Electro-mechanical interaction:
Growing trend of vibration issues
We have observed numerous vibration issues associated with rotordynamics in variable frequency drive systems across the industries. The trend is growing and this shall be a warning sign. There are multiple reasons for this trend. Here we name the most important ones from our perspective:
Lack of competence and experience
After several years of growth the VFD market was severely hit in 2015-2016. The depression took for about three years. Many manufacturers were forced to reduce the number of employees. Consequently, some experienced persons left the business. Due to that there is a general lack of competence and know-how. The actual trend in VFD market might help to slowly recover the competence level, but it will take a long time. The expertise is like a trust: it takes long to build it and can be lost within shortest time.
Price pressure
Due to persisting price pressure the end user became more open for cheaper products and less experienced manufacturers. These products do not have the same quality and performance as other proven equipment. And these manufacturers do not have the same skills to fix the vibration issues on site.
Experienced manufacturers tend to be more expensive and lost part of their market share to competitors with less knowledge and competence. Good engineers are not for free, but the market was not willing to pay for them. There will still be lot of equipment being commissioned that will run into vibration issues. At some point certain manufacturers might be banned and some of them might learn and understand the issues and resolve them. However, it is a learning curve.
Summary: End user nowadays receives significantly better VFD price, but may generally expect more technical issues.
Broader use of variable frequency drives
VFDs are increasingly used and take over applications where traditionally direct on-line (uncontrolled) motors dominated in the past. The torsional behavior of a drive system with VFD is naturally different than a system with a fix speed motor. The torque spectrum is different and there is a potential of torque amplification (as will be explained in upcoming posts). The industry might still not fully understand the mechanism of torsional interaction and is not able to define effective counter-measures.
Interdisciplinary topic
The electro-mechanical interaction, as the name already says, connects several disciplines: electrical engineering (electric machines, power electronics and drives), control engineering and mechanical engineering or rotordynamics, respectively. Sometimes there is a wall between electrical and mechanical disciplines and that is where the issue starts. These two engineering worlds need to talk to each other and have a fruitful exchange. It might require some patience and empathy from both sides.
Responsibility matrix
Imagine a medium size variable frequency drive system, e.g. frequency controlled compressor in a refinery. There is the end user who will operate the plant. In some cases it might be several users having shares in the project. Then there is the EPC taking care of engineering, procurement and construction. The compressor is designed and manufactured by the OEM and the OEM purchases the drive system from a VFD manufacturer. Who is finally responsible for proper integration?
The EPC usually issues specifications that define the torsional behavior. The OEM performs a torsional analysis of the entire shaft string considering the inputs from VFD and motor manufacturer (torque fluctuation in normal operation, transient torques in case of a fault). However, experience shows that it does not always properly work. Moreover, if the torsional mitigation shall be done correctly, it shall be an iterative process and some information shall flow back to VFD manufacturer as well (this part usually fails).
Preparation of commissioning
Commissioning done in ‘trial and error’ mode is typically not much appreciated by the user. Instead, the manufacturer shall have rules defining the key control parameters. These parameters shall be defined during execution, i.e. prior to site arrival. There is always time pressure on site to complete the tasks as fast as possible. Therefore it is even more important to prepare things well in advance.
On site verification
As already mentioned in our introduction (click HERE to access), torsional vibration is difficult to measure and the excessive torsional stress might remain undiscovered until mechanical failure appears. Therefore it is beneficial to make as much verification on site in early stage as possible. Commissioning phase is the perfect time for such verification. The VFD might provide a valuable information that can be used for diagnostic purpose. Unfortunately, this option is seldom used.
Expertise in rotordynamics
Luckily there are still several skilled companies around us focusing on rotordynamics and solving lateral and torsional vibration issues. Some of these companies even develop their own software for modeling to rotor dynamic systems.
Possible companies to work with
We have our personal experience with following companies:
DELTA JS AG
— Delta JS is located in Technopark Zurich in Switzerland
— The company develops, maintains and markets the software MADYN 2000
— They provide training courses on regular basis
— We have worked with them on several projects
— link to company’s website: https://www.delta-js.ch/en
XDOT™ Engineering
— Active in research and development of special bearings and rotordynamics
— Company provides consulting services
— They organize regular training courses/seminars
— link to company’s website: https://xdotea.com/
Upcoming rotordynamics training
Do you want to expand your knowledge about rotordynamics? To understand the vibration problems you are facing and hopefully also find a solution?
The 2021 Spring Rotordynamics Training will take place in March.
The event is jointly organized by No Bull Engineering and Xdot Engineering.
Check out this website to learn more:
https://www.rotordynamicscourse.com/
The training is organized by recognized professionals:
Malcolm Leader, P.E. (President – Applied Machinery Dynamics)
Dr. Erik Swanson, P.E. (President – Xdot Engineering and Analysis)
Mark A. Corbo, P.E. (President – No Bull Engineering)
Each of the experts is covering one major topic of the training course.
Lesson learned
The learning from one project shall flow into the processes and general improvements. Instead, it is often considered as isolated problem for the specific site. It might be fixed in the end, but there is often no sustainable way how to mitigate similar issues in the future.
What is your experience? Do you see other reasons causing electro-mechanical issues such as torsional vibration? Let us know! You can use below contact form to submit your inputs.
References
[1] Introduction to electro-mechanical interaction, https://mb-drive-services.com/elmech-intro/
[2] Medium voltage AC drives, https://new.abb.com/drives/medium-voltage-ac-drives
