VFD motor -Dongchun

What is Motor Temperature Rise and Why Use F-Class Insulation with B-Class Temperature Rise Assessment?

When a motor starts running at room temperature (ambient temperature), it gradually heats up due to internal energy dissipation, causing its temperature to rise above the ambient temperature. This increase above the ambient temperature is known as motor temperature rise. It’s a crucial performance indicator that reflects the motor's heat dissipation ability and insulation efficiency. If the motor's temperature rise exceeds the specified limits, it could lead to insulation degradation, reduced efficiency, and even motor failure.

To further enhance motor safety and durability, a common strategy is to use F-Class insulation while assessing the motor based on B-Class temperature rise standards. This means that even though the motor is built with F-Class insulation materials (which have a maximum permissible temperature of 155°C), the motor’s temperature rise is still evaluated according to the stricter B-Class insulation standards (which allow a maximum temperature of 130°C). By adhering to this more stringent standard, the motor’s reliability and service life are significantly increased.

What is Motor Temperature Rise?

Motor temperature rise can be calculated using the following formula:

Motor Temperature Rise (K) = Motor Temperature (°C) - Ambient Temperature (°C)

For example, if a motor’s temperature is 80°C while the ambient temperature is 30°C, the temperature rise would be:

80°C - 30°C = 50K

The unit of measurement used here is Kelvin (K), where a 1K increase corresponds to a 1°C rise. By monitoring motor temperature rise, you can determine if the motor is operating within safe limits and decide whether cooling measures are necessary.

Why Assess Motor Temperature Rise Using B-Class Standards?

Assessing temperature rise using B-Class standards ensures a higher safety margin for the motor. Although F-Class insulation has a maximum permissible temperature of 155°C, evaluating the motor under the lower B-Class temperature rise standard (130°C) provides greater reliability and extends the motor’s service life. This “downgraded assessment” approach effectively prevents premature aging of insulation materials due to high temperatures and reduces the risk of motor failure caused by overheating.

Insulation Classes and Their Temperature Limits

The insulation class of a motor directly determines its temperature rise limit. The commonly used insulation classes are classified according to the thermal performance of the insulation material, as shown below:

  • A Class Insulation: Maximum allowable temperature 105°C
  • E Class Insulation: Maximum allowable temperature 120°C
  • B Class Insulation: Maximum allowable temperature 130°C
  • F Class Insulation: Maximum allowable temperature 155°C
  • H Class Insulation: Maximum allowable temperature 180°C

The table below provides a comparison of B, F, and H insulation classes:

Common Insulation ClassMax Permissible Working Temperature (°C)Ambient Temperature (°C)Temperature Rise Limit (K)
B1304080
F15540105
H18040125

From the table, we can see that B-Class insulation has a temperature rise limit of 80K, while F-Class and H-Class are 105K and 125K respectively. When we use F-Class insulation but assess the motor under B-Class standards, we need to strictly control the temperature rise to below 80K, ensuring a higher safety margin for the motor.

Benefits of Using B-Class Temperature Rise Assessment

  1. Enhanced Motor Reliability: Lower temperature rise reduces thermal stress on insulation materials, delaying their aging and extending the motor’s lifespan.
  2. Improved Performance in Harsh Environments: Motors assessed under B-Class standards perform better and run more stably in high-temperature, high-humidity, or dusty environments.
  3. Reduced Risk of Failure: If a motor’s temperature can be maintained well below the F-Class limit, the probability of failures caused by overheating is significantly lowered.

National Standards for Motor Temperature Rise

In China, to ensure motors can adapt to various environmental conditions, the national standard specifies that the cooling air temperature for motors should be set at 40°C. This standard allows motors to operate stably throughout the year in different regions. Once the maximum temperature of the cooling medium (e.g., air) is determined, manufacturers can define the motor’s temperature rise limits based on the insulation class and ambient conditions.

Methods for Measuring Motor Temperature Rise

Accurate measurement of motor temperature rise is essential for ensuring safe operation. The following are two common methods used:

  1. Resistance Method: This involves measuring the resistance of the winding after the motor has stopped and converting it to temperature. Although precise, it requires quick measurement to ensure accuracy.
  2. Embedded Temperature Sensor Method: This method involves embedding temperature sensors directly into the motor to measure temperature. While this method has lower errors, the sensor might not always be at the motor’s highest temperature point, so a margin of 5°C is usually considered.

What Happens if the Motor’s Temperature Rise Exceeds Limits?

If a motor’s temperature rise exceeds the allowable limits, it can lead to several problems:

  • Insulation Degradation: High temperatures accelerate the aging of insulation materials, reducing their insulating properties.
  • Reduced Efficiency: Overheating increases energy losses, causing a drop in efficiency.
  • Motor Damage: Continuous operation above the temperature rise limit can cause insulation failure, leading to motor burnout or total failure.

Real-World Example

For instance, if a motor’s nameplate specifies a permissible temperature rise of 90K and the ambient temperature is 35°C, the winding’s maximum temperature should not exceed:

35°C + 90K = 125°C

If the winding temperature exceeds 125°C, the insulation materials inside the motor could fail, causing severe damage.

However, if the motor is built with F-Class insulation and evaluated under B-Class temperature rise standards, the actual operating temperature will be much lower than the insulation’s maximum threshold, greatly enhancing the motor’s safety and reliability.

Conclusion: Enhancing Motor Safety with B-Class Temperature Rise Assessment

Although F-Class insulation allows a maximum temperature of 155°C, assessing the motor under B-Class temperature rise standards is a safer approach. This method effectively prevents premature aging of insulation materials due to high operating temperatures and reduces the risk of motor failure.

If you are looking for high-quality motors that meet stringent temperature rise standards, DONGCHUN Motor provides motors built with F-Class insulation assessed under B-Class temperature rise limits, suitable for various demanding applications.

VFD motor -Dongchun

Visit our website at www.iecmotores.com for more information on choosing the right motor for your needs.

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