GATE 2014 EE (EE1) – Question 47

Electrical Machines · DC machines: separately excited, series and shunt, motoring and generating mode of operation and their characteristics, speed control of dc motors · 2 marks · Numerical answer

A 15 kW, 230 V DC shunt motor has armature resistance 0.4 Ω and field resistance 230 Ω. At no load and rated voltage it runs at 1400 rpm drawing line current 5 A. Full-load line current is 70 A. Neglect armature reaction. Find full-load speed (rpm).

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Correct answer: 1239 to 1242

Explanation

Field current is230/230=1A, so armature currents are5−1=4Aat no load and70−1=69Aat full load. Back emfs are E0=230−4×0.4=228.4Vand Efl=230−69×0.4=202.4V. With constant flux, speed is proportional to back emf: **nfl=1400×202.4/228.4=1240.630rpm**.