# Three-phase transformer

Three-phase transformer - transformer, intended goals of galvanic isolation circuits the three phases while changing the voltage amplitude. Three phases, it is common knowledge, has introduced Dolivo-Dobrovolsky, but the patent could not obtain because the years ahead by Nikola Tesla.

## Thanks

Let us recall the remarkable author of the USSR before the war - Holuyanova Fedor. Simple story enjoyable hearing unprepared reader, rather than the best modern essay on the three-phase transformers.

## define

Three-phase power transformer average power - less than 33.3 MBA impedance short circuit does not exceed 25 - 0,3N / W%. N - rated power of the transformer (MVA), W - number of core rods.

Large three-phase power transformer - power up to 100 MVA, impedance above defined formula specified for the previous class products.

Distribution phase transformer - reduction capacity to 2.5 MVA, with separate windings and cooling type ON.

## Structure

The authors propose to commence consideration of the three-phase transformer with simplicity. It is assumed, readers familiar with the 220-volt circuits. Know how transformer work.

### Brief description of the single-phase transformer

You need to start with simple things: inductance coil around itself creates a vortex magnetic field. It extends along an axis, comes out at the North Pole. The figure shows the two coils of wire. Current flows from positive to negative, the direction of magnetic field lines is determined by the "thumb" rule. The trajectory is bent, resulting in the neighboring coil (a coil) poorly covered.

It requires the extent possible fully convey the magnetic flux, providing galvanic separation (current). When transforming the output voltage easily varies. It is used in the transmission of electricity to consumers.

Finally transported secondary field winding mandrel capable of a ferromagnetic alloy. Inside the material of the magnetic induction intensity repeatedly increases. It provides a tight linkage, of EMF induced in the output value becomes enormous. The core line of the field intensity along the axis of the permeate. Effect described above is obtained.

### The design of the three-phase transformer

Simply consider the three-phase transformer, providing three single-phase. Screen shows the stick-type specimen. Like the "armor" (the title belongs to the authors) means: winding are put on the bars. Combined, magnetic field lines closing yokes. The word does not imply the presence of a rod of circular cross section. Probably present before modern transformers practiced other handicaps.

A laminated core is made, by definition, not round. Technologically difficult. The transformer is fitted with a round core, round? Yes, round, covering a square area inferior round similar vein length. The simple fact, the utilization of modern materials transformer alien perfection. Rectangular core, the yoke, it is easier to assemble the charge plate.

The three-phase transformer is recommended to introduce three rod composed side to side to form a single central rod is not used.

Since phase shifted uniformly by an angle of 120 degrees relative to one another, the geometric vector sum will be zero. If we make the cores of single-phase transformers, the magnetic flux through the central part will not work. Speaker basis of work circuits with isolated neutral. Middle rod does not carry the magnetic flux, therefore, it can be thrown out of the design. The remaining part is so arranged:

1. Coils arranged on parallel rods.
2. Primary, secondary phase windings are on a single shaft.
3. The core is closed yokes.
4. According to distinguish two phases symmetrical structure:
• View from above - an equilateral triangle. The symmetry of the phase.
• View from above - single line. Asymmetry.

The symmetry of the phases means: inputs are equal. If the rods are arranged in a row along the length of the yoke between extremes greater than between two other couples. The magnetic flux will be shifted in phase, the signal will be distorted. Resistance core asymmetrically for the field. Currents causes inequality in the idle mode. The effect is amplified low-quality assembly, poor saturation of the iron yoke.

Armor-phase transformers actually delivered to each other, covered by a single-core, single-phase. Asymmetry is missing, primary, secondary windings are on the same rod. As the central flux yokes doubles core sectional area must be increased according.

The primary winding is divided in two, covering both sides of the secondary, as shown in the figure (primary - I, secondary - II). In armored transformers one distinct advantage - small currents of idling. It is believed due to a short stroke field intensity within the core. Deficiencies for three:

1. Greater weight at the same gear ratio, the same power.
2. The windings are difficult to repair because all sides are surrounded by the armor.
3. cooling conditions are worse, although the nominal volume is greater. The core is heated, working, reversal, relatively small eddy currents.

### cores

Laminated core steel sheets are typed. Smaller plate thickness, below are the eddy current loss, more laborious assembly. The layers were separated lacquer for mutual isolation. Preventing occurrence of eddy currents. Requirements to become, quite typical:

1. A large value of magnetic permeability provides amplification in the tens of thousands of times the induction field. Consequently, the first prerequisite for the operation of the transformer.
2. High resistivity is provided by silicon impurities (by weight - up to 4%). As a result, losses are reduced to 50% in heavily-doped samples.
3. Low coercive force, results in a low magnetic reversal losses (narrow hysteresis loop).

It has long been seen: the square area of ​​0.88 circle. Consequently, the most favorable will be the selected curve. Irrational complicate the production process, come in practice in another way: Low-power transformers are provided with square rods, medium - cruciate (see. rice), large -. circular. The end justifies the means, if the substation will no longer take care of the energy loss will be huge. Modest transistor receiver costs low. Savings - the loss is small. Rectangular core provides most favorable conditions for the heat sink, as characterized by a large volume.

Sometimes the corners of a dielectric insert holding the winding along the desired curve. The oily core transformers are sometimes provided with slits. It is assumed, circulating in the ducts, the fluid will cool the coil steel. The channels are fitted along the plates across. The second case is more productive for one simple reason. The ends of the plates are not covered by varnish, as in the direction of the Foucault currents (eddy) does not occur, the metal loses heat faster, distributed along the plate. The first method is easier to achieve in terms of the production process.

Wire bad lies straight edge of the core, is curved outwards at the corners crack varnish insulation. Imposes restrictions on the assembly process. In the operation of inevitable thermal variation of geometric dimensions, eventually worsens these effects. Consequently, rectangular coil has a smaller mechanical strength. Indeed said round rod due thicker increases the amount of winding of the yoke, is used due to frequent failures powerful three-phase transformers of a different design.

Despite the advantages of designs with symmetrical phases, more cores are placed side by side for obvious reasons: the simplified process. If the core rod, assembly overlap is only used for low-power designs, in other cases, the yoke is butt. At the armor the opposite - thin butt, other - overlap.

### Winding

In power transformers concentric coils disposed in one another, have a common axis. Alternating winding shown in the diagram above, for the sale of the broad masses of radio amateurs are not issued. When calculating pay attention to the calculation of the following parameters:

1. Mechanical strength (see. above), including a short circuit mode.
2. Dielectric strength lived in isolation.
3. Temperature modes (including the maximum).

The winding is performed circular, rectangular (sometimes transpose) wire. Separation of the single strands to a number of veins is performed by complementing measure burdening the core. It will reduce eddy currents. When the desired diameter of wire 3.5 mm replaced rectangular (TC 16.K71 - 108 - 94). Become too large gaps between the wires. Circular section is endowed with an advantage: easier to manufacture, more common in everyday life. The rectangular wire is used mostly for winding coils. Therefore, to make unprofitable, the process is more costly.

The rectangular conductor is larger than 8h25 mm transposed. Copper for electrical winding is taken, a minimum purity of 99.95%. Due to the high cost is often replaced by refined aluminum. Metal has a lower tensile strength, lower ductility, high resistivity. Wire insulation is made from the phone, transformer paper. Meets lacquer:

• RAS, a rectangular copper wire insulated transformer paper.
• PB, rectangular copper wire insulated with paper telephone.
• PTBU transpose copper wire, paper insulated.
• PTB transpose copper wire with a total paper insulation.

## winding Forms

1. The helical coil is spirally with oil cooling passages. The three-phase power transformers used for low voltages. It is placed between the layers lining.
2. Continuous coil was named for the way: one piece of copper wire wound in plural windings. Often, an external coil placed first performed after relining.
3. Bound winding, weaving through the adjacent coils characterized by high mechanical strength.
4. The cylindrical layered coil resembles a spiral, the windings are placed end to end without any intermediate cooling channels.
5. Disc coil winding is similar to a continuous, unlike more limited exclusion, affixed to each coil separately. Characterized by large mechanical strength.

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