Why High-Speed Machines?
What are high-speed machines?
Unfortunately, there is no clear definition as to what constitutes a 'high-speed' electrical machine (motor or generator). Furthermore, the limit between medium and high-speed also depends on the application - electric vehicle (EV) motors generally operate at higher speeds than industrial utility motors, but still fall well below speeds seen in compressor applications.
Nevertheless, there are three key figures that can be considered:
The most criterion for a high-speed machine is probably its number of revolutions per minute, or rpm.
Of all electrical machines, the single most common rpm is probably close to 3000 rpm, corresponding to a two-pole motor at operating at a 50 Hz electrical grid. By contrast, EV motors nowadays move between 0 and 15 000 rpm or so.
Now, what rpm would count as high-speed, then? Most experts would probably classify 20 000 rpm and above as high-speed; however, smaller numbers might also count in certain applications.
The next criterion relates to an actual speed, namely the speed at which the rotor surface travels as the machine rotates.
Traditional medium-speed machines typically operate in the low tens of meters per second range, while values starting from 60 m/s and ranging up to 200 m/s and above would be readily classified as high-speed.
Considering the surface speed also helps us understand why the rpm alone is not a sufficient quantifier. Indeed, the surface speed is proportional to the rpm times the rotor diameter. Meaning, a large diameter megawatt-level aviation motor prototype rotating at a modest rpm might have a surface speed approaching 150 m/s.
The third number of interest is the electrical frequency - the frequency of the currents and voltages that the machine is fed with.
Again, 50 Hz is probably the most common one, while high-speed applications (that is to say, applications generally classified as high-speed) begin from 450 Hz or so. At the moment, topical research is being conducted at the 1-4 kHz range, for high-powered (hundreds of kilowatts to low megawatts) motors and generators, for instance for aviation applications.
Why high-speed machines
There are two main reasons that can make high-speed machines an attractive choice:
- The application demands it
- Performance benefits
Application demands high-speed
An obvious reason to utilize a high-speed machine is that the application itself demands it. For examples, turbines and compressors naturally spin at very high rpms. Having the motor or generator spin at the same speed can simplify the design considerably, eliminating the need for a gearbox.
The second factor that makes high-speed machines attractive are the performance benefits that they can offer. Indeed, high-speed machines generally make it much easier to reach a good compromise between efficiency and power density.
This translates to reduced life-time energy consumption and initial raw material requirements.
Now, let us try to understand why high-speed machines can perform so well.
Now, please consider the nearby picture. It presents a permanent magnet mounted on a rotating shaft, with a single coil (red) wound around it and connected to an alternating current source.
From high-school physics, we may remember that feeding current to a coil makes a compass needle turn. Now, permanent magnet electric motors operate the in exactly the same way, with the coils in the stator making the rotor with its magnets turn.
Expressed in a bit more detailed way, feeding a current to the coil exerts a torque on the magnet, proportional to the current. From high-school mechanics we may remember that the power output is then the product of the torque and angular velocity, i.e. proportional to the speed.
At the same time, the same current also generates resistive losses in the coil, equal to RI^2.
Now, if we increase the rotation speed, this directly increases the power output proportionally to the speed. At the same time, the resistive losses and the motor size stay unchanged, resulting in increased power density AND efficiency.
Indeed, increasing the speed of an electrical machine is by far the most effective way of improving its power density without compromising its efficiency (barring any unexpected leaps on the material development front).
In reality, things are of course more difficult, bringing us to the next point.
Challenges of high-speed machines
One of the main obstacles against more widespread adoption of high-speed machines are the numerous engineering challenges included. Indeed, high-speed machines are very challenging to design, representing a highly multi-disciplinary problem.
The key design and simulation challenges include:
- Electromagnetic phenomena
- Especially high-frequency losses
- Electric phenomena
- High electrical frequencies, increasing the risk of e.g. insulation and bearing failures due to voltage stresses and leakage currents.
- Mechanical aspects
- Very high centrifugal stresses in the rotor with low safety margins.
- Risk of catastrophic vibrations
- Thermal aspects
- High-speed machines often feature a high power density, making heat extraction of critical importance.
Applications of high-speed machines
Traditionally, high-speed machines have been predominantly employed when required by the application itself, for example to drive turbocompressors.
In the near future, with the drive towards electrification, and energy and material utilization efficiency, some application areas include:
- Electric aviation
- High power motors with very high efficiency required
- Regional aviation: energy and operating-cost savings, re-vitalization of smaller & shorter routes in a realistic 10-20 year time-horizon
- National and international flights: fuel cost and emission reduction via e.g. turbo-electric systems
- Hydrogen economy
- Clean hydrogen for e.g. fertilizer production: very high-speed centrifugal compressors (3x compared to natural gas) needed to pre-compress large volumes of the atmospheric-pressure hydrogen coming from the electrolyzer
- Utilizing hydrogen for energy in high-speed turbines, e.g. for grid load balancing or isolated microgrids
- High-power heat pump systems for district heating and industrial applications
- Heat pumps may save up to 60-78 % energy compared to direct electric heating
- Supercritical CO2 generators
- Improved efficiency and greatly reduced size compared to steam generators, applications in e.g. nuclear power plants and microgrids