Receiving alternating current
For alternating-current obtaining industrial frequency (50 Hz) at power plants electromachine sinyokhronny alternating current generators are installed. In these machines, as well as in direct current generators, the phenomenon of elektromagyonitny induction is used. By means of the alternating current generator the meyoanichesky energy reported by the prime mover (the paroyovy or hydraulic turbine) will be transformed to alternating-current elektricheyosky energy.
The synchronous generator (fig. 1) consists of the motionless chayosta 1 — the stator and the rotating part 2 — the rotor.
The stator gathers from sheet electrotechnical steel. It has the form of the hollow cylinder. In grooves (kanaloobrazny hollows) made along the internal surface of the stator the insulated conductors connected among themselves and forming the stator alternating-current winding are laid.
The rotor represents the electromagnet excited by poyostoyanny current. This energizing current moves in the winding of the rotoyor through the copper rings strengthened on the rotor shaft. On kolyyoets the motionless brushes connected by wires to the activator — the small generator of poyostoyanny current slide.
The design of the rotor depends on the operating speed of rotation which the generator is expected.
For the high-speed generators rotated by steam turbines (turbine generators), rotors izyogotavlivatsya by not salient pole. For the low-speed generators rotated by hydraulic turbines (gidrogeyonerator), rotors are produced by salient pole (fig. 2).
The elementary on the device not salient pole rotor (3000 and 1500 about. / mines) represents the massive steel forging in which radial grooves magnetizing coil conductors become stronger metal wedges.
The magnetic flux of the rotor passes through the rotor body, two air intervals between the stator and the rotor and the stator core (see fig. 1). At rotation of the rotor this magnetic sequins F crosses conductors of the stator and induces in them pereyomenny EMF. As the magnetic flux in air gap направлен it is perpendicular to stator winding conductors, that, according to Faraday law of induction, in each of these conductors at rotation of the flow is induced by EMF.
е = BLV:
- where B — magnetic induction in the groove where there is this proyovodnik;
- L — the active length of the conductor, i.e. length of that part of the proyovodnik which is in magnetic field;
- V \circumferential speed, i.e. motion speed of the magnetic flux in relation to the conductor.
In most cases the circumferential speed of the generator podderyozhivatsya by the constant (v = const) and as length L too the postoyanyon, change of EMF е at rotation of the rotor is caused только by changes magnetic induktsiiv along the circle of the rotoyor. If this induction is distributed along the rotor circle sinusoidalno (In = by Bm sin and), then EMF changes in time too sinusoidalno (е = Em sin wt). This circumstance ispolyyozutsya at creation of alternating current generators to receive from them sine wave voltage.
To one turn of the bipolar rotor there corresponds one period of the EDE variable induced in stator obmotyoka conductors. If the stator has р steam of poles, then the turn of the royotor on the corner occupied by one couple of poles causes one period of change of EMF. In this case corresponds to one turn of the rotor р the EMF variable periods. The rotor does n of revolutions per minute; therefore, in the minute the EMF variable of the generator will have p • n of the periods. Number of the periods in the second of the alternating current generator, i.e. the frequency of its EDE variable, will be 60 times less:
But the generator has to give alternating current of the standard chastoyota of f = 50 Hz. Provided that p • n = 3000.
Thus, the maximum speed which the alternating current generator of industrial frequency can have sootvetyostvut to one couple polyusovr = 1. At this skorostin = 3000 about. / mines above-mentioned turbine generators work. Vyyogodno to build machines with the largest admissible speed as at the same power high-speed machines weigh less and have smaller dimensions.
Speed of the hydrogenerator is defined by speed the water dvizheyoniya therefore at construction of the hydroelectric station on the rivers with the medyolenny current it is necessary to supply rotors of hydrogenerators with the large number of poles. For example, generators of the Dnieper hydroelectric station of Lenin do 83,3 about. / min., and their rotors have 72 poles (i.e. p = 36).Top