Some aspects of effects of longitudinal and normal stress on power loss and flux distribution within a transformer core

An experimental 1.0 m Epstein square has been constructed in which all the limbs could be flattened to 1.5×105 N/m2 and two opposite limbs could be stressed longitudinally to ±13.8×106 N/m2 Power-loss measurements were made on cores of 46-grade and 56-grade grain-oriented silicon-iron subjected to combinations of longitudinal and normal stress. The power loss (at 1.5 T) increased by only about 10% owing to the normal compression, but linear stress increased the loss by up to 40% (at 9.65×106 N/m2 compression). When the applied linear stress was greater than about 5.8 × 106 N/m2, the normal stress restricted the power-loss increase. The effect of normal stress on a 45° mitred-overlap and a double-overlap joint were investigated. In the double-overlap joint, the flux did not deviate from the rolling direction of the laminations. Airgaps in the mitred-overlap joint and the length of the overlap were both found to have a pronounced effect on the corner loss. The lower-loss-grade material was found to have a lower stress sensitivity of power loss to both normal and longitudinal stress.