electrical machines - i

Code No: 07A30201 Set No. 1

II B.Tech I Semester Regular Examinations, November 2008ELECTRICAL MACHINES-I

(Electrical & Electronic Engineering)Time: 3 hours Max Marks: 80

Answer any FIVE QuestionsAll Questions carry equal marks

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1. In a rectangular electro magnetic relay, he exciting coil has 1200 turns. Crosssectional are of the core is A = 6 cm × 6 cm. neglect the reluctance of he magneticcircuit and fringing effects. With coil current kept constant at 2A, derive expressionfor force on armature as a function of air gap of length x. Find the work done bythe magnetic field when x decreases from 1 cm to 0.5 cm by integrating the force.

[16]

2. (a) Deduce an expression for the voltage induced in a DC generator.

(b) The lap wound armature of a 4 - pole generator armature has 51 slots. Eachslot contains 20 conductors. What will be the e.m.f generated in machinewhen driven at 1500 r.p.m. If useful flux per pole is 0.01Wb? [8+8]

3. (a) Explain the armature reaction in a D.C generator on no-load. Enumerate andexplain the method to overcome the adverse effects of the armature reaction.

(b) A 4-pole 50 KW, 250v wave wound, shunt generator has 400 armature con-ductors. Brushes are given a lead of 4 commutator segments. Calculate thedemagnetizing ampere turns/pole if shunt field resistance is 50 ohms. Alsocalculate extra shunt filed turns/pole to neutralize the demagnetization.[8+8]

4. (a) How do you determine the magnetization characteristics of a DC generator.

(b) A separately excited generator when running at 1200 rpm supplies a currentof 200A at 125 V to circuit of constant resistance. What will be the currentwhen the speed drops to 1000 rpm if the field current is unaltered? Armatureresistance is 0.04 ohm and the total voltage drop at the brushes is 2V. Ignorethe change in armature reaction. [8+8]

5. (a) Explain the drooping charectoristics of a D.C shunt generator

(b) A d.c series generator having an reternal charectristics which is a straightline through zero to 50V at 200A is connected as a booster between a stationbusbar and a seeder of 0.3Ω resistance. Calculate the voltage different betweenthe station busbar and the far end Ω the seeder at a curent of

i. 1200A and

ii. 50A. [6+10]

6. A 20 KW,250 V dc shunt motor has a full-load armature current of 85 A at 1100rpm.The armature resistance is 0.18 ohm.Determine:

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(a) the initial torque developed;

(b) the internal torque of the field current is suddenly reduced to 80% of its originalvalue;

(c) The steady motor speed in part

i. assuming the load torque to have remained constant. [16]

7. The speed of a 50 h.p series motor working on 500V supply is 750 rpm at full loadand at 90 % efficiency. If the load torque is made 350Nm and a 5Ω resistance isconnected in series with the machine, calculate the speed at which the machine willrun. Assume the magnetic circuit to be unsaturated and the armature and fieldresistance to be 0.5Ω. [16]

8. The Hopkinson’s test on two similar machines gave the following full load results.Line current =48ALine voltage =110VMotor armature current=230AThe field currents are 3A an 3.5A. Armature resistance of each machine is 0.035ohms.Calculate the efficiency of each machine assuming a brush contact drop of 1Vper brush. [16]

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1. (a) Describe the principle of energy conversion. From a consideration of the vari-ous energies involved, develop the model of an electro mechanical conversiondevice.

(b) Show that the torque developed in a doubly excited magnetic system is equalto the rate of increase of field energy with respect to the displacement atconstant currents . [8+8]

2. (a) Distinguish between lap and wave windings.

(b) The armature of a 2 pole, 200V generator has 400 conductors and runs at 300rpm. Calculate the useful flux or pole. If the number of turns in each field coilis 1200, what is the average value of the emf induced in each coil on breakingthe field, if the flux dies away completely in 0.1s. [8+8]

3. (a) How are demagnetizing and cross magnetizing ampere-turns/pole in a D.CMachines calculated?

(b) Determine AT/pole for each interpole of a 4 pole generator with 88 slots eachcontaining 900 amp - conductors. The interpole air gap is 0.01 m and fluxdensity in the interpole air gap is 0.3 T . The effects of iron parts of iron partsof the circuits and leakage may neglected. [8+8]

4. (a) How are the series and shunt windings arranged on the pole of a dc compoundmachine?

(b) How will you distinguish between series and shunt windings of a dc compoundmachine? [8+8]

5. Two short-shunt compound generators A and B running in parallel supply a loadcurrent of 140A at a terminal voltage 100 V. An equalizig bar connects the twomachines. The data regarding the machines are:Generator A: Ra=0.02 ohm; Rsh=80ohm; Rse=0.02 ohm.Generator B: Ra=0.05 ohm; Rsh=100 ohm; Rse=0.05 ohm; e.m.f generator B, 105V.Calculate:

(a) current in series windings

(b) armature currents

(c) current in equalizer

(d) e.m.f generated by generator A. [4+4+4+4]

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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 con-ductors. The flux per pole is 22 mWb when the motor is taking 50 A. Ironand friction losses amount to 1.0 Kw. Armature resistance is 0.2 ohm andseries 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) Draw and explain the speed-torque characteristics and torque charectristics ofdc shunt motor.

(b) A 240V, 50A,800 rpm dc shunt motor has armature circuit resistance of 0.2Ω.If load torque is reduced to 60% of its full-load value and a resistance of 2Ωis inserted in series with armature circuit, find the motor speed. Armaturereaction weakens the field flux by 4% at full load and by 2% at 60% of fullload. [8+8]

8. A 10kW 900 rpm, 400V dc shunt motor has armature circuit resistance (includingbrushes) of 1Ω and shunt field resistance of 400Ω. If efficiency at rated load is 85%,then calculate:

(a) The no-laoad armature current,

(b) The speed when motor draws 20A from the mains and

(c) The armature, eurrent, when the total(or internal) torque developed is 98.5Nm.Assume the flux remain constant. [16]

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1. (a) Prove that energy and coenergy in a linear magnetic system are given byidentical expressions.

(b) A 10 KW, 1500rpm d.c shunt generator has a time constant Lf/Rf of 0.5 secfor its field winding. Under normal operating conditions, the I

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f rf loss in thefiled winding is 600 watts. Compute the energy stored in the magnetic fieldproduced by the field winding, under normal operating conditions. [8+8]

2. What are the ordinary types of armature winding for dc machine? Explain theessential difference between them and give relative merits and the applications ofthe two types windings. [16]

3. (a) How demagnetizing and cross magnetizing ampere turns per pole are calcu-lated in a DC machine?

(b) Determine per pole, the number of

i. across magnetizing ampere turns

ii. demagnetising ampere turns

iii. series field turns to balance the back ampere turns in the case of a dcgenerator having the following data6000 conductors, total current 100A, 6 pole wave wound, angle of lead is100 , leakage coefficient = 1.3 [8+8]

4. (a) List the conditions for building up of a dc shunt generator.

(b) A d.c. shunt generator is supplying load connected to a bus - bar voltage of220 V. It has an armature resistance of 0.025 Ω and field resistance of 110 Ω.Calculate the value of load current and load power when it generates an emfof 230 V. Neglect the effect of armature reaction. Draw circuit diagram.[8+8]

5. (a) What are the reasons for failure of voltage build up in a self excited D.Cgenerator

(b) A shunt generator is to be converted into a level compounded generator bythe addition of a series field winding. From a test on the machine with shuntexcitation only, it to give 400V on no-load and 4.8A to give the same voltagewhen the machine is supplying its full load of 200A. the shunt winding has1200 turns/pole. find the noos series turns required per pole. [10+6]

6. (a) Draw and explain the dc Series motor characterstics.

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(b) The magnetization characteristic of a 4-pole dc series motor may be taken asproportional to current over a part of the working range; on this basis theflux per pole is 4.5 mwb/A. The load requires a gross torque proportional tothe square of the speed equal to 30 Nm at 1000 rev/min. The armature iswave-wound and has 492 active conductors. Determine the speed at whichthe motor will run and the current it will draw when connected to a 220 Vsupply, the total resistance of the motor being 2.0 ohm. [8+8]

7. A 250V dc series motor has armature and series field resistance of 0.25 and 0.15ohmsrespectively.

(a) Calculate the current for developing a torque of 80Nm at 1200 rpm.

(b) Calculate the percentage reduction in flux when the motor runs at 1800 rpmat half the current obtained in part (a). [8+8]

8. A 50 Kw, 440 V shunt generator having an armature circuit resistance includinginter-pole winding of 0.15 ohm at normal working temperature was run as a shuntmotor on no-load at rated voltage and speed. The total current drawn by the motorwas 5 A including shunt field current of 1.5 A. Calculate the efficiency of the shuntgenerator at 3/4th full-load. [16]

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1. (a) Prove that energy and coenergy in a linear magnetic system are given byidentical expressions.

(b) A 10 KW, 1500rpm d.c shunt generator has a time constant Lf/Rf of 0.5 secfor its field winding. Under normal operating conditions, the I

2

f rf loss in thefiled winding is 600 watts. Compute the energy stored in the magnetic fieldproduced by the field winding, under normal operating conditions. [8+8]

2. (a) With neat diagram give the constructional features of dc machine.

(b) An 6 pole lap wound generator armature has 720 conductors, a flux of 30 mwband a speed of 600 r.p.m. Calculate the e.m.f generated on open circuit. Ifthe same armature is wave wound, at what speed it be driven to generate 600volts? [8+8]

3. A 4-pole generator supplies a current of 143A. It has 492 armature conductors

(a) wave wound

(b) lap wound connected. When delivering full load, the brushes are given anactual lead of 100.

Calculate the demagnetizing ampere turns/pole. This field winding is shunt con-nected and takes 10A. Find the number of extra shunt filed turns necessary toneutralize this demagnetization. [8+8]

4. (a) Draw OCC of a dc shunt generator and define critical speed and critical resis-tance.

(b) A dc shunt generator has the following open circuit magnetization curve at itsrated speedField current ( A ) : 0.5 1.0 1.5 2 3 4EMF ( V ) : 180 340 450 500 550 570

The resistance of the field circuit is 200 Ω. The generator is driven at its ratedspeed. Find the terminal voltage on open circuit. (Use graph paper) [8+8]

5. (a) Explain how critical resistale and critical speed of a D.C. generator can beobtained.

(b) Draw the load charectristics of all the D.C generators. [8+8]

6. A 250v 4 pole shunt motor has two circuit armature winding with 500 conduc-tors.The armature circuit resistance is 0.25 ohms field resstance is 125 ohm and theflux per pole is 0.02wb.neglect armature reaction.find the sped and torque devel-oped if the motor draws 14 A from the mains. [16]

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7. (a) What are the factors effecting the speed of dc motors.

(b) State one advantage and one disadvantage in the application of each of thethree basic types of dc motors. [8+8]

8. A 200V shunt motor has Ra=0.1Ω, Rf=240Ω and rotational loss=236W. On fullload the line current is 9.8A with the motor running at 1450 rpm. Determine:

(a) the mechanical power developed

(b) the power output

(c) the load torque

(d) the full load efficiency. [4+4+4+4]

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Code No: R050210202 Set No. 1

II B.Tech I Semester Supplimentary Examinations, February 2008ELECTRICAL MACHINES - I



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1. (a) Give applications of singly excited system and explain their working.

(b) A U shaped lifting magnet made of cast steel is wound with an exciting coil of800 turns. It is required to lift a mass of 160 kg in air at a distance of 0.1 mm.The magnetic path length of magnet is 75 cm with a cross sectional area of 6cm2. Neglect the resistance of coil.. Calculate the minimum current requiredto lift the weight. The B-H curve data is as follows.B = 1.81 1.82 1.83 wb/m2

H = 2800 3000 3500 AT/m. [8+8]

2. (a) What do you understand by retrogressive and progressive windings? Whichwinding is generally preferred? Why? What are the advantages of retrogres-sive windings?

(b) Give the relation between all pitch(es) for a DC generator with wave winding.Explain the essential condition for Dummy coil requirement. [8+8]

3. (a) What are the causes of sparking in a d. c. machine? Explain how commutationis improved by use of inter poles.

(b) A 22.38 kW, 400-V, 2-pole wave-wound d.c., shunt motor has 840 armatureconductors and 140 commutator segments. Its full-load efficiency is 88% andthe shunt field current is 1.57A. If brushes are shifted backward through 1.5segments from the geometrical neutral axis, find the demagnetizing and dis-torting amp-turn /pole.

[8+8]

4. (a) Explain the method of build-up of the emf in a shunt generator

(b) What is classification of generators? Explain with relevant connection dia-grams. [8+8]

5. (a) Sketch the internal and external characteristics of DC shunt and series gener-ators. What are their fields of application?

(b) Discuss the need for parallel operation of generators. Explain the load sharingof DC Shunt generators operating in parallel. [8+8]

6. (a) How may the direction of rotation of the following DC motors be reversed?

i. Shunt

ii. Series

iii. Compound

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(b) Deduce the expression for torque developed in armature of a DC motor. Statethe factors on which the torque depends. [8+8]

7. (a) With Neat diagram Explain the construction and working of 4-point starter

(b) A DC shunt motor runs at 750 RPM from 250 V supply and is taking a fullload line current of 60 Amps. Its armature and field resistances are 0.4 Ω and125 Ω respectively and brush voltage drop is 2V, calculate,

i. The no load speed, for a no load line current of 6 Amp.

ii. The value of resistance to be added in series with armature to reduce fullload speed to 600 RPM. [8+8]

8. (a) Explain the various losses taking place in DC machines. With the help ofthese losses draw the power flow diagram for a DC Motor.

(b) A 440 V DC shunt motor takes a current of 3 A. at no-load. The armatureresistance including brushes is 0.3 Ω and the field current is 1 A. Calculatethe output and efficiency when the input current is 20 A. [8+8]

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1. With the help of neat diagram obtain the expression for the energy stored in amagnetic system for a simple attracted armature type relay. Explain the operationof system. [16]

2. (a) What is an equalizer connection? What is necessity of equalizer connection?

(b) An 8 pole DC generator has per pole flux of 40mWb and winding is connectedin lap with 960 conductors. Calculate the generated EMF on open circuitwhen it runs at 400 rpm. If the armature is wave wound at what speed mustthe machine be driven to generate the same voltage. [8+8]

3. (a) Explain about demagnetizing Ampere turn per pole and Cross magnetizingAmpere Turn per pole.

(b) What is the purpose of compensating winding? Explain in detail. [8+8]

4. (a) Draw the neat graph to show open circuit characteristic of a separately excitedDC generator? Why is a field regulator necessary for this machine.

(b) What is critical speed? Explain the significance of critical speed. [10+6]

5. Six DC generators are running in parallel, each having an armature resistance of0.15 ohm, running at the same speed and excited to give equal induce e.m.f(s).All generators share load equally at a terminal voltage of 500V. The total load is360KW. If the field current of one generator is raised by 5% and the speed remainsconstant, Calculate

(a) New Terminal Voltage

(b) Output of each machine. [16]

6. (a) Define torque. Derive the expression for torque developed by a D.C. motorfrom fundamentals.

(b) Determine the torque developed when a current of 30A passes through thearmature of a motor with the following particulars: lap winding, 310 conduc-tors, 4-pole, pole-shoes 16.2 cm long subtending an angle of 600 at the centre,bore radius 16.2 cm, flux density in air gap 0.7 tesla. [8+8]

7. (a) Compare the speed control of DC motor by rheostat control method and speedcontrol by semiconductor devices.

(b) A DC shunt motor is required to supply a load requiring a constant torque of120 Nm, over a speed of 500 to 2000 RPM.

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i. Explain the three methods of obtaining the required speed range.

ii. For each method above specify the base speed and power rating. [8+8]

8. (a) Explain the Testing method to separate rotational losses in DC machine.

(b) Two identical DC machines when tested by Back to back method gave thefollowing test results: Field currents are 2.5 A and 2 A. Line voltage is 220V. Line current including both field current is 10 A. Motor armature currentis 73 A. The armature resistance of each machine is 0.05 Ω. Calculate theefficiency of both machines. [6+10]

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1. (a) Explain the doubly excited system? Explain the various losses in it.

(b) Find an expression for the force per unit area between the plates of a parallelplate condenser in terms of the electric field intensity. Find the value of theforce per unit area when E = 3 * 106Volts/ meter, the break down strengthof air. [8+8]





4. What is the significance of residual magnetism? Explain the experimental deter-mination of OCC in laboratory? [16]

5. (a) Explain the procedure of withdrawing a DC generator that is already in par-allel with others and supplying the load.

(b) Eleven DC Shunt Generators are running in parallel. Each generator suppliesa load current o f 400A at 210V. The shunt field current of each generator is4A and the armature resistance of each generator is 0.04 ohm. If one generatoris suddenly switched off, determine the % change in terminal voltage, the totalload current being kept unchanged. [8+8]

6. (a) Discuss Armature reaction and commutation in DC motors. Explain theireffects on the performance of the motor and give remedies to their effects.

(b) A 6-pole DC motor has a wave connected armature with 87 slots, each slotcontaining 6 conductors. The flux per pole is 20 m.wb and the armature hasa resistance of 0.13 ohm when the motor is connected to 240V supply and thearmature draws a current of 80A driving a load of 15KW. Calculate

i. Speed

ii. Armature Torque and

iii. shaft Torque. [10+6]

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7. (a) What are the different speed control methods of DC Shunt motor? Explaineach method and enumerate advantages and disadvantages.

(b) A DC series motor is driving a fan load, whose torque varies a cube of speed.The total armature and series field resistance is 1 Ω. It takes 10 Amp from200 V mains and runs at 1000 RPM. Find the resistance to be connected inseries with the motor to make it run at 800 RPM. [6+10]



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1. (a) Explain the mechanical energy and work done in singly excited system whenactual displacement occurs?

(b) An electromechanical device has following self and mutual inductances.L11= L22= 1.6 + cos2θ andL12= 1 + 0.4cosθ. Find the expression for torque asa function of θ. Find Tav forθ = 30o. when coils are connected to a voltagesource of 160 sin 314t, find Tav for θ= 30o.

[8+8]





4. (a) With relevant connection diagrams, explain the different types of DC genera-tors and give their application.

(b) Explain voltage build-up process of self excited DC generator? [8+8]

5. (a) Distinguish between internal and external characteristic of a DC generator.How can the internal characteristic be derived from the external characteristicof a separately excited generator.

(b) A separately excited generator with constant excitation is connected to a con-stant load. When the speed is 1500 rpm, it delivers 120A at 500V. At whatspeed will the current be reduced to 60A ? Armature resistance is 0.1 ohm,Contact drop/brush is 1V. Armature reaction may be ignored. [8+8]

6. (a) Distinguish between motor and generator action. Derive the equation for theback e.m.f induced in a DC motor.


i. Speed

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7. (a) What are the different speed control methods of DC Shunt motor? Explaineach method and enumerate advantages and disadvantages.

(b) A DC series motor is driving a fan load, whose torque varies a cube of speed.The total armature and series field resistance is 1 Ω. It takes 10 Amp from200 V mains and runs at 1000 RPM. Find the resistance to be connected inseries with the motor to make it run at 800 RPM. [6+10]

8. (a) With neat diagram explain the Swinburne’s test conducted on DC shunt mo-tors. What are the advantages and limitations of this test? Enumerate allcalculations.

(b) 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 ar-mature current = 380 Amp. Field currents are 5 A and 4.2 A. Calculate theefficiency of each machine. Armature resistance of each machine = 0.02Ω.

[6+10]

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II B.Tech I Semester Supplimentary Examinations, November 2008ELECTRICAL MACHINES - I



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3. (a) Explain the effects of armature reaction in a d. c. generator.

(b) Determine per pole the number

i. of cross-magnetizing ampere-turns

ii. of back ampere-turns and

iii. of series turns to balance the back ampere-turns in the case of a.d.c.generator having the following data500 conductors, total current 100A, 4 poles, 2-circuit wave winding, angleof lead = 100, leakage coefficient = 1.3. [8+8]

4. (a) Explain experimental determination of critical field resistance for a self excitedgenerator?

(b) What are the causes and indication of over load generator. [8+8]

5. Six DC generators are running in parallel, each having an armature resistance of0.15 ohm, running at the same speed and excited to give equal induce e.m.f(s).All generators share load equally at a terminal voltage of 500V. The total load is360KW. If the field current of one generator is raised by 5% and the speed remainsconstant, Calculate

(a) New Terminal Voltage

(b) Output of each machine. [16]

6. (a) Distinguish between motor and generator action. Derive the equation for theback e.m.f induced in a DC motor.


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i. Speed



7. (a) With Neat diagram Explain the construction and working of 3-point starter

(b) A 2 ? pole lap wound DC shunt motor with 360 conductors operates at aconstant flux level of 50mWb. The motor armature has a resistance of 0.12 Ωand is designed to operate at 240 V, taking a current of 60 A at full load.

i. Determine the value of external resistance to be inserted in the armaturecircuit so that armature current does not exceed twice its full load valueat starting.

ii. The external resistance is completely cut out when the motor reaches itsfinale speed, with the armature current at the full load value. Calculatethe motor speed under these conditions. [8+8]

8. (a) Compare Swinburne’s test and Hopkinson’s test conducted on DC machines.List the advantages and limitations of both the methods.

(b) The Hopkinson’s test on two DC machines gave the following results for fullload: line voltage 250 V, line current = 45 A excluding field currents, motorarmature current = 385 A, field currents 5 A and 4 A. Calculate the efficiencyof each machine. Armature resistance of each machine is 0.015 Ω. [6+10]

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2. (a) Explain the construction of armature core. What type of material is used forarmature core? Explain the reasons.

(b) The armature of a DC generator is Wave wound with 2 poles. There are 40slots on the armature surface and 12 turns per coil. The armature winding isdouble layer winding. The resistance of each conductor is 10 milliohm, findthe resistance of armature and EMF generated, if flux per pole is 45mwb andgenerator is rotated at 350 rpm. Repeat calculation for Lap winding. [6+10]



[8+8]

4. What is critical speed? How do you draw magnetization characteristics in labora-tory. [16]

5. (a) Distinguish between:

i. DC shunt generator and

ii. DC compound generator.

Show and explain the nature of external characteristic of each of them. Alsogive applications of each of them.

(b) Eight DC Shunt Generators are running in parallel. Each generator supplies aload current o f 400A at 210V. The shunt field current of each generator is 4Aand the armature resistance of each generator is 0.04 ohm. If one generator issuddenly switched off, determine the % change in terminal voltage, the totalload current being kept unchanged. [8+8]

6. (a) Define torque. Derive from first principles, the torque equation of a DC motor.

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(b) Determine the torque developed when a current of 30A passes through thearmature of a motor with the following particulars: wave winding, 310 con-ductors, 4-pole, pole-shoes 16.2 cm long subtending an angle of 600 at thecentre, bore radius 16.2 cm, flux density in air gap 0.7 tesla. [8+8]

7. (a) Explain the functions of NVC and OLC in 4 Point starter. What are theadvantages and limitations of this starter on 3 point starter?

(b) The EMF developed in the armature of a shunt generator at 1155 RPM is240 Volts for a field current of 4.5 Amps and 255 V for a field current of 5Amps. The generator is now used as a motor on a 260 V supply and takes anarmature current of 75 Amps. Find the motor speed, when the field currentis adjusted to 4.8 Amps, total armature resistance is 0.12 Ω. [6+10]

8. (a) The following readings are obtained when performing a brake test on DC shuntmotor. Spring Balances, 08 kgs and 30 kgs. Diameter of drum = 42 cm. Speedof the motor = 1000 rpm. Applied voltage = 220 V. Line current = 50 A.Calculate out put power and efficiency.

(b) A 500 V DC shunt motor takes a current of 5 A on no-load. The resistancesof the armature and field circuit are 0.2 Ω and 300 Ω respectively. Find,the efficiency when loaded and taking a current of 125 A and the percentagechange of speed. [8+8]

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2. Explain the various pitch(es) related with armature winding. Derive the relationbetween each for lap and wave winding. [16]



[8+8]

4. What is critical speed? How do you calculate the critical speed in laboratory. [16]

5. (a) Explain the external characteristic of DC series generator. Why does theterminal voltage start decreasing after a certain value of load current? Alsogive its applications.

(b) A separately excited generator with constant excitation is connected to a con-stant load. When the speed is 1000 rpm, it delivers 120A at 500V. At whatspeed will the current be reduced to 60A ? Armature resistance is 0.1 ohm,Contact drop/brush is 1V. Armature reaction may be ignored. [8+8]



7. (a) Explain the functions of NVC and OLC in 4 Point starter. What are theadvantages and limitations of this starter on 3 point starter?

(b) The EMF developed in the armature of a shunt generator at 1155 RPM is240 Volts for a field current of 4.5 Amps and 255 V for a field current of 5Amps. The generator is now used as a motor on a 260 V supply and takes an

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armature current of 75 Amps. Find the motor speed, when the field currentis adjusted to 4.8 Amps, total armature resistance is 0.12 Ω. [6+10]



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1. (a) Obtain the expression for the mechanical energy developed in electromechan-ical energy conversion devices (Single excited system).

(b) In a electromechanical relay excited from a voltage source, the current andflux linkages are related as i = (1.2λ)2+3.8λ(10.2x2); x < 1, find the force onthe armature as a function of λ. [8+8]



3. (a) Discuss armature reaction and commutation in a dc motor.

(b) A 2-pole lap connected motor armature has 880 conductors and delivers 120A.The brushes have been displaced through 4 angular degrees from the geomet-rical axis. Calculate

i. demagnetizing amp-turns/pole

ii. cross-magnetising amp-turns/pole

(c) the additional field current for neutralizing the demagnetization of the fieldwinding has 1100 turns/pole. [8+8]

4. What is critical field resistance? How do you draw the open circuit characteristicin laboratory. [16]

5. (a) Discuss in detail the characteristics and applications of DC Shunt, series andcompound generators

(b) Explain the conditions necessary for the parallel operation of DC generators.[10+6]



7. (a) Why we need to control the speed of D.C. Motor? Explain the torque-speedrelation of DC Motor.

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(b) A 230 V DC series motor has resistance of 0.2 Ω. At a speed of 1800 rpm, ittakes a current of 40 A. Find the resistance to be added to limit the speed to3600 RPM, when the current is 10 Amp. Assume that flux is linear to currentbetween 10 Amps to 40 Amps. [8+8]

8. (a) With neat diagram explain the Back to Back test conducted on two DC ma-chines? Give all calculations assuming equal constant losses.

(b) A Hopkinson’s test on two identical shunt machines gave the following results:Input voltage 500 V, Input current 15 A, output current of generator 120 A,Field current of generator 4A, field current of motor 3 A. Armature resistanceof each machine 0.06 Ω. find efficiency of motor and generator. [8+8]

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