Engineering: A Very Short Introduction by David Blockley

Author:David Blockley [Blockley, David]
Language: eng
Format: epub
ISBN: 9780199578696
Publisher: Oxford University Press
Published: 2012-01-15T07:00:00+00:00

12. A rotating coil

Large-scale power was still not feasible, however, until the permanent magnets were replaced by electromagnets. Electromagnets are not permanently magnetic – they are iron bars that only become magnetic when an electrical current is passed through wire coils wound around them. The wire coils are called field windings. The clever part was that the DC generator (dynamo) was self-excited, i.e. some of the current being generated was also used to create the electromagnets. It worked because the iron bars retained some residual magnetism – just enough to enable the dynamo to produce some output voltage, causing current to flow in the field windings and fully magnetize the bars.

Werner von Siemens produced the first big improvement in the efficiency of dynamos when in 1856 he developed an iron cylinder armature (the rotating part) with slots containing the field windings (see Figure 13). The AC was converted to DC using a split ring commutator to give the output voltage V as it varies through time t as shown in the figure. Soon after, in Italy in 1860, Antonio Pancinotti produced a machine with a solid ring armature that could be used as a generator (i.e. converting mechanical to electrical energy) or a motor (i.e. converting electrical to mechanical energy). About 10 years later, a Belgian working in Paris, Zénobe Théophile Gramme, used Pancinotti’s idea of a ring armature, but instead he formed it from iron wire. He obtained, for the first time, a more or less continuous DC by tapping the wire at very short intervals around the ring and connecting them to a multi-segmented commutator. In doing so, he effectively smoothed out the humps in the variation of voltage V through time t in Figure 13, and the greater the number of segments the smoother the DC. His machines were improved and made in several countries and used mainly for lighting. Siemens in Germany and England, R. E. B. Crompton in England, Thomas Edison in the USA, and many others contributed applications in factories, agriculture, and locomotives – the first electrical train was opened in the ‘deep’ London Underground in 1890. A new era in the generation and use of electricity was ushered in – electrical engineering moved from infancy to adolescence.

Up to this time, DC power was preferred, but unfortunately it could only be supplied over relatively short distances. AC generators were being made but could not be widely used because it was difficult to synchronize the wave form of their outputs. They were therefore only operated singly. The real breakthrough for AC came when in 1888 Nikola Tesla, an Austrian-Hungarian who had worked for Edison, announced that he could make a rotating magnetic field from AC currents which are out of step. His induction alternator consisted of an electromagnet (the rotor – shown in Figure 14 just as a bar magnet for simplicity) rotated, by an external source, within several separate pairs of coils of copper wire – in Figure 14 just two pairs of single wire coils are shown set 90 degrees apart.

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