Transformers with five leg cores

It is often necessary to adjust the transformer dimensions to existing machine enclosure. The construction of 5-column core allows to reduce the transformer height.
5-column core contains two unwound external columns which constitute the extension and connection of the lower yoke with the upper one. The application of such magnetic core in 3-phase transformer allows to reduce the section of yokes to about 60% of yoke section in typical 3-column core, maintaining full transformer power and unchanged dimensions of columns.
This way the transformer height is reduced by about 80% of the height of one yoke.

General diagram of 5-column core.

The diagram above presents a core of 3-phase 5-column transformer. Yoke parts of various lenght (m`, m``, n), and various cross-sectional area (Ajm`, Ajm``, Ajn) are distinguished. 

In practice several various constructions of 5-column core are applied:

  • Equal cross-sections of all yoke sections, amounting to 50-58% of column cross-section (example of designation: 50/50)
  • The cross-section of yokes between main columns is about 58%, and the cross-section of return yokes is about 45% of the column cross-section (designation 58/45).
  • The cross-sections of all horizontal sections of yokes amount to about 58% of the column section, and the cross-sections of vertical sections of return yokes are about 45% of the column cross-section (designation 58/58-45).

In 5-column transformers produced by our company we often apply the same cross-sections of all yoke sections and they are equal to the half of column cross-sections – 50/50 proportion. In such solution the maximal induction of column yokes is higher by about 5% and smaller by 5-10% in closing yokes in relation to the maximal induction in columns.

50/50 dimensional proportion of core ensures the maximal savings on material and optimal adjusting of transformer size to the existing body of machine.
The yokes closing themselves in 5-column core can constitute a low-reluctance way for the fluxes of third harmonic and its multiple. The flow of third harmonics fluxes would cause a strong deformation of fluxes in wound columns, which would result in non-sinusoidality of phase voltages. In order to prevent that the primary or secondary transformer winding should be linked in the triangle (D), or the primary winding should be linked in the star, with neutral conductor (YN).

ELHAND 5-column transformer, type ET3oGH.