The performance of electric motors depends on the type of insulation paper used in its windings. If the thickness of paper in the winding insulation layer is thin enough to cause a short circuit in the windings, it could be the cause of an electric motor working only for months instead of years. It is one of the crucial steps in motor manufacturing that is not given sufficient importance by many people.
In this guide, we discuss different types of insulation paper used in electric motor winding, their temperature class, and how to select one suitable for your requirement.
Why Insulation Paper Matters in Motor Winding
Motor windings generate heat, vibration, and electrical stress during operation. The insulation paper placed between conductors, slots, and phases has to withstand:
- Thermal stresses due to constant operating temperature and short term overloads.
- Dielectric stresses due to voltage differential between windings and the body of the motor.
- Mechanical stresses due to vibrations, heat and winding tension.
- Chemical stresses due to varnish, oil and moisture.
Paper insulations are graded into five thermal classifications: E (120°C), B (130°C), F (155°C), H (180°C), and C (above 180°C). Choosing the right classification is the initial step even before the comparison of different papers.
1. Aramid (Nomex-type) Insulation Paper
Aramid paper has been set as the standard material for motor winding insulation, especially for Class H (180°C) applications. The advantages of aramid paper include high dielectric strength, high mechanical resistance, and heat resistant, hence its suitability for traction motors, servo motors, and high voltage rotating machines.
There are two types of aramid paper, calendered and uncalendered:
- Calendered aramid paper this has high density, smoothness and high dielectric strength, hence perfect for use as slot liners and as phase separators.
- Uncalendered aramid paper This has high porosity hence allows deep impregnation with resin and varnishes. Hence, improving bonding and thermal conductivity.
Some of the types of aramid papers used are DuPont Nomex and Metastar aramid papers, calendered and uncalendered such as Pure Nomex Aramid Paper 410 and Uncalendered Aramid Paper YT 511.
2. NMN Insulation Paper (Aramid + Polyester + Aramid)
The NMN paper is a laminate composite paper wherein a polyester film layer is put between two aramid paper layers. With this design, motor manufacturers can enjoy the benefits of using the dielectric property of polyester film while having the thermal and mechanical resistance of aramid paper on both sides.
NMN Insulation Paper is extensively utilized in Class H motor windings because it has a good balance of cost, dielectric property, and thermal property especially in slot insulation, phase paper, and ground wall insulation of medium to large motors.
3. NHN Insulation Paper
NHN Insulation Paper is yet another aramid laminate that utilizes aramid paper along with a heat-resistant film or another layer of aramid material and is used for purposes requiring good thermal stability in addition to NMN. It is mostly recommended for use in motors and generators exposed to constantly high temperatures like steel mills and cement industries.
4. AHA Insulation Paper (Laminated Aramid + Polyimide)
In very difficult thermal conditions, for applications classified as Class C, having temperatures greater than 180°C, the AHA Insulation Paper consists of aramid paper laminated onto a polyimide film core. The polyimide film is known to maintain its dielectric and mechanical characteristics under extreme temperatures (up to 240°C or more).
5. Slot Insulation Paper (FDR / Garlam / Grafilex / Triplex)
Slot Insulation Papers are multi-layered lamination products designed with the intention of lining the slots of motors and generators so as to insulate the copper windings from the laminated steel core. Such types of slot insulation papers are usually made from combinations of Polyester films, Presspaper and Mylar, referred to as FDR, Garlam, Grafilex or Triplex, depending on the composition of each particular brand.
The slot papers have to be flexible enough so as to be able to withstand punctures and to fit into the slots of the motors without breaking apart.
6. Insulation Kraft and Crepe Papers
In case of thermal classes E and B, kraft and crepe insulating paper prove economical and practical choices. The kraft paper acts as a good dielectric material for use in general purpose transformers and low voltage motors, whereas crepe paper with its flexible and wrinkled nature serves best in those areas where it needs to be wrapped around coils.
7. Diamond Dotted Paper (DDP)
The Diamond Dotted Paper is a specific type of insulation which is produced by laminating two layers of polyester film or press paper using epoxy or polyester as an adhesive, where the adhesive forms a pattern of dots in diamond shape. In this way, air and varnish can penetrate the area that remains uncoated, but mechanical bonding takes place at the dots.
DDP is generally used for slot insulation and phase insulation in motors, generators, and transformers.
8. Polyester Film-Based Insulation
Though polyester is not technically “paper,” it is often laminated with paper as well as used as phase and slot insulation in Class B and F motors. Polyester film provides good electrical strength, dimensional stability, and moisture resistance at a lower price than aramid.
How to Choose the Right Insulation Paper for Your Motor
When selecting insulation paper for motor winding, consider:
- Class of operating temperature – Select the class of temperature of the paper (E, B, F, H, C) according to the operating temperature of the motor.
- Class of voltage – High voltage motors need papers with higher dielectric breakdown voltage and laminates such as NMN or AHA.
- Handling method – Automatic winding lines may demand tougher papers, while hand wound coil motors will require more flexible papers.
- Impregnation – Impregnation quality of uncalendered papers and diamond dotted papers is better.
- Environment of usage – If the motor is subjected to oil, moisture, or any other chemicals, the paper must have additional resistances.
There is no single best insulating paper for the winding of a motor since the selection of an appropriate material will depend on the thermal class, voltage ratings, mechanical needs, and budget available. Be it the inexpensive Kraft paper and Crepe paper used in Class E/B motors to expensive materials like Aramid, NMN, NHN, and AHA laminates which are used in Class H and Class C motors respectively, each material has its own unique purpose for ensuring the efficiency of the motor over time.
Harnawa Insulations is one of the top manufacturers of electrical insulation materials with more than 25 years of experience and ISO 9001:2015, UL, CE, and RoHS certification for all its products.
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