History of development of electric motors
The asynchronous electric motor - the electrical asynchronous machine for transformation of electrical energy in mechanical.
The principle of operation of the asynchronous electric motor is based on interaction of the rotating magnetic field arising when passing three-phase alternating current on stator windings with the current induced by the field of the stator in rotor windings therefore there are mechanical efforts forcing the rotor to rotate towards rotation of magnetic field provided that rotating speed of the rotor of n is less than rotating speed of the field n1. Thus the rotor makes asynchronous rotation in relation to the field. For the first time the phenomenon called magnetism of rotation was shown by the French physicist D.F. Aragon (1824). It showed that the copper disk strengthened on the vertical axis begins to rotate if to rotate over it the permanent magnet. 55 years later, on June 28, 1879, the English scientist U. Bailey received rotation of magnetic field serial connection of windings of 4 rod electromagnets to the source of the direct current. In M. Des Prez's works (France, 1880 — 1883), I. Thomson (the USA, 1887), etc. describes the devices based also on properties of the rotating magnetic field. However strict scientific statement of essence of this phenomenon for the first time practically at the same time was also independently of each other given in 1888 by the Italian physicist Mr. Ferraris and the Croatian engineer and N. Tesl's scientist.
The two-phase asynchronous electric motor was invented N. Tesla in 1887 (the English patent No. 6481), he made the public message about this invention in 1888. This type of the asynchronous engine did not gain distribution generally because of bad control characteristics. In 1889 M.O. Dolivo-Dobrovolsky tested the first-ever three-phase asynchronous engine designed by him in which he applied the rotor like "squirrel wheel" (the German patent No. 51083), and placed the winding of the stator in grooves on all circle of the stator. In 1890 Dolivo-Dobrovolsky invented the phase rotor with rings and starting arrangements (patents English No. 20425 and the German No. 75361). In 2 years the rotor design called "the double squirrel cage" which began to be applied, however, widely only since 1898 thanks to works of the French engineer P. Bushero who provided the asynchronous electric motor with such rotor as the engine with special control characteristics was offered them. Thanks to the simplicity of the device, reliability in operation engines of this kind are the most widespread electrical machines in the world. Phase windings of the stator of the electric motor connect in the star or the triangle (depending on mains voltage). If in the passport of the electric motor it is specified that windings are executed on tension 220/380 B, then at inclusion it in network to linear stress of 220 V of the winding is connected in the triangle, and at inclusion in network of 380 V - in the star.
Schemes of connection of windings of the stator of the three-phase asynchronous engine: and - in the star, - in the triangle, in - in the star and the triangle on the terminal guard of the electric motor. The scheme of turning on of the asynchronous electric motor with the phase rotor: 1 - the stator winding, 2 - the rotor winding, 3 - contact rings, 4 - brushes, R - resistors. For veering of rotation of the shaft of the asynchronous engine it is necessary to change the rotation sense of magnetic field of the stator. For this purpose two any wires connecting the stator winding to the power line are enough to trade places.
Hidden side of history
History of creation of the engine of the alternating current based on the invention of the rotating magnetic field is much more dramatic and even the detektivna, as well as any history of the real invention. Direct current motors were already operated at full capacity. Concentration of industrial facilities far from places of production of the electric power demanded its transfer on the increasing and long distances. But transfer of the direct current on such distances led to huge losses. Such transfer would be reasonable only at use of tension in tens of thousands of volts. But it was impossible to receive such tension in direct current generators. Then came up with the idea alternating-current transfers with the subsequent its transformation.
Using single-phase generators with mechanical switching of the ends of coils of the rotor (the collector, brushes), began to make alternating current of low tension, then transformers to raise it to any required size, to transfer to distance by high tension, and on site consumption to reduce again to demanded and to use in current collectors. But … there was the problem of alternating-current straightening in constant for use in engines again that led practically to the same losses, as in lines by transfer of the direct current there were no alternating-current electric motors Yet. And at the beginning of the 1880th years the electric power was consumed mainly for power needs. Electric motors of the direct current were used to the drive of the most different machines more and more widely. To create the electric motor which could work at current without rectifiers became the primal problem of electrical equipment. In search of new ways it is always necessary to look back. Whether was in the history of electrical equipment something such that could prompt the way to creation of the alternating-current electric motor? Search in the past was crowned with success. Remembered that else in 1824 Aragon showed the experiment which laid the foundation for the set of fruitful researches. It is about demonstration of "magnetism of rotation". The copper (non-magnetic) disk was fond of the rotating magnet. There was the idea: whether it is impossible, having replaced the disk with winding rounds, and the rotating magnet the rotating magnetic field, to create the alternating-current electric motor? Probably, it is possible, but how to receive rotation of magnetic field? These years many alternating-current different methods of application were offered. The conscientious historian of electrical equipment will have to tell the names of the different physicists and engineers trying to create in the mid-eighties alternating-current electric motors. He will not forget to remind of Bailey's experiences (1879), Marcel Des Prez (1883), Bradley (1887), of Venstrom's works, Hazelvander and many others. Offers, undoubtedly, were very interesting, but any of them could not satisfy the industry: their electric motors were or are bulky and uneconomical or difficult and unreliable. All of them were based on mechanical switching of windings of electromagnets that it was even more expensive and more unreliable, than straightening.
Search of the solution of this task, creation of the rotating magnetic field, there began Nikola Tesla. It went in the way and proposed the radical solution of the arisen problem. In Budapest in the spring of 1882 Tesla clearly imagined that if somehow to carry out the power supply of windings of magnetic poles of the electric motor two different alternating currents differing from each other only in phase lag, then alternation of these currents will cause variable formation of northern and southern poles, or rotation of magnetic field. The rotating magnetic field has to carry away also the winding of the rotor of the machine. Tesla the first, in 1882, it is independent of anybody, constructed the source of the two-phase current (the two-phase generator of the sinusoidal current with phase shift 900) and the same two-phase electric motor, having laid its stator windings so that the rotating magnetic field was created, and that carried out the idea independently and independently and for the first time. Creation of the alternating-current engine also consists in it. Then it still as well as nobody in the world, had ideas of stray currents of interinduction, and its couple "generator engine" with the all-metal stator and the rotor strongly overheated. But it was that main and desperate breakthrough in electrical equipment described by it in the patent No. 6481 for 1887 where Tesla, having theoretically considered all chances of phase shift, stopped on shift in 90 °, that is on the two-phase current, but described the possibility of application of the rotary field and for multiphase systems. On the basis of this description then also Dolivo-Dobrovolsky worked on the three-phase system. But Tesl was not the only scientist who remembered experience of Aragon and found the solution of the important problem. In the same years in the field of alternating currents the Italian physicist to Galilee Ferraris, the representative of Italy on many international congresses of electricians researched (1881 and 1882 in Paris, 1883 in Vienna and others). Preparing lectures on optics, he arrived at the idea of the possibility of statement of the experience showing properties of light waves.
For this purpose Ferraris strengthened on thin thread the copper cylinder which two magnetic fields shifted at an angle in 90 ° affected. At inclusion of current in the coils which are alternately creating magnetic fields in one, in other of them (besides by means of mechanical switching of windings of these coils M. H.), the cylinder under the influence of this field turned and twisted thread therefore rose by some size up. The device it perfectly modelled the phenomenon known under the name of polarization of light. Ferraris also did not assume to use the model for any electrotechnical purposes. It was only the lecture device which wit consisted in skillful application of the electrodynamic phenomenon for demonstrations in the field of optics. But Ferraris was not limited to this model. In the second, more perfect it managed to reach rotation of the cylinder with the speed up to 900 revolutions per minute. But beyond certain limits how current intensity increased in the chain, creating magnetic fields (in other words how the spent power increased), it was not possible to reach increase in speed. Calculations showed that the power of the second model did not exceed 3 watts. Undoubtedly, Ferraris, being not only the optician, but also the electrician, could not but understand values of the experiments conducted by it. However, by its own recognition, and did not come to its mind to apply this principle to creation of the alternating-current electric motor. The biggest that he assumed, - to use it for measurement of current intensity, and it even began to design such device. On March 18, 1888 in the Turin academy of Sciences Ferraris gave the report "the Electrodynamic rotation made by means of alternating currents". In it he told about the experiences and tried to prove that receiving in such device of efficiency over 50 percent is impossible. Ferraris was sincerely convinced that, having proved inexpediency of use of alternating magnetic fields for practical purposes, he renders to science big service. Ferraris's report outstripped Nikola Tesl's message in the American institute of electroengineers. But the application submitted for obtaining the patent in October, 1887 confirms the undoubted priority of Tesl before Ferraris.Top