TRANSFORMERS 385 



low-voltage, primary and secondary, the A.I.E.E. standardiza- 

 tion rules read as follows: 



" The terms high-voltage and low-voltage are used to distinguish 

 the winding having the greater from that having the lesser number 

 of turns. The terms primary and secondary serve to distinguish 

 the windings in regard to energy flow, the primary being that which 

 receives the energy from the supply circuit, and the secondary 

 that which receives the energy by induction from the primary." 



The terms primary and secondary are, however, often confused, 

 and in order to avoid any misunderstanding it is preferable to 

 use the terms high-voltage and low-voltage instead of primary 

 and secondary. 



In every symmetrical three-phase circuit there are two voltages 

 which should be clearly distinguished: 



(1) The voltage between lines, called the " delta- voltage " 

 and (2) the voltage from line to neutral, called the " Y-voltage." 

 Under balanced conditions 



Y-voltage = delta-voltage divided by Vs, and 



Delta-voltage = Y-voltage times \/3. 



Transformers designed to be suitable for use in either delta or 

 Y-connection have, as a rule, on the name plates the line voltages 

 which apply for both connections. The line voltage resulting 

 from Y-connection is followed by the letter " Y " for example, 



10 000 



if the transformer voltage is given ^ ^; this signifies that 



17,oUu i 



both voltages are line voltages but the latter is the voltage result- 

 ing when the transformer is connected in Y. The symbol " Y " 

 is used as an abbreviation to indicate that sufficient insulation has 

 been provided so that the transformer may be connected in Y for 

 the line voltage with which the letter is used, but this symbol 

 should not be confused with " Y-voltage." The expressions 

 " delta-voltage " and " Y-voltage " are often loosely used for 

 'Voltage when connected in delta " and " voltage when con- 

 nected in Y " and misunderstandings are often caused thereby. 

 If, however, the facts are kept clearly in mind that a " Y " in the 

 voltage rating of a transformer stands for " Y-connection " and 

 that " Y-voltage " is only a part of the line voltage, there should 

 be no cause for misunderstanding. 



The transformer voltage depends, of course, on the nature of 

 the system. The primary voltage of the step-up transformers is, 



