1. The problem statement, all variables and given/known data 2. Relevant equations Efficiency = o/p divided by i/p I/P KW divided by input voltage gives input current Field Current = voltage divided by field resistance armature current = total current - field current Back Emf = V - IaRa T proportional to Ia Back Emf proportional to flux and N 3. The attempt at a solution For 1st part: we have efficiency = 0.75 = o/p divided by input o/p = 18KW So we get i/p = 24KW So Vt * (Total current) = 24kW Vt = 200V So Total current = 24000/200 = 120A Field current = voltage / Rf = 200/100 = 2A --- (constant all the time) Now to limit starting current to three times we have, Total current must be 120 * 3 = 360A Field current = 2A Armature current = 358A At start Eb = 0 Terminal Voltage = Armature Current times Ra So 200 = 358 times Ra This gives Ra = 0.558 Given Ra = 0.3 So we need to add extra 0.26 ohm So for first part answer is B But I am not able to solve second part. At full load Torque is same irrespective of starting resistance since torque depends on load. In shunt motor Torque proportional to Ia So Ia is same in both cases Back Emf is proportional to Speed times flux Flux is same in both cases of armature resistance as Field Current is same. So Eb1/Eb2 = 1500/(Nx) Eb1 = 200 - 118 * 0.3 = 164.6 Eb2 = 200 - 188 * 0.56 = 133.92 So Eb1 / Eb2 = 1.23 So 1.23 = 1500 / Nx Nx = 1220 rpm But that is not even in the options. Where am I wrong, or is the question wrong?