High-speed drives:
Introduction
Since a long time we wanted to write more about high-speed drives, their theory, design, application etc. Finally we manage to publish this short post providing an introduction into high-speed drives. In line with our blog, the focus will be on medium voltage and medium/high-power drives.
Introduction
Variable frequency drives (VFD) gain increasing popularity across many industries. The various applications have very different speed ranges from ultra-low speed machines such as large mills in mining and minerals industry, low speed rolling mills in metals industry, medium speed pumps and fans up to high speed turbomachinery in oil and gas sector. The motor is connected with the load machine either directly or through a gear [1].
In turbomachinery a competing technology to VFD is a gas turbine or steam turbine. The turbine rotates with high speed and is directly coupled to the load machine (typically compressor) without any gear. Due to environmental aspects, easier controllability, reduced maintenance and other motivations there is a trend to replace such turbines with all electric drive system. In some cases both conventional and high-speed technologies can be used.
What is high-speed? Where does it start?
Before we go into more details we shall define what a high-speed electric drive is and where is the border between conventional drive system and high-speed drive system. There is no official classification to author’s knowledge so for simplicity we will consider as “high-speed” any drive with motor speed above 3’600 rpm, i.e. above a speed that a 2-pole machine can reach when supplied from 60 Hz network. Of course, things are not black and white and this classification is very rough. It might make sense to introduce also a “semi high-speed” category in addition.
Semi high-speed can be seen as speed range that a motor with traditional sleeve bearings or tilting bearings can reach. Such speed might go up to approx. 6’000 – 6’500 rpm. For higher speeds magnetic bearings are usually (but not necessarily) applied.
Of course, we consider medium voltage motors with shaft power in the range of several megawatt (MW). Low power motors have smaller diameter and can more easily reach higher rotational speeds. Larger motors with bigger diameter (shaft height) are more challenging to build as the circumferential speed becomes very high.
Why to go high-speed?
As motivation for high-speed drive technology these advantages are usually named:
– Direct drives (elimination of speed increasing gear)
– Increased efficiency (elimination of gear losses)
– Increased flexibility
– Oil-free technology in case of magnetic bearings (environmental aspect)
– Reduced carbon emissions
– Compact design (less footprint and weight)
– Low maintenance and wear-free operation
One significant motivation for high-speed technology against conventional drive system is the elimination of the speed-increasing gear. It allows to build the drive system more compact.
Increased efficiency is frequently named as advantage. How large the efficiency gain is might not be that clear. Obviously, the gear losses are eliminated. At the same time a high-speed motor tendentially has somewhat lower efficiency that a conventional one. Overall, the efficiency of a high-speed system is expected to be somewhat better than conventional electric drive system, but the gain is probably rather small. However, when comparing efficiency of high-speed electric drive system with system driven by gas or steam turbine, the efficiency gain is large and the benefits are undoubtable.
High flexibility is a benefit against turbine driven systems. The electric drive systems allow wide range of speed control, faster response and much easier integration into upstream control system (process control).
Oil-free technology is definitely an advantage as environmental concerns are taken more seriously. The gear usually requires quite large quantity of oil for lubrication. This can be eliminated by eliminating the gear as such. If the motor is equipped with magnetic bearings instead of sleeve or tilting bearings then the system can be realized completely oil-free.
Reduced carbon emissions are linked with higher efficiency as already explained. The benefit is especially considerable in retrofit cases where a steam or gas turbine is replaced with high-speed electric drive.
Compact design with reduced footprint is again important e.g. for retrofit market (brown field) where the available space is often restricted. Reduced weight is additional advantage e.g. in offshore installations.
Low maintenance and larger maintenance intervals help to increase the availability and profitability. It might be one of the key economical factors driving the decision towards electric high-speed technology against (more traditional) turbine driven solutions.
Motor design for high-speed drives
High speed motor available as induction and synchronous. Design differs from conventional one mainly in rotor construction [2-7].
♦ Semi high-speed synchronous machines
-Turbomotors derived from turbogenerators (known e.g. from thermal power plants)
♦ High speed squirrel cage induction machines
– 2-pole asynchronous machine with squirrel cage rotor
A) Solid rotor construction [5]
- superior stiffness and good thermal stability
- three principal variants of solid rotor:
-Smooth solid
-Slitted solid
-squirrel cage solid
B) Laminated rotor construction
- superior electrical properties,
- challenging mechanical integrity
-Special laminated rotor with tie rods and end shafts [6]
What are the applications for high-speed drives?
An important market for high-speed electric drive systems are compressors in chemical, oil and gas industry. The electric high-speed drive inherently requires a VFD and it provides high level of flexibility and wide range of speed control. Other application field might be high-speed pumps (medium speed range, semi high-speed). And finally test benches for various purposes. It is likely that more applications for high-speed drives will appear in the next few years. The market might see an interesting growth driven by the already mentioned advantages, especially the environmental aspects. Other trends, such as turbine to electric, will surely boost the use of high-speed electric drives.
Outlook
In the next post we will look at the VFD design aspects when being used in a high-speed application as well as the field experience with high-speed electric drives.
As always, this post is just scratching the surface. To learn more, subscribe to one of our premium plans and/or attend our webinars.
References
[1] To Gear or not to Gear? https://mb-drive-services.com/to-gear-or-not-to-gear/
[2] G.J. Neidhöfer, A.G. Troedson, “Large converter-fed synchronous motors for high speeds and adjustable speed operation: Design features and experience”, IEEE Trans. on Energy Conversion, Vol. 14, No. 3, September 1999
[3] Challenges of high-speed machine design, https://mb-drive-services.com/challenges-of-high-speed-machine-design/
[4] M. Ahrens, U. Bikle, R. Gottkehaskamp, H. Prenner, “Electrical Design of High-Speed Induction Motors of up to 15 MW and 20000 rpm,” IEE Conf. on Power Electronics, Machines and Drives, paper no. 487, pp. 381-386, 2002.
[5] Pyrhönen, J. Nerg, P. Kurronen, U. Lauber, “High-speed, 8 MW, solid-rotor induction motor for gas compression, proceedings of the 2008 International Conference on Electric Machines
[6] T. Maufrey, J.-F. Pradurat, L. Durantay, J. Fontini, “Comparison of 5 different squirrel cage rotor designs for large high speed induction motors”, PCIC Europe, Istanbul 2013, pp. 33-41
[7] Uzherov, A. Smirnov, C.H. Park, J.H. Ahn, J. Heikkinen, J. Pyrhönen, “Design aspects of high-speed electrical machines with active magnetic bearings for compressor applications”, IEEE Trans. on Ind. Electronics, Vol. 64, pp. 8427 – 8436, 2017
[8] M. Bruha, K. Pietiläinen, A. Rauber, High Speed Electric Drives – Perspective of VFD Manufacturer, HSTED, May 2020
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High-speed drives: VFD perspective - MB Drive Services · May 22, 2023 at 2:09 pm
[…] the introduction to high-speed drives [1] we have briefly described the design variants of semi high-speed and high-speed motors. The impact […]
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