R7210206-electrical Machines - I

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Code No: R7210206

II B.Tech I Semester(R07) Supplementary Examinations, May 2009 ELECTRICAL MACHINES-I (Electrical & Electronics Engineering) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ????? 1. (a) Describe doubly excited magnetic system. (b) Discuss torque - load angle characteristics of reluctance motor.

[8+8]

2. (a) Why is a commutator and brush arrangement necessary for operation of a DC machine? (b) A 6 pole lap wound DC armature has 66 slots with 20 conductors/slot. The ratio of pole arc to pole pitch is 0.7.the length of pole shoe is 0.3m. If the air gap flux density is 0.3wb/m2 and the emf generated in the armature is 490 V, find the speed at which it runs. [8+8] 3. (a) Explain with help of neat sketches the phenomenon of commutation in D.C. machines. State and discuss the methods adopted for minimizing the sparking at the brushes? (b) A compensated generator has an interpole air gap of 0.082 m and a flux density in the interpole air gap of 0.36T the ratio of pole arc to pole pith is 0.65. If the armature ampere turns per pole is 16200, determine the ampere turns/pole for the compensating winding and for the interpole winding. [8+8] 4. The OCC of a dc generator when separately excited and driven at 1100 rpm is given by Field amperes: 0.5 1.0 1.5 2 2.5 3 3.5 O.C. voltage( V ): 61.5 123 165 198 220 238 253 If the generator is shunt connected with total field circuit resistance of 80 Ω determine (a) The no load emf and (b) The out put resistance when the terminal voltage is 200 V, if the armature resistance is 0.1 Ω [16] 5. (a) What are the factors which will effect the voltage build up in self excited D.C. generators (b) A separately excited generators, when running at 1000 rpm supplies 200A at 125V. What will be the load current when the speed drops to 800rpm if If unchanged? Given that the armature resistance is 0.04Ω. and B.C.D is 2V. [6+10] 6. A 250v 4 pole shunt motor has two circuit armature winding with 500 conductors.The armature circuit resistance is 0.25 ohms field resstance is 125 ohm and the flux per pole is 0.02wb.neglect armature reaction.find the sped and torque developed if the motor draws 14 A from the mains. [16] 7. A 230V d.c. shunt motor has an armature-circuit resistance of 0.4Ω and field resistance of 115Ω. This motor drives a constant torque load and takes an armature current of 20A at 800 rpm. If motor speed is to be raised from 800 to 1000 rpm, find the resistance that must be inserted in the shunt-field circuit. Assume magnetization curve to be a straight line. [16] 8. A 7.46 kW, 250V shunt motor takes a line current of 5A when running light. Calculate the efficiency as a motor when delivering full load output, if the armature and field resistance are 0.5Ω and 250Ω respectively. At what output power will the efficiency be maximum? Is it possible to obtain this output from these machine? [16] ?????

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Code No: R7210206

II B.Tech I Semester(R07) Supplementary Examinations, May 2009 ELECTRICAL MACHINES-I (Electrical & Electronics Engineering) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ????? 1. (a) Explain the principle of energy conversion of electromechanical system. (b) Derive an expression for the energy stored in a magnetic field.

[8+8]

2. (a) Give the materials and functions of the following parts of a DC machine: i. Field poles ii. Yoke iii. Commutator iv. Commutating poles v. Armature (b) The armature of a 6 - pole generator has a wave winding containing 664 conductors. Calculate the generated e.m.f when flux per pole is 60 mWb and the speed is 250 rpm. Find the speed at which the armature must be driven to generate an e.m.f. of 550 V if the flux per pole is reduced to 58 mWb. [8+8] 3. (a) Explain the following methods of improving commutation i. Resistance commutation ii. E.M.F commutation (b) A 75kw, 4-pole wave wound dc generator has 72 armature conductors. The brushes are given an actual lead of 90 at full load. Calculate i. Cross magnetizing AT/pole ii. Demagnetizing AT/pole and iii. Series turns required to neutralize the demagnetizing effect. [8+8] 4. (a) What are the essential conditions for voltage build up of a self exited shunt generator. (b) A long shunt compound generator delivers a load current of 50 A at 500 V. Its armature resistance is 0.05 Ω, series field resistance of 0.03 Ω and shunt field resistance of 250 Ω. Calculate the generated emf neglecting brush drop? [8+8] 5. (a) Draw and Explain the load characteristics of compound generators. (b) A long shunt compound generator has a shunt field winding of 1000 turns per pole and series field winding of 4 turns/pole and resistance of 0.05 ohm. In order to obtain the rated voltage both at no-load and full load as shunt generator, it is necessary to increase field current by 0.2 A. The full-load armature current of the compound generator is 80A. Calculate the diverter resistance connected in parallel with series field to obtain flat compound operation. [8+8] 6. (a) Explain the basic performance equations for a dc motor. (b) A 4-pole, 250 V series motor has a wave-connected armature with 1254 conductors. The flux per pole is 22 mWb when the motor is taking 50 A. Iron and friction losses amount to 1.0 Kw. Armature resistance is 0.2 ohm and series field resistance is 0.2 ohm. Calculate: i. the speed ii. the BHP iii. the shaft torque and iv. the efficiency at this load. [8+8] 7. (a) Derive an expression for torque developed in a D.C. motor from the first principles (b) A 200V D.C. shunt motor running at 1000 rpm takes an armature current of 17.5A. it is required to reduce the speed to 600rpm. what must be the value of resistance to be inserted in the armature ciruit if the original armature resistance is 0.4Ω. Take armature current to be constant. [10+6] 8. Explain Back-to-Back test of evaluating the efficiency of a D.C Machine. ?????

[16]

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Code No: R7210206 II B.Tech I Semester(R07) Supplementary Examinations, May 2009 ELECTRICAL MACHINES-I (Electrical & Electronics Engineering) Time: 3 hours

Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ?????

1. (a) Show how mechanical energy output can be determined in the multiple excited nonlinear systems. (b) In a rectangular electromagnetic relay the exiting coil has 1000 turns. Cross sectional area of the core is A = 5 c.m2 .neglect reluctance of the magnetic circuit and fringing effects. Find the mechanical energy output when the armature moves such that the air - gap decreases form 1cm to 0.5 cm with flux linkages kept constant at the corresponding Coil current of 2A.

[8+8]

2. (a) Name the different parts of a dc machine. Briefly describe the function of each part. What is the purpose of laminating the armature core of a dc generator? (b) Calculate the different pitches for a 4 pole lap wound dc machines having 4 slots. Each slot has 2 coil sides.

[8+8]

3. (a) Explain Linear commutation, under commutation and over commutation in a dc machine? (b) A 4 pole dc generator has wave wound armature with 722 conductors which develops 50A on full load. Calculate the total ampere - turns per pole.

[12+4]

4. (a) How are the series and shunt windings arranged on the pole of a dc compound machine? (b) How will you distinguish between series and shunt windings of a dc compound machine? [8+8] 5. Write short notes on the following: (a) Parallel operation of DC generators (b) Cross connection of field windings.

[8+8]

6. Explain how torque is developed in a dc motor? What will happen when a dc series motor is started without a load connected to it?

[16]

7. Explain the need and necessity of starters in dc machines. Explain the working of a 3-point starter with neat diagram.

[16]

8. A 220V shunt motor has an armature resistance of 0.2Ω and field resistance of 110Ω. The motor draws 5A at 1500rpm at no load. Calculate the speed and shaft torque if the motor draws 52A at rated voltage.

[8+8] ?????

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Code No: R7210206

II B.Tech I Semester(R07) Supplementary Examinations, May 2009 ELECTRICAL MACHINES-I (Electrical & Electronics Engineering) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ????? 1. (a) Define field energy and co - energy. Give the significance of coenergy in the derivation of torque or force in an electro mechanical energy conversion device. (b) In a rectangular electromagnetic relay the exiting coil has 900 turns of a resistance 1.5 ohm. Cross sectional area of the core is A=3 c.m2 .neglect reluctance of the magnetic circuit and fringing effects. If the coil is exited with an ac voltage of 50 Hz frequencies , having a peak to peak value of 200 V and the armature is held at a distance of 1 c.m. , find the average force on the armature. [8+8] 2. (a) Describe briefly various parts of a DC machine. (b) An 8 pole lap wound generator armature has 960 conductors, a flux of 40 mwb and a speed of 400 r.p.m. Calculate the e.m.f generated on open circuit. If the same armature is wave wound, at what speed it be driven to generate 400 volts? [8+8] 3. (a) Explain in detail the function of compensating windings. (b) A compensated 4 pole lap wound D.C. generator has an armature current of 180A and 480 armature conductors. Calculate the AT/pole for the compensating winding to prevent distortion of the main field over the pole arc. Assume that ratio of pole arc to pitch = 0.7. [8+8] 4. The OCC of a dc generator when separately excited and driven at 1100 rpm is given by Field amperes: 0.5 1.0 1.5 2 2.5 3 3.5 O.C. voltage( V ): 61.5 123 165 198 220 238 253 If the generator is shunt connected with total field circuit resistance of 80 Ω determine (a) The no load emf and (b) The out put resistance when the terminal voltage is 200 V, if the armature resistance is 0.1 Ω [16] 5. In a 110 V compound generator, the resistance of the armature, shunt and series windings are 0.06, 25, and 0.04 ohms respectively. The load consists of 200 lamps each rated at 55 w, 110 V. Find the e.m.f generated and armature current when the machine is connected: (a) long shunt (b) short shunt (c) How will the ampere-turns of series winding be changed if in: i. a diverter of resistance 0.1 ohm be connected in parallel with the series winding? Ignore armature reaction and brush contact drop. [16] 6. (a) Explain the voltage equation of a DC motor and derive the condition for obtaining maximum power. (b) A 25-Kw, 250-V D.C shunt generator has armature and field resistances of 0.6Ω and 100Ω respectively. Determine the total armature power developed when working as a motor taking 25 Kw input. [8+8] 7. (a) Describe the methods for speed control of dc series motors. (b) A dc series motor, running a fan at 1000 r.p.m., takes 50A from 250V mains. The armature plus field resistance is 0.6Ω. If an additional resistance of 4.4 is inserted in series with the armature circuit, find the motor speed in case the field flux is proportional to the armature current. [8+8] 8. A dc shunt motor, with armature circuit resistance of 0.1Ω, runs at 1600 rpm while taking an armature current of 100A from 230V dc source. The friction and windage loss is 300W, no-load core losses are 1200W and the total I2 R loss is 2500W. Stray loss equals 1% of the output. Find the shaft torque of the motor and its efficiency. [8+8] ?????