208 



ELEMENTS OF ELECTRICAL ENGINEERING. 



the components of E" are -fa x 1,000 volts and T a ^ x 800 volts 

 parallel to and perpendicular to the current respectively ; and the 

 value of I" is of course 40 amperes. The student should 

 represent these details in a clock diagram. 



101. Relation between core flux and primary voltage. The 

 electromotive force in abvolts induced in each turn of wire of the 

 primary coil of a transformer is at each instant equal to the rate 

 of change of the core flux (, so that the total induced electro- 

 motive force in abvolts is equal to N' dfyjdt, and this induced 

 electromotive force is equal and opposite * to the primary voltage 

 e f at each instant (resistance of primary coil negligibly small), 

 therefore 



e ' = N '~di W 



where e' is expressed in abvolts. 



Example of core flux for a particular non-harmonic electromotive 

 force. The relation between core flux and primary voltage is 

 simplest for the case in which the electromotive force curve is 

 rectangular like the full-line curve in Fig. 176. Suppose, for 



Fig. 176. 



example, that a 5O-volt storage battery is connected through a 

 reversing switch to the primary coil of a transformer, imagine 

 that the switch is reversed 25 times per second, and let it be re- 

 quired to determine the curve of which the ordinates represent 

 the varying value of the core flux, the primary coil having 100 

 turns of wire. 



*In fact the induced electromotive force should properly be considered equal to 

 N f d$\dt so that, placing e f equal and opposite to the induced voltage, we have 

 equation (i). 



