CHAPTER VI 

 TRANSFORMERS 



177. The Constant-potential Transformer. The constant- 

 potential transformer consists of one magnetic circuit interlinked 

 with two electric circuits, the primary circuit which receives energy 

 and the secondary circuit which delivers energy. Its function is 

 to transform electric power from low voltage and large current to 

 high voltage and small current, or the reverse. In step-up trans- 

 formers the primary is the low-voltage (L.V.) side and the second- 

 ary is the high-voltage (H.V.) side. In step-down transformers 

 the primary is the high-voltage side. 



In the following discussion letters with the subscript 1 will be 

 used to represent primary quantities and with the subscript 2 to 

 represent secondary quantities. 



Fig. 265 represents a transformer. The core is made up of 

 thin sheets of iron or steel of high permeability with small hyster- 

 esis and eddy current loss. 



The primary winding consists of n\ turns in series and has a 

 resistance of r\ ohms, a self-inductive or leakage reactance of Xi 

 ohms and thus an impedance of z\ = r x -f j%i ohms. 



The secondary winding consists of n% turns in series. Its re- 

 sistance is TZ ohms, its reactance is Xz ohms and its impedance is 

 ?2 = 7*2 + j%2 ohms. 



When an alternating e.m.f. EI is impressed on the primary 

 winding with the secondary open, a current 7 flows in the pri- 

 mary and produces an alternating flux through the core of maxi- 

 mum value 3>. The current /o is called the exciting current of 

 the transformer and consists of two components (Fig. 266) IM 

 in phase with the flux <I>, called the magnetizing current, and I c 

 in quadrature ahead of the flux and in phase with the impressed 

 e.m.f., called tne core-loss current. The product of EI and I c 

 is the power wasted in the core loss, that is, in supplying the 

 hysteresis and eddy current losses of the transformer. The ex- 

 citing current, therefore, lags by an angle do, which is less than 

 90 degrees, behind the impressed e.m.f. Cos0 is the no-load 

 power factor of the transformer. 



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