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In the previous column, we explained how motor heat generation affects performance and lifespan, as well as how rated output changes depending on operating patterns. In this final installment, we will explain the mechanisms and role of motor cooling, along with its impact on rated output.

 

1. Mechanisms of Motor Cooling

To explain the basic concept of cooling, let’s use the analogy of chilling a watermelon. Placing a watermelon in a cool room, submerging it in a water bucket, or exposing it to running water—these are common cooling methods in daily life. The reason submerging a watermelon in water cools it faster than simply placing it in a room is that water has a higher thermal conductivity than air. Running water further accelerates cooling because fresh cold water is continuously supplied. In addition, the size of the watermelon also affects the cooling process. Larger watermelons take longer to cool because they have higher heat capacity (specific heat × mass), making their temperature more stable to change.

Now let’s apply this concept to motors: Motors generate heat due to energy losses, and without an efficient heat dissipation mechanism, their temperature will continue to rise. To help visualize the relationship between heat transfer and temperature rise, we can use the analogy of a glass with a hole, where liquid flows in and out. 

 

    Liquid inflow: Represents the heat generated by the motor.

    Glass base area: Represents the motor’s heat capacity.

    Liquid level height: Represents the motor’s temperature.

    Glass upper edge: Represents the motor’s maximum allowable temperature.

    Hole size: Represents the motor's cooling capacity.

If cooling performance is low (i.e., the hole is small), the liquid level rises quickly, indicating a rapid increase in temperature. Conversely, if cooling performance is high (i.e., the hole is large), temperature rise is slower. Additionally, if the base area of the glass is larger, even with the same inflow and outflow rate, the change in liquid level height (temperature change) is smaller. Finally, if liquid overflows from the glass edge, it indicates the motor has overheated. This principle is similar to the mechanism used when cooling a watermelon.

 

2.  The Role of Motor Cooling

The internal components of a motor (e.g., coils, bearings, resin materials) have temperature resistance limits. Exceeding these limits can shorten the lifespan of components and, in the worst case, lead to catastrophic issues such as failure or fire, as explained in the previous column. Therefore, managing and operating motors at appropriate temperatures is crucial. Cooling is a key factor for ensuring efficient and safe motor operation, and careful consideration of cooling methods is required during system design.

Below are general motor cooling methods and their characteristics. Selecting the optimal method that suits the application and design requirements is important: 

Common Motor Cooling Methods and Their Characteristics

(1) Natural Air Cooling:

A simple cooling method where heat is passively dissipated through the motor’s surface or heat fins.

(2) Forced Air Cooling:

Use a fan to actively evacuate heat to the external environment.

(3) Water Cooling:

Circulating cooling water to remove heat, ideal for applications requiring continuous high output.

(4) Oil Cooling:

Uses cooling oil within the motor to facilitate internal heat transfer. Suitable for extremely high-output and harsh environments, more so than water cooling.

 

3. Impact on Rated Output

By implementing powerful cooling systems, efficient heat dissipation can reduce temperature increases, allowing the motor to achieve higher rated output and longer operating times. However, depending on the selected cooling method, requirements for water or oil management may arise, increasing system costs and maintenance burden.

Our Toyota Super Drive AC Motor proposes a simple and maintenance-friendly natural air-cooling system, leveraging its high efficiency and low energy loss characteristics. With natural air cooling, motor installation layout plays a critical role, such as directing driving wind toward the motor or maintaining appropriate distance from other heat-generating components. Appropriate cooling can enhance the practical rated output of the motor while improving safety and reliability.

 

Throughout this three-part series, we have explained various aspects related to motor rated output. We hope these insights contribute to a deeper understanding of motor operation. Moving forward, we will continue to support our customers’ solutions through the development and sales of CN components, contributing to the realization of a sustainable society.