Problem:
Consider the three-phase Y-connected circuit that we analyzed in the lectures. Suppose $r_s = 1$,
$L_s = 0.8$ mH, and $M = -0.2$ mH. At $t = 0$, the circuit is energized by a voltage source with
$e_a = 10$ V, $e_b = 15$ V, and $e_c = -19$ V (all dc). Derive an expression for the neutral wire current
as a function of time, and sketch this waveform.