Basics of Electrical Transformers Construction

The windings of industrial electrical transformers are not placed on separate legs (as shown in Figure 11-1). A more efficient method, which reduces flux leakage, is to place the windings on top of one another (Figure 11-5). If a transformer is wound as shown in Figure 11-1, much of the flux produced by the primary current cannot reach the secondary winding. The leakage flux will induce a back voltage in the primary, causing a primary reactance drop. Similarly, much of the secondary flux will not reach the primary and, therefore, will not neutralize the primary flux but will produce a reactance drop in the secondary. The overall effect is similar to connecting a reactance in series with each winding.

Figure 11-1

 

Figure 11-5

Transformer with two high-voltage windings and two low-voltage windings. The windings may be connected in series or in parallel, depending upon the voltage ratings and the desired results.

 

In addition to reactance, each winding has resistance. The reactance and resistance of the windings may be represented as shown in Figure 11-6. In each winding of a transformer, there is an  drop and an IR drop. The    drop is kept to a minimum by placing the primary and secondary windings on the same leg of the core. Frequently, they are made cylindrical in form and placed one inside the other. Another method is to build up thin, flat sections called pancake coils. These sections are sandwiched between layers of insulation. Figure 11-7 shows the cylindrical method; Figure 11-8 shows the pancake method. In the cylindrical method, the low-voltage winding is placed next to the core and the high-voltage winding is placed on the outside. This arrangement requires only one layer of high-voltage insulation placed between the two windings.

 

Figure 11-7

Cutaway view of a transformer. The windings are made in the form of a cylinder and placed inside one another.

 

Figure 11-8

Cutaway view of a transformer. The windings consist of thin, flat sections sandwiched between layers of insulation.

The above article is an extract from our Fast Trax® Magenta Course for Industrial Electricity. For more information on that course CLICK HERE.