Correct Answer - Option 3 : 478.3 rpm
V = 200 V, R1 = Ra + Rse = 1 ohm, Ia = 15 A
N1 = 1000 rpm
R1 = Ra + Rse = 7 Ω
Eb ∝ Nϕ and ϕ ∝ Ia
\(\frac{{{E_1}}}{{{E_2}}} = \frac{{{I_{a1}}}}{{{I_{a2}}}} \times \frac{{{N_1}}}{{{N_2}}}\)
E1 = 250 – 20 × 1 = 230 V
Given that Ia1 = Ia2
E1 = 250 – 20 × 7 = 110 V
\(\Rightarrow \frac{{230}}{{110}} = \frac{{1000}}{{{N_2}}}\)
⇒ N
2 = 478.26 rpm