The literature reports various transformer model core studies on local values of the building factor (BF), which consider the core as a 2-D single-package object. For the first time, we report 3-D BF analyses of a three-phase core that exhibits three different widths of packages. The inner BF values were determined by taking advantage of 25 channels of 2.5 mm width through the entire core. BF profiles along the channels were measured by means of a thermistor sensor that was controlled by a 3-D scanning system. For a nominal induction of 1.7 T, the results indicate that the main package of maximum width and thickness behaves similarly to one-package cores, with minimum BF in outer limbs and maximum BF (however, of reduced intensity) in the T joint. On the other hand, outer packages of reduced width and thickness show distinctly reduced BF, especially in the T joints. The results indicate that 2-D studies are not representative of peripheral packages. They show specific performance, especially due to the usual combination of circular limbs with semicircular yokes. The resulting regional off-plane z-flux seems to have a balancing function which reduces the effects of overlaps. However, low BF in the peripheral packages does not indicate good performance. Rather, it indicates poor exploitation of the core material.

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