The aim of the transformer cores is to offer an easy path for the flux. The cores are made of laminations of silicon steel to minimise the eddy current and hysteresis losses. Each lamination is insulated from each other by means of varnish or impregnated paper or enamel to minimise the eddy currents and silicone is added to minimise the hysteresis losses. The thickness of the lamination varies from 0.35 to 0.5 mm. It must be kept in mind that the assembly of the lamination should be such that there is no air gap left.
According to the design of the cores these are:
Core type. In this type of cores the magnetic flux has only one magnetic path. The two windings are placed on the two limbs. Sometimes the windings are placed on the single limb to avoid the magnetic leakage. In this case the cores are surrounded by the winding as shown in Fig. 18.2. In case of three phase core type construction there are three limbs of equal width.
(a) Core type (b) Shell type single phase
Shell type. In this type of cores there are two magnetic paths. The width Of the central limb is twice that Of the side limbs, because the total flux passes through the central limbs and half the flux is passing through the side limbs. The windings are placed on the central limb and the winding is surrounded by the cores.
Fig. 18.2. (c) Core type three phase care assembly
In case of shell type construction of three phase transformer, the windings are placed on the three limbs, but the outer two limbs are there to shell it as shown in Fig. 18.9. The size of each limb is same. In five limb construction the cross-section in yoke can be reduces to 30% than the limbs but the cross-section of limbs is increased by a little percentage.
Berry type. In this case the distributed paths of the magnetic field are used. Generally the core type construction is used. One limb of all the cores passes through the center of the windings. The length of the core inside the coil is less than the length of limb outside the winding. These are used to obtain the variable voltage. In case of the small transformers, simple rectangular limbs and coils are used. The coils can be either rectangular or circular. The rectangular has more number of weak points (bends) than circular one. To avoid the air gap in case of circular coil, the cruciform of cores setup is used. It demands the use of two or more size stempings. The different sizes depends upon the number of core stepings.
