[No. 6] The Periodic Table of Electric Machines

When the archaeologists dig us up in 5,000 years’ time, they will find a large number of electric motors and generators.Who knows if they will understand what they find?They will surely marvel at some of the winding configurations, and they might even rediscover high-energy magnets.When they find very large machines, they will ask “how big were those people?”, and when they find wristwatches and nano-motors, they will ask “how small were those people?”And did the large people and the small people inhabit the Earth at the same time?

The figure shows the family of electric machines in a “sat-nav” cartoon.Going forwards in one direction, we approach the machines of the future on the right.Going backwards, we find the machines of the past on the left.Machines have their periods or eras, so it seems appropriate to call it the Periodic Table of electric machines.

Going forwards, or even standing still, we cannot avoid seeing the machines of the present day.Which ones attract our attention, and which ones are the most important?

The induction motor must be somewhere in the centre, with a large “footprint” or profile.Induction motors convert nearly half the electrical energy that is generated in industrial countries.So they are important in connection with energy, including renewable energy and climate change.The induction motor is also at the heart of the AC power system, as it has been since the days of Tesla and the development of interconnected 3-phase power networks.It is difficult to displace it from this important central position.

Looking a little to the left, and taking a historical perspective, the AC synchronous machine is the main source of electrical power in interconnected networks and also in isolated power systems (such as, for example, on aircraft or oil-drilling platforms).It is also the main player in renewable energy generation in wind turbines and in hydro-electric power.In these and other applications it has been injected with new technology using high-energy permanent magnets and power-electronic converters.

To the right of the induction motor the future is clearly marked with the permanent-magnet (PM) brushless motor, which is now mainly an inverter-fed AC motor.It has a high profile because of electric and hybrid vehicles, and many other recently-developed applications in automation, robotics, domestic appliances and all kinds of actuators.

Some readers may be surprised to see the DC motor sitting near the middle of the road next to the induction motor.Well, I must admit a personal bias, and a love of the DC motor, partly because of its huge contribution to humanity in the field of variable-speed drives on railways, in elevators, paper mills, and many other areas.Small PM DC motors are still made today in huge numbers for automotive auxiliaries, and this is not going to change.Also, from a theoretical point of view, the DC motor gave us the basis for field-oriented control of AC machines, and it deserves its place in the hall of fame even if someone moves it to the left-hand side of the road.

The left-hand side is packed full of fascinating, complex, ingenious machines such as the rotary converter, the Schräge motor, the Winter-Eichberg motor, and many others: all obsolete.They will never return unless power electronics goes out of fashion.The universal AC commutator motor is over there too, hanging on for dear life in mains-fed power tools and some domestic appliances.

The right-hand side is equally well populated with interesting concepts, some of which are obsolete now, long before anyone risks manufacturing them to prove it.But in that nursery, who knows what we will find?To borrow a slogan from a famous motor manufacturing company (A.O. Smith), “Expect something special”.

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Prof. Miller was educated at the universities of Glasgow and Leeds, U.K., and served an industrial apprenticeship with Tube Investments Ltd. He worked for G.E.C. in the U.K. and General Electric in the United States. From 1986-2011 he was professor of electric power engineering at the university of Glasgow, where he founded the Scottish Power Electronics and Electric Drives Consortium. He has published more than 200 papers and 10 books and 10 patents, and he has given many training courses. He has consulted for several industrial companies in Europe, Japan and the United States. He is a Life Fellow of I.E.E.E. and in 2008 he was awarded the Nikola Tesla award.

The Green Book: “Design of Brushless Permanent-Magnet Machines”