John Hopkinson was outstanding British consulting electrical engineer and expert witness. He worked on electromagnetic occurrences in dynamos and the theory of dielectrics. Hopkinson devised the three-phase system for the transmission of electricity.
He was IEE President in 1890 and 1896. John Hopkinson born in 1849, was the eldest son of a Manchester engineer of the same name who had married the daughter of a prosperous cotton spinner. There were thirteen children by the marriage, of whom ten reached maturity (five sons and five daughters).
He was fortunate in that being brought up in Manchester he had good schooling and in 1865 entered Owens College in that city. Owens College, which went on to become the University of Manchester, was an excellent institution in which to study.
After showing great abilities in mathematics Hopkinson was awarded ascholarship to allow him to continue his study of that subject at Trinity College Cambridge. He entered Cambridge in 1867 and graduated with amathematics degree in 1871. Whilst studying for the mathematics tripos at Trinity College, he worked concurrently for a BSc at London University. In Cambridge he was coached by E.J.Routh, emerging in 1871 as Senior Wrangler. He was captain of the boat club and also won the mile race in the athletics sports.
Although his scholarship would have allowed him to continue his mathematical studies at Cambridge, Hopkinson decided to put hismathematics to practical use in engineering. There had been no chair of engineering at Owens College when Hopkinson studied there, but it is interesting to note that Osborne Reynolds was appointed to such a chair while Hopkinson was studying at Cambridge.
Chance Brothers and Company
On leaving Cambridge he started work in his father’s factory until he was appointed engineering manager of the optical works of Chance Brothers and Company, glass makers and lighthouse engineers of Birmingham in 1872. There he studied the problems of efficient ways of shining lights from a lighthouse and, in particular, he recommended the use of flashinggroups of lights.
John Hopkinson married Evelyn Oldenburg, with whom he had been in love since his undergraduate days. In 1877 he moved to London as a consulting engineer. By this time there were three children, Bertram, the eldest, having been born in 1874. Money was short but he soon built up a lucrative practice in the law courts as an expert witness on patents and other scientific matters.
John Hopkinson converted two of the rooms in his house into laboratories and there conducted experiments in electricity and magnetism. Hopkinson’s application of Maxwell’s electromagnetic theories to the analysis of residual charge and displacement in electrostatic capacity led to his election as a fellow of the Royal Society in 1877. In 1878 Hopkinson founded his own electrical engineering company.
Collaborating with Edward Hopkinson, one of his brothers, he applied the theory of electricity and magnetism to the development of electric motors. He used his mathematical expertise to give a general theory of alternating currents and he applied this theory to the operation of alternating current generators in parallel. Hopkinson invented the three-wire system (three phase) for electricity generation and distribution, a system that he patented in 1882.
He presented the principle the synchronous motors (1883). Hopkinson improved the design and efficiency of electric generators, as well as the study of condensers and the phenomena of residual load. Hopkinson was appointed professor of electrical engineering at King’s College London in 1890. At the same time he became director of the newly founded Siemens Laboratory. He was President of the Institution of Electrical Engineers (IEE) on two occasions: in 1890 and 1896.
3-Phase Generator (or Motor) Principles
All 3-phase generators (or motors) use a rotating magnetic field. In the picture to the left we have installed three electromagnets around a circle. Each of the three magnets is connected to its own phase in the three phase electrical grid. As you can see, each of the three electromagnets alternate between producing a South pole and a North pole towards the centre.
The letters are shown in black when the magnetism is strong, and in light grey when the magnetism is weak. The fluctuation in magnetism corresponds exactly to the fluctuation in voltage of each phase. When one phase is at its peak, the other two have the current running in the opposite direction, at half the voltage. Since the timing of current in the three magnets is one third of a cycle apart, the magnetic field will make one complete revolution per cycle.
Synchronous Motor Operation
The compass needle (with the North pole painted red) will follow the magnetic field exactly, and make one revolution per cycle. With a 50 Hz grid, the needle will make 50 revolutions per second, i.e. 50 times 60 = 3000 rpm (revolutions per minute). In the picture above, we have in fact managed to build what is called a 2-pole permanent magnet synchronous motor.
The reason why it is called a synchronous motor, is that the magnet in the centre will rotate at a constant speed which is synchronous with (running exactly like the cycle in) the rotation of the magnetic field. The reason why it is called a 2-pole motor is that it has one North and one South pole. It may look like three poles to you, but in fact the compass needle feels the pull from the sum of the magnetic fields around its own magnetic field. So, if the magnet at the top is a strong South pole, the two magnets at the bottom will add up to a strong North pole.
The reason why it is called a permanent magnet motor is that the compass needle in the centre is a permanent magnet, not an electromagnet. (You could make a real motor by replacing the compass needle by a powerful permanent magnet, or an electromagnet which maintains its magnetism through a coil (wound around an iron core) which is fed with direct current). The setup with the three electromagnets is called the stator in the motor, because this part of the motor remains static (in the same place). The compass needle in the centre is called the rotor, obviously because it rotates.
Synchronous Generator Operation
If you start forcing the magnet around (instead of letting the current from the grid move it), you will discover that it works like a generator, sending alternating current back into the grid. (You should have a more powerful magnet to produce much electricity). The more force (torque) you apply, the more electricity you generate, but the generator will still run at the same speed dictated by the frequency of the electrical grid.
You may disconnect the generator completely from the grid, and start your own private 3-phase electricity grid, hooking your lamps up to the three coils around the electromagnets. (Remember the principle of magnetic / electrical induction from the reference manual section of thisweb site). If you disconnect the generator from the main grid, however, you will have to crank it at a constant rotational speed in order to produce alternating current with a constant frequency. Consequently, with this type of generator you will normally want to use an indirect grid connection of the generator.
In practice, permanent magnet synchronous generators are not used very much. There are several reasons for this. One reason is that permanent magnets tend to become demagnetised by working in the powerfulmagnetic fields inside a generator. Another reason is that powerful magnets (made of rare earth metals, e.g. Neodynium) are quite expensive, even if prices have dropped lately.
Hopkinson’s work brought him into contact with most of the leaders of science and industry. Visitors to his house on Wimbledon Common included Lord and Lady Kelvin, Sir Benjamin Baker, builder of the Forth Bridge and the Assouan Dam and Sir William Crookes, the chemist.
When James Stuart, the first Professor of Mechanism and Applied Mechanics in Cambridge resigned, he tried to persuade Hopkinson to take his place, but he said he had too many commitments to take it on. Instead he became Professor of Electrical Engineering in King’s College London. However he continued to take an interest in Cambridge and accepted an invitation to become a member of the Enquiry Syndicate appointed to look into the crisis over the engineering workshop at Cambridge.
John Hopkinson died in 1898 in a mountaineering accident in Switzerland, on Mount Petite Dent de Veisivi, together with one of his sons and two of his three daughters.