Noida Institute of Engineering & Technology, Gr. NoidaQuestion Bank of EMEC-1 (NEE 301)

B.Tech EN III Sem

Q-1 Explain the concept of electromechanical energy conversion with neat diagram.

Q-2 For a singly excited magnetic system, establish relationship b/w magnetic field energy & co-energy. Develop also the relation for the magnetic stored energy in terms of reluctance.

Q-3 Define energy & co-energy. What is the significance of co-energy? Show that the field energy in a linear magnetic system is given by

Wf = ½ Li2 = ½ ψ i = ψ2/2L where ψ is flux linkage.

Q-4 Describe the principle of virtual works& hence shows that the magnetic force fe is given by

fe=−∂Wfld∂ x

(φ, x) = - ∂Wfld∂ x

(λ, x)

Where λ is flux linkage.

Q-5 Derive the following relationship for field energy Wf¿∫0

λ

i d λ where λ is flux linkages.

Q-6 With suitable diagram, give the constructional feature of a D.C machine.

Q-7 A 4 pole, lap wound, D.C generators has 42 coils with 8 turns per coils. It is driven at 1120 r.p.m. If useful flux per pole is 21 mWb, calculate the generated e.m.f. Find the speed at which it is to be driven to generate the same e.m.f. as calculated above, with wave wound armature.

(Ans. E = 263.424 V, N= 560 r.p.m)

Q-8 Define armature reaction. How it affect the main field flux. How it can be reduced?

Q-9 Describe the working principle explaining how the commutator converts alternating voltage into direct voltage.

Q-10 Explain commutation process in D.C. machines. What are the causes of poor commutation? Describe the role of interlopes & compensating winding in dc machine.

Q-11 A 6- pole lap wound D.C. shunt machine has 40 slots and 8 conductors/slot and each conductor carries a current of 30 A. The machine core diameter is 0.4 m, length is 0.3 m and average value of flux density is 0.3 T. The machine is driven at 1500 rpm. Determine emf, armature current and power developed.

(Ans. Ia=180A, Eg=150.792V, Pm = 27.14 kW)

Q-12 A dc series generator has armature resistance of 0.5 ohm & series field resistance of 0.03 ohm. It drives the load of 50A. If it has 6 turns & total 540 coils on the armature & is driven at 1500 rpm, calculate the terminal voltage at the load. Assume 4 pole, lap winding, flux per pole as 2mWb & total brush drop as 2V.

(Ans. Vt= 295.5V)

Q-13 State & explain No load magnetization characteristics, External & internal characteristics of dc generator.

Q-14 Draw and explain the performance characteristics of different types of dc generators.

Q-15 What is back emf? Explain the significance of a back emf.

Q-16 Derive an expression for the torque developed in a dc motor.

Q-17 A dc series motor is running with a speed of 800 rpm while taking current of 20A from the supply. If the load is changed such that the current drawn by it increased to 50A calculate the speed of motor on new load. The armature & series field winding resistance are 0.2 ohm & 0.3 ohm respectively. Assume the flux produced is proportional to the current. Assume supply voltage as 250V.

(Ans. N= 300rpm)

Q-18 A 4 pole dc motor runs at 600 rpm on full load & takes 25A at 400V. The armature is lap wound with 500

conductors & flux per pole is given by φ =1.7 X 10 -1√ I Wb, where I is the motor current. If the supply voltage

and torque both are halved, calculate the speed at which the motor will run. Neglect stray losses.

(Ans. N= 372.22rpm)

Q-19 A 4-pole, lap wound long shunt compound generator has 1200 armature conductor. The armature, series field & shunt field resistances are 0.1 ohm, 0.15 ohm & 250 ohm, respectively. If flux per pole is 0.075Wb, calculate the speed at which the machine should be driven so that it can deliver the load of 50kW at 500V. Take overall voltage drop due to brushes contact as 2V.

(Ans. 351.67rpm)

Q-20 Draw the speed characteristic of dc shunt, series & compound motor in same diagram and compare them. Which of the characteristic is more suitable for traction purpose and why?

Q-21 Explain why starter is required for starting a dc motor. Explain 3 point starter.

Q-22 Describe a 4-point starter. Compare and distinguish it with a three point starter.

Q-23 A 250V dc shut motor has an armature resistance of 0.5 ohm and a field resistance of 250 ohm. When driving a constant torque load at 600 rpm the motor draws 21A. What will be the new speed of the motor is an additional 250 ohm resistance is inserted in the field circuit?

(Ans N=1150 rpm)

Q-24 Explain the methods of speed control of dc shunt motor with the help of neat diagrams.

Q-25 Explain Ward-Leonard method of speed control in dc motors.

Q-26 A 200 V shunt motor has Ra = 0.1ohm, Rf = 240 ohm and rotational loss 236 W. On full-load, the line current is 9.8 A with the motor running at 1450 rpm.

Determine: 1) The mechanical power developed

2) The power output

3) The load torque

4) The full load efficiency.

(Ans. Pm= 1785.164W, Pout= 1549.1604W,Tl=10.20 Nm,%Ƞ=79.03 %)

Q-27 With neat circuit diagram explains how you will conduct Swinburne’s test. Also show how to calculate the efficiency when the machine is running a) as a motor b) as a generator. Also state the merits & demerits of this method.

Q-28 Explain back to back test (Hopkinson test) as two identical dc machine and calculate the efficiency of the machine as generator & motor. Mention the advantages of this test over the other tests.

Q-29 A 100 kVA, 1100/200 volt single phase transformer has the following parameter; r1= 0.1 ohm, x1 =0.3 ohm, r2=0.004 ohm, x2 = 0.012 ohm Find equivalent resistance and leakage reactance as refer to high voltage winding & low voltage wdg.

(Ans 0.221ohm, 0.663 ohm)

Q-30 Draw a phasor diagram to represent conditions in a single phase transformer supplying load at:

1) Unity p.f.

2) Lagging p.f. and

3) Leading p.f.

Q-31 A 100 kVA, 2200/220 V transformer has leakage reactance drop of 8% and resistance drop of 2%. Find voltage regulation at full load and 0.8 p.f. lagging. Find also the p.f. at which regulation will be zero.

(Ans. VR = 6.4 %, cos φ= 0.9701 leading)

Q-32 Define Voltage Regulation of a two wdg transformer & explain its significance. Also derive the condition for zero VR.

Q-33 A 600kVA, single phase transformer when working at u.p.f. has an efficiency of 92% at full load and also at half load. Determine its efficiency when it operates at unity power factor and 60% of full load.

(Ans 92.32 %)

Q-34 A 200kVA, single phase transformer has an efficiency of 98% at full load 0.8 p.f. lag. If the maximum efficiency occurs at three quarters full load, calculate the iron loss and full load copper loss.

(Ans Pi=1175.51W, Pcu= 2089.8W)

Q-35 Find the all-day efficiency of the transformer having maximum efficiency of 98.5% at 20 kVA, u.p.f and loaded as follows:-

11 hours: 5kW, 0.7 p.f. Lagging

6 hour: 8kW, 0.8 p.f. Lagging

7 hour: No load

The maximum efficiency occurs at 80% of full load.

(Ans 96.174%)

Q-36 A 100 kVA, 1000/220 V, 1-phase transformer gave the following test result:

OC test (hv): 1 kV, 0.8 kW, 2 A

SC test (lv): 15V, 1.5 kW, 455 A

Determine parameters of equivalent circuit referred to hv side and efficiency at full load, 0.8 pf lagging.

(Ans. 97.2 %)

Q-37 With neat circuit diagrams, explain the open-circuit and short-circuit test conducted on single-phase transformer. How to obtain regulation and efficiency from these tests?

Q-38 Explain the back to back (Sumpner) method of testing of two identical single phase transformers.

Q-39 What is the difference b/w 3 single phase transformer bank and a 3-phase transformer unit?

Q-40 With proper connection and phase diagrams describe the different ways of connecting three phase transformer.

Q-41 A 3 phase step down transformer is connected to 6600 volts mains and it takes 10 A. calculated the secondary line voltage, line current, and output for the following connections a) Delta-Delta b) Star- Star c) Star- Delta d) Delta –Star

Turns ratio/phase is 1.2. Draw connection diagrams.

( Ans. 550 V, 120 A, 114.3 kVA)

Q-42 Draw the connection diagram for open delta system and show that S open∆S closed∆

= 1√3

Q-43 With the help of phasor diagram explains how 2 phase supply can be obtained from 3 phase supply using Scott connection.

Q-44 Explain 3 phase to 6 phase conversion.

Q-45 What is meant by three phase transformer group? What is the significance of these groups.

Q-46 What is current inrush phenomenon in transformer?

Q-47 Explain how harmonic are produced in transformers even when the supply voltage be purely sinusoidal. Which orders of harmonics are usually prominent? What is done to neutralize the effect of third harmonic voltage in high voltage star-star connected transformer?

Q-48 What is an autotransformer? Give the application of it. Also States the merits and demerits of autotransformer?

Q-49 An autotransformer is used to supply a resistive load of 5 kW at 400 V. supply voltage is 400 V. Neglecting losses calculate the currents in various part of the winding. Find the percentage copper saving effect to use of transformer instead of equivalent two winding transformer.

(Ans : 11.375 A, 1.125 A, 12.5 A, 91%)

Q-50 Explain the need for parallel operation of single-phase transformer. Give the condition to be satisfied for their successful operation.

Q-51 Deduce the expression for the load shared by the two transformers with equal voltage ratios.

Q-52 A 500 kVA transformer with 0.01 pu resistance and 0.05 pu reactance is connected in parallel with a 250 kVA with 0.015 pu resistance and 0.04 pu reactance. The secondary voltage of each transformer is 400 V on no load. Find how they share a load of 750 kVA at 0.8 pf lagging.

(Ans. SA = 471.1 <-40.3° kVA, SB = 280.9 <-31.06° kVA)

Q-53 Two 2200/110 V transformers are operated in parallel to share a load of 120 kVA at 0.8 pf lagging. Transformer A is rated 100 kVA, 0.8 % resistance and 10 % reactance. Transformer B is rated as 60 kVA, 1 % resistance and 5 % reactance. Find the load carried by each transformer.

(Ans. SA = 55.1 <-40.49° kVA , SB = 65.08 <-33.79° kVA)Q-54 Two coupled coils have self and mutual inductance of L11= 2+1/(2x), L22= 1+1/(2x), L12=L21= 1/(2x), over a certain range of linear displacement x. the first coil is excited by a constant current of 20 A and the second by a constant current of -10 A. find

a) Mechanical work done if x changes from 0.5 to 1 m.b) Energy supplied by each electrical source in part (a).

Q-55 A doubly excited rotating machines has the following self and mutual inductances.rs= 40 Ω, rr= 2 Ω, Msr= 0.08 cos r, Ls= 0.16 H, Lr= 0.04+ 0.02 cos r. r is the space angle between coil axes. The rotor is revolving at a speed of 100 red/sec. For is=10 A and ir= 2 A dc. Derive expressions for the instantaneous voltages applied to the stator and rotor windings.

Q-56 Show that the torque developed in a doubly excited magnetic system is equal to the rate of increase of field energy with respect to displacement at constant currents.

Q-57 For a doubly excited magnetic field system, various inductances are,L11= (4+cos 2).10-3 H, L12 = 0.15 cos H, L22 = (20 + 5 cos 2) H. Find the torque developed if i1 = 1 A

and i2 = 0.02 A.

Q-58 Derive an expression for the torque in a doubly excited system having salient type of stator as well as rotor. State the assumption made.

Q-59 Explain the advantages of analysing energy conversion devices by FIELD ENERGY CONCEPT.

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