University of Mumbai 2007-5th Sem B.E Electrical and Electronics Engineering Electrical Machine-I - Question Paper

(2)    Attempt any four of remaining.    lA

(3)    Assume necessary data if required and Justify the same.    * '

1.    (a) Explain with the help of neat diagram, four point starter in details. State advantages of four 10

point starter over three point starter.

(b) Explain mechanical forces developed in large transformers.    10

2.    (a) Explain excitation phenomenon in transformers    10 (b) Explain phenomenon of oscillating neutral with suitable phasor diagram. 10

3.    (a) A 200V dc series motor runs at 1000 rpm takes 20A, combine resist of arm. and series field 10

is 0.4 Q calculate resist to be inserted so as to reduce the speed to 800 rpm assuming torque proportional to the square of speed and linear magnetisation curve.

(b) Explain armature reaction in DC machines.    10

4.    (a) Starling from energy balance equation, obtain expression for electromagnetic torque for 10

doubly excited system in terms of angular rates of change of self and mutual inductances of stator and rotor windings.

(b) Hopkinson test on two shunt machines gave following results    10

Line voltage 250V, line current excluding field currents 50A, motor arm. current 380A, field currents 5A and 4.2 A. Calculate efficiency of each machine if arm.resist of each machine is 0.02 n.

5.    (a) Explain working of interpoles and compensating wdg used in DC machine.    10 (b) A 230V dc shunt motor takes an arm. current of 3 : 33A at rated voltage and at a no load 10

speed of 1000 rpm. The resist of arm. current and field circuit are 0.3 fl and 160 O resp. The line current at full load and rated voltage is 40 A. Calculate at full load, speed and developed torque in case the armature reaction weakens the no-load flux by 4%.

8. (a) With the help of neat diagram, explain how back to back test is conducted on transformers. 10

(b) Two three phase transformers, rated at 500 KVA and 450 KVA resp., are connected in parallel 10 to supply a load of 1000 KVA at 0.8 pf lag. The per phase resist and per phase leakage reactance of the 1⁸¹ transformer are 2.5% and 6% re9p. and of second transformer 1.6% and 7% resp. Calculate the KVA load and power factor at which each transformer operates.