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GATE 2014 EE (EE2) – Question 46

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 250 V DC shunt machine has armature resistance 0.6 Ω and field resistance 125 Ω. On 250 V mains, it runs separately as generator and motor with line current 50 A in either mode. Find generator-speed/motor-speed ratio.

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Correct answer: 1.22 to 1.32

Explanation

The shunt field current is250/125=2Ain both modes,so flux is taken constant. Generator armature current is50+2=52A,giving Eg=250+52×0.6=281.2V. Motor armature current is50−2=48A,giving Em=250−48×0.6=221.2V. Since E∝speed,Ng/Nm=281.2/221.2=**1.27125**.