suitable for automated winding processes.

These properties make enameled wire the preferred choice for small to medium-sized transformers, motor windings, and inductive components, especially in high-power-density applications where space is limited.

● Technical Parameter Comparison

The table below shows key parameters of common enameled wires:

Type	Cote de température	Tension de claquage	Applications typiques	Norme de conformité
Polyester Enameled	130 ° C	≥ 5 kV/mm	General transformers	IEC 60317-13
Polyurethane Enameled	120 ° C	≥ 3 kV/mm	High-frequency coils	IEC 60317-20
Polyimide Enameled	220 ° C	≥ 7 kV/mm	Environnements à haute température	NEMA MW 35C
Composite Enameled	180 ° C	≥ 6 kV/mm	Variable-frequency motors	IEC 60317-56

2. Cotton Covered Wire

Cotton covered wire is a traditional insulated wire that uses cotton or synthetic fiber yarn wrapped around the conductor as insulation.

● Structure and Manufacturing Process

Cotton covered wire is produced using a "wrapping" process, where yarn is spirally wound around the conductor at a specific angle and tension. Multiple layers are usually applied to achieve sufficient insulation thickness (0.2–0.5 mm). To enhance insulation performance, the yarn is often impregnated with insulating oil or resin.

The insulation performance of cotton covered wire depends on:

(1)The yarn»s inherent insulating properties.

(2)Wrapping density and uniformity.

(3)The performance of the impregnating material.

 

● Features and Limitations

(1) Advantages of Cotton Covered Wire
Cotton covered wire exhibits excellent corona resistance. Its fibrous structure evenly distributes electric fields, reducing partial discharges and increasing corona inception voltage by 30–50% compared to enameled wire of the same thickness. This makes it ideal for high-frequency and high-voltage environments.

Additionally, its porous structure provides superior heat dissipation, lowering winding temperatures by 15–20°C through enhanced air convection, making it suitable for high-power-density equipment.

The thick yarn layer also offers robust mechanical protection, effectively cushioning vibrations and impacts. Even if partially damaged, the insulation does not immediately fail, making it a "fail-safe" choice for high-vibration environments like mining machinery and rail transit.

(2) Disadvantages of Cotton Covered Wire
The main drawback is low space efficiency. The thick insulation increases the wire diameter by 50–100% compared to enameled wire, resulting in a winding fill factor of only 0.3–0.5, significantly increasing device size and weight.

Another issue is moisture absorption. In 60% humidity, insulation resistance can drop tenfold, leading to fiber hydrolysis and mold growth over time, necessitating additional impregnation treatments. Manufacturing is also slower, with wrapping speeds below 50 m/min and limited automation, increasing labor costs. Complex joint handling further reduces production efficiency.

These limitations restrict cotton covered wire to niche applications, such as antique electrical repairs and high-voltage testing equipment.

 

3. Silk Covered Wire

Silk covered wire is similar to cotton covered wire but uses natural or synthetic silk fibers for insulation, primarily in high-precision and high-frequency electronic devices.

● Material Properties and Manufacturing Process

Silk covered wire employs fine silk (natural or synthetic, e.g., aramid) wrapped at high density, with insulation thickness typically ranging from 0.1 to 0.3 mm. To enhance performance, the following treatments are often applied:

(1)Degreasing:Removes impurities from silk fibers.

(2)Impregnation:Uses special resins to improve insulation.

(3)Revêtement de surface :Adds protective layers against mechanical damage.

The micro-fibrous structure of silk creates uniformly distributed tiny air gaps, giving the wire unique dielectric properties.

● High-Frequency Performance and Applications

Silk covered wire excels in high-frequency applications due to:

(1)Low dielectric loss (tanδ<0.01).

(2)Stable permittivity across frequencies.

(3)Precise capacitance control via wrapping density.

These properties make silk covered wire ideal for high-frequency transformers, RF coils, and precision measurement devices, especially where parasitic parameters must be tightly controlled.

 

4. Film Insulated Wire

Film insulated wire uses polymer films (e.g., polyester, polyimide) wrapped or extruded around the conductor, offering a high-performance solution for modern electrical equipment.

● Film Materials and Structural Design

Common film materials include:

(1)PET film: 12-50μm thick, cost-effective.

(2)PI film: Withstands temperatures above 250°C.

(3)PTFE film: Excellent chemical resistance and low friction.

The insulation structure typically includes:

(1)Inner shielding (optional).

(2)Main insulating film (single or multiple layers).

(3)Outer protective layer (anti-scratch).

 

The theoretical breakdown voltage can be calculated as:
V_bd = E_bd × d × K

Où :

V_bd: Breakdown voltage (kV).

E_bd: Material dielectric strength (MV/m).

d: Total insulation thickness (mm).

K: Structural coefficient (0.7-0.9, accounting for interface effects).

● Advantages and Applications

Film insulated wire’s high performance stems from its precision-engineered film structure. With molecularly aligned polymers and thickness control within ±1 μm, its dielectric strength reaches 15 kV/mm—three times that of enameled wire. Special treatments like plasma pre-treatment and nano-interfacing (e.g., SiO₂ doping) further enhance adhesion and heat resistance.

Multifunctional designs integrate conductive, insulating, and protective layers, while nano-fillers (e.g., Al₂O₃) improve thermal conductivity. This enables:

(1)Automotive drive motors to endure 200°C and 100,000+ hours of corona resistance.

(2)Aerospace power systems to operate in extreme temperatures (-65°C 260°C) with NASA certification.

(3)5G base station transformers to achieve ultra-low permittivity (εr = 2.3) and loss (tanδ< 0.001), minimizing signal attenuation.

5. Comprehensive Comparison and Selection Guide

To help engineers choose the right insulated wire, the table below compares key metrics:

Propriété	Fil émaillé	Cotton Covered Wire	Silk Covered Wire	Film Insulated Wire
Épaisseur d'isolation	0.02 – 0.1 mm	0.2 – 0.5 mm	0.1 – 0.3 mm	0.05 – 0.2 mm
Max Température	220 ° C	105 ° C	150 ° C	250 ° C
Résistance diélectrique	5–10 kV/mm	3–6 kV/mm	4–8 kV/mm	10–15 kV/mm
Perte diélectrique	Moyenne	Haute	Faible	Très Bas
Indice des coûts	1.0	1.5	3.0	2.0-5.0

 

En résumé

Cotton covered wire, silk covered wire, film insulated wire, and enameled wire each have unique properties and applications. Enameled wire, with its balanced performance and cost efficiency, remains the top choice for most transformer and reactor applications. Cotton and silk covered wires retain value in specialized fields, while film insulated wire excels in high-temperature, high-frequency, and high-reliability scenarios.

Advances in materials science and manufacturing-such as composite insulation and nano-modified materials-are driving innovation in winding wire technology. Engineers should consider electrical, thermal, and mechanical requirements alongside cost when selecting insulated wires, while staying updated on international standards to ensure high-performance, reliable electromagnetic designs.

 

 

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