Cooling Solution

cooling solution

Virtual prototyping shaves time off the development of a small electric motor.

This article was prepared by staff writers in collaboration with outside contributors.

When management decided to create a new 3.7-inch-diameter motor, Emerson engineers knew that their toughest challenge would be keeping temperatures within the motor low enough to avoid breaking down the insulation that could eventually cause the motor to fail.

The company normally uses an insulation system that requires a maximum temperature rise of 85°C above ambient. When the design team assembled their first prototype using a traditional ventilation system design, as they expected, it tested out well above the specification at 125°C above ambient.

St. Louis-based Emerson Motor Co., a producer of motors and drive systems, has moved to a new design methodology, computational fluid dynamics, to create a virtual prototype of each iteration of the motor design. The original model can usually be created in a matter of days compared with a month for a physical prototype.

Emerson's tub motor, shown here built of prototype parts, increased cooling efficiency by having its fan draw air, rather than push it.

After the initial model has been produced, new design iterations can typically be created in hours or even minutes if they represent simple changes. CFD computes engineering results, such as airflow velocity, direction, and temperature, at all points in the model, compared with the relatively few points that can be measured in a physical test.

Peter Bostwick, an engineering specialist in CFD and
heat transfer for Emerson, modeled the initial prototype using software called CFX from AEA Technology of
Waterloo, Ontario.

The original design looked much like the company's larger motors. The fan would draw air axially through holes in the fan cover. The fan cover redirected the air around the shell that enclosed the windings.

Bostwick created a multiple frame of reference model, which uses a rotating frame for the spinning fan and a stationary frame for the rest of the motor.

Because of several simplifying assumptions that Bostwick made to reduce run-time, the first iteration had to be calibrated to the test results. From this point on, the simulation matched physical testing results within 5 percent.

By depicting the airflow throughout the motor design, the CFD results provided insights that helped Bostwick improve the design. He changed the end shields so that air could flow through the space around the windings. He modified the fan geometry to boost the flow through the stator slots and air gap. The new fan geometry had a second set of blades, with the first set intended to provide circulation around the outside of the motor shell and the second promoting flow inside the motor.

"Everyone was happy with the new design, but management required a change to reduce manufacturing costs," Bostwick said.

"They decided to remove the shell that enclosed the windings. The design had to be substantially modified because without a shell, much of the component attachment changes and the end shields had to be totally rethought." Bostwick said he developed a new way to solve the cooling problem within two weeks.

The biggest challenge was that the shell was no longer there to confine the airflow around the windings. So Bostwick designed a concentric cylinder for the end shield that serves the same purpose by drawing the air into the motor along the exterior of the stator and across the windings.

Analysis showed that the fan could be made considerably smaller and that it would work more efficiently by changing to pulling rather than pushing air into the motor. CFD analysis showed that the temperature rise was only 57°C at the rated load using this new approach.

According to Bostwick, the project was completed in about eight months.