Everyone knows that the degree of heat generated by an electric motor is measured by "temperature rise" rather than "temperature". When the "temperature rise" suddenly increases or exceeds the maximum operating temperature, it indicates that the motor has malfunctioned.
The following provides a basic explanation of some fundamental concepts.
Insulation class of insulating materials
Insulating materials are classified into seven grades according to their heat resistance: Y, A, E, B, F, H, and C. Their maximum operating temperatures are 90, 105, 120, 130, 155, 180℃, and above 180℃, respectively.
The so-called limiting operating temperature of insulation materials refers to the temperature of the hottest spot in the winding insulation during motor operation within its designed expected lifespan. Based on experience, Class A materials can have a lifespan of up to 10 years at 105℃ and Class B materials at 130℃. However, in reality, ambient temperature and temperature rise will not consistently reach the design values, so the typical lifespan is 15–20 years. If the operating temperature consistently exceeds the material's limiting operating temperature, insulation aging will accelerate, and the lifespan will be significantly shortened. Therefore, temperature is one of the main factors affecting the winding lifespan of a motor during operation.
Temperature rise
Temperature rise is the temperature difference between the motor and the environment, caused by the motor's heat generation. During operation, the motor core is in an alternating magnetic field, generating iron losses; the windings, when energized, generate copper losses; and there are other stray losses. All of these contribute to an increase in motor temperature. On the other hand, the motor also dissipates heat. When heat generation and heat dissipation are equal, an equilibrium is reached, and the temperature stabilizes at a certain level. When heat generation increases or heat dissipation decreases, this equilibrium is disrupted, the temperature continues to rise, widening the temperature difference. This then increases heat dissipation, leading to a new equilibrium at a higher temperature. However, the temperature difference, i.e., the temperature rise, is now greater than before. Therefore, temperature rise is an important indicator in motor design and operation, signifying the degree of heat generation. A sudden increase in motor temperature rise during operation indicates a motor malfunction, blocked airflow, or excessive load.
Relationship between temperature rise and other factors
Theoretically, the temperature rise of a normally operating motor under rated load should be independent of the ambient temperature, but in reality it is still affected by factors such as ambient temperature.
(1) When the temperature drops, the temperature rise of a normal motor will decrease slightly. This is because the winding resistance R decreases, and the copper loss decreases. For every 1°C drop in temperature, R decreases by approximately 0.4 %.
(2) For self-cooled motors, the temperature rise increases by 1.5 to 3°C for every 10°C increase in ambient temperature. This is because the winding copper losses increase with rising air temperature. Therefore, changes in air temperature have a greater impact on large motors and enclosed motors.
(3) For every 10% increase in air humidity, the temperature rise can decrease by 0.07 to 0.38 ℃ due to improved heat conduction, with an average of 0.19 ℃.
(4) The altitude is based on 1000m. For every 100m increase , the temperature rise increases by 1% of the temperature rise limit.
Extreme operating temperature and maximum permissible operating temperature
Generally speaking, the extreme operating temperature for Class A is 105℃, and the maximum permissible operating temperature for Class A is 90℃. So, what is the difference between the extreme operating temperature and the maximum permissible operating temperature? Actually, this is related to the measurement method. Different measurement methods will reflect different values, and thus have different meanings.
(1) Thermometer method
The measurement results reflect the local surface temperature of the winding insulation. This figure is on average about 15°C lower than the actual highest temperature of the winding insulation, i.e., the "hottest spot." This method is the simplest and most widely used in the field for small and medium-sized motors.
(2) Resistance method
The measurement results reflect the average temperature of the entire winding copper wire. This value is 5–15°C lower than the actual maximum temperature, depending on the insulation class. This method involves measuring the cold and hot resistance of the conductor and calculating the average temperature rise using relevant formulas.
(3) Buried thermometer
During the test, a copper or platinum resistance thermometer or thermocouple is embedded in the windings, core, or other components where the expected temperature is highest. The measurement results reflect the temperature at the contact point of the sensing element. This method is commonly used to monitor the operating temperature of large motors.
The temperature measured by various methods will have a certain difference from the actual maximum temperature. Therefore, the "maximum allowable operating temperature" of the insulation material must be obtained by subtracting this difference from the "limited operating temperature".
Temperature limits of various parts of the motor
(1) The temperature rise of the iron core in contact with the winding (thermometer method) shall not exceed the temperature rise limit of the winding insulation in contact (resistance method), namely 60℃ for Class A, 75℃ for Class E, 80℃ for Class B, 100℃ for Class F, and 125℃ for Class H.
(2) The temperature of rolling bearings should not exceed 95℃, and the temperature of sliding bearings should not exceed 80℃. Too high a temperature will cause changes in the oil quality and damage the oil film.
(3) In practice, the temperature of the casing is usually determined by whether it is hot to the touch.
(4) The surface stray loss of the squirrel-cage rotor is very large and the temperature is high. Generally, it should be limited to not endangering the adjacent insulation. Irreversible color-changing paint can be applied in advance to estimate the risk.
Troubleshooting motor overheating
When the motor temperature exceeds the maximum operating temperature, or the temperature rise exceeds the specified limit, or the temperature rise, although not exceeding the specified limit, suddenly increases under low load, it indicates a motor malfunction. The methods for diagnosis and troubleshooting are as follows:
(1) The temperature rise under rated load does not exceed the temperature rise limit, but the motor temperature exceeds the maximum allowable operating temperature only because the ambient temperature exceeds 40℃. This phenomenon indicates that the motor itself is normal. The solution is to artificially lower the ambient temperature. If this is not possible, the load must be reduced.
(2) Temperature rise exceeds the nameplate specification under rated load. In any case, the motor is faulty and must be stopped for inspection, especially if the temperature rise suddenly increases.
External causes include: low grid voltage or excessive line voltage drop (more than 10%), excessive load (more than 10%), and improper coordination between motor and machinery.
Internal causes include: single-phase operation, inter-turn short circuit, phase-to-phase short circuit, stator grounding, fan damage or loose fastening, blocked air duct, bearing damage, stator-rotor rubbing, motor and cable joint overheating (especially copper-aluminum or aluminum-aluminum connections), motor corrosion or moisture, etc.
Furthermore, while theoretically all motors can rotate in both directions, some motors have directional fans; if the fan is reversed, the temperature rise will be significantly higher. In short, troubleshooting must be done for each specific situation.
★Supplementary information is attached (a table comparing the allowable temperature rise values of windings for different insulation grades).
