Challenges of high-speed machine design
In turbomachinery applications the conventional solution is a 4-pole machine with nominal speed in the range 1’500 – 2’000 rpm and a speed-increasing gear. However, the alternative is a gearless high-speed drive. Such concept may offer several advantages (compact design with small footprint, absence of gear, potential improvement in efficiency, retrofit projects replacing a steam turbine with electric driver) but is also associated with couple of challenges concerning the design of a high-speed machine. Let’s have a look!
1. Surface speed and centrifugal forces
As the rotational speed increases, the circumferencial speed goes up as well. The centrifugal forces acting on parts such as end windings increase significantly. That represents a challenge for the mechanical integrity.
Practical designs therefore usually limit the diameter of rotor to a level where surface speed can still be handled. In conventional turbo-generators the maximum rated speed is 3’600 rpm (60 Hz, 2-pole machine). That is already a huge challenge and many 60 Hz generators are built as 4-pole machines with 1’800 rpm. In VFD driven machines the stator frequency can theoretically be set almost arbitrary. The VFD is normally not the bottleneck and can supply the machine with a frequency of 250 Hz or higher without any major issues. The machine windings must then withstand severe centrifugal loading.
In high-speed range the preferred rotor design is a solid one due to its superior mechanical behavior. Yet, there are also special solutions with laminated rotor asynchronous machines. Such design is much more complex. The advantage is a better power factor of the machine. For synchronous machines the solid rotor is used exclusively as there is no benefit from laminated construction.
2. Increased losses and cooling
Increased stator frequency leads to increased losses. The hysteresis losses are proportional to the frequency. Eddy current losses even grow with the square of the frequency. Moreover, there is skin effect that increases the effective resistance of the windings and the corresponding winding losses.
However, that is just part if the challenge. While the losses increase with machine speed, the physical size of the machine decreases. That leads to a situation of higher losses, lower surface area for cooling and generally higher power density. The only way to solve the issue is the use of highly efficient cooling system.
Water is an excellent cooling medium. Conventional water cooled machines only use water in the external circuit. Internally they are air cooled using an air-to-water heat exchanger. At high power density a more efficient solution is required. The stator winding is directly water cooled, i.e. the hollow conductor of the winding is cooled by a deionized water. On the rotor direct water cooling would be very complex. The solution for large high-power generators is hydrogen as cooling medium. Thermally it is not as good as water but it is much easier to handle during operation (see our previous article here about VFD cooling – similar applies to the motor design).
Special solutions exist for pipeline integrated compressor drives. There the motor is cooled by the process gas itself.
3. Bearing technology
Bearings need special attention to support the operation range at high speeds. Up to certain speed fluid film sleeve bearings are still possible. Important is to properly examine the bending modes of the system. For highest speeds the preferred solution is an active magnetic bearing (AMB) system. The AMB is way more complex. But it can provide benefits such as virtually no mechanical wear (→ lifetime, maintenance intervals), oil-free infrastructure (environmental), active damping control etc.
There are references with oil type bearings going as high as 12’000 rpm.
4. Critical speeds
Critical speeds have major impact on drive’s performance. In ideal case all critical speeds are outside of the operating speed range with suitable separation margin. For VFD applications a sub-critical rotor design is the preferred choice. The machine always operates below the first critical speed and the rotor can be considered as stiff. A conventional 4-pole machine almost always fits this category. In contrast, high-speed 2-pole machines can often be super-critical. In such case the machine needs to cross one or more critical speeds during each and every start-up. Moreover, the super-critical design may limit the minimum operation speed and restrict the speed range. Large turbo-generators typically have several critical speeds below the rated speed. On the other hand, as they are fix-speed machines the crossing of those critical speeds is just a short transient. In variable speed application it needs to be analyzed whether the machine could operate continuously at such speed (depending on damping of that vibration mode) or whether some restricted speed ranges (speed windows) need to be implemented.
Highest speeds are mostly realized with 2-pole asynchronous machine using a solid rotor technology. Despite the high speed, the rotor has excellent vibration performance and allows very wide speed range (e.g. 30% to 110% rated speed).
5. Power-speed capability
The maximum speed of the machine needs to be set in relation with the output power. For low power applications it is not so difficult to reach very high speed. But in high-power industrial applications the situation looks quite different. As already mentioned, most limitations come from the mechanical design:
- Centrifugal forces on rotating parts limit rotor diameter
- Bending crritical speeds limit the core length
That basically means that with increasing power the machine dimensions and volume get more and more restricted which then also limits the achievable output power. The power-speed capability curve has following shape:
Example of high-speed synchronous machine capability
While at 4’000 rpm the shaft power can go up to 70 MW, at 6’000 rpm the maximum output is limited to roughly 20 MW. Then at 8’000 rpm the output is restricted to less than 10 MW.
Summary
High-speed machine solutions offer several benefits from the system perspective: compact design with reduced footprint, elimination of speed-increasing gear, elimination of lube oil system etc. In addition, the VFD technology supports this trend and medium voltage VFDs with output frequencies up to 250 Hz or higher are considered as proven solution these days. Yet, the main challenge is the machine design itself. In this blog post we have informed about some of the topics that the designer of high-speed machine needs to deal with.
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References
[1] To gear or not to gear? https://mb-drive-services.com/to-gear-or-not-to-gear/
[2] Power, speed and torque, https://mb-drive-services.com/power-speed-and-torque/
[3] Variable speed motors and critical speeds, https://mb-drive-services.com/variable-speed-motors-and-critical-speeds/
[4] T. P. Holopainen, P. Jörg, O. Liukkonen, Comparison of Two- and Four-Pole VSD Motors up to 4000 RPM, 45th Turbomachinery and 32nd Pump Symposia (TPS), Houston TX, September 12 – 15, 2016
[5] T. Mauffrey, J.-F. Pradurat, L. Durantay, J. Fontini, Comparison of 5 different squirrel cage rotor designs for large high speed induction motors, PCIC Europe, 2015
