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For a given compound DC machine, Net field current obtained from characteristic is equal to 7.5 A, where shunt field current is equal to 5 A, armature current is 505 A, demagnetizing current equal to 0.95 A and shunt field winding of 1000 turns at rated speed of 1000 rpm. What will be the series field turns?(a) 7(b) 8(c) 5(d) 9I got this question during an internship interview.The above asked question is from Graphical Representation of External Characteristics in section DC Generator of DC Machines

Answer»

Correct answer is (a) 7

Easiest explanation: From the EXCITATION balance EQUATION, If + [Nse/Nf] Ia – Ifd =If (NET).

5 + 505(Nse /1000) – 0.95 = If (net).

Calculating for Nse by substituting If (net), we GET Nse = 6.8 that is 7 turns.



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