234 ELEMENTS OF ELECTRICAL ENGINEERING. 



tion is thus vitiated by magnetizing current decreases with increase 

 of transformer load. 



The so-called potential transformer. When it is desired to 

 operate a transformer to transform a voltage in an accurately 

 known ratio so as to enable a low-reading voltmeter to measure 

 a high voltage, the effect of coil resistances and the effect of 

 magnetic leakage must be reduced to a minimum. 



The so-called current transformer. When it is desired to 

 operate a transformer to transform a current in an accurately 

 known ratio so that an ammeter need not be connected in the 

 main circuit of an alternator, the effect of core reluctance (mag- 

 netizing current) must be reduced to a minimum. 



A transformer which is used to supply a voltmeter should be 

 fairly large so that the current delivered to the voltmeter may be 

 a light load for the transformer. 



A transformer which is used to supply an ammeter should be 

 small so that the current delivered to the ammeter may be a heavy 

 load for the transformer. In saying that the transformer should 

 be small it is understood that it is to be designed to take a small 

 magnetizing current, and mere smallness does not ensure the 

 relization of this condition. 



115. Effect of core reluctance. Magnetizing current. The mag- 

 netizing current of a transformer is not harmonic,* but it is as- 

 sumed to be harmonic in all calculations of transformer action ; 

 and its power component M p and its wattless component M w 

 may be calculated from the known details of design of the trans- 

 former as follows : 



Power component M p . Knowing the thickness of laminations, 

 kind of iron, volume of iron, and frequency of the alternating 

 current used, the total core loss P K + P % of the transformer may 

 be calculated as explained in Art. 103. This total core loss is 



* IH the induction motor the magnetizing current is very nearly harmonic if the 

 supply voltage is harmonic, because of the large and constant magnetic reluctance of 

 the air gap. 



