Economic design of a 50 kVA distribution transformer. Part 2: Effect of different core steels and loss capitalisations

A range of 50 kVA transformers has been designed by computer for minimum total cost, defined as the purchase price plus the capitalised cost of losses. Two core steels are compared, Unisil 28M4 and Nippon 23 ZDKH, and in each case four loss capitalisations have been taken. For optimal designs based on available prices for materials and labour, a lower total cost is found for 23ZDKH except when the capitalisation of losses is very low. However, in that case the calculated flux density exceeds 1.7T and may be impracticable. A higher purchase price was found for 23ZDKH than for 28M4 in all the designs optimised for total cost except for the highest loss capitalisation considered. In that case 23ZDKH gave both lower total cost and lower purchase price. Cost comparisons between designs using the two core steels were made both for mineral oil and for a more expensive fluid of low flammability (Midel 7131). Identical cost relations were found except that where cost benefits arose from using 23ZDKH they were slightly larger for the more expensive fluid.