Question

A shunt machine is running as a motor off a 500 - V system , taking an armature current of 50 A. if the field current is suddenly increased so as to increase the flux by 20 per cent , calculate the value of the current that would momentarily be fed back into the mains . neglect the shunt current and assume the resistance of the armature circuit to be 0.5 ohms.

Answer 1

Given quantities:

$U = 500V$ $I=50A$ $\varPhi_2= 1.2\varPhi_1$ $X_L = 0.5\varOmega$

$R = \frac{U}{I} = 10\varOmega$ $X_L = wL \to L = \frac{X_L}{w}$

$\varPhi = LI$

$\frac{R}{w}I_2 =1.2* \frac{X_L}{w}I_1 \to I_2= \frac{1.2X_LI_1}{R} = \frac{1.2*0.5*50}{10}=3A$