CHAPTER IV 



NO-LOAD CURRENT, POWER FACTOR AND EFFICIENCY 



\\ 1 1 EN the secondary circuit of a transformer is open (/.<., 

 when the transformer is unloaded) there is a certain current 

 flowing through the primary circuit from the source of supply, 

 and this is commonly termed the "no-load" current. The 

 "no-load" current is the resultant of two components, namely, 

 the current necessary to magnetise the core to the required 

 density and Ahe core loss current. The calculation of the 

 magnetising current will ^e taken up first. In the case of 

 the 20-kw transformer the procedure is as follows : 



Magnetic density in core =11,5 kilolines per sq cm. 



Corresponding specific magnetomotive force in ampere 

 turns per centimeter of length of magnetic circuit (from 

 Fig. 10, on p. 34) = 6,0 ampere turns per cm. 



Mean length of magnetic circuit (from Fig. 36, on p. 58) = 

 147 cm. 



Total magnetomotive force required to send the flux round 

 the magnetic circuit = (6,0 X 147 =) 890 ats. 



The primary winding comprises 3100 turns. Consequent^, 

 in order to provide the required magnetomotive force of 

 890 ats, the crest value of the magnetising component of the 

 no-load current must he equal to 



= 0,286 ampere. 



gnetising compo 

 = 0,202 ampere. 



The virtual value of the magnetising component works out at 

 * 



T 



Obviously the above result involves the assumption that the 

 mngnetising component is sinusoidal in form. 



