Constant-current regulating transformer characteristics: Special tests show radical differences from conventional assumptions

It has generally been assumed that the total losses of a constant-current regulating transformer remain practically constant for all loads. Performance and test calculations have accordingly heretofore been made according to well known conventional methods from the segregated losses determined from test or design data, assuming total losses constant. Extensive tests described in the paper show, however, that these losses are not necessarily even approximately constant, but for some transformers vary greatly with the load, the total losses at light loads greatly exceeding those at full load. The tests further show that the increased losses produce increased temperature rises at light loads, which may be sufficiently excessive to endanger the apparatus, although the same transformer may be able to carry full load with normal temperature rise. The increased total losses and temperature rises at light loads are shown to be due largely to the increase of stray load losses with decrease of load, caused mainly by the leakage flux inducing excessive circulating currents in the laminations and structural parts of the transformer, such as the cage. Exploring coils and iron filings were used to investigate the amount and direction of the leakage flux. Performance specifications should be based on the actual total losses for all loads and not on the conventional assumption of constant total losses, as has been done in the past. It is pointed out that the characteristics described depend somewhat on the design, other features besides liberality in the use of materials being of importance. Consequently, measures should be taken by designers to correct these undesirable characteristics as much as possible. Users not only desire the most highly efficient apparatus consistent with cost, but must have apparatus of the highest reliability under all possible conditions, which cannot be expected of constant-current transformers endangered by excessive temperatures at light loads. The characteristics discussed are, therefore, not only of theoretical interest, but also of considerable practical interest to both designer and user.