Why torsional vibration in variable speed drive systems deserves an attention?
Introduction
I am an electrical engineer, same as probably most of the readers of our blog. The electro-mechanical interaction within the drive system is therefore not so easy to understand. Yet, it is crucial to avoid vibration issues, accelerated mechanical wear and even component damages and failures due to excessive torsional vibration.
Back in 2020 we have published a post about growing trend of torsional vibration issues [1]. Five years later I can confirm that the prediction was absolutely correct. Installations with increased vibration levels are reported
Is VFD the bad guy?
When a torsional vibration in a variable speed drive system occurs, the users tend to blame the variable frequency drive (VFD) to be the root cause. But is VFD really the bad guy?
It is similar like asking whether electricity is good or bad: It powers electric motors, supplies the lights, charges our laptops, tablets and smartphones. But it also kills people when not used carefully.
Electricity is a good servant but bad master. Likewise, the VFD offers abundant benefits (energy savings, smooth start, precise speed and torque control, improved dynamics etc) and new possibilities how to operate and optimize industrial processes. However, if the integration is not done properly, there is a risk of electro-mechanical interactions that can reduce the lifetime of the shaft components or even lead to a failure.
Open loop or closed loop control?
The speed of a motor supplied from a VFD can be adjusted using an open loop method (sometimes called “scalar” although this is not 100% correct) or by using a closed loop control.
In an open loop scheme the inverter adjusts the output frequency (i.e. motor stator frequency) and the motor is naturally changing the shaft speed accordingly. In case of an asynchronous motor there is an inherent slip. The slip can be compensated in some open loop schemes or might be neglected (especially if precise speed control is not required). In contrast, a closed loop system utilizes a speed feedback, either by measuring the motor speed with a sensor (encoder) or by estimating the speed using a mathematical model. This feedback signal is the actual speed which is compared with the reference speed and the difference of them (speed error) enters the speed controller.
The IEC standard states the advantages of closed loop control. However, the industrial experience shows a bit different pattern…
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IEC recommendation versus practical experience
IEC 61800-4 in one of the annexes states:
Closed loop speed control is recommended for the dynamically demanding applications such as rolling mill drives and for applications where continuous low speed operation with high torque is required. Moreover, high performance closed loop control may be necessary for some applications in order to increase the inherently low mechanical damping of the torsion resonances.
Does it match the industrial practice? When we do a bit of research we will find several torsional vibration issues reported in the literature where the root cause is the closed-loop electromechanical interaction. The experience shows that while a closed loop control can theoretically provide an additional damping, in most cases it is the opposite: there is an amplification inside the closed loop system. And open-loop control is in some cases the preferred solution.
I have troubleshooted and optimized numerous variable speed drive systems ranging from few hundred kW up to multi-megawatt range that suffered from an increased torsional vibration. And I must say that the “standard” closed loop speed control seldom increases the overall damping. Vice versa, dynamic speed control tends to amplify the torsional vibration through the closed loop interaction. When not recognized, it leads to accelerated mechanical wear, more frequent maintenance, trips (when reaching trip level of vibrations) or a fatal damage (especially when no monitoring is in place).
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What applications are affected?
Torsional vibration can affect practically any application. The question is more about the severity of torsional vibration and possible consequences.
For example, turbomachinery is traditionally sensitive to torsional vibration. However, there is also a sophisticated system of instruments and sensors to monitor the vibration. An elevated level of vibration indicates that the system is not in a perfect shape and a torsional oscillation may be going on. When the vibration level reaches a trip level, the protection system trips the whole drive train to avoid any major damages.
However, torsional vibration can affect simpler applications, such as e.g. fan drives. Such driven equipment often does not contain extensive vibration monitoring system. Thus, there is a risk that torsional vibration is present over long time without any detection until a catastrophic failure happens.
Minimizing the risk of torsional vibration
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Final message
If I leave you with anything from this post, then it is following:
- Do not underestimate the topic of torsional vibration
- Involve the right specialists
- Follow best practice guidelines
- If possible, verify and validate any simulation results with measurements
- Act early to prevent major damage
References
[1] Growing trend of vibration issues, MB Drive Services, January 2020, available online, https://mb-drive-services.com/trend-of-vibration-issues/
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