1 Introduction
Thermal overload is one of the most common electrical stresses faced by winding equipment such as motors, transformers, and inductors during operation. When the winding current exceeds the rated value, the copper loss increases sharply, causing the winding temperature to rise. This overheating state, exceeding the enameled copper wire insulation temperature, is called thermal overload. Thermal overload will significantly accelerate the aging of the enameled copper wire insulation, shorten the service life of the winding, and may even lead to instantaneous insulation breakdown and equipment burnout.
A thorough understanding of the impact mechanism of thermal overload on enameled copper wire is of great significance for wire selection, winding design, overload protection configuration, and operation and maintenance. This paper, based on standards such as NEMA MW 1000-2018, IEC 60317 series, IEC 60034-1, and IEEE 1776, systematically analyzes the causes of thermal overload, the thermal aging mechanism of enameled copper wire, thermal life assessment methods, overload protection strategies, and engineering improvement measures.
2 Causes of Thermal Overload
The root cause of thermal overload is that the winding current exceeds the design rating, leading to unexpected copper losses and temperature rise. Common causes include:
Excessive mechanical load is a direct cause of thermal overload. When the load driven by the motor exceeds its design capacity, the motor output torque increases, the stator current rises, copper losses increase, and the winding temperature rises. Short-term mechanical overload can cause the winding current to reach 1.2 to 2 times the rated value within minutes.
Abnormal voltage is also a common cause of thermal overload. When the grid voltage is too low, the motor increases the current to maintain output power; when the grid voltage is too high, the increased iron loss indirectly leads to an increase in winding temperature. Three-phase voltage imbalance can cause negative sequence current, resulting in additional copper losses.
Failure of the ventilation and cooling system is a cause of latent thermal overload. Problems such as fan damage, heat sink blockage, reduced coolant flow, and increased coolant oil temperature can all weaken heat dissipation capacity, causing thermal overload in the windings even under rated load.
Frequent starts and stalling are causes of dynamic thermal overload. When a motor is started directly, the starting current can reach 5 to 7 times its rated value, and the stall current can reach 5 to 10 times its rated value. Under frequent start-stop conditions, the winding temperature fluctuates repeatedly in a short period, accelerating thermal fatigue.
Harmonic currents and the skin effect are significant in variable frequency drive systems. The high-frequency harmonic currents output by the PWM inverter generate additional copper losses in the conductor. The skin effect causes the current to concentrate on the conductor surface, increasing the local current density and thus increasing copper losses.
3 enamel coating Thermal aging mechanism
3.1 Chemical Processes of Thermal Aging
Under long-term thermal stress, enamel coatings undergo complex chemical changes. Oxidative degradation of the molecular chain is the core process: upon contact with oxygen, the CH bonds on the main chain of the enamel coating are oxidized to form peroxides. These peroxides decompose to produce free radicals, which further attack the molecular chain, leading to chain breakage and a decrease in the molecular weight of the enamel coating. Polyester and polyurethane enamel coatings are more sensitive to oxidative degradation; polyimide and polyamide-imide enamel coatings, due to the presence of aromatic ring structures in their main chains, exhibit better resistance to oxidative degradation.
Hydrolysis of ester, amide, and imide bonds is another important process. In humid environments, water molecules attack the polar functional groups in the enamel coating, leading to bond breakage and a decrease in molecular weight. Polyester enamel coatings, due to their higher ester bond content, experience more significant hydrolytic aging.
Copper ion catalysis is a unique mechanism in the thermal aging of enamel coatings. At high temperatures, copper ions diffuse from the conductor surface into the enamel coating, acting as a catalyst to accelerate its oxidative degradation. Copper ion-catalyzed oxidative degradation can shorten the enamel coating’s lifespan by 30% to 50%.
3.2 Physical Changes During Thermal Aging
The enamel coating undergoes significant physical changes during thermal aging. Its color gradually changes from light yellow and brownish-yellow to dark brown and black; the surface loses its luster, becomes brittle, and is easily peeled off; the coating thickness decreases due to the release of volatiles; the enamel coating peels off from the conductor interface; and microcracks and pores develop inside the coating.
The physical changes caused by the thermal aging of the enamel coating lead to a decline in dielectric properties. The dielectric breakdown voltage decreases by 30% to 70%; the dielectric loss tangent increases by 1 to 2 orders of magnitude; the volume resistivity decreases by 1 to 2 orders of magnitude; and the insulation resistance decreases significantly.
3.3 Correspondence between thermal aging and thermal level
The thermal rating temperature of an enamel coating is a core indicator of its heat resistance performance. The thermal rating temperature refers to the temperature at which the enamel coating achieves a design life of 20,000 hours. NEMA MW 1000-2018 specifies the following thermal rating systems for enamel coatings: 105 (Class A), 120 (Class E), 130 (Class B), 155 (Class F), 180 (Class H), 200 (Class N), 220 (Class R), 240 (Class C) and above.
According to Montsinger’s rule of thumb, for every 10 K increase in operating temperature of enameled wire, the insulation life is approximately halved. Conversely, for every 10 K decrease in operating temperature, the insulation life is approximately doubled. During the design phase, enameled wire with a thermal rating 10 to 20 K higher than the actual operating temperature should be selected to cope with the cumulative effects of short-term thermal overload.
4 The Effect of Thermal Overload on Enameled Copper Wire
4.1 Effects of Short-Term Thermal Overload
Short-term thermal overload refers to an overload condition lasting from a few seconds to tens of minutes. The main effects of short-term thermal overload on enameled copper wire are: the winding temperature exceeds the thermal stage temperature of the enamel coating within a short period of time, accelerating the thermal aging of the enamel coating; the copper conductor resistance increases with temperature, further increasing copper losses and forming positive feedback; the difference in the coefficient of linear expansion between the enamel coating and the copper conductor leads to thermomechanical stress, and frequent short-term overloads can cause the enamel coating to peel off.
Typical scenarios for short-term thermal overload include: direct motor start-up or reverse rotation, where the stall current can cause the windings to reach 150 to 200 degrees Celsius within 5 to 30 seconds; short-term motor overload, operating at 1.2 to 1.5 times the rated current for 10 to 60 minutes; short-term transformer overload, operating at 1.3 to 1.5 times the rated capacity for tens of minutes; and frequent motor start-stop, where the winding temperature fluctuates repeatedly under the impact of the starting current.
According to IEC 60034-1, the motor should be able to operate for a short time at 1.2 times the rated current, with the corresponding short-time temperature rise not exceeding the enamel coating thermal grade temperature. In actual engineering, 1.2 times overload for 10 to 30 minutes is the typical operating limit of the motor.
4.2 Effects of Long-Term Thermal Overload
Long-term thermal overload refers to an overload condition lasting from several hours to several years. The effects of long-term thermal overload on enameled copper wire are mainly manifested in the following ways: The enameled coating operates continuously at high temperatures close to or exceeding the thermal stage, accelerating the rate of molecular chain oxidation and degradation; the cumulative effect of thermal oxidation causes a continuous decrease in the molecular weight of the enameled coating, leading to a gradual degradation of dielectric strength; under long-term thermomechanical stress, the enameled coating develops microcracks and interface peeling, resulting in mechanical strength degradation; and aging products of the enameled coating deposit at the conductor interface, forming a weak layer with low dielectric strength.
Typical scenarios for long-term thermal overload include: the motor running under heavy load for a long time, with the winding temperature exceeding the thermal level of the enamel coating by 10 to 20 K for an extended period; the transformer running under long-term overload, with the winding hot spot temperature remaining consistently high; the aging of the ventilation and heat dissipation system leading to a decrease in heat dissipation capacity, but the load not being reduced accordingly; and the ambient temperature remaining consistently high, resulting in an accumulation of winding temperature rise.
Prolonged thermal overload will significantly shorten the actual service life of enameled wire. According to the Arrhenius model, for every 10 K increase in operating temperature above the thermal stage temperature, the service life of the enameled wire is reduced by about half; for every 20 K increase, the service life is reduced to about a quarter. The cumulative effect of long-term thermal overload can reduce the actual service life of enameled wire from 20 years to 5 to 10 years.
4.3 Effects of Instantaneous Thermal Overload
Transient thermal overload refers to an overload condition lasting less than a few seconds, such as a short-circuit current surge. The effects of transient thermal overload on enameled copper wire are mainly manifested as follows: the short-circuit current can cause the winding to reach 200 to 300 degrees Celsius within 0.1 to 1 second; the enamel coating undergoes transient thermal breakdown at such high temperatures; the enamel coating melts, blisteres, and peels off instantaneously; and the copper conductor anneals at high temperatures, resulting in a decrease in mechanical strength.
Protection against instantaneous thermal overload relies primarily on the rapid disconnection of protective devices such as fuses and circuit breakers. These devices should disconnect the circuit before the enamel coating reaches its instantaneous breakdown temperature to prevent irreversible damage to the windings.
5 Thermal Life Assessment Methods
5.1 Arrhenius Accelerated Aging Model
The core of thermal lifetime assessment is the Arrhenius accelerated aging model. This model assumes that enamel coating aging is a chemical reaction process, and the relationship between the aging rate and temperature conforms to the Arrhenius equation:
Lifetime L equals a constant A multiplied by an exponential function exp, where the exponent is the activation energy E divided by the product of the gas constant R and the absolute temperature T.
Where A is a constant, E is the activation energy, R is the gas constant, and T is the absolute temperature. By conducting accelerated aging tests at ≥ 3 temperature points to determine the failure time of the enamel coating, the design life at the actual operating temperature can be extrapolated.
According to IEEE 1776 and IEC 60216, accelerated thermal aging tests should be conducted at three or more temperature points, each lasting until the enamel coating fails or a predetermined endpoint is reached. The test data are plotted as Arrhenius curves, and extrapolation yields the temperature corresponding to a 20,000-hour lifespan, i.e., the thermal stage temperature.
5.2 Short-term overload cumulative damage model
The cumulative damage from short-term thermal overload can be evaluated using the Miner linear cumulative damage model. This model accumulates the damage generated by each overload event proportionally, and the enamel coating is considered to have failed when the accumulated damage reaches 1.0.
The Miner model is suitable for evaluating the cumulative effects of frequent short-term overloads and is an important tool for setting motor overload protection. In practical applications, cumulative damage should be calculated based on parameters such as the enamel coating thermal level, winding thermal time constant, and typical load cycles.
5.3 Winding Hotspot Temperature Monitoring
Winding hotspot temperature is a key parameter for assessing the effects of thermal overload. Hotspot temperature can be measured using temperature sensors, embedded thermocouples, infrared thermal imaging, etc. The winding temperature rise limits specified in IEC 60034-1 are average values; hotspot temperatures are typically 10 to 15 K higher than the average. Winding hotspot temperatures exceeding the enamel coating thermal stage temperature will lead to significantly accelerated aging.
Modern motors and transformers are often equipped with online temperature monitoring systems to provide real-time feedback on winding hot spot temperatures and provide data support for thermal overload protection.
6 Overload Protection Strategy
6.1 Types of Overload Protection Devices
There are various types of motor overload protection devices. Inverse-time overcurrent relays adjust their breaking time according to the current magnitude; the larger the current, the faster the breaking time, making them suitable for continuous overload protection. Thermal relays operate based on the principle of bimetallic strip bending under heat, suitable for overload protection of small and medium-sized motors. Electronic overload protectors are implemented using current transformers and microprocessors, offering high accuracy and configurability, suitable for large motors. Temperature sensor protection operates based on winding temperature signals, directly monitoring hotspot temperatures, offering the highest accuracy.
Transformer overload protection devices include oil level temperature protection, winding temperature protection, and pressure relief protection. Oil-immersed transformers achieve overload protection by monitoring oil level and winding temperatures. Dry-type transformers directly monitor hot spot temperatures using winding temperature sensors.
6.2 Overload Protection Setting Principles
Overload protection settings should balance equipment protection with operational continuity. If the set current is too low, malfunctions may occur under normal operating conditions, affecting production continuity. If the set current is too high, protection may fail, and cumulative thermal overload damage may lead to premature equipment failure.
Setting principles: The protection action time should be less than the thermal life of the enamel coating under overload temperature; the protection setting value should take into account the enamel coating thermal level, winding thermal time constant, and typical load cycle; after setting, on-site testing should be conducted for verification.
6.3 Thermal Overload Protection at the Design and Operational Levels
Design aspects: Select enameled wire with a thermal stage temperature 10 to 20 K higher than the actual operating temperature; optimize winding heat dissipation design to ensure thermal balance; configure comprehensive overload protection devices.
Operational aspects: Avoid prolonged overload operation; regularly inspect the ventilation and heat dissipation system; regularly calibrate overload protection devices; establish operational records and analyze thermal overload history and aging trends of the enamel coating.
7 Engineering Improvement Measures
7.1 enameled wire Selection Optimization
Optimization of wire selection for thermal overload scenarios includes: using wires with higher thermal class, such as Class 180 (H) or Class 200 (N), instead of Class 155 (F); using enamel coating systems with excellent thermal shock resistance, such as polyester imide or polyamide imide; using fiberglass-coated wires, as the fiberglass layer can effectively suppress enamel coating peeling caused by thermomechanical stress and improve thermal overload tolerance; and using enamel coatings with excellent resistance to thermal oxidation to reduce the catalytic effect of copper ions.
7.2 Winding Design Optimization
Thermal overload protection at the winding design level includes: optimizing conductor cross-sectional area, reducing current density, and reducing copper losses; optimizing winding structure design to improve heat dissipation efficiency; using vacuum pressure impregnation process to improve the overall thermal conductivity of the winding; and adding temperature sensor configuration to achieve real-time monitoring of hot spot temperatures.
7.3 Cooling System Optimization
Optimizing the heat dissipation system can significantly improve the thermal overload resistance: optimized fan design for air-cooled motors increases airflow and air pressure; optimized cooling water circuit design for water-cooled motors ensures uniform distribution of cooling water; optimized heat dissipation channel design for dry-type motors improves natural cooling efficiency; and regular cleaning of heat sinks and inspection of cooling medium quality are essential.
7.4 Operation and Maintenance Optimization
Thermal overload protection at the operation and maintenance level: Establish equipment operation records to record temperature, load, and fault history; conduct preventive tests such as insulation resistance, dielectric loss, and partial discharge regularly; regularly calibrate temperature sensors and overload protection devices; establish an aging trend analysis model for enamel coating to predict remaining service life.
8 Conclusion
Thermal overload is one of the main causes of insulation aging in enameled copper wire, and it has a decisive impact on the reliability and service life of winding equipment. The causes of thermal overload include mechanical overload, abnormal voltage, heat dissipation failure, frequent start-stop cycles, and harmonic currents, among other factors. Thermal aging involves chemical processes such as oxidative degradation, hydrolytic fracture, and copper ion catalysis. The effects of thermal overload can be categorized into three modes: instantaneous thermal stress from short-term overload, cumulative aging from long-term overload, and instantaneous breakdown from transient overload.
Thermal life assessment primarily relies on the Arrhenius accelerated aging model and the Miner cumulative damage model. Overload protection strategies include relay protection, setting optimization, and comprehensive protection at the design and operational levels. Engineering improvements should be comprehensively promoted from multiple dimensions, including wire selection, winding design, heat dissipation systems, and operation and maintenance.
In engineering practice, appropriate wire types and insulation systems should be selected based on the load characteristics, operating environment, and maintenance conditions of the specific application scenario. Through strict process control, overload protection configuration, and operation and maintenance management, the risk of insulation failure caused by thermal overload can be minimized, thereby improving the reliability and service life of electrical equipment.
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