49. Transformers. Eatio of transformation 50. Constant potential transformer 

 51. Examples of transformer construction 52. Calculation of e.m.f. induced 

 in transformer winding 53. Losses in transformer. Hysteresis loss 54. Cal- 

 culation of eddy-current loss in core. Discrepancy between calculated and observed 

 core losses 55. Copper losses. Best dimensions of core 56. Heating of trans- 

 formers 57. Star and mesh connections for transformers. Comparison of single- 

 phase and polyphase transformers for polyphase circuits 58. Auto-transformer 

 or compensator 59. Phase transformers 60. Choking coils. 



49. Transformers. Ratio of transformation 



ONE of the most important advantages of alternating over con- 

 tinuous currents is the extreme ease with which the transformation 

 from a low to a high voltage, or vice versa, may be accomplished. 

 Such transformations are effected by means of transformers, whose 

 efficiency exceeds that of any other known apparatus. 



A transformer consists essentially of a laminated iron core sur- 

 rounded by two windings : a primary winding or primary, which is 

 supplied with alternating currents, an alternating magnetic flux being 

 thereby produced in the core ; and a secondary winding or secondary, 

 in which an alternating e.m.f. is induced by the alternating flux. By 

 connecting the terminals of the secondary to an external circuit, a 

 current may be obtained in this circuit. 



In order to reduce the ampere-turns necessary to produce the 

 required magnetization to as low a value as possible, the core of a 

 transformer is arranged so as to form a closed magnetic circuit, i.e. 

 the path of the magnetic flux lies entirely in iron, air-gaps being 

 avoided. 



By far the greater part of the alternating magnetic flux in a well- 

 designed transformer will become linked with both primary and 

 secondary. A certain small fraction of it, however, representing 

 magnetic leakage lines, will become linked with the primary alone. 

 As will be seen at a later stage, it is important to adopt a form of 

 construction which will reduce the magnetic leakage to the smallest 

 possible amount. 



As an approximation, wo may provisionally assume that magnetic 

 leakage is negligible. From this it immediately follows that the 



