Electric Motor Temperature Limits: Protecting Performance

1. What is the appropriate operating temperature for an engine?

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

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

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

2. How hot will the engine burn?

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

3. Temperature limits of various parts of the engine

1. The temperature rise of the iron core in contact with the winding (thermometer method) must 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 bearings must not exceed 95°C and the temperature of the sliding bearings must not exceed 80°C. High temperatures can cause changes in oil quality and damage the oil film.
3. In practice, enclosure temperature is usually measured by the standard that it should not be hot to the touch.
4. The squirrel cage rotor presents high surface loss and high temperature, generally limited by not compromising the adjacent insulation. This can be estimated by applying irreversible color change paint in advance.

4. Engine temperature and temperature rise

The degree of engine heating is measured by “temperature rise” and not by “temperature”. When the “temperature rise” suddenly increases or exceeds the maximum working temperature, it indicates that the engine is defective. 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 limiting working temperatures are 90, 105, 120, 130, 155, 180°C and above 180°C. °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 usually Class F, while the copper wire may use Class H or even higher to improve its quality.

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

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

According to experience, Class A material has a service life of 10 years at 105°C, and Class B material has a service life of 10 years at 130°C, but in real situations, neither room temperature nor temperature increase will reach the design. value over a long period of time, so the general useful life is 15 to 20 years.

If the operating temperature exceeds the material's working temperature limit for a long period of time, the insulation will age more quickly and the service life will be significantly reduced.

Therefore, temperature is one of the main factors that affect the life of an engine during operation.

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

The allowable temperature rise refers to the limit of temperature rise of the electric motor compared to the surrounding environment. In electrical equipment such as generators, the 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 resistances, and electrical equipment that uses different insulating materials will have different abilities to withstand high temperatures.

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