Why do variable speed drives fail and how do we test them?: Page 3 of 5

May 02, 2017 //By Kari Tikkanen
Why do variable speed drives fail and how do we test them?
Freezing cold, intense heat, vibration, corrosive chemicals… It’s impossible to know what sort of dangers drives might face. The answer is to design drives for durability and subject them to rigorous testing.

Reliability Demonstration Testing (RDT)

RDT testing is carried out to confirm that the product’s expected life meets or exceeds the target, and it is generally carried out as part of R&D.

Factors that must be known or determined to design ALT/RDT tests are the product’s expected reliability at end of life, its mission profile, required confidence level, and the allowed stress levels identified in HALT tests. The tests expose the product to the typical stresses it will experience over its entire lifetime. Most common stresses for drives are temperature and temperature cycling. To reduce the time needed to complete the testing, stress levels that exceed specified operating conditions are used.

Depending on the confidence level required and the reliability levels expected, 7-20 samples are typically needed. The product can be launched on the market following successful RDT testing, but the tests will continue in the form of ALT programs. These follow-on tests confirm the validity of the model and may provide opportunities for total life cycle cost reduction.

At ABB we RDT test complete drives in a special reliability container where they are exposed to drastic stresses. Typically, 10 years of life can be simulated in a few months.


Accelerated Life Testing (ALT)

ALT testing is used to determine the product’s expected lifetime. It is very similar to RDT, except that various stress levels are used and the tests are continued until failure. The difference between RDT and ALT testing is that in RDT we don’t know what the actual life is because the units are not supposed to fail. We just know that it should survive so many years or longer. ALT tests will give us estimate of the real life. RDT tests also assume certain failure models and material constants. ALT testing provides us with the model and material constants.

Depending on the expected confidence and reliability levels, ALT testing typically requires 7-60 samples. Large sample sizes and a significant test time are often needed, especially if the activation energy or other coefficients used by the model are not known.

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