The first question asked when choosing a conductor for a winding is usually this: "Is Class 180 enough, or should I move up to Class 200?" The answer cannot be given without knowing what the class number means. Contrary to popular belief, the thermal class is not the "highest temperature the conductor can withstand"; it is a temperature index based on a defined thermal-life assumption.
How Is the Thermal Class Defined?
The thermal classification of insulation materials is done under IEC 60085. For enamelled conductors, the temperature index that forms the basis of the class is determined by the IEC 60172 method: samples are aged at different temperatures, their electrical strength is measured at set intervals, and from the resulting life-temperature curve the temperature corresponding to the accepted life is read. So a "Class 200" conductor is qualified not for an instantaneous 200 °C, but to preserve its insulation function throughout long-term service.
The class number should therefore be read not as a "red line" but as a reference around which the design is built.
Common Classes and Typical Film Structures
| Class | Letter | Temperature index | Typical film structure | Typical application |
|---|---|---|---|---|
| 130 | B | 130 °C | Polyurethane / modified polyester | Small coils, relays, voice coils |
| 155 | F | 155 °C | Polyester | General-purpose motor and transformer windings |
| 180 | H | 180 °C | Polyesterimide | Industrial motors, dry-type transformers |
| 200 | N | 200 °C | Polyesterimide + polyamide-imide top coat | High-load motors, generators, wind turbines |
| 220 | R | 220 °C | Multi-layer film, predominantly polyamide-imide | Compact traction motors, e-mobility |
| 240 | S | 240 °C | Polyimide | Aerospace, special high-temperature windings |
The double-layer structure seen in the table matters: in Class 200 and 220 products the base layer provides flexibility and adhesion, while the polyamide-imide film on top raises abrasion resistance and thermal stability. This is why two products of the same class can behave differently in winding.
How Is Winding Temperature Accounted For?
To find the correct class, the real temperature the conductor will see must be thought of as the sum of three components:
- Ambient temperature: The environment in which the machine operates. Inside panels, in enclosed volumes or in hot climates, this value is higher than expected.
- Winding temperature rise: The average rise due to copper losses and cooling efficiency.
- Hot-spot allowance: The hottest region of the winding stays above the average winding temperature; this difference is added separately in the design.
If the sum of the three components approaches the index of the chosen class, moving up one class is necessary. A large gap is not always good news either: an unnecessarily high class can bring film thickness and cost increases.
Temperature and Life: The Ten-Degree Rule
The thermal ageing of insulation materials is a chemical process and accelerates exponentially with temperature. The practical rule widely used in the industry says: an increase of about 8–10 °C in the continuous operating temperature roughly halves the expected life of the insulation. This relationship explains why the choice of thermal class is so decisive.
In numbers: a winding designed with a Class 155 insulation system, running continuously 20 °C above the intended level, drops to a quarter of its life. The reverse also holds — lowering the winding temperature by 10 °C through better cooling roughly doubles the life. The thermal-class discussion therefore cannot be separated from the cooling-design discussion.
This rule is not a design formula but an order-of-magnitude approach. The precise assessment must be made using the product's IEC 60172 temperature index and the real load profile of the application.
Three Common Mistakes
1. Taking the class as an absolute temperature limit
The class index points to continuous service, not to a short-term overload temperature. The overload scenario must be assessed separately.
2. Forgetting the whole system
The thermal class of the winding is not set by the conductor alone; the impregnating varnish, slot insulation, inter-phase separator and connection materials must support the same class. The weakest link in the chain sets the class of the system.
3. Ignoring the frequency converter
In inverter-fed motors, fast-rising impulses create partial-discharge stress on the film. Here the decisive factor is not only the thermal class but the film thickness and impulse resistance.
Summary
- The thermal class rests on the IEC 60085 classification and the IEC 60172 temperature index.
- Class selection is based on the sum of ambient + winding rise + hot-spot allowance.
- Class 200 and 220 products usually have a multi-layer film structure.
- The winding's thermal class must be supported by all components of the system.
Our technical team can support you in determining the class suited to your application.
