The rotating magnetic field
One of the most important advantages of three-phase system yavyolyatsya simplicity of obtaining the rotating magnetic field. On the basis of the rotating magnetic field the device of the most widespread electric motors, asynchronous engines of the three-phase current and also often used synchronous motors works. Besides, by means of the rotating magnetic field many measuring apparatuses and devices of regulation and management are put in action.
By consecutive veering постоянного current in two coils which axes form the corner 90 ° it is possible to force the magnetic needle to turn gradually in preyodela 360 °. But it is easy for variables which will change the direction to replace the switched direct current. At the same time it is necessary that current veerings in two coils occurred not at the same time. To this requirement udovletvoryayot two alternating currents, dephased one concerning another on the quarter of the period. In fig. 1 the system from two identical coils which axes form the corner 90 ° is shown. For giving of bigger uniformity to magnetic field each of coils is separated into two parts.
As currents are rather dephased on the quarter of the peyoriod, magnetic induction in the fields excited by them of the dolyozhna to be also dephased one in relation to another. The sinusoid and the kosiyonusoida meet this condition of phase lag.
If the instantaneous value of induction in the field of the first coil:
- B1 = W of cos? t;
that instant znayocheniye of induction in the field of the second katushyoka has to be:
- B2 = W of sin? t;
here Bm — the amplituyoda of the magnetic indukyotion identical in both coils.
Being put in the middle of the device, two variation magyonitny fields obrazuyot the resulting magnetic field in which inyoduktion will be:
- Cutting =? B21 + B22;
as the directions of fields of coils vzayoimno perpendikuyolyarna (see fig. 1). Having substituted in expression of Vre3 of B1 and B2 value as functions of time, we will receive:
- Cutting = W of Vsin2? t + cos2? t = W;
Therefore, the resulting magnetic field of the device Cutting constantly in size though it also consists of two peyoremenny magnetic fields.
Let's define position of the resulting field in the proyostranstvo now. In relation to the axis of the first coil this field obrayozut the corner determined by the condition:
- tga = B2/B1 = sin? t/cos? t=tg? t;
on the basis chegoa =? t, i.e. the corner formed by the axis of the rezulyyotiruyushchy field in relation to the coil axis evenly izmeyonyatsya also during one alternating-current period:
- and =? t = 2? / TxT=2?;
i.e. the field does the whole revolution.
In the second the field delayetf turns, and field speed in minutun = f • 60
Thus, with the standard industrial frequency число field turns in the minute will make:
- п = 50-60 = 3000 RPM;
The described system is called as the two-phase rotating magnetic field.
For its excitation the two-phase system of alternating currents is necessary. Such system demands for transmission of energy not less than three wires (see fig. 1). Vectors of two line currents/l of the considered system form the corner 90 ° therefore the current vector in the general wire / about is defined as the gipoyotenuz of the isosceles rectangular triangle. On the osnoyovaniye of what current
- /0 =? 2i2l =? 2il.
It is much more favorable to receive the rotating magnetic ate by means of the three-phase system of currents as it was predloyozheno M.O. Dolivo-Dobrovolsky. For receiving the three-phase rotary field three identical coils (fig. 2) which axes form corners on 120 ° are necessary.
It is possible to count that in this case the resulting field Cutting = 1,5 vm, i.e. it is constant in size too.
This field rotates in the plane of axes of coils with the angular skoyorost?, as well as the considered two-phase field is higher. Let's compare conditions of two-phase and three-phase rotary fields now. At two-phase system two proyovod expected the line current/l, and the third wire, rasschiyotanny current intensity are necessary? 2/. Magnetic induction in вращающемся the two-phase field is equal to W. At the three-phase system of the neobkhoyodima three identical wires expected everyone current intensity/l, and induction in the rotary field has the size of 1,5 W.
Therefore, the two-phase system requires the bigger seyocheniye of wires, and the rotary field is created 1,5 times more weakly, than in three-phase system. For these reasons the two-phase current invented before three-phase (the Czech engineer Tesla), is applied only in some special devices now.Top