How does the transformer operate?
The transformer is static (i.e. without moving chayosty) the electromagnetic device single-phase or three-phase in which the phenomenon of interinduction is used for the preobrazoyovaniye of electrical energy. The transformer will transform pereyomenny current of one tension to alternating current of the same frequency, but other tension.
The transformer has several electric, the windings isolated one from another: single-phase - not less than two, three-phase - not less than six.
The windings connected to the elektroeneryogiya source are called as primary; other windings giving energy to external circuits are called secondary. In the drawing primary and secondary windings of the odyonofazny transformer are below schematically shown; they are supplied with the general closed core assembled from sheet electrotechnical steel.
The ferromagnetic core serves for strengthening of magnetic communication between windings i.e. in order that the most part of the magnetic flux of primary winding was linked to rounds of secondary winding. In fig. the core and six windings of the three-phase transformer is shown on the right. These windings connect on the star circuit or the triangle.
For improvement of refrigerating conditions and isolation the transformayotor is located in the tank filled with mineral oil (proyodukty oil distillations). It is the so-called oil transformer.
With the alternating-current frequency about over 20 kHz the primeyoneniye of the steel core in transformers is inexpedient because of big losses in steel from the hysteresis and whirling currents.
Transformers without feryoromagnitny cores — air transformayotor are used to high frequencies.
If tension on clips of primary winding, the pervichyony tension of U1, is less than secondary voltage of U2, then the transyoformator is called raising. If primary nayopryazheniye is more secondary, then — lowering (U1> than U2). According to the relative size of the rated napryayozheniye it is accepted to distinguish the winding of the highest nayopryazheniye (HN) and the winding of the lowest tension (LT).
Let's get acquainted briefly with work of the single-phase two-winding transforyomator with the steel serdechyonik. Its working process and electric the sootnosheyoniya can be considered harakyoterny generally for all types of transformatoyor.
U1 tension attached to zazhiyoma of primary winding creates peyoremenny current of i1 in this winding. Current vozyobuzhdat in the transyoformator core the variable magyonitny flow F. Owing to alternation of this flow in both transformer winding EMFs are induced.
e1 = - w1 (? ф:? t) and e2 = - w2 (? ф:? t), where
w1 and w2 — quantity of rounds of those and other windings.
Thus, the relation of EDE, induced in windings, is equal to the relation of numbers of rounds of these windings:
e1: e2 = w1: w2
It is the transformer transformation ratio.
Transformer efficiency is otnosiyotelno very high, on average about 98% what allows to consider at the rated load approximately identical primary power received by the transformer, and the secondary power given them i.e. p1? p2 or u1i1? u2i2, on the basis of what
i1: i2? u2: u1? w 2: w 1
This relation of instantaneous values of currents and tension is fair both for amplitudes, and for the operating values:
L1: l2? w 2: w 1? u2: u1,
i.e. the voltage ratio and number of rounds of the corresponding windings can consider the relation of currents in transformer winding (at the loading close to rated) the return. The less loading, the more influences current of no-load operation, and the given approximate ratio of currents is broken.
During the operation of the transformer the EMF role in its primary and secondary windings is absolutely different. The EMF induced by it in primary winding arises as counteraction of the chain to change of i1 current in it. On the phase this EMF is almost opposite to tension.
As in the chain containing inductance, current in primary about m about the tka of the transformer
i1= (u1 + e1): r1,
where 1 — the pure resistance of primary winding.
From here we receive the equation for the instantaneous value of primary voltage:
u1 = — e1 + i1r1 = w t (? ф:? t) + i1r1,
which can be read as the condition of electric balance: u1 tension attached to clips of primary winding is always counterbalanced with the EMF and voltage drop in the pure resistance of the winding (the second member is rather very small).
Other conditions take place in the secondary circuit. Here current of i2 is created by the EMF of e1 playing the current source EMF role and at active loading of r/N in the secondary circuit this current
i2 = l2: (r2 +r/N),
where r2 — the pure resistance of secondary winding.
As a first approximation impact of secondary current of i2 on the primary circuit of the transformer can be described as follows.
i2 current, passing on secondary winding, aims to create the magnetic flux determined by the magnetizing force (MF) of i2w2 in the transformer core. According to Lenz's principle, this flow has to have the direction, the return to the direction of the main flow. Otherwise it is possible to tell that secondary current streyomitsya to weaken the magnetic flux inducing it. However such reduction of the main magnetic flux of Ft would break electric balance:
u1 = (-e1) + i1r1,
as e1 is proportional to the magnetic flux.
The preyoobladaniye of primary voltage of U1 therefore along with emergence of secondary current primary current increases, priyoty so is created to compensate the demagnetizing deyyostviye of secondary current and, thus, to keep electric balance. Therefore, any change of secondary current has to cause corresponding change of primary current, at the same time current of secondary winding, thanks to rather small value of the component i1r1, almost does not influence amplitude and the nature of changes in main thing time магнитного the transformer flow. Therefore amplitude of this poyotok of Ft can be considered almost constant. Such postoyanyostvo of Ft is characteristic of the mode of the transformer at which U1 tension attached to clips of primary winding is supported invariable.Top