electrical mechines i syllabus

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Code 9A02308 1II B.Tech I semester (R09) Regular Examinations, November 2010

ELECTRICAL MACHINES-I(Electrical & Electronics Engineering)

Time: 3 hours Max Marks: 70Answer any FIVE questions

All questions carry equal marks? ? ? ? ?

1. (a) In a rectangular electromagnetic relay, the exciling coil has 1500 turns of resistance 1Ω .The cross-sectional area of the core A=5cmx5cm, Neglecting reluctance of the magneticcircuit and fringing, if the coil is excited with 50Hz a.c having a peak to peak value of 100vand the armature is held at a distance of 1cm, find the average force on the armature.

2. (a) Derive the expression for the voltage induced in a dc generator

(b) The lap wound armature of a 4 pole generator has 51 slots. Each slot contains 20 conduc-tors. What will be the emf generated in machine when driven at 1500 rpm? The usefulflux per pole is 0.01wb

3. (a) Explain how demagnetizing and cross magnetizing ampere-turns/pole are calculated in dcmachines.

(b) Calculate the ampere turns for each commutating pole of an 8 pole with 107 slots, eachcontaining 1000 ampere conductors. The interpole air gap is 1.2 cm. The flux density inthe air gap is to be 0.32 T. Neglect iron parts and leakage.

4. (a) Explain the process of voltage building in self excited generator. Also mention the possiblecauses of failure of excitation.

(b) A short shunt compound generator delivers a load current of 30 A at 220 V and hasarmature, series, field and shunt field resistance of 0.05Ω , 0.03Ω and 200Ω respectively.Calculate the induced emf and the armature current. Allow 1.0v per brush for contactdrop.

5. (a) Draw and explain the load characteristics of a series generator

(b) Two shunt generators, each with a no load voltage of 200v are run in parallel. Theirexternal characteristics can be taken as straight lines over this operation range. Generator1 is rated at 200 kw and its full load voltage 190v. Generator 2 is rated at 100kw at185v. Calculate the bus bar voltage when the total load is 3000A. How is the load dividedbetween the two?

6. A 200 volts d.c. series motor runs at 1000 r.p.m when operating at its full load current of 30A.The motor resistance is 0.5Ω and the magnetic circuit can be assumed unsaturated. What willbe the speed if

(a) The load torque is increased by 44%

(b) The motor current is 20A.

7. (a) What is a starter? Explain the necessity of starter in d.c..motors.

(b) Explain the method to control the speed of a d.c. motor above its rated speed.

8. A 440v d.c. shunt motor takes a no load current of 25A. The resistance of the shunt field andthe armature are 550 Ω and 1.2 Ω respectively. The full load line current is 32A. Find the fullload output and the efficiency of the motor.

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1. In a rectangular electromagnetic relay, the exciting coil has 1200 turns. Cross-sectional area of the coilA=6cmx6cm . Neglect the reluctance of the magnetic circuit and fringing effects. With coil currentskept constant at 2A, derive expression for force on armature as a function of air gap of length x. Findthe work done by the magnetic field when x decreases from 1cm to 0.5cm by integrating the force.

2. (a) Explain the action of commutator in dc generators.(b) A 4 pole d.c. generator has a wave wound armature with 792 conductors. The flux per pole is

0.12 Wb. Determine the speed at which it should be run to generate 240v on no load.

3. (a) What is armature reaction? Explain the effects of armature reaction. Also explain the methodsto overcome the effects of armature reaction.

(b) A 500 volts, 4-pole, wave wound, 750rpm d.c. shunt generator supplies a load of 195A. Thearmature has 720 conductors and shunt field resistance is 100Ω . Find the demagnetizing ampereturns per pole, if the brushes are given an actual lead of 100 at the load. Also calculate the extrashunt field turns required to neutralize the demagnetization.

4. (a) What is self excited generator? Explain the process of voltage building in self excited generator.(b) What are the causes of failure to excite a self excited generator? Also suggest the remedial

measures.

5. (a) Explain the necessity of parallel operation of generators.(b) Two d.c. compound generators, A and B with an equalizing bar, supply a total load of 500A.

The data relating to the machine are as follows.Armature resistance, RA =0.05 ohms,RB =0.03 ohms; series field winding , RSA = 0.02 ohms,RSB =0.01 ohms. Generated emf EA =463V, EB=470vCalculate :

i. The current in each armatureii. The current in each series winding

6. (a) A 250V d.c shunt machine has line current of 80A. It has armature and field resistance of 0.1Ωand 125Ω respectively. Calculate power developed in armature when running as

i. generator andii. motor.

7. (a) What is a starter? Explain the necessity of starters in dc motors.(b) Explain the Ward-Leonard system of speed control of d.c. motors.

8. The no load test of a 45kw, 230v d.c. shunt motor gave the following results:Input current =14AField current =2.55ABrush drop =2VEstimate full load current and efficiency.

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1. (a) Explain the principle of energy conversion. From the consideration of various energies involved,develop the model of an eletromechanical conversion device.

(b) Prove that energy and coenergy in a linear magnetic system are given by identical expressions.

2. The armature core of a 4 pole dc machine has 31 slots each designed to accommodate 4 coil sides of asimplex wave winding. The winding has total of 496 conductors. Find

(a) total numbers of coils(b) turns per coil(c) commutator pitch(d) back, front and total pitches(e) number of commutator segments.

3. A 4 pole lap wound d.c. generator delivers full load current of 400A. It has shunt field current of 12Aand 123 commutator segments in the commutator ring of the machine. If the brushes are advanced by3 commutator segments full load , find

(a) the demagnetizing AT/Pole(b) the cross magnetizing AT/Pole

4. In a 110V, d.c. compound generator, the resistance of the armature, shunt field and series field are0.06Ω , 25Ω ,and 0.4Ω respectively. The load consists of 200 lamps each rated at 55W, 110V. Find thetotal emf generated and the armature current when the machine is connected in

(a) long shunt and(b) short shunt

5. (a) List and explain the applications of various types of degenerators.(b) Two shunt generators each with an armature resistance of 0.01Ω and field resistance of 25Ω run

in parallel and supply a total load of 3000A. The emfs are respectively 220V and 240V. Calculatethe bus bar voltage and output of each machine.

6. A 4 pole d.c. shunt motor takes 22.5 A from a 250V supply Ra =0.5Ω and RSH =125Ω . The armaturesis wave wound with 300 conductors. If the flux per pole is 0.02Wb, calculate

(a) speed(b) Torque developed(c) Cross power developed.

7. (a) Explain the scheme to control the speed of a dc shunt motor below its rated speed.(b) A 220 V d.c. shunt motor runs at 500rpm taking a current of 16A from the field winding in 220

ohms. The motor drives a load whose torque is constant. Find the extra resistance to be addedto the field winding to increase the speed to 700rpm.

8. A 200 V d.c. shunt machine has armature and field resistances of 0.2Ω and 200 Ω respectively. Theline current in 40A. Find i) Output as generator ii) Input as a motor iii) Power developed in armatureand iv) Copper losses in both the cases.

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1. For a doubly excited magnetic system, the inductances are approximated as follows: L1=11+3cos2θH;L2=7+2 cos2θ H; M=11 cosθ H. The coils are energized with d.c. I1=0.7A, I2=0.8A, Find

(a) the torque as a function of θ and its value when θ=-500

(b) the energy stored in the system as a function of θ

(c) the maximum energy stored.

2. (a) With a neat diagram explain the constructional features of a dc machine.(b) A six pole lap wound dc generator armature has 720 conductors, a flux of 30mwb and running at

a speed of 600rpm. Calculate the e.m.f. generated on open circuit. If the same armature is wavewound, at what speed it is to be driven to generate 600 volts?

3. (a) Explain the armature reaction in a dc generator. Also explain the methods to overcome the effectsof armature reaction.

(b) A 4 pole 50kw, 250v wave wound, shunt generator has 400 armature conductors. Brushes aregiven a lead of 4 commutator segments. Calculate the demagnetizing AT/pole if shunt fieldresistance is 50Ω. Also calculate extra shunt field AT/pole to neutralize the demagnetization.

4. (a) Draw and explain occ of a dc shunt generator. Also define the critical field resistance and criticalspeed.

(b) A shunt generator has an induced voltage on open circuit of 127V. When the machine is on load,the terminal voltage is 120V. Find the load current if the field circuit resistance is 15Ω and thearmature resistance is 0.02Ω. Ignore armature reaction.

5. (a) Draw and explain the load characteristics of a DC shunt generator.(b) Eight shunt generators are running in parallel. Each generator supplies a load current of 400A at

310V. The shunt field current of each generator is 0.04Ω. If one generator is suddenly switched off,determine the percentage change of terminal voltage, the total load current being kept unchanged.

6. A 20kw, 250V dc shunt motor has a full load armature current of 85A at 1100rpm. The armatureresistance is 0.18Ω. Determine:

(a) the initial torque developed(b) the internal torque if the field current is suddenly reduced to 80% of its original value(c) the steady motor speed assuming the load torque to have remained constant.

7. What is the need for speed control of electric motors? List and explain different speed control methodsfor dc motors.

8. A 200v shunt motor has Ra=0.1Ω, Rf=240Ω and rotational loss= 236w on full load the line currentis 9.8A with the motor running at 1450rpm. Determine:

(a) the mechanical power developed(b) the power output(c) the load torque(d) the full load efficiency.

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