Turn Insulation

Turn insulation is the first insulation layer in electrical machines and transformers. It electrically separates individual conductor turns from one another and prevents short circuits within the winding pack.

Put simply: turn insulation is the „coating“ around the wire, which ensures that current flows only where it should, through the winding, not between adjacent conductors.

In electric motors, generators and transformers, it is the basis for dielectric strength, efficiency and service life. Without intact turn insulation, machines would fail after a short time due to short circuit, partial discharge or thermal overload.

Structure and working principle

Turn insulation is applied directly to the conductor. Typical variants:

• Enamelled copper wire (magnet wire): standard in motors, coils and transformers. The enamel is usually a polyimide, polyesterimide or polyurethane varnish.
• Multilayer enamel systems: combinations for higher dielectric strength and thermal classes.
• Fibre or film wrappings: for high-voltage and special applications, additional tapes of Nomex, polyester or mica.
• Sleevings and extruded insulations: glass fibre sleevings, Kapton sleevings or extrusion coatings for special requirements.

Turn insulation must be thin, flexible and at the same time electrically robust. It is thus a technical balancing act between minimum size and maximum safety.

Materials and thermal classes of insulation

The selection depends on thermal class (IEC 60085), voltage level and field of application:

• Insulating enamels: polyesterimide, polyamideimide, polyurethane, epoxy, polyimide.
• Aramid paper (Nomex): as wrapping for higher voltages.
• Polyester films (Hostaphan, Mylar): mechanical stability and good insulating properties.
• Polyimide films (Kapton): very high thermal and electrical loading capacity (up to class H 180 degrees C and higher).
• Mica tapes: unmatched dielectric strength, standard in high-voltage machines.
• Glass fibre sleevings: impregnated with resins or silicone for high mechanical and thermal stress.

Typical thermal classes of insulation:

• Class B (130 degrees C)
• Class F (155 degrees C)
• Class H (180 degrees C)
• Higher classes with specialty polymers and mica composites.

Manufacture and impregnation

1. Wire enamelling: wire is passed through enamel baths and baked thermally.
2. Additional wrapping: for high voltage, taping with Nomex, polyester or mica.
3. Insertion into slots: consideration of minimum bending radius and abrasion stress.
4. Impregnation: impregnating varnish or resin fills pores and improves partial discharge resistance. Processes:
   • Trickle process
   • Vacuum pressure impregnation (VPI)
   • Resin-rich systems
5. Curing: defined temperature profile for resin or varnish.

Requirements and tests

Electrical

• High dielectric strength between adjacent turns.
• High insulation resistance.
• Absence of partial discharge at higher voltages.

Thermal

• Resistance to continuous operation in the respective thermal class.
• Resistance to ageing through thermal cycles.

Mechanical

• Abrasion resistance during winding processes.
• Resistance to vibrations and centrifugal forces in operation.

Chemical

• Resistance to oils, resins, varnishes and moisture.

Tests

• Voltage test (HiPot test).
• Dielectric strength according to IEC standards.
• Partial discharge measurement.
• Thermal ageing tests.

Applications

• Electric motors: universal in low-voltage and high-voltage motors.
• Generators: for continuous operation at high temperatures.
• Transformers: turn insulation as basic protection against winding short circuits.
• Inductive components: chokes, coils, relays.
• E-mobility (hairpin windings): special enamel and film insulation for high-voltage on-board networks.

Advantages and challenges

Advantages

• Compact construction thanks to thin enamel layers.
• High voltage and temperature resistance.
• Economic mass production.
• Long service life with correct material selection.

Challenges

• Very thin layers: sensitive to mechanical damage.
• Risk of partial discharge at high voltage.
• Dependence on varnish quality and impregnation process.
• Repairs are often not possible, a complete rewind is necessary.

GOBA Takeaway

Turn insulation is the foundation of electrical insulation systems. It determines whether a motor or transformer functions reliably or fails due to winding short circuit. Its quality depends strongly on the enamels, films and wrappings used, as well as on the impregnation process. For designers and buyers the rule applies: only matched material systems and process-reliable manufacture guarantee the required voltage and service life resistance.