The completed magnetic circuit of the core-type transformer is in the shape of the hollow rectangle, exactly as shown in Fig, 2 in which I0 is the no-load current and Φ is the flux produced by it. N1 and N2 are the number of turns on the primary and secondary slides respectively.
Fig. 2. Magnetic circuit of a core-type transformer.
The core is made up of silicon-steel laminations which are, either rectangular or L-shaped. With the coils wound on two legs the appearance is that of Fig. 3. If the two coils shown were the respective high and low-side coils as in Fig.3, the leakage reactance would be much too great. In order to provide maximum linkage between windings, the group on each leg is made up of both high-tension and low-tension coils. This may be seen in Fig. 4, where a cross-sectional cut is taken across the legs of the core. By placing the high-voltage winding around the low-voltage winding, only one layer of high-voltage insulation is required, that between the two coils. If the high-voltage coils were adjacent to the core, an additional high-voltage insulation layer would be necessary between the coils and the iron core.
Fig. 5 shows the coils and laminations of a core-type transformer with a cruciform core and circular coils.
Fig. 6 shows the different types of cores used in core type transformers.
Rectangular cores [Fig. 6 (a)] with rectangular cylindrical coils can be used for small size core-type transformers. For large size transformers it becomes wasteful to use rectangular cylindrical coils and so circular cylindrical coils are preferred. For such purposes, ‘square cores’ may be used as shown in Fig. 6 (b) where circles represent the tubular former carrying the coils. Evidently a considerable amount of useful space is still wasted. A common improvement on the square core is to employ a
‘cruciform core’ [Fig. 6 (c)] which demands, atleast, two sizes of core strips. For very large transformers, further core stepping is done as in Fig. 6 (d) where atleast three sizes of core plates are necessary. Core stepping not only gives high space factor but also results in reduced length of the mean turn and the consequent I2R loss. Three stepped core is the most commonly used although more steps may be used for very large transformers as shown in Fig. [6 (e)].