Special-Use Electronic Assemblies Require Customized Tests

How do you demonstrate the quality level and reliability of an electronic assembly that has a unique application and is deployed into a harsh operating environment? The answer: A customized test program that properly demonstrates and evaluates electronic components and assemblies.

Consider a stator assembly used in the chain case of a motorcycle. A stator is the stationary part of a motor, dynamo, turbine, or other working machine about which a rotor turns. In a motorcycle, the stator assembly is the stationary part of the alternator that provides power to the motorcycle when it is operating. The stator assembly also helps maintain the charge on the motorcycle battery.

Certainly, the stator-assembly operating environment is not typical. Temperatures in excess of 400° F can be experienced by the stator assembly, and it must operate in hot chain case oil. The assembly also endures rather significant vibration and shock as well as high current and voltage transients.

Most industry standards do not specifically address the extreme operating environments experienced by the stator assembly. So to properly demonstrate the quality levels and the reliability of the stator assembly, it was necessary to develop a customized, rigorous testing regimen.

Gathering Field Data

Knowing the motorcycle field environment and the operating environment of the stator assembly are critical pieces of information when selecting appropriate materials and components and developing a test program. The availability of field data allows the engineer to customize a test program that contains realistic environments which can be simulated and accelerated in the lab.

In the case of the stator assembly, to augment system models and analytical simulation data, thermocouples, accelerometers, and other sensors were instrumented to the motorcycle at appropriate locations, and data was gathered from the field. With this data, a test program was developed. By developing such simulated and accelerated test programs, manufacturers can speed up the product development cycle while improving the long-term reliability and robustness of their products at the same time.

Stator-Assembly Testing Regimen

After field data was gathered and a test regimen developed, laboratory testing began. The test program for the stator assembly appears in Table 1. Some of the more interesting and challenging tests, which were part of the entire test regimen, are presented.

Oil-Leak Testing

The oil-leak test was conducted to evaluate the sealing properties of the stator lead wires, grommets, and crimps. The stator assembly had to meet a specified head pressure of hot (225° F) chain case oil. This environment was not necessarily adverse for the stator assembly itself. However, the test was a good check of the capability of the stator lead wires, which exited the assembly, to prevent wicking of chain case oil.

To perform this test, a container with the stator assembly was welded to the stator casting, and hot oil was poured into the container. This provided the specified head pressure and the hot oil environment. Then, the stator lead wires were checked for oil leaking from them. This exposure continued for four hours.

The stator wires were not allowed to wick any oil. This was a very difficult test to pass because the capillary action of the wire facilitated oil migration through the cable assembly. As a result of this testing, crimps and seals were improved, and the performance of the stator assembly was enhanced.

High-Temperature Soak/Vibration Voltage Stress Testing

Particularly difficult was the high-temperature soak/vibration voltage stress test. In this test, the stators were mounted on an aluminum fixture that simulated the mounting of the stator in the motorcycle. The aluminum fixture was secured to a shaker table.

An accelerometer was placed near the mounting point of the stator. The accelerometer was located in approximately the same location as the placement used for gathering the field data.

The actual field data was programmed into the vibration controller. The samples were wired so that a voltage potential of more than 2,000 V DC, limited to 10-mA leakage current, could be applied between the winding and the stator stack. A specialized chamber was lowered over the units under test (UUTs), and the temperature was adjusted to a specified maximum value. At that point, the DC test potential and the field vibration levels were applied.

During testing, the leakage current of the UUTs was monitored and verified to not exceed specification. As was the case with the oil-leak test, standard equipment did not exist for this test. A special chamber was built to control the extreme temperature conditions while the UUTs were being vibrated. Special insulation was used to protect the shaker from the high temperature.

High-Current Soak Testing

Also challenging was the high-current soak test, which really was a current/temperature cycling test. It involved passing high current through a number of stator assemblies connected in series. The current was allowed to flow through the windings until the temperature of the stator windings reached a specified high temperature level, at which time the current was turned off. The stator windings were allowed to cool to the minimum operating temperature, and then the current was turned back on.

Preliminary testing was performed prior to initiating the high-current soak test to verify that all of the stator windings connected in series were responding to the input current stimulus in the same way. This was done through a data-collection and closed-loop control system, which cycled the power on and off based on the thermal time constant of the stator windings.

The closed-loop control system tested many stators at the same time and completed the testing more efficiently than if it were on one unit at a time. In essence, the high-current soak was really a high-current, temperature-cycling test that resulted in temperatures in excess of 300°F.

The power cycling continued for 40 h. It was interrupted every hour to apply a high DC test potential between the winding and the stator stack to check for any degradation in the dielectric properties of the materials used in the construction of the stator assembly.

Conclusions

The unique operating environments of the stator assembly posed a difficult challenge for simulating the operating environments and testing the stator assemblies in the laboratory. A customized test program was needed to effectively evaluate and assess the quality and reliability levels of the stator assemblies.

For design engineers, the testing helped to confirm and optimize the selection of materials and components used in the stator assembly. It also helped optimize the process used to manufacture the stator assemblies.

A logical, strategic approach was used to develop and carry out the testing program. The end results were the optimization of materials, manufacturing processes, and product robustness. Even though the customized testing program was somewhat more costly to conduct than a typical, standardized test routine, the return on investment and the dividends that were paid in the form of increased product quality and reliability and reduced field returns were well worth the added cost. The net effect was a positive effect on the bottom line.

References

1. Schutt, J., “Considerations for Vibration and Shock Testing of Electronic Assemblies,” Electronic Packaging & Production, December 1996, Vol. 36, No. 13.

2. Schutt, J., “A Common Sense Approach to Good Testing,” Test Engineering and Management, August/September 1998, Vol. 60, No. 4.

3. MIL-STD-202F Test Methods for Electronic and Electrical Component Parts.

4. MIL-STD-810D, Environmental Test Methods and Engineering Guidelines.

About the Author

Jeff Schutt has been the general manager of Trace Laboratories since its inception in 1988. Currently, he is president-elect of the Institute of Environmental Sciences and Technology and a member of several industry-related associations and societies. Mr. Schutt holds a master’s degree in business administration from the University of Wisconsin and a B.S. in physics from Marquette University and has completed course work at the Illinois Institute of Technology toward an electrical engineering degree. Trace Laboratories, 1150 W. Euclid Ave., Palatine, IL 60067, (847) 934-5300.

William J. Lubitz is a principal engineer at the Harley-Davidson Motor Co. He has 38 years experience in the design, development, and test of electronic control systems for the aerospace and automotive industries. Mr. Lubitz is a NARTE-certified EMC and ESD engineer and a graduate of the Michigan Technological University with a B.S. in electrical engineering. (414) 616-1226.

Jess E. Favela is a senior project engineer in the Electrical/Electronics Systems Group at Harley-Davidson. He has nine years of experience in designing and testing analog and digital electronics and control systems. Prior to this position, Mr. Favela specialized in the development of stabilized sighting systems used on land combat platforms for a military defense contractor. He holds a B.S. in electrical engineering from California State University and has completed graduate course work in control systems engineering at Oakland University. (414) 616-1381.

Harley-Davidson Motor Co., 11800 W. Capitol Dr., P.O. Box 25527, Wauwatosa, WI 53225-5527.

Table 1

 

1. High-Voltage Shock

2. High-Temperature Soak/Vibration, Voltage Stress

3. Temperature Shock, Voltage Stress

4. High-Current Cycle

5. High-Current Soak

6. Lead Bend

7. Lead Pull

8. Hot Oil Soak

9. Oil Leak

10. Performance Verification Checks*

10.1 Dielectric Strength

10.2 Insulation Resistance

10.3 Induced Voltage Test

10.4 Winding Resistance

Copyright 1998 Nelson Publishing Inc.

October 1998

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