[No. 4] The Historic Claw-Pole Machine

My grandson is interested in mathematics and history. Maybe it is an unusual combination, but both these subjects are helpful in looking forwards as well as backwards. In engineering we are often looking forwards, to find new and better ways. It has always seemed to me to be a good idea to look backwards for new ideas! If it works in history and mathematics, then why not in engineering? At least, by studying the works of previous generations, we can understand their logic and ingenuity.

An example is the claw-pole alternator. The first long-distance three-phase AC transmission system in the world was demonstrated in 1891 at the Frankfurt International Electrotechnical Exhibition, where over 100 kW was transmitted over 175 km at 13 kV from a hydropower plant at Lauffen am Neckar. The generator was a claw-pole machine built by Oerlikon in Switzerland. Shortly afterwards, the Johnson & Phillips (J&P) company was installing single-phase steam-powered alternators for lighting systems in England, also with the claw-pole configuration with its “monopole” field. AC had arrived.

Today, millions of claw-pole alternators are used as automotive generators. Of course the size is smaller (about 1 kW), and the the speed is higher. We can hardly overstate the historical importance of this machine.

Another example is the bicycle “Dynohub” of the Sturmey-Archer company, illustrated in the figure. This had a 20-pole cast magnet in Alnico 3-5 (very expensive!) and a monopole armature winding between two “imbricated” claw-poles. The magnet rotated inside the hub of the spoked wheel. It dates from about 1935. At a speed of 25 km/h, the output was about 3 W. With tungsten filament bulbs, the lighting left a lot to be desired! The generator was protected by its own synchronous impedance, which limited the current when riding fast downhill. For the designer, this machine would be quite difficult to analyse. It is very three-dimensional, and the magnet characteristics are much less linear than those we enjoy today. Eddy-current losses are also significant.

What is the significance of this machine today? It certainly shows how far we have come. Machines of similar size are now manufactured as drive motors for electric bicycles. They are not claw-pole machines, and they use far stronger magnets than was dreamed of in 1935. The increase in power is of the order of 50 to 100 times.

We find the claw-pole motor in a few other applications, notably in small timer motors (hysteresis motors) widely used in automatic valves in central-heating systems. These tiny machines are also of historic interest, dating back to the 1920s and made for many years with high precision in large numbers.

Fig. 1 Claw-pole alternator components (historic Sturmey-Archer Dynohub)

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.