Insulation Class of High-Voltage Three-Phase Asynchronous Motors
The insulation class of high-voltage three-phase asynchronous motors is a core technical indicator defined based on the maximum allowable operating temperature of the insulation materials. It directly determines the motor's operational reliability, service life, and suitability for different working conditions. The classification standards follow the International Electrotechnical Commission (IEC) standard IEC 60085:2019 Electrical Insulation – Thermal Evaluation and Designation and the Chinese national standard GB/T 11021-2014 Electrical Insulation – Thermal Classification (which is identical to IEC 60085). Additionally, the temperature rise limits of motors must comply with GB 755-2019 Rotating Electrical Machines – Rating and Performance and IEC 60034-1:2017 Rotating Electrical Machines – Part 1: Rating and Performance.
I. Core Definition and Classification Logic of Insulation Class
The classification of insulation class is primarily based on the maximum allowable temperature at which the insulation system can operate over the long term without thermal aging failure. This temperature comprises two components:
Maximum heat resistance temperature of the insulation material(T<sub>max</sub>) – thehighest critical temperature at which the insulation material maintainsits insulating properties. Exceeding this temperature accelerates agingand shortens the motor’s service life.
Temperature rise limit of the motor (ΔT) – the difference between the winding temperature duringoperation and the ambient temperature (standard ambient temperature is40°C), determined by the insulation class.
Due to their high operating voltages (typically 3 kV, 6 kV, 10 kV), high winding losses, and greater heat dissipation challenges, high-voltage three-phase asynchronous motors rarely use low heat-resistance insulation classes. The main classes applied are Class B, Class F, and Class H, with Class C used in some special operating conditions.
II. Common insulation classes and technical parameters for high-voltage three-phase asynchronous motors
Insulation Class | Maximum allowable temperature Tmax (°C) | Standard temperature rise limit ΔT (K) (measured by resistance method, ambient temperature 40°C) | Typical insulation material system | Applicable scenarios for high-voltagemotors |
Class B | 130 | 80 | Mica flakes, glass fiber cloth + phenolic resin binder; epoxy resin impregnated glass fiber insulation structure
| Mainly used in low-voltage motors. In high-voltage motors, it is only applicable for niche scenarios with light load, low loss, and natural cooling, and has gradually been replaced by Class F.
|
Class F | 155 | 105 | Modified epoxy resin, polyesterimide resin impregnated mica tape; glass fiber reinforced polyester film composite insulation
| Mainstream insulation class for high-voltage three-phase asynchronous motors, suitable for general industrial drives such as fans, pumps, and compressors, balancing reliability and cost.
|
Class H | 180 | 125 | Silicone rubber resin, polyimide resin impregnated mica tape; polyimide film-mica composite insulation; glass fiber reinforced silicone insulation structure
| Used in high-load, high-temperature environments, such as metallurgical rolling mill motors, mine crusher motors, high-temperature kiln induced draft fan motors; also the preferred class for high-voltage variable frequency speed regulation motors.
|
Class C | ≥200 (no clear upper limit, determined by material properties)
| No fixed limit value, determined by product design verification | Polyimide film, ceramic fiber, quartz fiber insulation; inorganic binder cured insulation structure | Special extreme working conditions, such as high-temperature furnace motors and motors for aerospace and military applications. Very rarely used in high-voltage civil motors. |
III. Explanation of Key Technical Points
Relationshipbetween insulation class and motor service life
The thermal aging of insulation materials follows Arrhenius' law: forevery 10°C increase in temperature, the insulation life is approximatelyhalved. For example, if a Class F insulated motor operates for extendedperiods at temperatures exceeding 155°C, its service life drops sharply;whereas Class H insulation maintains stable performance at highertemperatures.Standardequivalency and verification points
IEC 60085 and GB/T 11021 are identically adopted (equivalent) standards, with fully consistent clauses, allowing direct cross‑verification.
GB 755-2019 explicitly states that the winding temperature rise limit for high-voltage motors shall be determined based on the insulation class and cooling method (e.g., IC411 self‑ventilated, IC611 forced air cooling). For actual selection, refer to the specific clauses of the standard.
Certificationrequirements for high-voltage motor insulation systems
The insulation system of high-voltage three-phase asynchronous motors mustbe verified through type tests, including thermal endurance tests,dielectric strength tests, damp heat tests, etc., to ensure that theinsulation performance under rated conditions complies with the standards.Insulation systems of high-voltage motors from international brands suchas ABB, Siemens, and Toshiba have passed certifications according tospecialized standards like IEC 60034-18-31.
IV. Selection Principles
General industrialapplications (fans, pumps, conveyors):
Class F insulation is preferred, offering the best cost‑performance ratio.High‑load, high‑temperature,variable frequency drive applications (steel rolling, mining,petrochemical):
Choose Class H insulation to enhance motor reliability.Special extremehigh‑temperature applications:
Custom Class C insulated motors are required, along with dedicated coolingsystems.
