777 /: TKAXWHtMER 17.-) 



cycle or in a timo T 2 sec. where T is the period or tho time re- 

 quired for the wave to complete one cycle. The time T/2 is 

 obviously equal to l/(2/) sec. From equation (74), Vol. I, page 

 t he average induced emf . becomes 



' = -N 10-' volts 



= -4fN<t> max 10- 8 volts 



Since with a sine wave the ratio of effective to average volts 

 i< 1.11 (see page 10, Par. 5), the effective induced emf. is 



E = 4A4fN<t> max 10- 8 volts 



If tho flux varies other than sinusoidally with the time, a 

 factor kj called the form factor must be substituted for 1.11 in 

 the above equation. 



The maximum flux <t> max = B max A, where B nax is t he maximum 

 flux d< nxity and A is the core cross-section Equation (46) may 

 then be written: 



E = 4.44fNB maz A10- 8 volts (47) 



This equation is the more convenient to use, as will be shown 

 later. 



The core of a 60-cycle transformer has a cross-section of 20 

 sq. in. and tin- maximum flux density in the core is 60, 000 lines per square inch. 

 There are 700 turns in the primary and 70 turns in the secondary. What is 

 the rated voltage of the primary and of the secondary? 



Ei= 4.44 X 60 X 700 X 60,000 X 20 X 10- 8 = 2,230 volts. Ans. 

 ,= 4.44 X 60 X 70 X 60,000 X 20 X 10- 8 = 223 volts. Ans. 

 Also 



10 = 223 volts. Ans. 



78. Ampere-turns. Figure \7'2 shows a transformer having a 

 primary and a secondary winding. The directions of the llux. 

 of the voltages and of the currents, as indicated on the figure. 



.isting at the instant when the upper primary liin 

 tive. Assume first that there is no load on tin- secondary. 

 I'ndrr the.M- conditions ;i very small current flows in The primary. 

 usually from .'i to S p-r cc\\i . of the rated current. This no-lnad 

 ciirn-nt can } resolved into two components, one supplying t he 

 no-load losses, and th<- other in quadrature with the first and 



