The greatest technical achievement of the late XIX century was the invention of the motor industry. This compact, cost-effective, user-friendly engine soon became one of the most important elements of production, displacing other types of engines everywhere where only it was possible to deliver an electric current. A great disadvantage of the former steam engine has always remained low efficiency, as well as the difficulty of transmission and "crushing" of energy derived from it. Usually one big machine served several dozen machines. Movement from it is supplied to each workplace by mechanical means using pulleys and endless belts. Thus there is a huge unnecessary energy loss. Electric drive does not have these shortcomings: it has a high efficiency because its shaft can be directly receive the rotational movement (whereas in the steam engine it was converted from a reciprocating), and "break up" the electrical energy was much easier. Losses in this is minimal, and labor productivity increased. In addition, with the introduction of electric motors for the first time it has the opportunity not only to supply any machine by its own engine, but also put a separate drive for each of its host.
Electric motors appeared in the second quarter of the XIX century, but it took several decades before creating favorable conditions for their widespread introduction into production.
One of the first advanced electric motors, powered by a DC battery, created in 1834 by Russian electrical engineer Jacobi. This engine has two sets of U-shaped magnets, of which one group (four U-shaped electromagnet) is located on the fixed frame. These pole pieces are arranged asymmetrically - elongated in one direction. The motor shaft was a brass disk, two parallel connected four electromagnets posed equidistant from one another. By rotating the shaft moving electromagnets were fixed against the poles. The latter were alternately polarity: the positive, the negative. By rotating disk electromagnets guides departed, fortified on the machine shaft. On the motor shaft was planted switch that changed the direction of the current in the moving electromagnets during each quarter turn of the shaft. Windings of the electromagnets fixed frame are connected in series and a current flows around the battery in the same direction. Windings of the electromagnets and the rotating disk were connected in series, but the current direction is changed in them eight times per revolution. Therefore, the polarity of the electromagnets also changed eight times per revolution, and these electromagnets alternately attract and repel electromagnets fixed frame.
Let us assume that the mobile solenoids occupy a position in which each pole against the fixed magnet pole of the same name is rolling; wherein each fixed magnet will repel the opposite drum magnet and attract nearby with the opposite pole. If the fixed pole magnets were asymmetrical, the device might not work, because the effect of the different magnets to balance each other. But due to the protrusion of the pole pieces fixed magnet attracts each toward the closest clockwise rotation is weaker than the other, because it is close to it first, and the latter is removed. After a quarter of a turn (in the engine Jacobi - through one-eighth) one against the other will be in opposite poles, but at this point the switch changes the direction of the current in the movable magnet, and one against the other would again like poles, as in the beginning of the movement. As a result, the movable magnets are again prepared to push the same direction, and so on ad infinitum, until the current remains closed.
The switch was a very important and deeply thought-out part of the engine. It consisted of four metal rings mounted on the shaft and isolated from it; Each ring has four notches which correspond to the 1/8 portion of the circumference. The recesses were filled with insulating wooden inserts; each ring has been shifted by 45 degrees with respect to the previous one. On the circumference of the ring sliding lever is a kind of brush; the second end of the lever was immersed in an appropriate vessel with mercury, which is summed to the conductors from the battery (mercury compounds were the most common while contact devices).
Disks, impaled on the motor shaft rotates with it. On the rim of the disc slipping metal levers, which, falling on the non-conductive portion of the disk, interrupt the electric circuit, and in contact with the metal - bringing up her. Disk layout was such that at a time when the opposite poles meet, contact levers crossed the brink of wood metal and it changed direction in the windings of the electromagnets. Thus, at each turn of the ring torn four times the electrical circuit.
As already noted, the Jacobi engine for its time was the most advanced electrical device. Also in 1834 a detailed report on the principles of his work was presented to the Paris Academy of Sciences. In 1838 Jacobi improved its electric motor and a position on the rowing bot, with ten companions made small sailing along the Neva River at a rate of 4 to 5 km / h. The source of the current it is a powerful battery of electrochemical cells. It is clear, however, that all these experiments were purely symbolic - until had not been invented and put into production a perfect electric generator, electric motors could not find widespread use as feed them from the battery was too expensive and unprofitable. In addition, for various reasons, which we will discuss in the following chapters, DC motors have received only limited use. A much more important role in the production of electric motors working with alternating current, to the consideration of which we now turn.
The strength and direction of the AC, as we know, are not permanent. The strength of his first increases from zero to some maximum value and again decreases to zero, then the current changes direction, increases to some of the negative maximum and again decreases to zero. (Time for which the current value varies from one to the other of the positive peak, called the period of current oscillations.) This process is repeated with great frequency. (For example, fifty times the current flowing in the lighting network in 1 second in one direction and fifty times in the opposite direction.) How is the current behavior will be reflected in the motor? First of all, it should be noted that the direction of rotation of the motor is independent of the current direction, because at AC polarity change not only in the anchor, but at the same time in the windings, causing attraction and repulsion continue to operate in the same direction as before. It seemed to be followed by that of the engine does not care how the current constant or variable - it feeds. However, it is not. With frequent magnetization reversal electromagnets (several tens of times per second) in which eddy currents that slow down the rotation of the armature, and much heat it. Motor power is sharply reduced, and eventually it crashes. For AC motor requires a special design. The inventors did not immediately able to find her. First of all, a model was developed so-called synchronous AC motor. One of the first of these engines built in 1841 by Charles Wheatstone.
Assume that the fixed part of the motor (stator) is designed as a four terminal venetseobraznogo electromagnet poles arranged alternately is designated according to their polarity letters N and S. In between the armature rotates (or rotor) in the form of a star wheel, the spokes of which eight are permanent magnets. Their invariable poles denote letters n and s. Assume that in an alternating current electromagnet. Then the ends of the cores of the electromagnet alternately changing its polarity. Let us imagine that at some point against each stator pole of the electromagnet is titled rotor pole. Push the wheel and inform him the speed at which each spoke n pass the distance between two adjacent cores N and S in the period of time equal to that during which the cores retain their polarity is unchanged, that is, in a period of time equal to half of the AC cycle supplying the electromagnets. Under these conditions, all the while moving from the core needles N to S all core cores reverse it magnetization, causing the further it moves borehole will again experience rejection from the core, left behind, and pull from the core to which it is approaching.
Worked on the principle of a synchronous motor consists of a ring-shaped multi-pole magnet, the polarity of which is changed by an alternating current, and the star of the permanent magnet, which was planted on the shaft and rotated as described above. For excitation of the permanent magnet direct current required, which was converted by the switch from a working AC. The switch was another purpose: it was used to start the engine, because in order to maintain the synchronous motor rotor speed he needed to report certain initial velocity. When the first circuit through indulged constant current, allowing the engine started to work as a DC motor, and is set in motion. As long as the engine has not received the required speed, switch, changing his direction in moving the electromagnets. Upon reaching speeds corresponded simultaneous move, the movable magnet in the pole has not changed, and the engine started to work as a synchronous AC motor.
This system had serious defects, except that the synchronous motor required for its launch additional overclocking engine, he had another flaw - Overload synchronicity of its stroke broken, magnets started to brake rotation of the shaft, and the engine stopped. Therefore, synchronous motors are not widely known. The real revolution in electrical occurred only after the invention of an asynchronous (or induction) motor.
The action of the induction motor will be understood from the following demonstration, which was conducted in 1824 by the famous French physicist Argo.
Let NS horseshoe magnet is hand in rapid rotation around the vertical axis. Above the poles installed a glass plate, which supports the island on which planted a copper circle. During rotation of the magnet induced currents induced in the circle, and the magnetic field formed by them will interact with the lower magnet, and the circle starts to rotate in the same direction as the lower magnet.
This phenomenon is used in the induction motor. Only instead of a rotating permanent magnet it used several fixed magnets which enable, disable and change its polarity in the sequence. Let us illustrate this by the following example.
Suppose that I, II, III and IV - two poles is four electromagnets between which is placed a metal needle. Under the influence of the magnetic field, it is magnetized and becomes along the magnetic field lines of the electromagnets, leaving, as it is known, from their north pole and a south incoming. All four poles disposed circumferentially at the same distance from each other. First, current is supplied to the II and III. The arrow remains fixed on the center axis of the magnetic field lines. Then, a current is supplied to the second electromagnet. Here like poles will be close by. Now the middle rail magnet lines of force will run from the middle of the distance between I and II in the middle between III and IV, and an arrow rotated 45 degrees. Disable the first electromagnet and leave only active poles II and IV. The field lines are directed from III to IV, whereby the needle has rotated by 45 degrees. Re-enable the first electromagnet, but this will change during the current movement, so that the polarity of the first magnet will change - the arrow will turn another 45 degrees. After turning off the second electromagnet arrow will move another 45 degrees, that is, make a half-turn. It is easy to understand how to get her to make the second half of the circle.
contact device described in general terms corresponds to the engine Bailey, invented in 1879. Bailey made a double solenoid four crosswise arranged poles, which he could magnetise with the switch. Over the pole he set a circle of copper, hung on the edge. By changing the polarity of the magnet, off and on, he made a circle to rotate in the same way as it occurred in the experience of the Argo. The idea of this engine is extremely interesting, because in contrast to the DC motors or synchronous motors, there is no need to supply current to the rotor. However, the form in which it was created Bailey induction motor can not be use: switching electromagnets it occurred under the influence of a complex collector, and, besides, it had a very low efficiency. But in order for this type of motor I have received the right to life, but remained a step, and it was made after the appearance of the art multi-phase currents. In fact, multi-phase currents and have received the application, primarily due to electric. To understand what is, for example, a two-phase current, imagine two mutually independent conductor, which take place two absolutely identical variable current. The only difference between them lies in the fact that they do not simultaneously reach their maximum. About such currents say they are shifted relative to each other in phase, and if these currents are fed to a single appliance, said that the current two-phase feeds. Accordingly, there can be three-phase current (if the power to the unit is derived from three identical currents shifted relative to each other in phase), four-phase current, etc. For a long time the technique was only used a conventional alternating current (which became similar to the multiphase currents called single-phase). But then it turned out that the multi-phase currents in some cases much more convenient single-phase.
In 1888, Italian physicist and Ferraris Yugoslav inventor Tesla (who worked in the US) discovered the phenomenon of rotating electromagnetic field. Its essence is as follows. Take two coils consisting of an equal number of turns of insulated wire, and place them mutually perpendicular so that one coil is included in the other. Now imagine that the current flows around the coil 1 and coil 2 i1 - i2 current i1 and i2 advances in phase by a quarter period. This, as we have said, means that the current i1, reaches a positive maximum at the moment when the current i2 is zero. If we mentally cut the coil horizontal plane in half and we will look at them from above, see section four sides of the two coils. We place between the magnetic needle, and will observe its movement. Coil through which alternating current flows, as is known, are electromagnets. Their magnetic field will interact with the arrow by turning it. Let us now consider the position of the magnetic needle, which coincides with the vertical axis of the coils at different time axis. At the initial time (t = 0), the current in the first coil is zero, and the second passes through the negative peak (the current direction will be denoted, as in electrical engineering - point and the cross, the cross means that the current is sent from the observer plane drawing, and the point - that the current is directed toward the observer). At time t1 the currents i1 and i2 are equal to each other, but one has a positive direction, and the other - negative. At time t2 the current value i2, it descends to zero, and the current i1 reaches a maximum. The arrow at the same time turn another 1/8 turn. Tracing the development process in this way, we note that at the end of the period of change of a current magnetic needle completes a full rotation around its axis. Then the process is repeated. Consequently, by means of two coils fed by two currents which are shifted relative to each other in phase by a quarter period, it is possible to obtain the same effect of the change of magnetic poles is achieved in a motor Bailey, but there is obtained a much simpler without any switch, and without the use sliding contacts, since current reversal manages itself. This effect was in the name of electrical uniformly rotating magnetic field. On the basis of Tesla designed the first ever two-phase induction motor. In general, he was the first who began to experiment with multi-phase currents and successfully solved the problem of generating such currents.
Since obtain a two-phase current of a single phase was not easy, Tesla built a special generator, which immediately gave the two currents to the difference in the 90-degree phase (that is, with a lag of a quarter period). In this generator between magnet poles rotate two mutually perpendicular coils. At the time when the coils of one coil located at the poles and induces therein a current is maximal, the coil windings are different between the poles (on the neutral line), and the electromotive force was zero in them. Therefore, two currents generated in these coils are also shifted in phase relative to each other by a quarter period. In a similar manner it was possible to obtain three-phase (using three coils at an angle of 60 degrees to each other), but considered most economical Tesla two-phase system. In fact, the multiphase current systems require a large number of wires. If the engine is running at normal AC (single-phase) current, it requires only two lead wires, it runs on a two-phase - already four, three-phase - six, etc. The ends of each coil have been deduced on the rings are located on the generator shaft. The motor rotor also has a winding in two at right angles to each other locked-in (ie, without any connection to an external electrical circuit) coils.
Tesla's invention marked the beginning of a new era in Electrical and led to his keen interest in the world. In June 1888 the company "Westinghouse Electric Company" bought him for a million dollars all patents on the two-phase system and offered to organize the production of induction motors in its factories. These engines went on sale the following year. They were much better and more reliable than all the pre-existing models, but not widespread, as were very poorly designed. The stator winding is carried out in them in the form of coils, impaled on a pole protruding. Unsuccessful been and rotor design in the form of a drum with two mutually perpendicular, closed the coils. This significantly reduces the quality of the engine at the moment of start-up and during operation.
Soon, the Tesla induction motor has been significantly redesigned and improved Russian electrician Dolivo-Dobrovolsky. Expelled in 1881 for political reasons from the Riga Polytechnic Institute, Dolivo-Dobrovolsky went to Germany. Here he finished Darmstadt Higher Technical School and joined the major German electrical company AEG in 1887. The first important innovation that made Dolivo-Dobrovolsky in the induction motor, the rotor was to create a wound "in the form of a squirrel cage." In all earlier models of induction motors rotors were unsuccessful, and therefore the efficiency of these motors has been lower than that of other types of electric motors. (Ferraris, mentioned above, has created an asynchronous ac motor with an efficiency of about 50%, and thought it was the limit.) It is very important to play here the material of construction for the rotor, because he was supposed to meet at once two conditions have a low electrical resistance (to induced currents can flow freely through its surface) and have a good magnetic permeability (to magnetic field energy is not being wasted in vain). From the viewpoint of reducing the electrical resistance of the best structural solution could be a rotor as a copper cylinder. But copper is a poor conductor for the magnetic flux of the stator and the efficiency of the engine was very low. If the copper cylinder was replaced with a steel, the magnetic flux is drastically increased, but since the electrical conductivity of steel is less than copper, the efficiency was low again. Dolivo-Dobrovolsky has found a way out of this contradiction: he performed a rotor in the form of a steel cylinder (which reduced its magnetic resistance) and drilled on the periphery of the latter channels began to lay the copper rods (which reduces the electrical resistance). At the end parts of the rotor, these rods are electrically connected with each other (closed on themselves). Decision-Dolivo Dobrowolski turned out well. After he received in 1889 a patent for a rotor, its structure fundamentally changed up to the present time.
After that Dolivo-Dobrovolsky began to think about the design of the fixed stator part of the motor. Tesla's design seemed to him irrational. Since the efficiency of the electric motor depends on how well the stator magnetic field of the rotor is used, then it follows that the more closed magnetic lines of the stator into the air (i.e., do not pass through the rotor surface), the greater the loss of electric power and the less efficiency. To avoid this, the gap between the rotor and the stator should be as small as possible. Tesla Motors, from this point of view was far from perfect - protruding coils on the stator poles create too big a gap between the stator and the rotor. Moreover, in the two engine did not work uniform rotor. On this basis, Dolivo-Dobrovolsky saw before him two tasks: to increase engine efficiency and achieve greater uniformity of its work. The first task was simple - it was enough to remove the protruding poles of an electromagnet, and distribute them across the winding circumference of the stator to the engine efficiency has increased at once. But how to solve the second problem? Uneven rotation can be significantly reduced only by increasing the number of phases two and three. But was this way rational? Get a three-phase current, as already mentioned, did not present much difficulty. Build a three-phase motor, too, it was not difficult - it is enough to place on the stator coils of three instead of two, and each of them to connect the two wires to the corresponding generator coil. This engine had to be in all respects better than the two-phase motor of Tesla, except for one thing - it required for its six power cables instead of four. Thus, the system became too unwieldy and expensive. But maybe, there was a possibility to connect the engine to the generator somehow different? Dolivo-Dobrovolsky spent sleepless nights over the schemes of multi-phase circuits. On the sheet of paper he sketched more and more options. Finally, the solution completely unexpected and brilliant in its simplicity, has been found.
Indeed, if we make a branch from the three points of the ring oscillator of the armature and to connect them with three rings, on which slide the brush, then the armature between the poles of rotation on each brush will be induced by one and the same magnitude current but with a shift in time that is necessary to turn moved along an arc corresponding to an angle of 120 degrees. In other words, in the circuit currents are shifted relative to each other and in phase by 120 degrees. But this system is three-phase proved inherent in another very interesting feature, some had no other system of multiphase currents - at any moment of time, the sum of currents flowing in the same direction, is here the value of the third current, which flows in the opposite direction, and the sum of all three currents at any given time is zero.
For example, at time t1 the current i2 passes through the time a positive maximum, and the values of currents i1 and i3, having a negative value, and reach half maximum equal to the sum of the current i2. This means that at any time one of the conductors of the system transmits the same amount of current in one direction, which is transmitted along the other two in the opposite direction. Consequently, the opportunity to use each of the three wires in a conduit for discharging the other two, connected in parallel, instead of six and only three wires do!
To explain this very important point, let us turn to the ideal scheme. Let us imagine that in a circle, rotating around its center, through three connected by a conductor, which take place three AC phase-shifted by 120 degrees. With its rotating each conductor is on the positive, on the negative side of the circle, and the transition from one part to another current changes direction. This system provides a completely normal flow (circulation) currents. In fact, at some point in time the conductors I and II are connected in parallel, and III diverts current from them. Some time later, II moves to the same direction, where III; now II and III are operated in parallel, and a common wire I discharge current. Next III moves to the side where still I; Now II assigns the amount that the sum I and III together. I then proceeds to the side where still II, etc.
In this example, nothing is said about the current sources. As we remember, this is the source of three-phase generator. It depicts a generator winding three coils. In order for current flow occurred in the manner described by us, these coils may be incorporated into the circuit in two ways. We can, for example, place them on three sides of a triangle, for example the left; thus, instead of three of its sides, we obtain three coils I, II and III, in which induced currents are phase-shifted by 1/3 period. We can also move the point of application of electromotive forces on the ends of the parallel conductors. If we put here for our coils, we get another connection. Triangles, is now serving a three-conductive compounds for the left ends of the coils can be shrunk to a single point. These compounds, of which the first is called the "triangle", and the second - a "star", are widely used in motors and generators.
His first three-phase asynchronous motor Dolivo-Dobrovolsky has constructed in winter 1889. As therein annular stator armature DC machine was used with 24 half-slots. Given Tesla's mistakes Dolivo-Dobrovolsky dispersed their windings in the grooves across the circumference of the stator, which made a more favorable distribution of the magnetic field. The rotor was cylindrical with windings' in the form of a squirrel cage. " The air gap between the rotor and the stator is only 1 mm, which in those days was a bold decision, as is usually done over the gap. Rods "squirrel cage" had no insulation. The standard DC generator, converted into a three-phase generator is used as a three-phase power source as described above.
The impression made by the first starting the engine to guide AEG, was enormous. For many, it became clear that a long thorny path of creating industrial motor finally passed through. According to its technical performance engines Dolivo-Dobrovolsky superior to all the then existing motors - having a very high efficiency, they are excellent for all modes to be reliable and easy to handle. Therefore, they immediately became widespread around the world. From that time began the rapid adoption of electric motors in all areas of production and the widespread electrification of the industry.