Solutions for variable speed control: Introduction
This post introduces the most common solutions for variable speed control in industrial applications. Before starting with the topic we want to refresh the meaning of some commonly used abbreviations.
Variable speed drive (VSD)
– A general term for anything that varies the speed of motor driven load (AC drive, DC drive, eddy current clutch drive, hydraulic variable speed drive etc.). The speed is controlled electrically or mechanically.
Variable frequency drive (VFD)
– A smaller subset of VSD. It is an equipment controlling the speed of AC motor by altering frequency and voltage of the motor. Therefore, VFD is used for purely electrical speed control of motors. For example, DC motor drive is a VSD, but not a VFD. Note that VFD refers to the power electronics device, not to the electric motor.
Adjustable speed drive (ASD) has similar meaning like VFD. Other terms like Static frequency converter (SFC) are less clearly defined. SFC often refers to a power electronic converter with fix output frequency (e.g. grid intertie from 50 Hz to 60 Hz or vice versa). However, the LCI drive is sometimes also called SFC in customer specifications or single line diagrams. The exact meaning is usually clear from the context.
So what are the solutions for controlling the speed of driven load? There are several and we focus on those actively used in the industry today.
1. Variable frequency drive
The entire blog is dedicated to variable frequency drives and many things have already been said. VFDs receive increasing popularity due to energy saving potential, precise speed and torque control (static and dynamic), low starting current, high input power factor, high availability, low maintenance, very high flexibility, easy integration into complex overriding control system, positive experience linked with growing installed base and gradually decreasing price level. This technology is clearly leading the market and is perfectly scalable from low-voltage micro-drives up to medium voltage drives rated approx. 100 MW [1]. Considering the focus on energy efficiency [2], local emissions and digitalization the VFDs have significant growth potential.
2. Hydraulic drive
The solution is based on hydrodynamic coupling (fluid coupling) in combination with electric motor. The solution is based on hydro-kinetic principle. Basically the motor shaft operates direct on-line with approx. rated speed and the fluid coupling provides certain slip. This slip shall not be confused with a slip of asynchronous motor. The slip of fluid coupling is the difference in speed between impeller and runner [3]. Therefore, the output speed is always lower than the input speed (most VFDs, besides cycloconverters in special applications, do not have such limitation). It can reach relatively high efficiency in a narrow speed range just below the nominal speed. When the speed range gets wider, the efficiency at lower speeds drops sharply [4]. As a mechanical solution it is inherently quite robust. However, it requires additional space in the machine room and besides this special coupling a fairly large lube oil system and oil re-cooler are required. Hydraulic drive provides the capability to control the speed in narrow range, but does not solve the issue with motor starting current known from direct on-line motors (starting current means stress for the grid; starting transient torque means stress for mechanical shaft line). Also, hydraulic drive has poor power factor at reduced speed resulting in higher reactive power consumption (exact opposite of VFD). A torsional analysis of a shaft string including hydrodynamic coupling is usually more complex and seems to be rather project specific.
A more detailed comparison “VFD vs fluid coupling” is provided in another post in this series – see ref. [5].
3. Gas turbine driver
Gas turbines can also be used as prime movers. It might be a convenient solution in case the customer produces gas and has his own ‘cheap’ fuel. Gas turbines can reach high power rating and high speed. It means that when used as compressor drivers there is no need for a gearbox. However, gas turbine normally cannot start from standstill and an auxiliary system is needed to accelerate the turbine to minimum speed. Afterwards the gas turbine is fired and accelerates on its own. The dynamics is very slow. Of course, the tightening regulations on emissions and noise make it very difficult to install new gas turbines. In this sense an electric driver has clear advantages. There is a trend to replace gas turbines by electric drivers as part of program to reduce carbon footprint. Gas turbine also requires extensive maintenance – inspections are typically once per year and can easily require a shutdown for 4 to 8 days [6, 7].
4. Steam turbine driver
Similar like gas turbines also steam turbines can be used as prime movers. Historically it is actually one of the oldest industrial prime movers as such utilized since end of 19th century. The best known examples are steam turbines in coal-fired or nuclear power plants. Those turbines have huge ratings. Industrial companies might use much smaller steam turbines as drivers, especially when their process produces steam as byproduct. Same as in previous case, the regulations on emissions motivate many end users to replace steam turbines with electric drivers. Steam turbines also reach high speeds and speed increasing gear is not needed. They are not suitable for low speed applications or applications with dynamic requirements. For references see e.g. [8] on retrofits of steam turbines with electric drives and [9] as recent news from petrochemical industry.
5. DC motor drive
Older electric solution for variable speed was the use of DC motors. The control is relatively simple (easier than AC drives), but DC motors are more complex for manufacturing (compared to squirrel cage induction machines or permanent magnet machines). The inherent slipring contact with brushes and mechanical commutator require more intensive maintenance. Therefore, these drives are becoming rare, at least in new installations. Some users might still prefer these drivers due to their past experience. To be fair we shall add that new development of materials of brushes (composite material instead of pure carbon) extended the service intervals and made these machines for user friendly. However, it does not change the trend making DC motor drive solutions rare in the future.
With regards to digitalization and automation the VFD has advantages that other drive technologies cannot keep up with. It is very easy to integrate into a larger system, provides variety of fieldbus protocols and allows highest control accuracy and advanced diagnostics (electric signals much easier to measure and post process compared to mechanical quantities). VFD is ideally scalable in power from micro-drives in range of some watts up to large medium voltage drives over 100 MW.
It is also the most universal solution fitting almost any application while the alternative solutions might be a good fit just for specific cases. Gas or steam turbine might make sense to drive a compressor, but no one would use such type of driver e.g. for a rolling mill.
Pros and cons of above variable speed drive solutions will be discussed in greater detail in upcoming blog posts of this series. Therefore, stay with us!
References
[1] ABB Drives webpage, https://new.abb.com/drives (accessed on 24th September 2019)
[2] What is an AC drive, https://www.youtube.com/watch?v=JJbZSCSCIDg&feature=youtu.be (accessed on 24th September 2019)
[3] Fluid coupling principle, https://www.coalhandlingplants.com/fluid-coupling/ (accessed on 24th September 2019)
[4] M. Sirovy et al., “Variable speed pumping in thermal and nuclear power plants: Frequency converter versus hydrodynamic coupling”, IEEE PEDS, Singapore, 5 – 8 December 2011
[5] Comparison of VFD and fluid coupling, “VFD versus fluid coupling”, http://mb-drive-services.com/comparison-of-vfd-and-fluid-coupling/
[6] H. Devold, T. Nestli, J. Hurter, “All electric LNG plants”, ABB Process Automation Oil & Gas, https://library.e.abb.com/public/9e770a172afc8d7ec125779e004b9974/Paper%20LNG_Rev%20A_lowres.pdf (accessed on 24th September 2019)
[7] F. Kleiner, S. Kauffmann, “All electric driven refrigeration compressors in LNG plants offer advantages,“ Gastech 2005.
[8] G. Schwarz, R. Gillmann, “Benefits of replacing steam turbines with electric drives and what to consider”, PCIC Europe 2019
[9] Petrochemical companies form Cracker of the Future Consortium and sign R&D agreement, https://www.brightlands.com/news/2019/petrochemical-companies-form-cracker-future-consortium-and-sign-rd-agreement (accessed on 24th September 2019)
