Pole-face losses due to open slots on grooved and ungrooved faces

The losses occurring at the pole face owing to open slots in solid and laminated faces were investigated by the authors some years ago using a specially built homopolar machine with a uniform air-gap, and their empirical formulae were published.2 Since then it has been pointed out that the results were liable to certain errors arising out of stray losses associated with bearing drag, and the authors therefore investigated the nature and extent of these errors and corrected their previously published data in the light of further investigation. In the present paper, the results have been presented in the form of curves in preference to formulae for greater convenience in use. In their previous experiments the authors had not studied the effect of circumferential grooves in solid pole-faces, but in their present investigation exhaustive tests have been carried out on solid poles having grooves of varying number, width and depth, and their effects on the pole-face losses have been determined. It has been found that while, in general, with high values of s/g, circumferential grooves are very effective in reducing pole-face losses, they have the opposite effect of increasing the losses for values of s/g below about 1.5. The results have also shown that to increase the number of grooves and deepen them is not as effective as it might at first appear.