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CFD Helps Develop Smaller Jetted Tub Motor
Posted Tue May 07, 2002 @05:15PM
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Application Emerson Motor Company, USA, recently used CFD to reduce the size of electric motors used to power jetted bathtubs. Emerson engineers knew their challenge would be to maintain low winding temperatures and when the first prototype ran hot, their suspicions were confirmed. Excessive winding temperatures can lead to breakdown of the electrical insulation and premature motor failure.

The established approach would have been to change the design by either adding copper and steel or changing the ventilation method, build a new prototype and evaluate the temperature reduction achieved. This process required approximately ten iterations, each taking close to a month, in order to obtain an acceptable design. Because of the slow pace and high cost of this approach, Emerson decided to use CFD instead. Emerson engineers chose CFX-TASCflow from AEA Technology to create a virtual prototype based on the original design.

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In this device, the fan draws air axially through holes in the cover then ejects it radially, from where it is turned along the exterior of the shell and around the motor windings, cooling these and other internal components. CFX-TASCflow depicted the airflow throughout the motor, immediately providing new insights that helped improve the design. Engineers changed the end shields and fan geometry to force air to circulate around the windings as well as to allow air to be drawn through the stator slots. The new fan had a second set of blades, the first providing circulation around the outside of the motor shell, the second promoting flow inside the motor. Emerson optimized the geometry of each of these components and produced a design that exceeded thermal requirements. The temperature reduction was significant enough to allow us to expand the product line from a 1/3 hp motor to a 3/4 hp motor and still remain below temperature limits. A final prototype demonstrated that the temperature prediction was within 5C of the measured performance. This process was completed in less than two months because of CFXs capability to provide solutions quickly.

Everyone was happy with the new design but management decided to remove the shell that enclosed the windings to reduce manufacturing costs. The design had to be substantially modified, but, because engineers had already gained a clear understanding of the airflow through the motor, the cooling problem was solved within a couple of weeks. Engineers designed a concentric cylinder for the end shield that serves to contain the airflow against the stator, drawing it across the windings and into the fan before it is exhausted from the motor. CFX showed that the fan could be made considerably smaller and that it would work more efficiently if it were changed to pull rather than push air through the motor.

CFD results were used to guide the entire design process and prototypes were built only at key decision points to validate the analysis. CFD dramatically reduced our time to market by providing understanding that the engineers couldn't have gained from experiments and by eliminating the time-consuming building of prototypes. The project was completed in about eight months while the trial-and-error method would have taken several years. The superior insight provided by CFD helped Emerson to produce a considerably better design; the new design has optimal ventilation, which allows for minimal copper and steel usage thereby reducing costs. The virtual prototyping method clearly provides a superior way to develop electric motors.

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Related Links
  • AEA Technology
  • CFD to reduce the size of electric motors used to power jetted bathtubs
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