Engineering BE Electrical Engineering Sem 4 December 2019 2019 Electrical Machines I Question Paper - Mumbai University | munotes

Questions asked in this paper

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1. Q2 Attempt any THREE questions from remaining
2. Q3 Figures to right indicate full marks
3. Q1 Attempt any Four questions
   • a) How the back emf (Eb) makes the DC motor a self regulating machine. 5
   • b) Explain the electro-mechanical energy conversion. 5
   • c) Briefly explain the Swinburne’s test for DC machine. 5
   • d) Explain the conditions for parallel operation of single phase transformer. 5
   • e) Why the terminal voltage of DC shunt generator falls when it is loaded. 5
4. Q2 A) Explain the concept of singly excited machines and derive the expression for the 10 marks
   • B) Derive the expression of copper saving in Auto-Transformer. 10
5. Q3 Derive the expressions for Demagnetizing Amp-turns (ATd/pole) and cross magnetizing Amp-turns (ATc/poles) for armature reaction 10 marks
   • B) Two identical dc shunt machine, when tested for Hopkinson’s test, gave the following Field currents SA and 4A If the armature resistance of each machine = 0.025 calculate the efficiency of each 10
6. Q4 A) Explain the Sumpner’s test for single phase transformer. 10 marks
   • B) Two single phase transformers shared.a load of 400 KVA at 0.8 lagging. Their equivalent impedances referred to secondary windings are (1 + j2.5) and (1.5 + j3) ohms Calculate the load shared by each transformer 10
7. Q5 — A) Explain the necessity of starter and hence explain the working of three point starter. 10 marks
   • B) A 7.46 kw, 220v, 900 rpm shunt motor has full load efficiency of 88% and armature resistance of 0.08 while shunt filed current of 2A If the speed. of this motor is reduced to 450 rpm by inserting a resistance in armature circuit. The load torque remains constant, find the motor output efficiency and the extra resistance inserted in armature circuit 10
8. Q6 Write short note on each 20 marks
   • A) Electrical braking in separately excited DC motor
   • B) Commutation process in DC Generator

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