Explain the ‘V’ curves of synchronous motors.

What types of motor would you employ for the following types of loads? State reasons foryour choice:—(i) Hoist (ii) Rolling mill (iii) Textile mill (iv) Refrigeration and Air-conditioning

Briefly enumerate the different types of starters used for induction motors.

A d.c. shunt motor, whose armature resistance is 0.5 ohm, is to be starred by a controllerwhich keeps the armature current between the limits of 60 A and 80 A, the supply voltagebeing 400 V. Calculate the number and values of the resistance steps. Prove the theory of themethod of calculation you employ.15

Explain, with the help of neat sketches, how interpoles improve commutation in a d.c.machine. How are the interpole windings connected in (i) d.c. motors, (ii) d.c. generator?

Explain why asynchronous motor does not start by itself.

Using synchronous-impedance method, determine the voltage regulation of a 2000-voltsingle-phase alternator supplying a load current of 100 A at rated voltage and a power factorof (i) unity, (ii) 0.8 leading, and (iii) 0.707 lagging. The test results available are as follows:The full- load current of 100 A is produced on short-circuit by a field excitation of 2.5 A, ane.m.f of 500 V is produced on open circuit by the same excitation, and the armatureresistance is 0.8 ohm.

Describe briefly the construction of (i) woundrotor, (ii) deep-bar and (iii) double-cage typeinduction motors and explain how in each case a high starting torque is obtained

Using double-revolving-field theory, develop an equivalent circuit for single-phase inductionmotor.

Explain

Techniques adopted for braking of d.c. motors

Hunting phenomenon in synchronous machines.

Purpose and action of a delta-connected tertiary winding in Y-Y connected transformers

A 400-V. 3-phase, 50-Hz 6-pole Y-connected induction motor, when running light at normalvoltage and frequency takes power input of 1.2 kW and the line current is 8.25 A. With therotor locked and 180 V applied to the stator, the power input is 5.25 kW and the line currentis 45 A. The stator resistance measured between two line terminals when hot is 0.9 .Draw the approximate equivalent circuit of the motor and calculate the net mechanicalpower output and torque when working at a slip of 4%.

Distinguish the torque-speed characteristics of induction motors meant for (i) continuousrunning under full-load and (ii) a.c. servomotorIndicate, in each case, the design criteria and the means adopted for achieving the desiredperformance.

Derive the expression for the synchronizing power of a synchronous generator with acylindrical rotor when connected to an infinite bus-bars.Discuss the effect of an excitation failure.

What is understood by ‘synchronous condenser’? Explain its operation and application

Explain how a salient pole machine can develop torque without field excitation

Explain what is meant by ‘Synchronous Impedance’ of an alternator.

Find the synchronous impedance and reactance of a single- phase alternator in which a givenfield current produces an armature current of 250 amperes on short-circuit and a generatede.m.f. of 1500 volts on open circuit. The armature resistance is 2 ohms. Hence calculate theterminal p.d. when a load of 250 amperes at 6.6 kV and lagging power factor 0.8 is switchedoff.

A 3-phase alternator having a reactance of 10 ohms has art armature current of 220 amperesat unity power factor when running on 11 kV constant frequency bus-bars. If the steamadmission be unchanged and the e.m.f. raised by 25% determine graphically or otherwise thenew value of machine current and power factor

Discuss the relative merits and disadvantages of single cage and double cage inductionmotors. If the standstill impedance of the outer cage of a double cage machine is 0.3 + j 0.4ohm and of the inner cage is 0.1 + j 1.5 ohms, compare the relative currents and torques ofthe two cages(i) at standstill;(ii) at a slip of 5%.

An induction motor with a star-delta starter is supplied through a feeder from V volts, 50 Hzmains. Coming to line drop, the starling current is found to be the same with star as well asdelta connections. From a short-circuit test on the motor, when delta connected, the followingdata were obtained; V = 200 V; I = 125 A; power factor = 0.4 (line values). Determine theresistance of the feeder. If a second feeder of the same cross-section be rim in parallel withthe original feeder, find the percentage increase in the starting torque obtained with eachconnection

What is ‘on load tap changing facility’ in a power transformer? What is its application inpower system control?

Two identical synchronous machines of rating 100 MW, 50 Hz operating in parallel have thefollowing characteristics:Machine 1 : Speed droop R = 4%Speed changer set to give 50% rated load at rated speed.Machine 2 : Speed drop R = 4%Speed changer set to give 75% rated load at rated speed.(i) Determine the load taken by each machine for a total load of 150 MW and thefrequency of operation.(ii) What adjustment should be made by the speed changers of the machines to share theload as in (i) but with a frequency of 50Hz?

With proper circuit diagram, explain the working of shaded pole single phase inductionMotor

Describe the star-delta method of starting a three-phase induction motor

A 100 HP induction motor is coupled to an industrial drive whose torque varies directly asthe square of the speed. An e.m.f. of 100 V per phase is induced in the rotor winding of theinduction motor at standstill. The full-load slip of the induction motor is 5 per cent. Calculatethe resistance to be added in the rotor circuit to reduce the speed by 20 per cent. Neglect themechanical losses and rotor reactance

The maximum efficiency of a 100 kVA single phase transformer is 98 per cent and occurs at80 per cent full load at 0.8 p.f. If the leakage impedance of the transformer is 5 per cent, findthe voltage regulation at rated load at 0.75 p.f. lagging

A synchronous motor is receiving 30 per cent of the power that is capable of receiving froman infinite bus. If the load on it is doubled, calculate the maximum value of the rotor angleduring swinging of the motor around its new equilibrium position.

Explain the speed-current torque-current and speed-torque characteristics of DC series motor

A three phase star connected synchronous generator is rated at 1.5 MVA, 11 KV. Thearmature effective resistance and synchronous reactance are 1.2ohm and 25 ohmsrespectively per phase. Calculate the percentage voltage regulation for a load of 1.4375 MVAat (i) 0.8 p.f. lagging and (ii) 0.8 p.f. leading. Also find out the p.f. at which the regulationbecomes zero

In the case of three phase induction motor derive the ratio of maximum torque to full loadtorque as

Tm / TF1 = ½ (β2+sFL2)/βSFL where β= y2/x2

With necessary circuit diagram briefly describe Sumpner’s test. Explain how the test resultsare useful in determining equivalent circuit and efficiency of the transformer

The following two synchronous machines arc operating in parallel.Machine A 50 MW 6% speed regulationMachine B 50 MW 3% speed regulation(i) Determine the load taken by each machine for a total load of 80 MW when the speedchangers are set to give rated speed at 100% rated output.(ii) The speed changers of machine A is so adjusted that 80 MW is equally shard. Findthe output of machine A for rated speed and also its percentage speed at no load.

A 220 V D.C. shunt motor takes 22 A at rated voltage and runs at 1000 rpm. Its fieldresistance is 100 and armature circuit resistance is 0.1. Compute the value of additionalresistance required in the armature circuit to reduce the speed to 800 rpm when(i) the load torque is proportional to speed(ii) the load torque varies as the square of the speed

Two identical D.C. machines when tested by Hopkinson method gave following test results:Field currents are 2.5 A and 2 A. Line voltage is 220 V. Line current including both the fieldcurrents is 10 A. Motor armature current is 73 A. The armature resistance of each machine is0.05.Calculate the efficiency of both the machines

Explain the working of a transformer at no load and loaded conditions.

A 10 KVA, 440 V, 50 Hz, 3 phase alternator has the following O.C.C.Field current(Amp)

1.5 3.0 5.0 8.0 11.0 15.0

Terminal voltage(vol)

150 300 440 550 600 635

With full load zero p1 load applied, an excitation of 14 A produced a terminal voltage of 500V. On short circuit, 4A excitation was required to circulate full load current. Using MMFmethod determine the full load percentage regulation for 0.8 p.f. lagging and 0.8 p.f. leading

When a three phase supply is given to a three phase winding a rotating magnetic field ofconstant amplitude will be produced. Justify the above statement

Explain how the parameters of 3 phase induction motor can be obtained from the test results

Explain the speed-current, torque-current and speed- torque characteristics of DC shuntMotor

A 3.5 MVA, slow-speed, 3—phase synchronous generator rated at 6.6 kV has 32 poles. Itsdirect and quadrature axis synchronous reactances as measured by the slip test are 9.6 and 6respectively. Neglecting armature resistance determine the regulation and the excitation emfneeded to maintain 6.6 kV at the terminals when supplying a load of 2.5 MW a 0.8 pflagging. What maximum power can the generator supply at the rated terminal voltage, if thefiled becomes open-circuited?

A 3-phase induction motor has a starting torque of 100% and a maximum torque of 200% ofthe full-load torque. Find slip at maximum torque.

Describe a series (universal) motor. Give its cross-sectional view when connected to acsupply. Describe its performance characteristics

A squirrel-cage induction motor has a slip of 4% at full load. Its starting current is five timesthe full-load current. The stator impedance and magnetising current may be neglected; therotor resistance is assumed constant.(i) Calculate the maximum torque and the slip at which it would occur.(ii) Calculate the starting torque.Express torques in pu of the full-load torque.

What do you understand by a synchronous condenser? Explain with the help of phasordiagrams its operation and application.

The emf per turn of a 1-phase 2200/220 V, 50 Hz transformer is approximately 12 V.Calculate(i) the number of primary and secondary turns, and(ii) the net cross-sectional area of core for a maximum flux density of 1.5 T.

Find the resistance of the load which takes a power of 5 kW from a dc shunt generator whose

external characteristic is given by the equation V = 250 – 0.5 IL .

Briefly describe the phenomenon of ‘hunting’ in a synchronous machine. How is it remedied?

A single phase transformer has percentage regulation of 4 and 4.4 for lagging power factor of0.8 and 0.6 respectively. The full load copper loss is equal to iron loss. Calculate(i) The lagging power factor at which full load regulation is maximum.(ii) The full lead efficiency at unity power factor.

Why are the compensating winding and the Interpole winding used in a universal motor forac operation? Draw the schematic connection diagram of all the stator windings and thearmature or an ac-operated series (universal) motor.

Derive the torque equation of a dc machine

The following test results were obtained on a 7.5 kW, 400 V, 4 pole, 50 Hz, delta-connectedinduction motor with a stator resistance of 2.1 /phase.No load 400 V, 5.5 A, 410 WRotor blocked 140 V, 20 A, 1550 WObtain the approximate equivalent circuit model. Also estimate the breaking torquedeveloped when the motor running with a slip of 0.05. has two of its terminal suddenlyinterchanged.

What is meant by similar polarity ends of the two windings of a single phase transformer ?How are these ends identified?

A is 15 kVA, 2300/230 V, 50 Hz single phase transformer gave the following test data:Open Circuit Test V0 = 2300 V, I0 = 0.21 A,W0 = 50 WShort Circuit Test VS = 47 V, IS = 6.0 A,WS =160 W(i) Find the equivalent circuit referred to high voltage side.(ii) Calculate the full load voltage regulation at 0.8 pf lagging when the load voltage isheld at 220 volts.(iii) What is the efficiency at half the rated load at unity of ?(iv) Find the maximum efficiency and corresponding output power.

State reasons of using tertiary windings in a transformer.

Explain with the help of suitable diagrams, how rotating magnetic field is produced in a 3-phase induction motor.

A 25 hp, 400 V, 50 Hz four pole star connected induction motor has the followingimpedances per phase in ohms referred to the stator side:RS

= 0.641, Rr = 0.332XS

= 1.106, Xr = 0.464 and Xmag = 26.30

Rotational losses are assumed constant and are 1.1 kw and core losses are assumednegligible. If the slip is 2.2% at rated voltage and frequency, find:(i) speed(ii) Stator current(iii) Power factor(iv) Output and input power(v) Efficiency of the motor

Show that the starting torque of a single phase induction motor is zero.

Sketch and explain the Open Circuit and Short Circuit characteristics of a synchronousmachine.

Two synchronous generators are supplying a common load. Generator 1 has a no loadfrequency of 51.5 Hz and regulation of 1 MW/Hz. The total load is 2.5 MW at 0.8 pf lagging.(i) At what frequency, are the generators supplying this load and how much power issupplied by each generator?(ii) An additional load of 1 MW is attached to this system. What will be the newfrequency and power generation of each alternator?(iii) How much is the governor set-point of generator 2 to be adjusted to bring the systemfrequency at 50 HZ for 3.5 MW system load?

Compare the speed-current characteristics of various types of D.C. motors.

Describe Sumpner’s (back to back) test used for transformer. Indicate its advantages.

A 1-phase, 30 KVA, 2300/230 V. 50 Hz transformer gave the following results on tests:Leakage impedance in the high voltage (HV) winding = 0.55 + j 0.65 .Leakage impedance in the low voltage (LV) winding = 0.0055 + j 0.0065 .Shunt branch admittance as seen from LV side = (0.003 – j 0.02) .(i) Draw the equivalent circuits of the transformer referred to HV and LV sides.(ii) The transformer is connected to 2200 volts at the sending end and delivers ratedcurrent at 0.8 p.f. lagging to a load of 0.3 + j2 on the LV side. Draw the phasordiagram and hence or otherwise determine the voltage at the load end and theefficiency of the transformer. Assume core lossed to be 160 W and ignore voltagedrops due to exciting current.

Describe with the help of a neat sketch and phasor diagrams a system through which a 2-phase supply can be obtained from a 3-phase system.

Explain the Torque-Slip characteristics of a 3-phase induction motor. Starting with theexpression for torque as a function of slip. Show that the value of maximum torque is

independent of rotor resistance.

A 3-phase, 400 V, 50 Hz star connected induction motor gave the following test results:No load : 400 V, 7.5 A, 0.135 power factorBlocked rotor : 150 V, 35 A, 0.44 power factorThe ratio of standstill leakage reactances of stator and rotor is estimated as 2. If the motor isrunning at a speed of 960 rmp, determine(i) the net mechanical power output(ii) the net torque(iii) slip and(iv) efficiency of motorAssume stator and rotor copper lossed to be equal.

What is the need of starters for induction motors? Briefly describe various techniques usedfor the starting of 3-phase induction motors.

What is meant by infinite bus bars? State the conditions required to be satisfied forconnecting a synchronous generator to an infinite bus bar. Explain how the instant ofsynchronizing can be determined.

A generator has synchronous reactance of 1.7241 p.u. and is connected to a very largesystem. The terminal voltage of the generator is 1/2 00 p.u. and the generator is supplying tothe system a current of 0.8 p.u. at 0.9 p.f. lagging. Neglecting resistance. Calculate:(i) internal voltage(ii) active and reactive power output of the generator(iii) the power angle and reactive power output of the generator if the excitation of thegenerator is increased by 20% keeping active power constant.

A 5 KW, 230 V, shunt motor has an armature resistance of 0.5 and a field resistance of230. At no load the motor runs at a speed of 1000 rpm and draws a current of 3 A. At fullload and rated voltage, the current drawn in 23 A and the armature reaction causes a drop of2% in the flux. Determine:(i) full load speed (ii) full load torque

SECTION A1. (a) With necessary circuit diagrams explain the open-circuit test and short – circuit testconducted on single. Phase transformer. Justify that the copper loss in the open-circuit testcondition and the iron loss in the short – circuit test condition are normally neglected12

(b) A 500 kVA single phase transformer with 0.012 p.u. resistance and 0.06 p.u. reactance, isconnected in parallel with a 250 kVA single phase transformer with 0.014 p.u. resistance and0.045 p.u. reactance to share a load of 600 kVA at 0.8 power factor logging. Find the 2VAand power factor shared by each transformer.12(c) A 220 V d.c. shunt motor takes 20A at rated voltage and runs at 1000 rpm. Its field circuitresistance is 100 and armature circuit resistance is 0.1. Compute the value of additionalresistance required in the armature circuit to reduce the speed to 800 rpm when(i) the load torque is proportional to speed(ii) the load torque varies as the square of the speed.162. (a) What are the conditions to he satisfied for a d.c. shunt generator to built up? Explain theprocess of building up of emf in a d.c. shunt generator.10(b) A 10 kVA, 440 V, 50 Hz. three phase alternator has the following OCC:-Field current (amp.) Terminal voltage (Volts)1.0 1003.0 3005.0 4408.0 55011.0 60015.0 635With full load zero power factor load applied, an excitation of 14 A produced a terminalvoltage of 500 V On short circuit, 4 A excitation was required to circulate the full loadcurrent. Using MME method, determine the full load percentage regulation for 0.6 p.f.lagging and 0.6 p.f. leading.20(b) Explain how to determine the direct and quadrature axis reactances of a salient polesynchronous machine.10I.E.S-(Conv.) - 1999

ELECTRICAL ENGINEERING3. (a) Describe with the help of an appropriate diagram the Star-Delta method of starting a threephaseinduction motor.10(b) A 400 V 6 pole, 50Hz, three phase star connected induction motor running at rated voltagelakes 7.5 A with a power input of 700W. With the rotor blocked and 150 V applied to thestator the input current is 35 A and the power input is 5000W. The rotor and the stator copperlosses are equal under the blocked rotor condition. The standstill leakage reactance of thestator and rotor as seen from the stator are estimated to be in the ratio of 1:0.5. Obtain theequivalent circuit of the induction motor. Calculate the net mechanical power output, torque,input power and the efficiently at a slip of 4%.20(c) Explain the operation of single phase capacitor start motor.10

2. (a) Two d.c. generators having rectilinear external characteristics operate in parallel. Onemachine has the terminal voltage of 270 V on no load and 220 V at the load current of 30 A.

The other has a voltage of 280 V at no load and 220 V at the load current of 30 A. Calculatethe output current and the bus voltage of each machine when(i) the total load current is 50 A(ii) the load resistance of 10 ohms 16(b) A 5 KVA, 220/110 V single phase transformer has the maximum efficiency of 96.97% atpower factor logging. Its core loss is 50 W and full load regulation at 0.8 p.f. lag is 5%. Findthe efficiency and regulation at 3/4 th full load, 0.9 p.f. lagging. At what p.f. at full load theregulation will be(i) maximum (ii) minimum, and (iii) zero?Calculate the maximum and minimum regulation?16(c) What do you understand by the term vector group of a transformer? Show the terminalconnections pf a 3-phase transformer having vector groups.(i) DY11(ii) Dd6, and the corresponding phasor diagrams83. (a) Discuss about the ‘open circuit’ and ‘short circuit characteristics of a synchronous generator.Draw the phasor diagram under short circuit condition. What do you understand by the term‘short circuit ratio”? Discuss how the short circuit ratio can be calculated from the twocharacteristic curves.12(b) A 2000 KVA, 11 KM 3-phase star connected alternator has a resistance of 0.3 ohm andreactance of 5 ohms per phase. It delivers full load current at a p.f. of 0.8 lagging and normalrated voltage. Compute the terminal voltage for the same excitation and load current at a 0.8p.f. leading.10(c) A 50 Hz, 3-phase induction motor designed for voltage V1 is switched on to 40 Hz supply ofvoltage V2. Find(i) the ratio of starting torques, starting currents and maxi- mum torques when V2 = 1.5V1.(ii) the ratio of V1 to V2 to give equal values of starting torques both at 40 Hz and 50 Hz.Ignore stator impedance and neglect rotor resistance.18

Why does the external characteristic of a d.c. shunt generator turn back as it is overloaded?(b) What are the terms air-gap powers, internal mechanical power developed and shaft power?

How are these terms related with, each other?

2. (a) (i) What are the no-load rotational losses in electrical machines? How can these bedetermined?(ii) Which of the losses in rotating electrical machines are influenced by the magnitude ofthe flux the load and the square of the load?Write a general expression for all the losses occurring in rotating electricalmachines and prove there from that maximum efficiency occurs when lossesproportional to square of current = constant losses,14(b) Explain the effect of armature reaction on the main field flux by using the developed view ofarmature-current sheet and poles of a d.c. machine. Hence, outline the bad effects of armaturereaction.Discuss how the resultant flux density waveform obtained above gets modified withthe use of interpoles.18(c) A 2200/220 1-phase transformer has maximum possible voltage regulation of 6% and itoccurs at a p.f. of 0.3, Find the toad voltage at full-load at p.f. at 0.8 lead.83. (a) A star-connected alternator is synchronized with an infinite bus of 11kV; its steam input isthen increased till its output power is 15 MW. Now, when its excitation e.m.f. is increased to130%, the synchronous machine starts operating at a p.f. of 0.8 lagging. Compute thesynchronous reactance of the machine. Neglect armature resistance.Determine the power factor, load angle and armature current of the machine beforethe excitation e.m.f. is increased.16(b) Explain how the desirable features of high-starting torque and low-operating slip are obtainedin a double-cage polyphase induction motorA double-cage motor has standstill impedance of 1 + j 1 and 0.3 + j 5 for its twocages. Compare the relative torques of the two cages (i) at standstill and (ii) at a slip of 0.0512(c) Why is it advantageous to use double revolving field theory for determining the runningperformance of a single-phase induction motor?Draw torque - speed characteristics of a single - phase induction motor based ondouble - revolving field theory and discuss about the magnitude of torque at zero speed andsynchronous speed.

Sketch waveform of rotor current under normal running conditions of the single-phaseinduction motor.

2. (a) The efficiency at unity PF of a 6600/384 V, 200 kVA single-phase transformer is 98%, bothat full-load and at half full-load. The PF at no-bad is 0.2 lagging and the full-load regulationat a lagging PE of 0.8 is 4%. Draw the equivalent circuit referred to LV side and insert allvalues.20(b) A 500 V, 25 HP DC shunt motor takes a current of 2.4 A while running light. The field andarmature resistances are 650 ohms and 0.57 ohm, respectively. Calculate full-load efficiency,assuming a brush drop of 2 V.10(c) Why is the starting current high in a DC motor? Explain the working of a four-point starterfor a DC machine.103. (a) A cylindrical rotor hydro-generator is feeding an active power of 0.25 p.u. into a largenetwork bus which is held at 1.0 p.u. voltage. The generator is overexcited the an inducedvoltage of 1.5 p.u. The synchronous impedance of the generator and the connecting link are j0.725 p.u./ phase and j0.11 p.u., phase respectively. Calculate the percent change in reactivepoweroutput measured at the network bus in each of the following cases:(i) If the turbine torque is increased by 100%, keeping the excitation of the generatorconstant(ii) If the turbine torque is held constant at the initial value, but the excitation is increasedby 20% (neglect saturation)20(b) Using double revolving field theory explains the working of a single-phase induction motor.10(c) A squirrel-cage induction motor has a starting current of six times the lull-load current at aslip of 0.04. Calculate the line current and starting torque in p.u. of full-load values for thefollowing methods of starting(i) Direct switching(ii) Auto-transformer starting with motor current limited to2.0 p.u.(iii) Star-Delta starting10

A 3-phase induction motor draws 1000 kVA at a p.f. of 0.8 lag. A synchronous condenser isconnected in parallel to draw an additional 750 WA at a p.f. of 0.6 lead. The p.f. of the totalload supplied by the mains is:

(i) Unity (ii) 0.707 lead (iii) 0.6 lag (iv) zero

A 3-phase synchronous motor has(i) High starting torque(ii) No starting torque(Hi) Low starting current(iv) Low starting torque

2. (a) Find the synchronous impedance and reactance of a single-phase alternator in which a givenfield current produces an armature current of 250 A, on short circuit and a generated e.m.f. of1500 V on open circuit. The armature resistance is 2.0 ohms. Calculate the terminal potential

difference when a load of 250 A at 6.6 kV at a lagging p.f. of 0.8 is switched off.15(b) A series motor of resistance 1.0 ohm between the terminals runs at 1000 r.p.m. at 220 V witha current of 15 A. Find the speed at which it will run when connected in series with a 4.5ohms resistance and taking a current of 10 A at the same supply voltage. Assume linearmagnetization characteristic.(c) Discuss briefly the different methods used for electrical braking of d.c. motors.103. (a) A 100 kVA transformer has its maximum efficiency of 98% at full load and unity p.f. Duringthe day it is loaded as follows12 hrs —20 kW at a p.f. of 0.5 lag6 hrs —20 kW at a p.f. of 0.9 lag6 hrs —20 kW at a p.f. of 0.8 lag.Calculate the “all day efficiency of the transformer.16(b) A 1200 V, 50 Hz star connected induction motor has a star connected slipring rotor with atransformation ratio of 3.75. The rotor resistance per phase is 0.016 ohm and leakageinductance of 0.8 mH/Phase. Neglect stator impedance. Find:(i) rotor starting current per phase with slip ring short circuited,(ii) the rotor p.f. at starting,(iii) the external resistance per phase required to obtain a starting current of 125 An thestator,(iv) the rotor current of 4% slip and(v) the rotor p.f. at 4% slip.16(c) Discuss the relative merits and demerits of single cage and double cage induction motors.

2. (a) A transformer is connected to a 1000 V. 50 Hz supply The total core loss is 1000 W, ofwhich 700 W are hysteresis and 300 W are eddy current loss, If the applied voltage is raisedto 2000 V and the frequency to 100 Hz, find the new core loss. 15(b) An alternator has an armature resistance of 0.3 and leakage reactance of 1.22. Thealternator supplies 100 A current to a feeder of resistance 1.512 and reactance 20. Thevoltage at the far end of the feeder is 3000 V. The load current having a p.f. of 0 8 laggingwith respect to this voltage. Find the terminal voltage of the alternator and the emf generated.10(c) A star connected squirrel-cage induction motor has following parameters and ratingRs = 2, Rv = 3, Xs = Xv = 3.5.V = 400 V, 50 Hz. 1380 RPM It is driven by a voltage source inverter which gives constantV/f control to IM. If the inverter frequency is variable from 10% of rated frequency to basefrequency, calculate(a) frequency for a speed of 1000 RPM and full mad torque(b) torque for a frequency of 35 Hz and speed of 950 RPM.153. (a) Derive the condition for maximum efficiency of a transformer.15(b) A series generator of total resistance 0.5 ohm is running at 1000 RPM and delivering 5 kW ata terminal p.d. of 100 V If the speed is raised to 1500 RPM and the power is adjusted to 8kW, find the new current and terminal voltage. Assume that the m/c is working on thestraight portion of the characteristic such that the flux is proportional to the current and theemf generated is proportional to product of flux and speed15

(c) A shunt wound motor runs at 500 RPM on a 200 V supply. Ra = 0.5 ohm, the current drawnis 30 A in addition to field current. What resistance must be placed in series with the armaturein order that the speed may be reduced to 300 RPM, the current in the armature remainingsame? If the load s channel so that the inserted resistance, the armature current reduces to 15A what wilt he the speed?10

2. (a) A 220 V d.c. compound generator has resistances of the armature, shunt and series windingsas 0.1 ohm, 50 ohms and 006 ohm, respectively. The load current is 100 A at 220 V. Find theinduced e.m.f. and armature current when the machine is connected as a (i) long shunt, (ii)short shunt. (iii) how will the ampere-\turn of the series winding will change if in (i) adiverter resistance of 0.1 ohm be connected in parallel with series winding? Ignore armaturereaction.12(b) Calculate efficiency, voltage at the secondary terminals and primary input current whensupplying full load secondary current at power factor (i) unity (ii) 0.8 lag, for a 4 kVA,230/460 V single phase transformer.The following are test resultsOpen circuit with 230 V applied to LV side – 0.6 A, 75 WShort circuit with 20 V applied to HV side - 10 A, 60 W.20(c) Summarize the conditions for proper parallel operation of (i) single-phase transformers, and(ii) 3-phase transformers.83. (a) A synchronous generator supplies rated power at 0.8 p.f. lagging. Its resistance andsynchronous reactance are 0.01 and 1.0 pu. Calculate the terminal voltage if the open circuitvoltage is 2.1 p.u. What is the minimum allowable excitation voltage?14(b) From the short circuit test on a 50 H.P., 440 V, 50 Hz, 6 pole star connected, 3-phaseinduction motor, the following data were obtainedV = 230 V, I = 140 A, cos = 035 (line values). The motor drives a load having a constanttorque of 300 N.m. Estimate the maximum percentage reduction in the supply voltagepossible before the motor stalls. Assume that the copper losses are equally divided betweenthe stator and the rotor. Neglect magnetizing current.14(c) Name the chief methods of speed control of a 3-phase induction motor. Explain briefly one ofthe most modem methods of speed control of an induction motor namely (v/f) control.12

2. (A) A 1500 kVA, 6.6/1.1 kV, 3 phase delta-star connected transformer has the following testresult:S.C. test: (Instruments placed on h.v. side)300 V, 131.21 A, 30 kwCalculate its percentage resistance, percentage reactance drops, percentage efficiency andpercentage regulation on full load at 0.8 power factor lagging. The iron loss during O.C. testwith rated voltage supplied is 25 kW.14(b) A star-connected 3-phase alternator delivers a 3-phase star- connected load at a power factorof 0.8 lagging. A wire connects the load and the alternator. The terminal voltage at no-load is2500 V, and at full-load of 1460 kW it is 2200 V. Determine the terminal voltage when itdelivers a 3 phase star-connected load having a resistance of 6 Q and reactance 8 (2 per phase

respectively. Assume constant current and field excitation.14(c) (i) Interpret the following instructions and mention their addressing mode and flagaffected, if anyXRA A; DAA; DCR M; ADD B.8(ii) What is the difference between Accumulator (ACC) and Instruction Register (IR)?43. (a) (i) With connection diagrams for both shunt and separately excited D.C. generators, drawthe important characteristics of these generators.6(ii) Calculate the rms value of the induced emf per phase of a 10 pole, three phase, 50 Hzalternator with 2 slots per pole per phase and 4 conductors per slot in two layers. Thecoil span is 150°. The flux per pole is 0.12 wb.8(b) Two alternators working in parallel supply a lighting load of 3000 kW and a motor loadaggregating to 5000 kW at a p.1 of 0.72. One machine is loaded up to 5000 kW at 0.8 p.f.lagging. What is the load and power factor of the other machine?14(c) Consider a two stage synchronous counter (both stages receive the pulses at the CK input). Ineach counter K = 1. If Q1 and J1 Q0, draw the circuit. From a truth table of Q0 and Q1 aftereach pulse, demonstrate that this is a 3 : 1 counter.12

2. (a) A 3 phase load of 40 kW, 380 V having a p.f. of 0.8 (lagging) receives its power through a 3phase auto transformer from a 3 phase 440 V 3 supply. The auto transformer is connectedin star. The neutral is NOT grounded. Neglect exciting current and impedance of thetransformer winding.(i) Draw the diagram depicting the above situation.(ii) Calculate the currents entering the load from the auto transformer.(iii) Calculate the currents entering the transformer from the supply side.(iv) Show the currents flowing in the transformer windings along with directions.3 + 4 + 4 + 4(b) A 230 V, D.C. Machine has Ra = 0.3 and Rsh 160 respectively. It is running as a motoron NO LOAD at 1000 RPM taking an armature current of 3.3 A at rated voltage. When themotor is run on FULL LOAD at rated voltage, the line current has a value of 40 Amps.Calculate the speed and torque developed for this condition, assuming that armature reactionweakens the no load flux by 4%.