Cooling Brushless DC Motors With Liquid
Brushless DC electric motors have become the preferred motor type for many electrically powered industrial drive systems. They offer the significant advantage of accurate variable-speed control over a range of load conditions, and they can produce full or nearly full rated torque.
Although variable-speed motors have been used for a number of years, earlier technologies generally could not produce nearly full-rated torque at small fractions of the motor's maximum speed. Instead, output torque and other measures of useful output often dropped dramatically as the speed was reduced. When an application required a variable-speed motor to produce high torque at a low speed, a motor capable of greater output had to be used.
The availability of variable-speed DC brushless motors capable of high output across a wide speed range has allowed equipment designers to specify smaller-capacity motors compared to those required with conventional technologies. However, such applications also present special heat-removal challenges. Because the motors are physically smaller, all the heat produced must be removed from a smaller surface area. Also, more motors fit into a given volume within an equipment enclosure, which increases the risk of overheating. Further, when lower-capacity motors are used, they must operate at a higher fraction of their full load output or maximum duty cycle. As a result, waste heat must be removed efficiently to avoid an excessive temperature increase, which could damage the motor or equipment in which it is installed, or could cause a fire or other safety hazard.
The Conventional ApproachThe conventional method of cooling medium-to-large electric motors used in common industrial equipment is to direct a forced airstream at the motor's exterior surface. Although this method can be effective, it has several disadvantages. For example, a substantial amount of space must be provided around each motor to allow enough air circulation, which limits the density of the equipment in that area.
Additionally, because air is not an efficient coolant compared to other cooling materials, the cooling air must strike the motor surface and circulate around it at a relatively high velocity. This requires the use of high-velocity blowers, which must be placed relatively close to the motor to be cooled. Because DC brushless motors might be required to operate at low speeds, they cannot drive their own blowers. Instead, a smaller additional motor typically is provided to drive the blower or fan blade. The additional motor requires energy and space, and generates still more heat.
Blowers also produce high-intensity noise and have rapidly moving mechanical components that can be hazardous to workers in the vicinity. In some environments such as materials processing and mining operations, the rapid air movement generated by blowers can cause gas or suspended particles to ignite or explode. When blowers are used, special care must be taken to comply with occupational safety and health regulations.
Another drawback to using blowers is that the cooling air must be drawn from and returned to the environment surrounding the equipment. Industrial facilities are rarely perfectly clean; thus, the environmental cooling air often carries particulates that can contaminate the equipment or the product being manufactured or processed. Many facilities install air-filtration devices to capture these particulates, but such devices are not always completely effective.
Finally, the heat produced within the motor must be conducted to the outermost surface of the motor housing before it can be transferred to the cooling air. Blower-cooled motors have traditionally been provided with cooling fins to increase their surface area, thereby providing improved thermal conductivity between the housing and the cooling air. However, the additional fin structure increases the mass, material cost and space requirements of the motor.
A Liquid AlternativeMotor housing and base designs have emerged that use integral liquid cooling to provide a cleaner, safer alternative for removing heat from high-powered electric motors. In these systems, the cast motor housing has at least one conduit tube cast in the housing for carrying a circulating heat removal fluid (typically water or glycol-water, although other fluids may be used). The conduit is arranged within the housing wall to extend along substantially the entire longitudinal and circumferential dimensions of the housing, and it can be formed as a multi-turn helix. In operation, fluid is circulated through the motor housing to absorb heat and then is circulated through a heat exchanger to expel the heat.
The same principle also has been used to create liquid-cooled mounting plates, which use a pump to circulate the water or other liquid from the plate through a radiator. These plates can be used to cool the large resistors that receive and dissipate the electric energy produced in DC electric motors with dynamic braking. The resistors, which are generally located remotely from the electric motor, convert the electric energy into heat energy. Instead of directing currents of air over the resistors to dissipate the heat produced -- a process that requires additional space for fans or blowers, as well as additional electricity -- facilities can mount the resistors to a liquid-cooled plate to achieve compact, efficient cooling. Chill plates with integral liquid cooling also can be used as a mounting surface for electronic components such as circuit board assemblies that require cooling to maintain optimal performance.
In a world of increasingly compact designs and high power densities, integral liquid cooling is meeting more stringent heat-removal requirements in applications such as lasers, high-powered electronics, telecommunications and semiconductor processing equipment. Advanced technology and flexible manufacturing capabilities allow cast-in thermal components to be designed in virtually any size and shape, including complex and challenging geometries. For high watt densities, when air-cooled heat sinks are inadequate, liquid-cooled cold plates can provide a high-performance heat transfer solution. PCE
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