3rd Sem 7 Marks Basic Elect Machine

BE 3rd SEMESTER QUESTION BANKBASIC ELECTRICAL MACHINEUNIT-1(b) Find the saving in copper by using the autotransformer instead of two winding transformer of same rating.7(c) A 10 KVA, 500/250 V, single phase transformer has its maximum efficiency of 94% when delivering 90% of its rated output at unity power factor. Estimate its efficiency when delivering its full load output at power factor of 0.8 lagging.7(d) Describe the four phasor groups pertaining to three phase transformers. Draw the connection schemes and phasor diagram for each group.7(b) A 30 KVA, single phase transformer has an iron loss of 457 W and copper loss of 125 W, when delivering half the full load. At what % of full load will the transformer have the maximum efficiency?7

(c) Derive an expression for saving of copper in auto-transformer as compared to an equivalent two winding transformer. Also, draw & explain phasor diagram of auto-transformer.7

(d) The following is the data pertaining to a 3-winding transformer:HV in star: 10 MVA, 33 KVMV in star: 7.5 MVA, 11 KVLV in star: 7.5 MVA, 3.3 KVThe per unit leakage reactances are :For Y equivalent circuit find the coil leakage reactance in pu & also in ohms.7(b) Draw and explain phasor diagram of single phase auto transformer. (Step-up).7

(c) Draw and explain DY11 connection of 3-phase transformer. 7(d) A single - phase three winding transformer gives the following data for short circuit test. 7Secondary shorted, excited: 120V, 30A, 750W. Tertiary shorted, excited: 135V, 30A, 810W.Tertiary shorted, excited: 30V, 125A, 815W. The ratings of and tertiary windings are respectively 3300 V, 100 KVA; 1000, 50 KVA; and 400V, 50 KVA. Find the resistances and leakage reactances of the star equivalent circuit. Also calculate their values for each winding. (b) The maximum efficiency of a 3300/440volt single phase transformer is 97% and occurs at full load at unity power factor. If the impedance is 9%, Calculate regulation at full load 0.8 power factor lagging.

(c) A 2000 KVA 6600/400 volt, 3-phase transformer is delta connected on the h.v side and star connected on the L.v.side. Determine its percentage resistance and percentage reactance drops, efficiency, regulation at full load 0.8 power factor leading. Given the following data: S.C. Test: 400 V, 175 A, 17 KW h.v side. O.C Test: 400 V, 150 A, 15 KW L.v side.

(d) A three winding transformer is star/star/delta has the following ratings: Primary: 10 MVA, 33 KVSecondary: 5 MVA, 11 KVTertiary: 5 MVA, 3.3 KVThe short-circuit tests on this transformer gave the following results. Secondary shorted, primary excited; 3000 V, 160 A, 100 KW. Tertiary shorted, primary excited: 200 V, 12 A, 1.25 KWTertiary shorted, secondary excited: 100 V, 40 A, 1.5 KWCalculate resistances and reactances in ohms of the star equivalent circuit of the three winding transformer. Also determine the p.v. values of leakage impedances.

b) Does the flux in a transformer core increases with load?1

2. What are the distinguish features of , in 3- connections. Compare their advantages and disadvantages. 7

3. Why is the wave shape of magnetizing current of a transformer is non-sinusoidal? Explain the phenomenon of inrush of magnetizing current. 7

4. For what purpose tertiary winding is used in transformer? Give the equivalent circuit diagram and any 3-applications of 3-winding transformer. 7(b) Find the saving in copper by using the autotransformer instead of two winding transformer of the same rating.

(c) A 10 kVA, 400/200 V, single phase, 50 Hz transformer has a maximum efficiency of 96% at 75% of full load at unity power factor. Calculate the efficiency of full load 0.8 power factor lagging.

(d) Explain vector grouping of transformers. Mention its usefulness. (b) Explain the behaviour of transformer in rush current in the following points: 7(i) Severety (ii) Doubling effect(iii) Time constant (c) (i) In three phase transformer what is zigzag connection? Also describe /z connection with the help of suitable vector diagram. 7

(ii) What are purposes of tertiary winding in a 3 phase transformer? Draw single phase equivalent circuit of 3 winding transformer.

(d) A three phase transformer bank consisting of three single phase transformer is used to step down the voltage of a 3 phase, 6600 V transmission line. If the primary line current is 10 A, calculate the secondary line current, line voltage and output kVA for the following connection: 7(i) Connection (ii) Connection The turn ratio is 12. Neglect losses.(b) Explain excitation phenomenon in transformers in detail.7

(c) A 10 kVA, 2500/250 V, single phase, two winding transformer is used as an auto transformer to raise the supply voltage of 2500 V to an output voltage of 2625 V. The LV winding of the two winding transformer consists of two equal parts of 125 V each. If both parts of the low voltage winding are used, determine: 7

(i) Auto transformer kVA output. (ii) kVA transformed and conducted.(d) A single phase three-winding transformer gives the following data for three short-ckt tests:7Secondary shorted; primary excited: 120 V, 30 A, 750 W.Tertiary shorted, primary excited: 135 V, 30 A, 810 W.Tertiary shorted; secondary excited: 30 V, 125 A, 815 w. The ratings of primary, secondary and tertiary windings are respectively 3300 V, 100 kVA; 1000 V, 50 kVA and 400 V, 50 kVA. Find the resistances and leakage reactances of the star equivalent circuit. Also calculate their values for each winding. . (a) List phase grouping of three phase transformers. What are uses of phase group? Draw winding connection diagram for Yd11 and Yd1 group. 8(b) What are applications of three winding transformer. How the circuit constants of three winding is determined? 8(c) A single phase 2400/240 V 120 kVA two winding transformer is connected as an autotransformer with additive and subtractive polarity. Determine the kVA rating of auto transformer in both cases. Also calculate the power transformed and power conducted. 8

(a) Explain the clock method of angle designation for representing 3 phase transformer. (b) Describe the four phaser groups pertaining to 3 phase transformers. Draw the connection schemes and phasor diagrams for each group. (c) A 3 phase, 1000 kVA, 6600 V/1100 V transformer in delta connected on the primary and star connected on theUNIT-2(b) How Scott connection are used to obtain two phase supply from three phase supply mains.7(c) A 500 KVA, single phase transformer (A) having 0.015 Pu resistance, 0.05 Pu leakage reactance is to share a load of 750 KVA at 400 V and at 0.8 p.f. lagging with another 250 KVA, single phase transformer (B) having 0.01 Pu resistance and 0.05 Pu leakage reactance. Their secondary no-load emfs are 405 V and 415 V respectively. 7Find:(i) Circulating current at no load.(ii) Current supplied by each transformer.(d) Describe the open delta connection of transformer and show that two transformer in open delta can be used to supply three phase power. Also explain the VA rating of open delta as compared to closed delta.7(b) Two 110 V, single phase furnaces take load of 500 kW & 800 kW respectively, at a power factor of 0.71 lagging & are supplied from 6600 V, three phase mains through a scott connected transformer combination. Calculate currents in 3 phase lines & also draw the phasor diagram.7

(c) Describe the Sumpners test with neat circuit diagram. Two similar 40 KVA, single phase transformers gave the following results when tested by the back to back test method, W1 in supply line measures 800 W, W2 in the secondary series circuit at rated current measures 1000 W. Calculate the efficiency of each transformer at full load unity power factor.7

(d) Two single phase transformers, one of 1000 KVA & other of 500 KVA are connected in parallel to the same bus-bars on the primary side; their no-load secondary voltages being 500 V and 510, respectively. The impedance voltage of the first transformer is 3% & that of second is 5%. Assuming the ratio of resistance to reactance the same & equal to 0.4 each. What will be the cross-current when the secondaries are connected in parallel & the secondary voltage is 480 V.7(b) Explain with circuit diagram, how losses can be separated.7(c) Two single phase furnaces A and B are supplied at 100 V by means of scott connection, from a 3-phase, 6 KVsystem. Furnace A is applied from the teaser transformer. Calculate the line currents on the three-phase side when:7(i) Each furnace takes 600 kW at p.f. 0.8 lagging. (ii) Furnace A takes 500 kW at u.p.f. and furnace B 600 kW at 0.8 p.f. lagging.

(d) Two transformers A and B ratings 500 KVA and 250 KVA are supplying a load of 750 KVA at 0.8 p.f. lagging. Their open circuit voltages are 405 V and 415 V respectively. Transformer A has p.u. resistance of 0.01 pu and 0.05 p.u reactance. Transformer B has 0.015 p.u . resistance and 0.04 pu reactance. Find circulating current. 7

(b) Three two winding transformers of equal voltage ratio have the following leakage impedances. Transformer T1: 100 KVA, 0.02 p.v.Transformer T2: 75 KVA, 0.03 p.v.Transformer T3: 50 KVA, 0.025 p.v.(i) How will they share a load of 225 KVA?(ii) If no transformer is to be over loaded, calculate the greatestKVA load that can be supplied by 3 transformers in parallel.

(c) In Scott connected transformer, teaser transformer supplies unity power factor load of 500 KW at 200 V and the main transformer supplies 0.8 power factor lagging load of 400 KW at 200 Volt. For a 3-phase input voltage of 6600 V, determine the py line currents and the phase angles between them.

(d) Two single phase transformers of ratings 10 KVA, 230/400 V, and 10 KVA, 230/410 V are employed for back to back test. The LV side is fed from 230 V supply mains, where a voltage regulator connected to the same 230 V mains feeds 25 A in series connected HV windings. For a core loss 200 watt in each transformer, calculate the readings of wattmeter connected on LV side. Each transformer has its reactance equal to the 3 times of its resistance. 2. Describe any one type of on-load tap changer with proper sequence of operation for changing the voltage. What precaution should be observed during the operation of on-load tap changer? 7

3. Two single phase transformer one of 100 kVA and other of 50 kVA are connected in parallel to the same bus bar of the primary side, their no load secondary voltages are 1000 V and 950 V respectively. Their resistances are 1.5% and 2.0% respectively and their reactances are 8% and 6% respectively. Calculate the no-load circulating current in the secondaries. 74. What is open delta system? In open delta transformer show that the secondary line voltage form a balanced 3-phase system of voltage, in case the supply voltages are balanced? 7(b) Two transformers are connected in parallel to supply a common load of 125 kVA at 0.8 power factor lagging. Rating of transformer A is 100 kVA and has resistance and reactance of 0.9% and 10% respectively. Rating of transformer B is 50 kVA and has resistance and reactance of 1.0% and 5% respectively. How will the two transformers share the common load?

(c) The full load copper loss and iron loss of a transformer are 920 watt and 430 watt respectively. Calculate: (i) Loading of the transformer at which efficiency is maximum. (ii) What would be losses for giving maximum efficiency at 0.85 of full load, its Total full load losses are unchanged?

(d) Three single phase transformers are connected in mesh. If one transformer is found faulty and removed, what will be the reduction in rating of each of the other transformers? What must be rating of each transformer in V-V connection to supply a three phase balanced load of 200 kVA? If a third similar transformer is included what will be the rated capacity of a set? (b) Explain three phase to two phase conversion by scott connection. 7

(c) Describe the open delta connection of transformer and show that two transformer in open delta can be used to supply three phase power. Also explain the VA rating of open delta as compared to closed delta. 7

(d) Two transformers of same voltage ratio rated at 315 MVA each are connected in parallel to supply a load of 700 MBA at 0.8 p.f. lagging. The per phase resistance and per phase reactance of first transformer are 2% and 11% respectively and of second transformer are 5% and 12% respectively. Calculate the load shared by each transformer. 7(b) Explain three to six phase conversion in detail. 7

(c) An OC test is conducted on the delta side of bank of three phase transformer give the following data: VT (V) : 214 171 128.4 85.6f (Hz) : 50 40 30 20pin (W) : 100 72.5 50 30 Determine the hysteresis and eddy current losses at: 7(i) 60 Hz(ii) 40 Hz

(d) The single phase transformers of ratings 10 kVA, 230/400 V and 10 kVA, 230/410 V, are employed for back to back test. The LV side is fed from 230 V supply mains whereas a voltage regulator connected to the same 230 V mains feeds 25 A in series connected HV windings. For a core loss of 200 W in each transformer, calculate the readings of the wattmeter connected on LV side. Each transformer has its reactance equal to the three times of its resistance. 7Q.2. (a) Show that in Scott connected transformers if secondary load is balanced, primary side will also be balanced neglecting magnetizing current.8(b) (i) In back to back test, one transformer may have more temperature rise than other. Explain: (ii) How will you separate the no load loss of transformer into its components?

(c) Two single phase transformers have kVA ratings of 200 kVA 100 kVA. The equivalent impedance of transformer having 200 kVA rating is Z = 1+j5 ohms while equivalent reactance of other is 10 . What should be equivalent resistance of 100 kVA transformer if each transformer is to supply the load in proportion to its kVA rating when operated in parallel? 8Q.2. (a) How Scott connections are used to obtain two phase supply from three phase supply mains?

(b) Two electric furnaces are supplied with single phase current at 80 V from 3 a phase. 11000 V system by means of two single phase Scott connected transformers with similar secondary windings. When the load on the main transformer is 800 kW and on the teaser transformer is 500 kW, determine the currents in the three phase lines at 0.5 pf lagging. (c) Describe the operation of on load tap changer in 3 phase transformer.

UNIT-3(b) Explain Armature reaction in a d.c. machine. Explain how the effect of armature reaction can be reduced.7(c) 250 V shunt motor on no-load runs at 1000 rpm and takes 5 amp. Armature and shunt field resistances are 0.2 and 250 respectively. Calculate the speed when loaded taking a current of 50 amp. The armature reaction weakens the field by 3%.7(d) Explain clearly how commutation takes place in a d.c. machine.7 b) Explain armature reaction & its harmful effects. Name the methods to improve it. 7

(c) Determine the number of turns on each commutating pole of a 6 pole machine if the flux density in the air gap of the commutating pole is 0.5 wb/m2 at full-load & the effective length of the airgap is 4 mm. The full load current is 500 A & the armature is lap wound with 540 conductors. Assume the number of ampere turns required for the remainder of magnetic circuit to be one-tenth of that of air gap.7

(d) The brushes of a certain lap connected 400 kW, 6-pole generator are given a lead of 180 electrical. Calculate: (i) The demagnetizing ampere-turns. (ii) The cross magnetizing ampere-turns. (iii) Series turns required to balance the demagnetizing component. The full load current is 750 A & total number of conductors are 900 & the leakage coefficient is 1.4.7

(b) What are the methods of attaining linear commutation? 7

(c) A 4 pole generator has a wave wound armature with 722 conductors and it delivers 100 A on full load. If the brush load is 8, calculate armature demagnetizing and cross-magnetizing amper turns per phase.7

(d) A 4-pole. 220V, dc shut motor has 540 lap would connected conductors. It takes 32A from supply mains and has an output of 5.6 kW, the field winding takes 1A. The armature resistance is 0.9 and flux per pole is 0.03 Wb. Calculate: 7(i) the output torque at shaft. (ii) Torque developed. (b) Give the concept of reactance voltage in dc machines. Discuss how the reactance voltage causes delayed commutation in dc machines. Explain how good commutation is achieved by minimizing the reactance voltage.

(c) A 6-pole lap wound dc generator has 240 coils of 2 turns each. Resistance of one turn is 0.03 ohm. The armature is 50 cm long and 40 cm diameter. Air gap flux density of 0.6 T is uniform over the pole shoe. Each pole subtends an angle of 40 mechanical. For armature speed of 1200 rpm find (i) generated emf at no load and (ii) the terminal voltage at full load armature current of 40 A.

(d) What do you understand by demagnetizing and cross magnetizing effects of armature reaction in a dc machine? Derive the expressions for demagnetizing & cross magnetizing ampere-turns per pole for a p pole dc machines. 2. Derive an expression for electromagnetic torque developed in d.c. machine using BIL concept? 7

3. Describe the process of commutation in D.C. machine. Hence show that e.m.f. induced in a coil undergoing commutation is given by the formula where is the armature current and is the commutating period; is the no. of parallel paths in d.c. machine. 7

4. A 22.38 kW, 440 V, 4 pole, wave wound d.c. shunt motor has 840 armature conductors and 140 commutator segments. Its full load efficiency is 88% and the shunt field current is 1.8 A. If the brushes are shifted backward through 1.5 segments from the geometrical neutral axis, find the demagnetizing and distorting amp-turns/pole. 7(b) Prove that torque developed in d.c. machine is directly proportional to the product of flux and armature current.

(c) Explain clearly how commutation takes place in a d.c. machine.

(d) A 6-pole lap wound shunt motor has 500 conductor in the armature path is of 0.05 . The resistance of the shunt field is 25 . Find the speed of motor when it takes 120 amperes from d.c. source of 100 V supply. Flux per pole is 20 m Wb. (b) Derive the expression for the torque in a d.c. machine using BIL approach. 7

(c) (i) Explain the purpose of compensating winding in a d.c. machine. (ii) What is ideal commutation? Give reasons for delayed commutation and write the names of method to improve commutation. 7

(d) A d.c. shunt generator driven by a belt from an engine run at 750 rpm. While feeding 100 kW of electric power into 230 V mains. When the belt breaks it continues to run as motor drawing 9 kW from the mains. 7At what speed would it run? Armature resistance = 0.08 Field resistance = 115 b)Explain the methods of attaining linear commutation in detail.7

(c) What is armature reaction? What are its effects? How will you neutralize its effects? 7(d) A 220 V, dc shunt motor runs at 500 rpm. When the armature current is 50 A, calculate the speed if the torque is doubled. 7Given that 0.2 . (b) An eight pole d.c. generator has 480 armature conductor with wave winding. The armature current is 200 A. Find the armature reaction demagnetizing and cross magnetizing ampere turns if (i) brushes are on geometric neutral plane (gnp) (ii) brushes are shifted by 6 electrical from gnp. 8(c) A d.c. series generator is running at 800 RPM and supplying a load of 6 kw at 120 V. The speed is increased to 1200 RPM and load is increased to 9 kW. The sum of armature and field resistance is 0.4 ohm. Find new value of armature current and terminal voltage. Q.3. (a) Prove that the torque developed by a DC motor is directly proportional to the flux per pole and armature current. (b) Explain Armature reaction in a DC machine. Explain how the effect of armature reaction can be reduced? (c) A 4 pole, lap wound separately excited generator has an ampere resistance of 0.4 ohm and is driven at 750 rpm. The armature has 720 conductors and the flux per pole is 0.03 wb. If the load resistance is 12 ohm, determine the terminal voltage of the machine.

UNIT-4(b) What is the need of starter? Explain the three point starter with all protecting devices.7(c) Explain the nature of no-load, external and armature characteristics of a d.c. shunt generator. Why does external characteristics of this generator turns back as the generator is over loaded. 7(d) A 240 V, 50 Amp, 800 rpm, d.c. shunt motor has armature circuit resistance of 0.2. If load torque is reduced to 60% of its full load value and a resistance of 2 is inserted in series with armature circuit, find the motor speed. Armature reaction weakes the field flux by 4% at full load and by 2% at 60% of full load.(b) A 60 kW, 240 V short shunt compound generator operates as a shunt generator, required an increase in field current of 3 A to provide an over compensated voltage of 275 V at rated load current of 250 A. The shunt field has 200 turns per pole & series field has 5 turns per pole with resistance of 240 & 0.005, respectively.(i) Calculate the required diverter resistance.(ii) If no load voltage of compound generator is also 240 V, calculate the air gap mmf/pole at no load & at full load.7

(c) Explain how back emf Eb acts like a governer in dc motor.7

(d) A shunt motor runs at 600 rpm from 250 V supply & takes a line current of 50 A. Its armature & field resistances are 0.4 & 125, respectively. Neglecting the effects of armature reaction & allowing 2 V brush drop, calculate:(i) no load speed if the no-load line current is 5 A.(ii) % reduction in the flux per pole in order that speed may be 800 rpm when armature current is 40 A.7b) Draw and explain the starter useful starting DC series motor.7(c) The OCC of separately excited DC generator driven at 1000 rpm is as follows:Field current: 0.2 0.4 0.6 0.8 1.0 1.2 1.4EMF Volts : 30 55 75 90 100 110 115If the machine is connected as a shift generator and driven at 1000 rpm and has a field resistance of 100. Find: (i) Open circuit voltage and exciting current. (ii) Critical resistance (iii) Resistance to induce 115 Volts on open circuit. 7

(d) A 10 kW, 250 V, dc shunt generator has total no-load rotational loss of 400 Watts. = 0.5 , = 250 . Calculate shaft power input and efficiency at rated load. Also calculate maximum efficiency and corresponding power output. 7(b) Explain how the shunt generator is self-protective against the accidental short circuit.

(c) A 250 volt, 15 KW, shunt motor has a maximum efficiency of 88% and a speed of 700 rpm, when delivering 80% of its rated output. The resistance of the shunt field is 100 . Determine the efficiency and speed when the motor draws a current of 78 A from the mains.

(d) A series motor runs at 500 rpm when taking a current of 60 A at 460 volt. The resistance of the armature circuit is 0.2 and that of the field winding is 0.1 . Calculate the speed when a 0.15 diverter is connected in parallel with the field winding. Assume the torque to remain unaltered and flux to be proportional to the field current. 2. Explain the nature of no-load, external and armature characteristics of a d.c shunt generator. Why does the external characteristics of this generator turns back as the generator is over loaded? 7

3. Explain why a starter is required for starting a d.c. motor. What do you understand by the drum controller for d.c. series motor? 7

4. A shunt generator delivers 50 kW at 250 V, when running at 400 r.p.m. The armature and field resistance are 0.02 and 50 respectively. Calculate the speed of the machine when running as a shunt motor and taking 50 kW input at 250 V. Allow IV per brush for constant drop. 7(b) What is critical resistance? Explain with reference to the series & shunt generator.

(c) A dc shunt motor running at 900 rpm takes a total current of 5 A from a 240 V dc supply. The armature circuit resistance is 0.75 and field circuit resistance 240 ohms. Calculate the value of series resistor to be inserted in the ampere circuit to reduce the speed to 700 rpm, the torque remaining constant.

(d) State and explain the various losses takes place in a d.c. machine. (b) (i) Derive and draw the characteristic of d.c. shunt and series motor with the help of basic equation. (ii) What are main reasons for necessity of starter in d.c. machines. Give example of starters. 7

(c)(i) Describe the various methods of speed control of d.c. motors. (ii) Explain the ward-Leonard speed control system. 7

(d) A 220 V, 7.5 kW d.c. series motor is mechanically coupled to a fan. When running at 400 rpm the motor draws 30 A from the main. The torque required by the fan is proportional to the square of speed. , . Neglect armature reaction and rotational loss. Also assume the magnetization characteristic to be linear. (i) Determine the power delivered to the fan and torque developed by the motor. (ii) Calculate the external resistance to be added in series to the armature circuit to reduce the fan speed to 200 rpm. 7(b) What is the need of starter? Explain three point starter, with all protecting devices. 7

(c) A shunt generator delivers 195 A at terminal voltage of 250 V. The armature and shunt field resistances are 0.02 and 50 respectively windage and friction losses equal to 950 W. 7Find: (i) Emf generated by the generator (ii) Total copper losses (iii) Output of Primemorer (iv) Commercial, mechanical and electrical efficiencies.

(d) The magnetization characteristics for a 4-pole, 110 V, 1000 rpm shunt generator as follows:7Field Current : 0 0.5 1 1.5 2 2.5 3A

OC Voltage : 5 50 85 102 112 116 120V

Armature is Lap-connected with 144 conductors. Field resistance is 45 ohms. Determine: (i) Voltage the machine will build up at no-load. (ii) Critical resistance (iii) The speed at which the machine fails to excite. (iv) Residual flux per pole.

(a) Explain the process of voltage build up of D.C. shunt generator. Hence explain how the shunt generator fails to self excite. 8(b) A 220 V shunt motor has armature resistance of 0.5 ohm. Armature takes 40 A on full load. By how much must the main flux be reduced to reduce the speed by 50% if developed torque is constant. 8(c) Draw the diagram of three point starter and explain its working. Explain how the drawback of this starter has been removed. 8Q.4. (a) Discuss various methods of speed control used for DC shunt motor. (b) Two separately excited DC generators having emfs of 230 V and 226 V; armature resistance of 0.5 ohm and 0.04 ohm respectively are operating in parallel. Find the load shared by the two generators when the load resistance is 0.5 ohm. (c) Describe the operation of 3 point starter.

UNIT-5(b) Explain the Hopkinsons test to determine the efficiency of d.c. machine.7(c) Describe the various methods of breaking in d.c. motor.7(d) Two d.c. shunt generators run in parallel to supply together 2,500 A. The machines have armature resistances of 0.04 and 0.025, field resistances of 25 and 20 and induced emfs of 440 V and 420 V respectively. Find the bus-bar voltage and output of each machine.7(b) Explain Swinburnes test with neat circuit diagram. 7

(c) The results of Hopkinsons test on two similar dc machines are as follows :Line voltage 220 V, Motor armature current 23 A, Generator armature current 20 A, Generator field current 0.4 A, Motor shunt field current 0.3 A, Armature resistance of each machine is 0.5 . Calculate the efficiency of each machine.7(d) Two shunt generators running in parallel have each armature resistance of 0.1 & field resistance of 100. Total current in external circuit is 220 A. The field are excited so that the emf induced in one machine is 270 V & the other is 250 V. Calculate the bus-bar voltage & output of each machine.7 (b) Explain Hopkinsons test in detail.7

(c) A 220 V, DC shunt motor at no load tak a current of 2.5 A. The resistances armature and shunt field are 0.8 and 200 respectively. Calculate the efficiency of the motor when the input current is 20 A. 7

(d) Explain in detail parallel operation of DC series generators. 7

(b) Hopkinsons test on two shunt machines gave the following test results for full load. Line voltage = 250 VLine current excluding field current = 50 AMotor armature current = 380 AField currents are 5A and 4.2 A.Calculate the efficiency of each machine. Armature resistance of both machines is 0 .02 .(c) The terminal voltage of shunt generator G1 falls from 500 Volt at no load to 470 V when delivering a current of 600 A. For a second generator G2, the figures are 505 Volt at no load and 470 V at 400 A. They are connected in parallel supplying a total load of 400 KW. Assuming that the voltage power characteristics are linear, determine the common bus bar voltage and the current supplied by each machine.

(d) Explain, why the series generators are not suitable for parallel operation? What remedial measures are taken for its successful parallel operation? 2. Explain briefly Hopkinsons test for determination of efficiency of d.c. shunt m/c. what are the main advantages of this test? 7

3. What is meant by braking of d.c. motors? Describe briefly any two methods of braking of d.c. shunt motor. 7

4. Two d.c. shunt generator are connected in parallel to supply a load of 1500 A. One generator has an armature resistance of 0.5 and e.m.f. of 400 V, while the other has an armature resistance of 0.04 and an e.m.f. of 440 V. The resistance of shunt fields are 100 and 80 respectively. Calculate the currents and supplied by individual generator and terminal voltage V of the combination. 7(b) With neat circuit diagram explain, how to perform Swinburnes test on a d.c. shunt motor.

(c) Explain the Hopkinsons test to determine the efficiency of d.c. machine.

(d) Two d.c. generators are connected in parallel to supply jointly a load of 2000 amperes. The machines have armature resistance of 0.04 ohm and 0.03 ohm, field resistance of 25 and 20 and give emf of 440 V and 420 V respectively. Determine the current supplied by each machine and common terminal voltage. (b) Explain Swinburnes test on d.c. machine. 7(c) Describe the various methods of breaking in d.c. motors. 7(d) A 30 kW 220 V d.c. shunt motor with a full load speed of 535 rpm is to be breaked by plugging. Estimate the value of resistance which should be placed in series with it to limit the initial breaking current to 200 A. What would be the value of initial breaking torque? 7Q.5. (a) Discuss Swinburne test to determine the efficiency of DC machine. (b) A 500 V, 20 kW, DC shunt motor took 2.5A when running light. Its armature resistance is 0.6 ohm and field resistance is 1,000 ohm. Determine its full load efficiency. (c) Describe the plugging operation employed to series motor.

secondary. The primary resistance per phase is 1.8 ohm and secondary resistance per phase is 0.025 ohm. Determine the efficiency on full load at unity power factor.

(b) Explain with figure. Hopkinsons Test carried on two dc shunt machines. 7

(c) A 220 V, dc shunt motor at no load takes a current of 2.5 A. The resistances of the armature and shunt field are 0.8 and 200 respectively. Calculate the efficiency of the motor when the input current is 20 A. 7

(d) Two dc shunt generators, operating in parallel, share a load current of 200 A. Each generator has an armature resistance of 0.10 and field resistance of 100 . Their no-load generated emfs are 250 and 245 volts. Calculate the bus bar voltage and the power output of each machine. 7. (a) Explain method of Electrical braking of d.c. motors. 8(b) Hopkinsons test on two identical shunt machines gave the following readings. Supply voltage = 240 V, field currents : 6 A and 5 A. Line current 40 A, Armature current of motor = 240 A. Armature resistance of each machine = 0.014 .Voltage drop per brush = 1V.Find efficiency of each machine. 8(c) Two shunt generators having armature and field resistance of 0.05 and 100 are operating in parallel. The emf induced in one machine is 250V, whereas that in another is 260 V supply a common load of 1500 A. Calculate terminal voltage and output of each machine. 8