Electric Motor Temperature Limits: Safeguarding Performance | MachineMFG

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Electric Motor Temperature Limits: Safeguarding Performance

Key takeaways:

1. Motor lifespan and efficiency are heavily influenced by operating temperature, with each motor insulation class (Y, A, E, B, F, H, C) having specific temperature limits that, if exceeded, can significantly reduce the motor's service life due to accelerated insulation aging.

2. The concept of "temperature rise" is critical for monitoring motor health, as it indicates the change in temperature from the ambient to the operating temperature, with specified limits for each insulation class to prevent damage, and sudden increases in temperature rise signaling potential malfunctions.

3. The design of electric motors incorporates a safety margin within the insulation classes, often testing materials at a lower class than their rating to ensure longevity, with the understanding that consistent operation at or near an insulation material's maximum temperature will hasten its deterioration and shorten the motor's expected lifespan.

1. What Is the Appropriate Operating Temperature for a Motor?

Generally, it is preferable that the temperature of the motor body does not exceed 80 degrees Celsius.

When the temperature exceeds this, the winding temperature inside the motor is also likely to be high, and it will definitely exceed 80°C.

Simultaneously, the body temperature will be transmitted to the motor shaft end, affecting the lubrication of the motor bearings.

2. How Much Temperature Will Burn Out the Motor?

Generally, if the motor’s insulation class is Class A, with an ambient temperature of 40°C, the outer shell temperature of the motor should be less than 60°C.

3. Temperature Limits of Various Parts of the Motor

  1. The temperature rise of the iron core in contact with the winding (thermometer method) should not exceed the temperature rise limit of the insulating material in contact with the winding (resistance method), i.e., Class A is 60°C, Class E is 75°C, Class B is 80°C, Class F is 100°C, and Class H is 125°C.
  2. The temperature of the rolling bearings should not exceed 95°C, and the temperature of the sliding bearings should not exceed 80°C. High temperatures can cause changes in oil quality and damage to the oil film.
  3. In practice, the temperature of the casing is usually gauged by the standard that it should not be hot to the touch.
  4. The squirrel-cage rotor has a large surface stray loss and a high temperature, generally limited by not jeopardizing the adjacent insulation. This can be estimated by applying irreversible color-changing paint in advance.

4. Temperature and Temperature Rise of the Motor

The degree of motor heating is measured by “temperature rise,” not “temperature”. When the “temperature rise” suddenly increases or exceeds the maximum working temperature, it indicates that the motor has malfunctioned. Below, some basic concepts are discussed.

Insulation class of insulating materials

Insulating materials are divided into seven classes Y, A, E, B, F, H, and C based on their heat resistance, and their limit working temperatures are 90, 105, 120, 130, 155, 180°C, and above 180°C respectively. Performance reference temperatures (°C) are A80, E95, B100, F120, H145.

Insulating materials can be divided into the following seven classes based on their thermal stability:

  1. Class Y, 90 degrees, cotton
  2. Class A, 105 degrees
  3. Class E, 120 degrees
  4. Class B, 130 degrees, mica
  5. Class F, 155 degrees, epoxy resin
  6. Class H, 180 degrees, silicone rubber
  7. Class C, above 180 degrees

For a commonly used Class B motor, the internal insulation material is often Class F, while the copper wire may use Class H or even higher to improve its quality.

In general, to extend service life, it is often stipulated that high-class insulation requirements should be checked at a lower class. For example, a common oil pump motor with Class F insulation is tested at Class B, i.e., its temperature rise should not exceed 120 degrees (with 10 degrees left as a margin to avoid individual motors exceeding the temperature rise due to unstable processes).

The so-called limit working temperature of the insulating material refers to the highest temperature at the hottest point in the winding insulation of the motor during operation, designed for its expected service life.

According to experience, Class A material has a lifespan of 10 years at 105°C, and Class B material has a lifespan of 10 years at 130°C, but in actual situations, neither the ambient temperature nor the temperature rise will reach the design value for a long period of time, so the general lifespan is 15-20 years.

If the operating temperature exceeds the material’s limit working temperature for a long period of time, the insulation will age faster, and the lifespan will be significantly shortened.

Therefore, temperature is one of the main factors affecting the lifespan of a motor during operation.

Temperature level of insulationAEBFH
Maximum allowable temperature (℃)105120130155180
Winding temperature rise limit (K)607580100125

The insulation class of an electric motor refers to the heat resistance grade of the insulating materials used, divided into Classes A, E, B, F, and H.

The permissible temperature rise refers to the limit of the temperature increase of the electric motor compared to the surrounding environment. In electrical equipment like generators, insulating material is the weakest link.

Insulating materials are especially vulnerable to high temperatures, which accelerate their aging and damage. Different insulating materials have different heat resistance, and electrical equipment using different insulating materials will have different abilities to withstand high temperatures.

Therefore, the maximum working temperature is usually specified for general electrical equipment.

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