An interview with Prof Ralph Kennel

Interview with Prof Ralph Kennel about digitalisation and electric drive technology. The age of motion driven by human labour is coming to an end – it is simply too expensive. Electric drive engineering and power electronics are therefore accordingly of great importance. This is the premise under which Professor Ralph Kennel heads the Chair of Electrical Drive Systems and Power Electronics at the Technical University of Munich.

What were your thoughts when Dyson advertised its digital motor?

Ralph Kennel: Allow me to respond with a little light-hearted humour. We always say when it comes to digital motors, either it works or it doesn’t. As you see then, the concept of a digital motor is not taken so seriously by us.

What do you understand by “digital drive engineering”?

R.K.: What we are talking about is a drive fitted with digital control. The electric machine has never been digital, nor will it ever be. Now the power converter was always digital. The only thing that has changed from analogue to digital is the control.

Which advantages does digital drive engineering offer?

R.K.: All possibilities that digital technology offers in terms of exchanging information are available. You get a lot of additional information from the drive, can transmit it externally and draw conclusions on this basis. This makes use of such drives much more conveniently and informatively.

What is the role of energy efficiency in this context?

R.K.: Energy efficiency is actually not a question of digital or analogue control engineering. However, the optimum working point energy-wise depends to a significant extent on external influences. This means that a lot of information is needed to find this optimum working point in the application. I can get this from a digital drive more readily than from an analogue drive. I therefore have more opportunities to actually be energy-savvy.

Where do you see the most important markets and applications for these informative drives?

R.K.: In the automotive sector in particular at the moment. The industrial sector has been doing this for a long time, with the drives networked all along via computerised numerical controls, which in turn had the required intelligence. Since the beginning of the 1990s, when the drives became digital, more and more of the intelligence has migrated to the drive. This step must still be taken in the automotive sector.

Is this also reflected by your institute? If not, which research ares are you currently involved with?

R.K.: On one hand, sensorless control is a big topic for us. What this means is that we determine the position of the rotor from the motor’s current and voltage values. We are also looking at what is known as predictive control – a modern form of control that can be used to process more information. Renewable energies are also on our radar at present: we are looking at how best to transmit energy from the source to the network. The last field of research we are looking at is contactless energy transmission. Many of these topics also affect electromobility in a marginal way, but the main focus of attention for us comprises industrial applications.

What can be understood by “predictive control”, then?

R.K.: A controller normally waits until a deviation occurs and only then responds. The predictive controller anticipates in advance what will happen. It operates virtually like a chess player, calculating what happens if it makes a particular move.

Can you give us an example of where it is used?

R.K.: In one particular project, for example, we controlled an impedance indirect converter predictively. These converters have the advantage that they are not susceptible to breaks, nor are they impacted by short circuits and open circuits. However, they are more difficult to control, because the individual variables have a much greater impact on each other. Conventional control engineering is simply overwhelmed in this respect. We can, however, solve this with predictive control, and we see lots of potential here for the future.

Contactless energy transmission sounds exciting. What work are you doing in this area?

R.K.: We are currently considering whether we could also transmit energy contactlessly for certain drive applications under the umbrella of 5G. This is of course difficult if the drive continually needs 10 kilowatts, but there are many small drives that only work intermittently. No cable is needed to supply them. The problem in this case is to bring the energy only to where it is actually needed and not to distribute it aimlessly in space.

What about the electric motors themselves – are they already fully developed?

R.K.: No – research is also under way in this area. But this has nothing to do with digital technology. It relates to physics.

What are the most exciting developments for you in the area of drive engineering at the moment?

R.K.: What few anticipated is the use of new power semiconductors, in other words silicon carbide. This will have a number of effects. For example, better power semiconductors should be possible as a result. Better generally means faster. This means that the switching frequencies will no doubt increase. However, faster in this sense also means that side effects will occur faster. More investment will -undoubtedly be needed here in order to get to grips with this. But this is the direction that we are moving in.

The prospects for drive engineering therefore remain exciting – there is no need to worry about the next generation …

R.K.: We are not only drive-engineering technicians, but also power-electronics technicians. To quote Barack Obama at this juncture: he once said something like “nothing works without power electronics”. The power-electronics technician will still be needed in the future. I therefore assume that our graduates will not be without work. Anyone with even a reasonable interest in this technology and the topic of energy should consider studying in this field.

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