tutorial emec-i dc machines

Rishi Kant Sharma TUTORIAL-2 SUBJECT: ELECTROMECHANICAL ENERGY CONVERSION –I SEMESTER: III YEAR: 2 nd year SESSION: 2014-15 SECTION: EE- SUBJECT CODE: UNIT-3: DC Machines-II Q.1: A 10 kW, 250 V shunt motor has an armature resistance of 0.5 ohm and field resistance of 200 ohm. At no load and rated voltage, the speed is 1200 rpm and the armature current is 3 A. At full load and rated voltage, the line current is 47 A and because of armature reaction, the flux is 4% less than it’s no load value. (a) What is the full load speed? (b) What is the developed torque at full load? Q.2: A 4 pole series wound series motor has 944 wave connected armature conductors. At a certain load the flux per pole is 34.6 mWb and the total mechanical power developed is 4 kW. Calculate the line current taken by the motor and the speed at which it will run with an applied voltage of 500 V. total motor resistance is 3 ohm. Q.3: A 250 V, 4 pole, shunt motor has two circuit armature winding with 500 conductors. The armature resistance is 0.25 ohm, field resistance is 125 ohm and the field flux per pole is 0.02 Wb. Armature reaction is neglected. If the motor draws 14 A from the mains, then compute (a) speed and the internal torque developed, (b) the shaft power, shaft torque and efficiency with rotational losses equal to 300 watts. Q.4: A 240 V, 50 A, 1500 rpm dc shunt motor has an armature resistance of 0.2 ohm. Neglect shunt field current. (a) For a four step starter, calculate the values of the various steps, with armature current not exceeding 1.40 pu. (b) If the counter emf starter is employed, find the voltages at which the contractors should close. (c) Sketch the appropriate variations of the armature current and speed, during the accelerating process of the motor. Q.5: A 200 V dc shunt motor takes 22 A at rated voltage and runs at 1000 rpm. Its field resistance is 100 ohm and armature resistance is 0.1 ohm. Compute the value of additional resistance in the armature circuit to reduce the speed to 800 rpm when (a) the load torque is independent of speed, (b) the load torque is proportional to speed, (c) the load torque varies as square of the speed, (d) the load torque increases as the cube of speed. Q.6: A 250 V dc shunt motor has armature resistance of 0.5 ohm and field resistance of 250 ohm. When driving a constant torque load at 600 rpm, the motor draws 21 A. what will be the new speed of the motor if an additional 250 ohm is inserted in the field circuit. Q.7: A 10 kW, 240 V dc shunt motor draws a line current of 5.2 A while running at no load speed of 1200 rpm from a 240 V dc supply. It has an armature resistance of 0.25 ohm and a field resistance of 160 ohm. Estimate the efficiency of the motor when it delivers rated load. Q.8: A 400 V, 20 kW dc shunt motor takes 2.5 a when running light. For an armature resistance of 0.5 ohm, field resistance of 800 ohm and brush drop of 2 V, find the full load efficiency. Q.9: two identical dc shunt m/c when tested by Hopkinson’s method gave following data: line voltage 230V, line current excluding both the field currents 30 A, motor armature current 230 V, field currents 5A and 4 A. if the armature resistance of each m/c is 0.025 ohm, calculate efficiency of both the machines. References: 1. P.S.Bhimbra, “Electrical Machinery”, Khanna publication India.

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Tutorial for under graduate students subject EMEC-I topic DC machines

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Rishi Kant Sharma TUTORIAL-2 SUBJECT: ELECTROMECHANICAL ENERGY CONVERSION I SEMESTER:III YEAR: 2nd yearSESSION: 2014-15 SECTION: EE-SUBJECT CODE: UNIT-3:DC Machines-II Q.1: A 10 kW, 250 V shunt motor has an armature resistance of 0.5 ohm and field resistance of 200 ohm. At no load and rated voltage, the speed is 1200 rpm and the armature current is 3 A. At full loadand rated voltage, the line current is 47 A and because ofarmature reaction, thefluxis4%lessthan itsnoloadvalue.(a)Whatisthefullloadspeed?(b)Whatisthe developed torque at full load? Q.2:A 4 pole series wound series motor has 944 wave connected armature conductors. At a certainloadthefluxperpoleis34.6mWbandthetotalmechanicalpowerdevelopedis4 kW. Calculate the line current taken by the motor and the speed at which it will run with an applied voltage of 500 V. total motor resistance is 3 ohm. Q.3:A250V,4pole,shuntmotorhastwocircuitarmaturewindingwith500conductors. The armature resistance is 0.25 ohm, field resistance is 125 ohm and the field flux per pole is 0.02Wb.Armaturereactionisneglected.Ifthemotordraws14Afromthemains,then compute(a)speedandtheinternaltorquedeveloped,(b)theshaftpower,shafttorqueand efficiency with rotational losses equal to 300 watts. Q.4: A 240 V, 50 A, 1500 rpm dc shunt motor has an armature resistance of 0.2 ohm. Neglect shuntfieldcurrent.(a)Forafourstepstarter,calculatethevaluesofthevarioussteps,with armaturecurrentnotexceeding1.40pu.(b)Ifthecounteremfstarterisemployed,findthe voltagesatwhichthecontractorsshouldclose.(c)Sketchtheappropriatevariationsofthe armature current and speed, during the accelerating process of the motor.Q.5:A200Vdcshuntmotortakes22Aatratedvoltageandrunsat1000rpm.Itsfield resistanceis100ohmandarmatureresistanceis0.1ohm.Computethevalueofadditional resistance in the armature circuit to reduce the speed to 800 rpm when (a) the load torque is independent of speed, (b)the load torque is proportional to speed, (c) the load torque varies as square of the speed, (d) the load torque increases as the cube of speed. Q.6: A 250 V dc shunt motor has armature resistance of 0.5 ohm and field resistance of 250 ohm. When driving a constant torque load at 600 rpm, the motor draws 21 A. what will be the new speed of the motor if an additional 250 ohm is inserted in the field circuit. Q.7: A 10 kW, 240 V dc shunt motor draws a line current of 5.2 A while running at no load speed of 1200 rpm from a 240 V dc supply. It has an armature resistanceof 0.25 ohm and a field resistance of 160 ohm. Estimate the efficiency of the motor when it delivers rated load. Q.8:A400V,20kWdcshuntmotortakes2.5awhenrunninglight.Foranarmature resistanceof0.5ohm,fieldresistanceof800ohmandbrushdropof2V,findthefullload efficiency. Q.9:twoidenticaldcshuntm/cwhentestedbyHopkinsonsmethodgavefollowingdata: line voltage 230V, line current excluding both the field currents 30 A, motor armature current 230V,fieldcurrents5Aand4A.ifthearmatureresistanceofeachm/cis0.025ohm, calculate efficiency of both the machines. References: 1. P.S.Bhimbra, Electrical Machinery, Khanna publication India.

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