High-temperature testing is a critical validation process for new energy vehicle (NEV) drive motors, verifying that winding insulation, permanent magnets, bearings, and overall motor performance can withstand the extreme thermal conditions encountered in electric and hybrid vehicle operation. This article explains the technical basis, applicable standards such as IEC 60034, GB/T 18488, ISO 16750, and IEC 60068, typical test samples, test items, methodologies, and acceptance criteria used by accredited third-party laboratories and motor manufacturers worldwide.
Technical Overview
Electric traction motors in battery electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs) operate in thermally aggressive environments. High power density designs, compact enclosures, elevated coolant temperatures, and proximity to power electronics expose the motor to sustained high temperatures, rapid thermal transients, and long-term thermal aging. High-temperature experiments are therefore essential to confirm that the motor system remains safe, reliable, and efficient throughout its design life.
High-temperature testing evaluates several critical failure mechanisms:
- Thermal aging and degradation of winding insulation systems, including magnet wire enamel, slot liners, phase insulation, and impregnation resins
- Irreversible demagnetization of neodymium-iron-boron (NdFeB) or other permanent magnets as temperature approaches the Curie point or exceeds the knee of the demagnetization curve
- Lubricant breakdown, viscosity reduction, and grease leakage in bearings operating above rated temperature
- Loss of performance, efficiency, and torque output at elevated winding and magnet temperatures
- Delamination, softening, or outgassing of potting compounds, plastics, sensors, and connectors
Relevant standards and reference frameworks include the IEC 60034 series (rotating electrical machines, including IEC 60034-1 for rating and performance and IEC 60034-18 for thermal endurance of insulation systems), IEC 60085 for thermal classification of electrical insulation, IEC 60068-2-2 for dry heat testing, ISO 16750 and its Chinese equivalent GB/T 28046 for road vehicle electrical and electronic equipment, and GB/T 18488.1 and GB/T 18488.2, which specify the general requirements and test methods for drive motor systems used in electric vehicles.
Testing Samples
Complete Drive Motor Assemblies
Full traction motor units, including stator, rotor, rotor position sensors, temperature sensors, bearings, seals, and housing, are tested as integrated systems. This approach captures thermal interactions between components and verifies system-level behavior under realistic thermal loading conditions.
Stator Cores and Winding Insulation Systems
Stator assemblies, hairpin or round-wire windings, and representative insulation system models are subjected to thermal endurance and thermal aging tests. These samples are used to verify the assigned thermal class (for example, Class F at 155 °C or Class H at 180 °C) in accordance with IEC 60034-18.
Permanent Magnet Rotors
Rotors equipped with NdFeB or samarium-cobalt magnets, either as complete rotor assemblies or as individually mounted magnet samples, are exposed to elevated temperatures to assess irreversible flux loss, magnet bond strength, and corrosion protection performance at temperature.
Components and Materials
Individual materials and sub-components may be tested separately, including slot liners, magnet wires, impregnation varnishes and resins, potting compounds, bearing greases, encoder and resolver assemblies, high-voltage terminals, and cable insulation.
Testing Items
Typical high-temperature test items for new energy drive motors include:
- High-temperature storage (dry heat endurance) at temperatures of 85 °C, 105 °C, 125 °C, 150 °C, or 180 °C per IEC 60068-2-2 and ISO 16750-4
- High-temperature operation under load, with the motor driven on a dynamometer at rated or peak torque while coolant and ambient temperatures are held at worst-case levels
- Continuous and short-time temperature rise testing to determine winding temperature rise above coolant temperature at rated load
- Thermal cycling and thermal shock between extreme low and high temperatures (for example, −40 °C to +140 °C or +180 °C) to induce controlled thermal fatigue
- Insulation resistance measurement at elevated temperature using a megohmmeter at 500 V DC or 1000 V DC
- Dielectric withstand (hipot) testing at high temperature and after thermal aging, typically at test voltages derived from the rated voltage of the insulation system
- Demagnetization resistance evaluation through back-electromotive-force (back-EMF) comparison, flux measurement, or torque constant verification before and after thermal exposure
- Performance mapping at elevated temperature, including efficiency, torque-speed characteristics, cogging torque, and no-load current
- Thermal endurance verification of the insulation system to confirm its assigned thermal class in accordance with IEC 60034-18 and IEC 60085
- Post-test visual inspection, dimensional verification, and bearing condition assessment
Testing Methods
High-temperature storage tests are performed in forced-convection thermal chambers or ovens with temperature uniformity typically maintained within ±2 °C or ±3 °C. Samples are stabilized at ambient conditions before and after exposure, and functional and dielectric checks are conducted after recovery to detect latent damage.
For temperature rise determination, the winding temperature is calculated using the resistance method. The winding resistance is measured at a known cold temperature, and the hot resistance is captured immediately after the load test. The average winding temperature is then calculated using the formula T2 = (R2 / R1) × (k + T1) − k, where k is 234.5 for copper and 225 for aluminum. The resulting value is compared against the thermal class limit of the insulation system.
Loaded high-temperature tests combine a calibrated dynamometer with programmable coolant conditioning so that the motor is exercised at specified torque, speed, and coolant temperature, typically up to 65 °C to 85 °C coolant, until thermal steady state is reached. Continuous data acquisition records winding temperatures, magnet temperatures via sensors or infrared methods, housing temperatures, power, and efficiency.
Thermal cycling tests follow defined profiles with controlled dwell times at each extreme, rapid transition rates, and a specified number of cycles, often several hundred, to simulate a full vehicle service life of repeated startup and shutdown events.
Acceptance criteria commonly require that the insulation resistance remains above the specified minimum (for example, greater than 1 MΩ or per the product standard), the hipot test shows no breakdown or excessive leakage current, the back-EMF constant or magnet flux reduction remains within the allowable limit (frequently less than 2 percent to 5 percent irreversible loss), and no mechanical damage, insulation cracking, or grease leakage is observed.
Applications
High-temperature experiments for new energy motors support a wide range of engineering and compliance objectives:
- Design validation and durability verification of traction motors for BEV, HEV, and PHEV passenger vehicles
- Qualification of e-axle and integrated drive unit (motor, reducer, and inverter) platforms
- Validation of commercial vehicle, bus, and heavy-duty truck drive motors with higher continuous thermal loading
- Testing of 48-volt belt starter generators and integrated starter generator (ISG) systems for mild hybrids
- Insulation system thermal class confirmation for new winding technologies such as hairpin (bar-wound) conductors and flat wire designs
- Supplier incoming inspection, production quality audits, and periodic type testing
- Failure analysis and root cause investigation of field returns or prototype anomalies
- Regulatory and homologation support, including test evidence packages aligned with GB/T 18488 and customer-specific specifications, generated by accredited third-party laboratories or in-house facilities
- Adaptation of motor platforms for adjacent applications such as electric two-wheelers, marine propulsion, aerospace actuators, and high-temperature industrial environments
FAQ
What temperature levels are typically used in high-temperature motor tests?
Storage temperatures commonly range from 85 °C to 180 °C depending on the insulation thermal class and installation location, with Class F (155 °C) and Class H (180 °C) systems being the most common in modern traction motors. Operating tests typically use worst-case coolant temperatures combined with full load until thermal steady state is achieved.
Which standards govern high-temperature testing of new energy drive motors?
The principal references are the IEC 60034 series, IEC 60085, IEC 60068-2-2, ISO 16750 (and GB/T 28046), and GB/T 18488.1 and GB/T 18488.2 for electric vehicle drive motor systems. UL and JIS standards may also apply depending on the target market and customer specification.
How is irreversible demagnetization detected after thermal exposure?
Demagnetization is typically quantified by comparing the back-EMF constant, magnet flux measured with a Helmholtz coil or fluxmeter, or the torque constant before and after the high-temperature exposure. A reduction beyond the specified threshold, often 2 to 5 percent, indicates irreversible magnet loss.
How long do high-temperature tests take?
High-temperature storage tests commonly run 48 to 1000 hours depending on the specification, loaded temperature rise tests continue until thermal equilibrium, which may take one to several hours, and thermal endurance verification of insulation systems can extend over weeks or months to establish a reliable thermal life curve.
Can high-temperature testing be combined with other environmental stresses?
Yes. Sequential or combined testing frequently pairs high temperature with humidity (such as damp heat at 85 °C and 85 percent relative humidity), vibration, salt mist, or voltage endurance to better replicate real-world multi-stress conditions in the vehicle under-hood or drivetrain environment.