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I (Klaus) was half way through writing our next Myth Buster article on factors that impact combined cycle power plant efficiency. Then one of Southwest Research Institute’s (SwRI) large variable frequency drives (VFD) for an electric motor driven (EMD) compressor failed — again. As usual, the vendor’s repair technician was on a different continent for training and wouldn’t be available to travel to our facility for another five days. SwRI operates several multi-MW VFD driven EMD compressors.

We also operate a large number of smaller sub-MW scale VFDs. Although we follow the manufacturer’s required operation and maintenance practices rigorously, there always seem to be problems and failures with the VFD or EMDs. With profound anger in my stomach, I decided to leave my esoteric thoughts on plant cycle efficiency factors for another day in the future.

Instead, I’m writing this myth about EMD/VFD maintenance, reliability, repair and spare part needs. In all fairness, VFD/EMDs are generally reliable. But SwRI completed a comprehensive user survey in this area several years ago. Funded by a consortium of pipeline companies, we statistically compared the reliability of EMD versus gas turbine and reciprocating engine-driven compressors. The results were surprisingly negative for EMD-driven compressors.

Electric motor driven compressors have lower than expected availability compared to conventional drivers, often because they just took a longer time to repair. The results from this study are available as a report from the Gas Electric Partnership.

A big problem with many studies, of course, lies in the fact that they usually don’t take the age of the equipment into account. This cuts both ways. A gas turbine installed in the 1960s, using relay logic control and wet seals for the compressors will quite possibly be harder to maintain and troubleshoot than a newer machine.

On the other hand, older equipment may be maintained by crews that have worked with them for decades. Which also means, that for any newer equipment, proper training of the operators should be an important part of operating these units.

Regardless of failure frequency or Mean Time Between Failures (MTBF), parts availability or availability of service technicians, one of the key problems is the time it takes to get drives repaired. In my experience, it often requires significant time to get a manufacturer’s technician to diagnose and repair the problem. And it can take a while to obtain the replacement part for what’s broken.

A lesson learned


The lesson learned for anyone operating and working with electric drives is to make sure that support is available. This should be a matter of thoughtful planning, not just dealt with on a fire drill basis when something goes wrong.

In all fairness, VFD manufacturers have come a long way to making more reliable drives and achieving higher availability for their equipment. If something ‘pops,’ it typically occurs during start-up and is associated with the power section of the drive. These power modules are typically provided as part of the start-up spare package.

Once running, VFD drives are reliable. But if something goes wrong, it is important to be prepared and to have an agreed upon plan for how to obtain timely service support.

To our readers: When writing the Myth Busters column, our goal is to provide arguments that are coherent and sometimes even cohesive. At the same time, we aim to be intentionally, although not abundantly forceful in our line of argument. The idea is to encourage discussion, debate and even dissent in some cases. In this spirit, we ask you to provide us with feedback on our topics through letters to the either the editor or to us directly. We are also happy to receive ideas for future Myth Buster columns. Is there a popular misconception in the turbomachinery field you wish to see skewered? Or a commonly held believe that you consider to be faulty? Let us know so that the concept can be dissected, discussed and disseminated.

Editor’s Note: Our Myth Busters just passed their 10-year anniversary as columnists for Turbomachinery International. Over the years, they have engendered a lively discussion among users on a wide range of topics. Their musings on potential myths have inspired a multitude of contributed articles either supporting their views, or providing a contrary opinion. Rainer and Klaus are warmly thanked for their valuable contribution to making the magazine the premier technical voice in the turbomachinery field. We look forward to working with them for many years to come.

Klaus Brun is the Machinery Program Director at Southwest Research Institute in San Antonio, Texas. He is also the past Chair of the Board of Directors of the ASME International Gas Turbine Institute and the IGTI Oil & Gas applications committee.

Rainer Kurz is the Manager for Systems Analysis at Solar Turbines Incorporated in San Diego, CA. He is an ASME Fellow since 2003 and the chair of the IGTI Oil and Gas Applications Committee. Any views or opinions presented in this article are solely those of the authors and do not necessarily represent those of Solar Turbines Incorporated, Southwest Research Institute or any of their affiliates.