148 THE DESIGN OF STATIC TRANSFORMERS 



transformers some firms employ deeply-corrugated cast-iron 

 cases. In some instances the height of the required case 

 leads to difficulties in the manufacture of these corrugated 

 castings. To overcome these difficulties such cases have some- 

 times been made in sections, which are bolted together with a 

 lead lining between adjacent sections. AVheii this plan is adopted, 

 the number of sections employed for a given transformer may 

 be roughly obtained by dividing the losses of the transformer 

 by the rate at which each section is known by experience to be 

 capable of dissipating heat. This plan, when applied to large 

 sizes, generally leads to a heavier and more expensive case than 

 when corrugated wrought-iron plate is employed. Cast iron 

 emits slightly more heat per unit of surface than either sheet 

 iron or boiler iron, and this has been put forward as an argument 

 in favour of cast-iron cases ; but the difference in this respect 

 is too slight to constitute a practical consideration. Any flaws 

 in a cast-iron case can be autogenously welded, or they can be 

 patched up with a blow-pipe or by other familiar workshop 

 methods. Nevertheless, these eventualities may add consider- 

 ably to the cost. The sides of cast-iron cases may be con- 

 siderably strengthened when external ribs are employed ; and 

 consequently resort to ribs extends the usefulness of the cast- 

 iron type of case. This is in addition to the circumstance that 

 by means of the ribs a much larger radiating surface is 

 available for heat emission. The minimum thickness prac- 

 ticable in cast-iron cases (due regard being given to the 

 danger of fracture during transit and to the initial difficulties 

 in casting) is some 8 mm, the range of thicknesses used in 

 practice varying between this figure and some 14 mm, accord- 

 ing to the size and the weight of the case. On the whole, the 

 disadvantages associated with the use of cast iron for trans- 

 former cases, taken in conjunction with the liability of fracture 

 during transit, are sufficient to make it generally preferable to 

 employ sheet-iron or sheet-steel cases for all but small-sized 

 transformers. 



