Do you know why electric motors need insulation? What are the key characteristics of motor insulation material?
a. Excellent corona resistance
b. Excellent electrical insulation
c. Great mechanical strength, especially high bonding strength
d. High thermal resistance (rated ≥180°C)
e. High thermal conductivity
f. Resistance to ATF oil and/or water
g. Halogen-free flame retardancy (to reduce fire risk), and good resistance to temperature shock (to adapt to climate changes)
What insulation materials are commonly used in automotive motors?
In new energy drive motors, common insulation materials include slot insulation paper, coating powder, injection molding, enameled wire, insulating varnish, potting compounds, and insulating sleeves.
Slot Insulation Paper
The most commonly used slot insulation papers are DuPont Nomex aramid paper and composite papers.
NOMEX 710/410 type aramid polyamide is a widely used insulation paper. It can be used as pure paper or layered composite paper. The composite paper has a multi-layer sandwich structure. The selection depends on operating conditions such as motor temperature, oil or water cooling, insulation level, and process.
Nomex 410 pure paper (oil-resistant)
NMN composite paper (cost-effective, not oil-resistant)
N: DuPont Nomex 410 insulation paper
M: PET polyester film
N: DuPont Nomex 410 insulation paper
NHN composite paper (high performance, not oil-resistant)
N: DuPont Nomex 410 insulation paper
H: PI polyimide film
N: DuPont Nomex 410 insulation paper
NSN composite paper (oil-resistant)
N: DuPont Nomex 410 insulation paper
S: PPS polyphenylene sulfide film
N: DuPont Nomex 410 insulation paper
NKN composite paper (oil-resistant)
N: DuPont Nomex 410 insulation paper
K: DuPont Kapton film (special PI)
N: DuPont Nomex 410 insulation paper
Slot insulation paper is usually tested for performance with the parameters shown in the following table.
| Item | Typical Value |
| Longitudinal Tensile Strength | 280 N/cm |
| Transverse Tensile Strength | 140 N/cm |
| Elongation in Machine Direction | 20% |
| Elongation in Cross Direction | 15% |
| Dielectric Strength at 20°C | 7 kV |
| Performance at Elevated Temperature | Tested at 200°C for 1 hour |
| Cleanliness Test | — |
| Oil Resistance Test | — |
Epoxy Resin
Epoxy resin is a primary material for motor powder coating. It is widely used in both stator slot and end-face insulation, as well as post-winding insulation for flat wire motors.
Common stator laminate insulation epoxy types include JY231(class F insulation) and 360ESD(class H insulation).
Flat wire winding ends often use brands like Solmalon.
Key characteristics of epoxy resin:
High heat resistance: Withstands up to 150°C, supports high-power-density motor cooling needs.
Strong mechanical strength: Hard, wear-resistant, and resists damage from vibration or stress.
Excellent dielectric properties: Stable insulation, suitable for high-voltage motors (e.g., 800V systems).
Limitations: Risks of chloride ion content, poor low-temperature performance, high curing shrinkage. Process improvements or modified resin are needed to overcome these.
Injection Molding
High-volume production lines, especially automated ones, often use integrated core injection molding. It improves cleanliness and enhances NVH performance in motors.
PA66 (Polyamide 66) is the most widely used material. It is a high-performance engineering plastic.
key properties:
Melting point: 252°C; decomposition temperature >350°C
Long-term heat resistance: 80–120°C, up to 250°C with glass fiber reinforcement
Density: 1.15 g/cm³; water absorption 2.5% (affects dimensional stability)
Strength: High rigidity and toughness; tensile strength above 80 MPa; wear and impact resistance
Chemical resistance: Withstands acids, alkalis, salts, and most organic solvents (except phenol and formic acid)
Oil resistance: Suitable for lubricated mechanical parts
Enameled Wire
Round copper coil wire in motors must meet voltage resistance levels, such as 500V or 1000V.
As drive motors run faster and EV systems increase in power density, the requirements for enameled wire performance are also increasing. Today, motors mostly use H-class (or higher) nano-particle-modified corona-resistant enameled wires.
Early designs used triple-layer coatings, but these had poor adhesion and short life.
The industry has shifted to dual-layer systems.
In 2000, DuPont launched a dual-layer corona-resistant enameled wire. The base coat is nano-modified polyester-imide; the top coat is PI and AI. Since launch, dual-layer wires have been widely used in drive motors. With the rise of oil-cooled motors, single-layer PI and AI enameled wires have grown in popularity due to their excellent oil and heat resistance.
Insulating Varnish
For round-wire stators, common insulation processes include vacuum impregnation (VI) and vacuum pressure impregnation (VPI). For flat-wire stators, drip varnish is usually used.
These varnishes are based on nano-modified resin systems. The base is modified polyester or polyester-imide. Nano additives enhance paint retention, heat resistance, and corona resistance.
Key features:
High paint retention
Strong bonding strength
Heat resistance: H (180°C) or C (200°C) insulation class
Corona and ATF oil resistance
In recent years, new resin systems have emerged that support winding heating cure, UV curing, and rotary drip varnish.
Among these, winding-current heating cure is the most efficient.It takes only a few minutes to reach gel state, and the full process can be done within an hour. This improves energy efficiency and production speed.
Insulating Sleeves
EV drive motors often use insulating sleeves over low-voltage wiring. These sleeves provide mechanical protection and insulation. Sleeves may be heat-shrink or non-heat-shrink, depending on wire fixing needs.
Material choices include silicone rubber and fluororubber. Selection depends on motor thermal rating and cooling method.
Oil-cooled motors should use fluororubber sleeves. Silicone rubber may swell when soaked in cooling oil, breaking insulation and causing failures.
Glass fiber sleeves are also common. They have excellent thermal and mechanical properties. However, some cooling oils may cause slight surface precipitation. Always do oil compatibility tests before use.
Potting Compounds
Potting is used for motor winding end encapsulation. The potting compound must strongly bond to windings and withstand thermal shocks and mechanical stress.
It should have high thermal conductivity to reduce surface temperature differences. This improves heat transfer and reduces internal stress.
Several potting products are available for EV motor use, such as Taihu TH-908, Rongtai 381-E, ELANTAS C88/E89, and Liding LD-107A/B.
Typical properties:
Thermal conductivity around 1.0 W/m·K; some up to 2.0 W/m·K
Low thermal expansion: ~3.5×10⁻⁶/°C
Good flowability: 400–800 mPa·s @ 90°C
UL94 V-0 flame retardancy
Do you now have a clearer view of the types and properties of motor insulation materials?
Insulation materials determine motor safety and reliability. They also affect lifespan and performance. Choosing the right materials for different conditions is critical in motor design and production.
Motor Insulation – Choose Lamnow
Lamnow is a motor lamination core manufacturer that provides motor insulation services. Whether it is epoxy coating, injection molding, slot insulation for stator cores, enameled wire and heat shrink tubing for windings, or varnishing after winding, we provide a full range of insulation to meet your motor needs.
In addition, we also perform various tests on motor insulation, such as withstand hitop test, resistance test, corona resistance test, etc. If you need motor stator and rotor core services, please feel free to contact us.
