96 ELECTRICAL ENGINEERING 



(a) the inductance of the field winding; 



(b) the time taken by the current to reach T V full value and % 



full value, after the field switch is closed; 



(c) the initial rise of voltage, if the field circuit carrying a 



current of 12.5 amperes is closed through a resistance of 

 25 ohms; 



(d) the energy stored in the magnetic field when the current is 



12.5 amperes. 



(a) The interlinkages of flux and turns with a current of 12.5 

 amperes or 1.25 c.g.s. units is 



6 X 500 X 6.3 X 10 6 ; 

 therefore the inductance is 



6 X 500 X 6.3 X 10 6 

 X = -- j~25 - = 15 ' 1 X 10 c '&- s - units ' 



or 



L = 15.1 henrys. 



(b) At time t after the field switch is closed the current is, by 

 equation 132, 



where / is the final value of the current = 12.5 amperes and R is 

 the resistance of the field circuit including the rheostat = 10 

 ohms; therefore, 



- 

 -e 1 



Let time for the current to reach rV of full value or 1.25 amperes 

 be ti sec., then 



1.25 = 12.5(1 - e -- 66 <0 



and ti = 0.16 sec. 



Let time for the current to reach J value or 6.25 amperes be fe sec., 



then 



6.25 = 12.5 (1 - e-o- 66 <') 



and tz = 1.05 sec. 



(c) If the field circuit carrying a current of 12.5 amperes is closed 

 through a resistance of 25 ohms, the initial value of the back 

 voltage due to inductance is, by equation 134a, 



eb = 125 l - = 437.5 volts. 



