Transformer core and widings

1. POWER TRANSFORMERS CORES The function of the transformer core is to provide a route for the magnetic flux and supports the primary and secondary windings.

The composition of a transformer core depends on such factors as voltage, current, and frequency. Size limitations and construction costs are also factors to be considered.

Commonly used core materials are air, soft iron, and steel. The type and characteristics of most commonly used in cores are:  High Resistance Steel (H.R.S.) – having 5% silicon; nominal thickness: 0.35 mm.  Cold Rolled Grain Oriented Silicon Steel – having 5% silicon; nominal thickness: 0.33 mm; actual trend.

A soft-iron-core transformer is very useful where the transformer must be physically small, yet efficient. The iron-core transformer provides better power transfer than does the air-core transformer.

To reduce iron losses (no-load losses), most transformer cores are made up of thin sheets of specially annealed and rolled silicone steel laminations that are insulated from each, other.

The molecules of the steel have a crystal structure that tends to direct the flux. By rolling the steel into sheets, it is possible to “orient” this structure to increase its ability to carry the flux.

A transformer whose core is constructed of laminated sheets of steel dissipates heat readily; thus it provides for the efficient transfer of power.

This construction provides an easy path for the magnetic flux and increases the amount of coupling between the windings.

These steel laminations are insulated with a non conducting material, such as varnish, and then formed into a core.

Figure 1 – Laminated core As the magnetic flux “cuts” through the core materials, small currents called “eddy currents” are induced.

As in any other electrical circuit, introducing a resistance (for example, insulation between the laminations), will reduce this current and the accompanying losses. If a solid piece of material were used for the core, the currents would be too high.

The actual thickness of the laminations is determined by the cost to produce thinner laminations versus the losses obtained. Most transformers operating at 50 Hertz have a lamination thickness between 0.3 and 0.5 mm.

The laminations must be carefully cut and assembled to provide a smooth surface around which the windings are wrapped.

Any burrs or pointed edges would allow the flux lies to concentrate, discharge and escape from the core.

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The laminations are usually clamped and blocked into place because bolting would interrupt the flow of flux. Bolts also have a tendency to loosen when subjected to the vibrations that are found in a 50 Hz transformer’s core.

It is important that this clamping arrangement remains tight; any sudden increase in noise or vibration of the transformer can indicate a loosening of core structure.

The two major configurations of iron-core transformers, which are shown in Figure 2, are:  Core type  Shell type

Figure 2 – Core construction In the core type, the windings are wrapped around the core, and the only return path for the flux is through the center of the core.

Since the core is located entirely inside the windings, it adds a little to the structural integrity of the transformer’s frame. This construction is desirable when compactness is a major requirement.

Shell type transformers completely enclose the windings inside the core assembly.

Shell construction is used for larger transformers, although some core-type units are built for medium and high capacity use.

The core of a shell type transformer completely surrounds the windings, providing a return path for the flux lines both through the center and around the outside of the windings.

Shell construction is also more flexible, because it allows a wide choice of winding arrangements and coil groupings.

The core can also act as a structural member, reducing the amount of external clamping and bracing required. This is especially important in larger application where large forces are created by the flux.

The lamination of the core material depends on the type of core construction, as shown in Figure 3.

Figure 3 - Core laminations

2. POWER TRANSFORMERS WINDINGS The windings consist of the current-carrying conductors wound around the sections of the core, and these must be properly insulated, supported and cooled to withstand operational and test conditions.

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Copper and aluminum are the primary materials used as conductors in power-transformer windings.

While aluminum is lighter and generally less expensive than copper, a larger cross section of aluminum conductor must be used to carry a current with similar performance as copper.

Copper has higher mechanical strength and is used almost exclusively in all but the smaller size ranges, where aluminum conductors may be perfectly acceptable.

In cases where extreme forces are encountered, materials such as silver-bearing copper can be used for even greater strength.

The conductors used in power transformers are typically stranded with a rectangular cross section, although some transformers at the lowest ratings may use sheet or foil conductors.

Multiple strands can be wound in parallel and joined together at the ends of the winding, in which case it is necessary to transpose the strands at various points throughout the winding to prevent circulating currents around the loop(s) created by joining the strands at the ends.

Individual strands may be subjected to differences in the flux field due to their respective positions within the winding, which create differences in voltages between the strands and drive circulating currents through the conductor loops.

Proper transposition of the strands cancels out these voltage differences and eliminates or greatly reduces the circulating currents.

The choice of the type winding is largely determined by the rating of the winding. Some of the common types of windings are:  Cross over type  Spiral type  Helical type  Continuous disc type Cross over type windings are suitable for currents not exceeding about 20 A. They comprise of circular cross section and are used for HV windings in small transformers in the distribution range.

A number of such coils are joined in series, spaced with blocks which provide insulation as well as duct for cooling.

Figure 4 – Cross over winding Spiral type windings are normally used up to 36 kV and low current ratings.

Strip conductors are wound closely in the axial direction without any radial ducts between turns.

Spiral coils are normally wound on a bakelite or pressboard cylinder.

Figure 5 – Spiral winding (single layer, wound on flat side) Though normally the conductors are wound on the flat side, sometimes they are wound on the edge. However, the thickness of the conductor should be sufficient compared to its width, so that the winding remains twist-free.

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Figure 6 – Spiral winding (edge wound) Spiral windings may be made as single layer or multilayer type.

Figure 7 shows a double layer spiral coil where an oil duct separates the two layers. For such a coil, both the start and the finish leads lie at one end of the coil and may at times prove to be advantageous for making the terminal gear.

Figure 7 – Spiral winding (double layer) Normally it is not necessary to provide ant transposition between the parallel conductors of a spiral winding as the lengths and the embracing of leakage flux are almost identical.

Helical type windings are used in low voltage windings of small and medium sized transformers.

The simplest helix coil consists of a single layer, formed by turns lying directly side by side, extending over the axial winding length. Each turn may comprise a single conductor or a number of conductors in parallel, the helix at each end of the coil being supported by a suitably shaped edge block to give adequate mechanical strength in an axial direction.

A number of conductors are used in parallel to form one turn. The turns are wound in a helix along the axial direction and each turn is separated from the next by a duct.

Helical coils may be single layer or double layer or multi-layer, if the number of turns is more.

Figure 8 – Helical winding (single layer – left; double layer -right) Unless transposed, the conductors within a coil do not have the same length and same flux embracing and therefore have unequal impedance, resulting in eddy losses due to circulating current between the conductors in parallel.

To reduce these eddy losses, the helical windings are provided with transposition of the conductors which equalize the impedances of the parallel conductors.

Continuous disc type windings are used for voltages above 36 KV. These coils consist of a number of sections placed in the axial direction, with ducts between them, as shown in Figure 9.

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Figure 9 – Continuous disc type winding Each section is a flat coil, having more than one turn, while each turn itself may comprise one or more conductors (usually not more than four or five), in parallel.

The conductor may be either a single rectangular strip or a number of rectangular strips in parallel, wound on the flat. This reduces considerably the risk of the strip twisting slightly in winding and thereby making an unsatisfactory disc.

The sections are connected in series, but without any joints between them. This is achieved by a special method of winding. It is not necessary to provide a cylindrical former for these coils, as these are self-supporting.

Each disc is mechanically strong and exhibits food withstand of axial forces.

Several types of windings are commonly referred to as “pancake� windings due to the arrangement of conductors into discs.

However, the term most often refers to a coil type that is used almost exclusively in shell-form transformers.

The conductors are wound around a rectangular form, with the widest face of the conductor oriented either horizontally or vertically.

Figure 10 illustrates how these coils are typically wound.

Figure 10 - Stacked pancake windings In core-form transformers, the windings are usually arranged concentrically around the core leg, as illustrated in Figure 11.

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Figure 11 - Concentric arrangement, outer coil being lowered onto core leg over top of inner coil.

Most distribution transformers for residential service are built as a shell form, where the secondary winding is split into two sections with the primary winding in between.

This so-called LO-HI-LO configuration results in lower impedance than if the secondary winding is contiguous.

The LO-HI configuration is used where the higher impedance is desired and especially on higher-kVA ratings where higher impedances are mandated by standards to limit short-circuit current. Core-form transformers are always built LO-HI because the two coils must always carry the same currents.

In three-phase transformers, windings may be installed in cores with 3, 4 or 5 legs.

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