DMD Insulation Materials for Dry-Type Transformers: A Comprehensive Guide

In dry-type transformers (especially in Class F 155°C systems), DMD Insulation Material is a highly cost-effective co...

In dry-type transformers (especially in Class F 155°C systems), DMD Insulation Material is a highly cost-effective composite insulating material. It consists of a layer of polyester film (M) sandwiched between two layers of polyester fiber non-woven fabric (D) with an adhesive.
The following are the specific application details of DMD as interlayer insulation:

1. Core Functions

Electrical Isolation: Primarily used between layers of high and low voltage windings to prevent interlayer voltage breakdown. The middle polyester film provides the core dielectric strength (voltage resistance).
Mechanical Protection: The outer non-woven fabric layers have excellent absorbency, allowing them to soak up insulating varnish or resin. Once cured, this integrates the coils into a solid unit, enhancing the ability to withstand short-circuit electromagnetic forces.
Physical Buffering: The non-woven fabric has a certain degree of loft, which protects the polyester film from being scratched by rough wire burrs during the winding process.

2. Common Specifications

Thickness: Typical interlayer thickness ranges from 0.15mm to 0.35mm (e.g., DMD100, DMD180).
Thermal Class: Standard DMD is Class F (155°C). If the transformer is designed for Class H (180°C), it usually needs to be upgraded to a modified version (such as prepreg DMD) or replaced with Nomex®.

3. Performance in Different Processes

Conventional Impregnated (Non-encapsulated): The non-woven layers of DMD fully absorb the impregnating varnish. After curing, this provides better thermal conductivity and reduces internal air gaps, thereby suppressing partial discharge.
Prepreg (Pre-impregnated DMD): This type of DMD comes pre-coated with B-stage resin. When the winding is heated after assembly, the resin in the DMD melts and bonds with the coils, eliminating the need for overall post-dipping and creating a compact structure.

4. Precautions for Use

Folding and Damage: Although DMD is tough, sharp folding can cause the internal polyester film to crack, leading to a significant drop in insulation performance.
Hygroscopicity (Moisture Absorption): The non-woven layers easily absorb moisture. If the transformer is stored in a humid environment, the DMD may soak up water; therefore, thorough pre-drying is required before casting or varnishing.
Creepage Distance: When used as interlayer insulation, the DMD width is usually slightly wider than the winding width (leaving a margin at the edges) to prevent flashover at the ends.

Comparison Suggestion:

If your design temperature rise is higher (exceeding 155°C), DMD may undergo thermal aging and become brittle. In such cases, it is recommended to consider Nomex® paper or NMN (Nomex-Mylar-Nomex) composites.