Modern electric motors employ higher electromagnetic loads to improve material utilization, and the single-unit capacity of motors is also increasing. Therefore, it is necessary to improve the cooling system of motors to enhance their heat dissipation capacity. Except for a few small or special motors, most motors use fans to force airflow for cooling.
Overview of Motor Cooling System Development: As early as 1928, synchronous condensers used hydrogen as a cooling medium were successfully developed to improve the power factor of power grids. Tests showed that, due to reduced mechanical friction losses and improved heat dissipation, hydrogen cooling could increase the capacity of motors of the same size by 20-25% or more, while also improving efficiency. In the following two decades, hydrogen cooling technology was rapidly adopted for steam turbine generators. After the 1950s, internal cooling systems were used in large-capacity motors, especially steam turbine generators. Internal cooling involves directly transferring heat generated in the conductor to the cooling medium without insulation. Initially, hydrogen was commonly used as the cooling medium in internal cooling systems; later, to further improve cooling capacity, liquids (water or oil) were also used as cooling media.
In 1955, the British company, Mowell Electric, completed the first 30,000 kW steam turbine generator with a water-cooled stator and a generally hydrogen-cooled rotor. The results were excellent; its electrical load increased 4-5 times compared to air-cooled generators, while the winding temperature rise did not exceed the allowable value. my country's electrical engineers have made significant achievements in the research and manufacturing of dual-water-cooled steam turbine generators, where both stator and rotor windings are water-cooled. Currently, in addition to further researching existing cooling methods, countries around the world are also researching and developing various new cooling methods, including those utilizing low-temperature superconducting technology. It is convenient to classify motor cooling systems according to the different cooling media. Currently, the most widely used system in motor manufacturing is the air-cooled system, which uses air as the cooling medium. Today, Ms. Can will mainly discuss air-cooled systems. Air-cooled motors have a simple structure and low cost; however, their disadvantages are poor air cooling effect and higher frictional losses in high-speed motors. There are many structural types of air-cooled ventilation systems; their basic characteristics will be explained from the following aspects:
● Open-circuit cooling (or free circulation) or closed-circuit cooling (or closed-loop circulation) motors: In open-circuit cooling motors, cooling air is drawn from around the motor, passes through the motor, and then returns to the surrounding environment. In closed-circuit cooling motors, the primary cooling medium circulates through the motor along a closed circuit; the heat in the primary cooling medium is transferred to the secondary cooling medium via structural components or a cooler.
●Radial, Axial, and Hybrid Ventilation Systems: Air cooling systems can be classified into three types based on the direction of airflow within the motor: radial, axial, and hybrid (radial and axial). Radial ventilation systems are widely used because they facilitate the use of airflow generated by rotor components (such as ductwork plates and magnetic poles). Axial ventilation systems allow for the installation of larger diameter fans to increase airflow. Their main disadvantages are uneven cooling along the axial direction and difficulty in utilizing the airflow from rotor components. In China, axial ventilation systems are generally only used in small and medium-sized DC motors. In reality, the terms "radial" or "axial" ventilation systems refer primarily to the cooling effect of the cooling medium within the motor; purely radial or axial ventilation systems are relatively rare.
Hybrid ventilation systems have both axial and radial channels, but often emphasize one. Large DC motors are hybrid systems with axial channels as the main channel, while some steam turbine generators widely use hybrid systems with radial channels as the main channel.
●Suction and forced air cooling systems can be categorized into two types based on whether the cooling air first passes through the motor's heat-generating components and then through the fan, or vice versa. Because the cooling air in a suction system comes into contact with the motor's heat-generating components first, and a larger diameter fan can be used, while in a forced air system the cooling air first passes through the fan and is heated by fan losses before contacting the motor's heat-generating components (in high-speed motors, fan losses can cause an air temperature rise of approximately 5°C), suction systems have a higher cooling capacity.
In DC motors, fans are usually installed at the non-commutator end. If a pressure cooling system is used, carbon dust from brush wear can be prevented from entering the motor. However, most of the airflow passes through the space between the stator poles, while less airflow blows onto the rotor, making it difficult to dissipate heat from the rotor which suffers more losses. In this case, appropriate measures should be taken, such as making full use of the air duct plates of the armature core and the blowing action at the winding ends for heat dissipation.
● External and Internal Cooling: Air-cooled systems generally employ external cooling, or so-called surface cooling. However, to improve cooling capacity, internal cooling is sometimes used, such as in the excitation windings of hydro generators, where air internal cooling can be employed. Air internal cooling has also been used in the rotor windings of small-capacity steam generators abroad. However, internal cooling systems are structurally complex and require extremely clean cooling gas; therefore, they are currently rarely used in motors that use air as the cooling medium.
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