Transformer Construction

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Core Type Transformer

The shape of a complete magnetic circuit of a core type transformer is similar to a hollow rectangle. The two sides of the rectangle are called, primary and secondary slides. The wires that carry the current are twisted a few times on the two sides of the hollow rectangle to produce flux.

 The core of this type of transformer is made of silicon and steel laminations. The shape can be either L- shaped or rectangular. The coils are wound on the two legs of the base structure.

  • The leakage reactance will be most if, the coils wound are high side and low side coils.
  • The wound on each leg can also be made of high tension and low tension coils, to provide high linkage among the coils.

Hollow rectangular cores are only used for small scale core type transformers. It proves to be wasteful for larger transformers. Hence, circular cylindrical coils are preferred foe a larger sized core type transformer.

Even though square cores may seem helpful but still there is considerable amount of space wasted in that core type. But the square shaped core can be modified by using cruciform core, which is dome by core stepping.  Core stepping;

  • Gives comparatively a higher space factor.
  • The lengths of the mean turns on the coils can be reduced
  • Considerable IIR loss or copper loss.

Three stepped core are the widely used core type transformers.

Shell-type transformer

Theshapes of the cores for shell type transformers are of E or F shaped. The cooper in this core is completely surrounded by iron. The shapes of E or F are later stacked to form a figure of ‘8’.

The windings are done on the centre leg of the core to reduce the leakage. High side winding and low side winding is placed adjacent. Low voltage coils are placed adjacent to reduce high voltage insulation.

Core type construction versus Shell type construction

  • The mean length of winding is more but the mean length of coil turns are less in core type.
  • The smaller cross section of iron in core type will require large number of turns to obtain a high amount of flux.
  • The insulation in core type construction is high, hence it is able to service for high voltages.
  • Shell type construction is better for mechanical services.
  • Good bracing of the coils provide good resistance to high mechanical forces.

The main factors to consider while choosing between core type and shell type are:

  • Voltage rating.
  • kVA reading.
  • Heat distribution.
  • Insulation stress.
  • Mechanical stress.

Spiral Core Transformer

The shape of the core for spiral core transformer is elliptical. It may consist of continuous strips of steel or short strips of steels. Long strips are folded to form a spherical shape. The short strips are accumulated to give an elliptical shape.

The flux always follows the skeleton of the iron core. Steel with high contents of silicon produced high magnitude of flux. The accumulation of the iron, reduces the loss per kg.


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