CONSTANT POTENTIAL TRANSFORMER 101 



e.m.f. induced by the alternating flux in each turn of the primary is 

 oqiuil to that induced in each turn of the secondary, so that the ratio 

 of the total primary to the total secondary e.m.f. is simply equal to 

 tlif ratio of the number of primary turns to the number of secondary 

 turns. Again, in order to avoid excessive loss by heating of the coils, 

 their resistances are always so chosen as to reduce the resistance drop 

 (I'vrii at full load) to a very small fraction of the p.d. (in both primary 

 mid secondary), so that the p.d.'s are nearly equal to the e.m.f.'s, and 

 we have, approximately 



primary p.d. primary turns 

 secondary p.d. ~~ secondary turns 



This ratio is sometimes spoken of as the ratio of transformation. 

 If, e.g., a transformer is required to transform from 10,000 to 2000 

 volts, then the primary will have to be wound with five times as 

 many turns as there are in the secondary, and the ratio of transforma- 

 tion will be 5 : 1. 



50. Constant Potential Transformer 



Let us suppose that the p.d. across the primary is maintained 

 constant.* When the secondary circuit is open, the primary current 

 adjusts itself to a value such that the e.m.f. due to the alternating 

 flux just balances (neglecting the very small resistance drop) the 

 primary p.d. Now, this balance of primary p.d. and primary (counter) 

 e.iu.f. must also still neglecting the resistance drop be maintained 

 when the transformer is loaded, i.e. when the secondary circuit is closed. 

 In order, therefore, to maintain the original value of the alternating 

 flux corresponding to the given primary p.d., the resultant ampere- 

 turns must be maintained constant. But this condition obviously 

 implies that in the primary there must, in addition to the current 

 wliich existed on open secondary circuit, be a further component of 

 current the load component which has a value such that the 

 ampere-turns due to it just suffice to wipe out the ampere-turns of 

 the secondary, leaving a constant value for the resultant ampere-turns. 

 We may speak of the inoperative primary ampere-turns (those balanc- 

 ing the secondary ampere-turns) as the load ampere-turns, and of the 

 open-circuit ampere-turns as the magnetizing ampere-turns. We- 

 then have the relation 



prim, load ampere-turns = second, ampere-turns 

 or 

 prim, load current x prim, turns = second, current x second, turns 



* A transformer whose primary is supplied at a constant p.d. is variously spoken 

 of as a " constant potential " and a " pressure " transformer. 



