EE1202 ELECTRICAL MACHINES-I question papers Anna university question papers Download

B.E/B.Tech DEGREE EXAMINATION, NOVEMBER/DECEMBER 2006
Fourth Semester
Electrical and Electronics Engineering
EE 1202 --ELECTRICAL MACHINES-I
(Regulation 2004)
Time: Three hours                                                    Maximum marks: 100
Answer ALL questions
PART A (10 x 2 =20 marks)

1.Write the applications of singly excited and doubly excited magnetic systems.
2.Explain the following terms with respect to rotating electrical machines.

(a) Pole-pitch

(b) Chording angle

3.What are the conditions to be fulfilled for a DC shunt generator to build up emf?
4.What are the two functions of a commutator in DC machines?
5.What is the need for starter in a DC motor?
6.Why is a DC series motor used to start heavy loads?
7.Why transformer rating is expressed in terms of KVA?
8.What are the various types of three-phase transformer connections?
9.Give the expression for the load current when the transformer operates at its maximum efficiency.
10.Define 'all-day efficiency' of a transformer.

PART B (5 x 16 = 80)

11 (i)Draw and explain the general block diagram of an electromechanical energy-conversion device. (8)
(ii)Explain with the help of neat diagram about the nature of the magnetic field produced when a three phase supply is connected across a three phase winding. (8)
(or)
(b) (i)Show that the net eletromagnetic torque developed is zero if the rotating electrical machine has different number of poles on its stator and rotor. (8)

(ii)Derive an expression for torque developed in a 2-pole round rotor machine and state the assumptions made.(8)

12.  (a)(i).Derive an expression for the emf generated in the armature winding of a DC machine. (8)
(ii).A long-shunt compound generator delievers a load current of 50 A at 500 V and has armature,series field and shunt field resistances of 0.05 Ω,0.03 Ω and 250 Ω respectively.Calculate the generated voltage and the armature current.Allow IV per brush for contact drop.
(or)
(b)(i).Explain the effect of armature reaction in a DC generator.How are its demagnetising and cross-magnetising ampere turns calculated?(8)

(ii)Two seperately-excited DC generators are connected in parallel and supply a load of 200 A.the machines have armature circuit resistances of 0.05Ω and 0.1Ω and induced emfs of 42 V and 440 v respectively.Determine the terminal voltage,current and power output of each machine.The effect of armature reaction is to be neglected.(8)

13 (a)(i) Describe the working of athree-point starter for a DC shunt motor,with the help of neat diagram. (8)

(ii)Explain the field-flux control and armature contol methods of speed control of a DC shunt motor.(8)
(ii)Explain the field-flux control and armature control methods of speed control of a DC shunt motor.(8)
(or)
(b) (i)Explain the speed-torque characteristics of DC shunt and DC series motors. (8)

(ii)A 230 V,DC shunt motor,takes an armature current at 3.33A at rated voltage and at a no-load speed of 1000 r.p.m.The resistances of the armature circuit and field circuit are 0.3Ω and 160Ω respectively.The line current at full load and rated voltage is 40A.Calculate,at full load,the speed and the developed torque in case the armature reaction weakens the no-load flux by 4% (8)

14. (i)Draw and explain the no-load phasor diagram of a single phase transformer. (8)
(ii).Derive expression for the current shared by two transformersoperating in parallel,with unequal no-load voltages.(8)
(or)
(b) (i)Obtain the equivalent circuit of a 200/400 V,50 Hz,single phase transformer from the following test data:

O.C.Test:200V,0.7 A,70 w-on L.V side

S.C Test:15 V,10 A,85 w-on H.V side (8)

(ii)Expalin the principle of operation of an auto-transformer (8)

15. (a) (i)Explain the various losses which takes place in a DC machine (8)
(ii)A 500 V,DC shunt motor takes a total current of 5 A when running unloaded.The resistance of armature circuit is 0.25Ω and the field resistance is 125Ω.Calculate the efficiency and output when the motor is unloaded and draws a current of 100A. (8)
(or)
(b) (i)Explain the Hopkinson,s test for determining efficiency of two similar DC shunt machines (8)

(ii)Explain how the efficiency of a single phase transformer is estimated from the open circuit and short circuit test (8)

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