Daniel Krähenbühl and Christof Zwyssig are doctoral students in the work group led by Johann Kolar, Professor and Head of the Power Electronic Systems Laboratory at ETH Zurich, which works on drive systems. Christof Zwyssig explains that "Although drive systems in themselves are an old research topic, we are working on one of their very specific forms. We study very small drive systems, whose main features are their unusually high rotation speeds and the fact that they can be controlled by power electronics with almost no losses." The research project has been running for three and a half years, and the first drive systems are now being installed in practical applications.

Half a million revolutions per minute

The research project began as an interdisciplinary collaboration inside ETH Zurich. The aim the researchers set themselves at that time was to build a portable combustion gas turbine with a volume of one litre and weighing one kilogram. It also had to yield 100 watts of electric power for a period of ten hours. The mechanical engineers scaled down a combustion gas turbine to small power and small size. From the technical viewpoint that means reducing the power and increasing the rotational speed – to 500,000 revolutions per minute. The electronic engineers had the task of developing a drive system that starts the turbine, generates electric power and is controllable via power electronics. To do this the motor needed to be built to withstand the high rotation speeds. Zwyssig explains: “At that time there was no motor fulfilling these specifications, either on the market or in research.”

Solar Impulse and drills

After the first drive system had been built, more areas of application emerged very quickly. The energy-efficient systems are now not much bigger than a matchbox and will some day be usable in turbomachines, compressors and turbines. One outstanding example is the mini-compressor that is to be used in the Solar Impulse Project, where it will supply oxygen to Bertrand Piccard and his pilots as they fly round the world in their solar aircraft. Miniature ultra-high speed drive systems can also be used in medical technology as drills for dentists and surgeons. Electrical engineering itself also needs the ability to drill holes on a micro scale.

Interdisciplinary work

The drive system developed by the scientists consists of three components: the machine, the power electronics and the software. Before a usable, durable prototype could be built it was first of all necessary to find suitable materials, machine types, electronics topologies and control methods that have low losses and achieve an efficiency of more than 90 percent at 500,000 revolutions per minute: amorphous iron, titanium housings and tiny ball bearings with balls smaller than a pinhead. Daniel Krähenbühl explains that “The drive system can already reach one million revolutions per minute,” but says the tiny cage containing the ball bearing’s balls is ruined in a very short time.

For Johann Kolar one essential challenge and appeal of the project lies in its interdisciplinarity. Kolar says "Our aim is to reveal, both theoretically and experimentally, the full breadth and depth of the questions of rotor dynamics, machine design, power electronics and sensor-free control." The future plan is for even more intense research in the area of mounting, i.e. the connection between the various mechanical elements. A non-contact magnetic bearing for 500,000 revolutions per minute is already in the test phase, and an air bearing for 1,000,000 revolutions per minute is planned as an alternative. One of the next aims is to make the systems even smaller and to implement what are known as Power MEMS (microelectronic mechanical systems).

Cornel Bartholet, Martin Bartholet and Christof Zwyssig won third place among 101 entrants in the Venture 2008 Business Plan Competition with a business idea based on these developments. This competition involves an initiative by ETH Zurich and McKinsey & Company, Switzerland, in which young Swiss entrepreneurs compete with their plans and if necessary receive support to found their business. The scientists now intend to enter the market in the autumn with their spin-off company “Celeroton”.

Engineering

The scientists’ highly compact miniature drive systems were installed first of all in compressor systems that convert electrical energy into mechanical energy. The processes are reversed in turbine-generator systems in which electrical energy is obtained from compressed air or other gases. In this system the researchers utilise the energy lost in processes in which a high pressure is generated first of all and then needs to be reduced again. The scientists developed a laboratory prototype of a highly compact, ultrahigh-speed rotary turbine-generator system measuring only 22 by 60 millimetres and achieving half a million revolutions per minute. The compressed air is passed around the generator first of all to cool it. Then it hits the guide vanes where it expands and is deflected to make it flow against the turbine blades at an optimum angle.
The turbine, which is coupled directly to the generator shaft, is accelerated to up to 500,000 revolutions per minute, depending on the inlet pressure and generator loading. The system is capable of generating up to 100 watts of electric power. The turbine-generator system developed by the scientists could replace flow control valves in gas pipelines for example, and could drive a generator via the turbine to produce electric power. It might also be possible to install them instead of conventional throttle valves in motor vehicle engines, because here again energy is lost in the throttle valve. The turbine could drive the generator as the intake air is throttled, again producing electrical energy. The energy recovery system could also be used particularly efficiently in industrial refrigeration plants.