EE1202 ELECTRICAL MACHINES-I NOVEMBER/DECEMBER 2007 question papers Anna university question papers Download

B.E/B.Tech DEGREE EXAMINATION NOVEMBER/DECEMBER 2007
Third Semester
(Regulation 2004)
Electrical and Electronice Engineering
EE 1202 -- ELECTRICAL MACHINES-I
(Comman to B.E (Part-Time)Second s\Semester Regulation 2005)
Time: Three hours                                                    Maximum marks: 100
Answer ALL questions
PART A (10 x 2 =20 marks)

1. What does speed voltage mean?
2. Give an example for each of single and multiple excited system.
3. Mention an application of a differentially compounded generator .
4.List out the possible reasons for a d.c generator fails to build up voltage when run at rated speed.
5. When you will say the motor is running at base speed?
6.A 4 pole dc motor takes a 50 A armature current.The armature has lap connected 480 conductors.The flux per pole is 20 MWb.Calculate the gross torque developed by the armature of the motor.
7. What is the function of tertiary winding in transformer?
8.Draw the no-load vector diagram of transformer.
9. Why polarity test has to be done in a transformer?
10. What precaution is nec3essary to be observed while performing brake test on a d.c series motor?

PART B (5 x 16 = 80)

11 (a)(i) What are full chorded and short chorded coils?(4)
(ii) Discuss to show that the sinusoidal current in any phase of an ac winding produces a pulsating mmf wave in space whose amplitude varies sinusoidally with time.(12)
(or)
(b) (i) Derive an expression for generated voltage of AC winding(8)

(ii) A 3 phase , 40 kW, 4 pole ,50 Hz induction motor has a winding (ac) designed for delta connection.The winding has 24 conductors per slot arranged in 60 slots.The rms value of the line current is 40 A.Find the fundamental of the mmf wave of phase -A when the current is passing through its maximum value.What is the speed and peak value of the resultant mmf/pole?(8)

12.  (a) (i)Derive and emf equation for dc generator.(4)
(ii)In a 110 V compound generator,the armature ,shunt and series windings are 0.06Ω,25Ω,and 0.04Ω respectively.The load consists of 200 lamps each rated at 55kW,110V.Find the total emf and armature current,when the machine is connected for (1) long shunt(2) short shunt.How will the ampere-turns of the series windings be changed,if in(1), a diverter of resistance 0.1Ω is connected across the series field.Ignore the armature reaction and brush drop.(12)
(or)
(b) (i)Eplain under communication in DC machines.(4)

(ii) Two shunt connected d.c generators are connected in parallel to supply a load of 5000A.Each machine has an armature resistance of 0.03 Ω and a field resistance of 60Ω but the emf of one machine is 600 V and that of the other is 640 V.Find the power supplied by each machine.(12)

13 (a) (i) Explain with neat diagram the workigng of a 3 point starter.(8)
(ii)A 4 pole ,220 V d.c shunt motor,lap wound,has 960 conductors.The flux per pole is 20 MWb.Determine the torque developed by the armature and the useful torque in N-m when the current drawn by the motor is 28 A.The armature resistance is 0.1Ω and the shunt field resistance is 125 Ω .The rotational losses of the machine amounts to 800 watts.(8)
(or)
(b) (i) Explain the armature diverter method of speed control.(4)

(ii) A 250 V dc shunt motor has Rƒ=150Ω and Rα=0.6Ω .The motor operates on no-load with a full field flux at its base speed of 1000 rpm with Iα=5A.If the machine drives a load requiring a rorque of 100 N-m,calculate armature current and speed of motor.If the motor is required to develope 10 kW at 1200 rpm,what is the required value of the external series resistance in the field circuit?(12)

14. (a) (i) Explain why in a real transformer V1: V2≠N1: N2 , where V1 and V2 are terminal voltages.(4)

(ii) Explain the term percentage impedance as applicable to transformer.(4)
(iii) The emf per turn of a 1-Ø,2200/220 V,50 Hz transformer is approximately 12V.Calculate(1) the number of primary and secondary trrns, and (2) the net cross-sectional area of core for a maximum flux density of 0.054 web. (8)
(or)
(b) (i) Explain how to ,two winding transformer can be connected in Autotransformer mode.(4)

(ii) A 20 kVA , 2000/200 V,50 Hz transformer is operated at no-load on rated voltage, the input being 150 Wat 0.12 p.f.Whwn it si operating at rated load, the voltage drops in the total leakage reactance and the total resistance are,respectively,2 and 1% of the rated voltage.Determine the input power and power factor when the transformer delivers 10 kW at 200 V at 0.8 pf lagging to a load on the LV side.(12)

15. (a) (i) Derive a condition to obtain maximum efficiency for dc machine(4)
(ii) The Hopkinson's test on two machines gave the following results for full load:Line voltage 250 V,line current ,excluding field current,50 A;motor armature current 380A;field currents 5 Aand 4.2 A.Calculate the efficiency of each machine.Armature resistance of each machine=0.02Ω.State the assumptions made.(12)
(or)
(b) Draw the equivalent circuit for a single phase 1100/220 V transformer on which the following results were obtained.

(i)1100V,0.5A,55W on primary,secondary being open circuited

(ii) 10V,80V,400W on L.V.side,H.V being short-circuited

calculate the voltage regulation and efficiency for the above transformer when supplying 100A at 0.8 p.f lagging.

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