Exam...Electrical Machnes-I

Dr. NNCE EEE/04 SEM SY&QB

- 1 -

SUBJECT CODE : EE2251

SUBJECT NAME: ELECTRICAL MACHINES - I

UNIT-I INTRODUCTION

Part –A (1 MARK)

1. The two windings of a transformer is

a. Conductivelylinked

b. Inductively linked.

c. Not linked at all.

d. Electrically linked.

2. A salient pole synchronous motor is running at no load. Its field current is switched

off.The motor will

a. Come to stop.

b.Continue to run at synchronous speed.

c. Continue to run at a speed slightly more than the synchronous speed.

d. Continue to run at a speed slightly less than the synchronous speed.

3. The d.c. series motor should always be started with load because

a. at no load, it will rotate at dangerously high speed.

b. it will fail to start.

c. it will not develop high starting torque.

d. all are true.

4. The frequency of the rotor current in a 3 phase 50 Hz, 4 pole induction motor at full

load speed is about

a. 50 Hz.

b. 20 Hz.

c. 2 Hz.

d. Zero.

5. In a stepper motor the angular displacement

a. can be precisely controlled.

b. it cannot be readily interfaced with micro computer based controller.

c. the angular displacement cannot be precisely controlled.

d. it cannot be used for positioning of work tables and tools in NC machines.

6. The power factor of a squirrel cage induction motor is

a. low at light load only.

b. low at heavy load only.

c. low at light and heavy load both.

d. low at rated load only.

7. The generation voltage is usually

a. between 11 KV and 33 KV.

b. between 132 KV and 400 KV.

c. between 400 KV and 700 KV.

d. None of the above.

8. When a synchronous motor is running at synchronous speed, the damper winding

produces

a. damping torque.

b. eddy current torque.

c. torque aiding the developed torque.

d. no torque.

9. If a transformer primary is energised from a square wave voltage source, its output

voltage will be

a. A square wave. b. A sine wave.


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c. A triangular wave. d. A pulse wave

10. In a d.c. series motor which type of the torque developed

a. rotational torque

b. electromagnetic torque

c. .both

d. none

11. In a 3 – phase induction motor running at slip ‘s’ the mechanical power developed in

terms of air gap power Pg is

a. (1-s)

b. (1-s)+Pg

c. (1-s)Pg

d. (s-1) Pg

12. In a 3 – phase induction motor the maximum torque

a. is proportional to rotor resistance r2.

b. does not depend on r2.

c. is proportional to square root of r2.

d. is proportional to square r2

13. In a d.c. machine, the armature mmf is

a. stationary w.r.t. armature.

b. rotating w.r.t. field.

c. stationary w.r.t. field.

d. rotating w.r.t. brushes.

14. In a transformer the voltage regulation will be zero when it operates at

a. unity p.f.

b. leading p.f.

c. lagging p.f.

d. zero p.f. leading.

15. The maximum power in cylindrical and salient pole machines is obtained

respectively at load angles of

a. 90 , 90 .

b.

c.

d.

16. The primary winding of a 220/6 V, 50 Hz transformer is energised from 110 V, 60 Hz

supply. The secondary output voltage will be

a. 3.6 V.

b. 2.5 V.

c. 3.0 V.

d. 6.0 V.

17. The emf induced in the primary of a transformer

a. is in phase with the flux.

b. lags behind the flux by 90 degree.

c. leads the flux by 90 degree.

d. is in phase opposition to that of flux.

18. The relative speed between the magnetic fields of stator and rotor under steady state

operation is zero for a

a. dc machine.

b. 3 phase induction machine.

c. synchronous machine.

d. single phase induction machine

19. The current from the stator of an alternator is taken out to the external load circuit

through

a. slip rings.

b. commutator segments.

c. solid connections.

d. carbon brushes.


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20. A motor which can conveniently be operated at lagging as well as leading power

factors is the

a. squirrel cage inductionmotor.

b. wound rotor induction motor.

c. synchronous motor

d. d. DC shunt motor.

Answers

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

b b a c a A a d a b c b c b d c C e c C

PART-B (2 MARK)

21. What is prime mover?AU-APRIL/MAY 2008

The basic source of mechanical power which drives the armature of the generator is

called prime mover.

22. Give the materials used in machine manufacturing AU-APRIL/MAY 2008

Three materials are used in machine manufacturing.

(i)steel – to conduct magnetic flux

(ii)copper – to conduct electric current

(iii)Insulation

23. What are the factors on which hysteresis loss depends? AU-NOV/DEC 2008

The hysteresis loss depends on the magnetic flux density, frequency f and the volume of

the material V.

24.What is core loss? What is its significance in electric machines? AU-NOV/DEC 2008

When a magnetic material undergoes cyclic magnetization, two kinds of power losses

occur on it – hysteresis and eddy current loss which together are known as core loss. It is

important in determining heating, temperature rise, rating and efficiency of transformers,

machines and other a.c run magnetic devices.

25.What is eddy current loss? AU-APRIL/MAY 2009

When a magnetic core carries a time varying flux voltages are induced in all possible

paths enclosing flux. Result is the production of circulating current in core. These induced

currents do no useful work are known as eddy current and have power loss known as

eddycurrent loss.

26.How are hysteresis and eddy current losses minimized? AU-APRIL/MAY 2009

Hysteresis loss can be minimized by selecting materials for core such as silicon steel & steel

alloys with low hysteresis coefficient and electrical resistivity. Eddy current losses are

minimized by laminating the core their moving parts.

27.How will you find the direction of emf using Fleming’s Right Hand Rule? AU-NOV/DEC 2009

The thumb, the forefinger and the middle finger of the right hand are held so that these

fingers are mutually perpendicular, then Forefinger – Field,Thumb – Motion,Middle finger-

I, current


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28.How will you find the direction of force produced using Fleming’s Left Hand Rule?

AU-APRIL/MAY 2010

The thumb , forefinger and middle finger of the left hand are held so that those fingers are

mutually perpendicular then

Forefinger - Field

Thumb - Motion(due to force)

Middle finger - I, current

29.Why do cylindrical rotor alternators operate with steam turbines? AU-APRIL/MAY 2010

Steam turbines are found to operate at fairly good efficiency only at high speeds. The high-

speed operation of rotor tends to increase mechanical losses, so the rotors should have

smooth external surface. Hence smooth cylindrical type rotors with less diameter and large

axial length are used for synchronous generators driven by steam turbines with either 2 or 4

poles.

30.Which type of synchronous generators are used in Hydroelectric plants AU-

MAY/JUNE 2012

As the speed of operation is low, for hydro turbines used in hydroelectric plants, salient pole

type synchronous generators are used. These allow better ventilation and also have other

advantages over smooth cylindrical type rotor.

31.What is the relation between electrical degree and mechanical degree?AU-OCT/NOV 2010

Electrical degree θe and mechanical degree are related to one another by the number of

poles P, the electrical machine has, as given by the following equation.

θe = (P/2) θm

32.What is the meaning of electrical degree? AU-APRIL/MAY 2011

Electrical degree is used to account the angle between two points in rotating electrical

machines.Since all electrical machines operate with the help of magnetic fields, the

electrical degree is accounted with reference to the polarity of magnetic fields. 180 electrical

degrees is accounted as the angle between adjacent North and South poles

33.Why short-pitch winding is preferred over full pitch winding? AU-APRIL/MAY 2011

a. Waveform of the emf can be approximately made to a sine wave and distorting

harmonics can be reduced or totally eliminated.

b. Conductor material, copper is saved in the back and front-end connections due to

less coil span.

c. Fractional slot winding with fractional number of slots/phase can be used which in

turn reduces the tooth ripples.

d. Mechanical strength of the coil is increased.

34.Write down the formula for distribution factor.AU-NOV/DEC 2011

Kd = sin (mβ/2) or Kdn = sin (mnβ/2)

35.Define winding factor. AU-NOV/DEC 2011

The winding factor Kw is defined as the ratio of phasor addition of emf induced in all the coils

belonging to each phase winding of their arithmetic addition.


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36.Name the main magnetic quantities with their symbols having the following units: webers,

Tesla ,AT/Wb, H/m. Nov-2013(R-08)

37.How will you minimize hysteresis and eddy current losses? Nov-2013(R-08)

38.What are the basic types of rotating electric machines? May-2013(R-08)

39.Draw the typical magnetization curve of ferromagnetic material. May-2013(R-08)

40.Define magnetic flux along with its unit? Nov-2012(R-08)

41.A coil of 1500 turns carrying a current of 5 amps produces a flux of 2.5 mWb Find the self-

inductance of the coil. Nov-2012(R-08)

42.Clearly define the MMF and EMF May-2012(R-08)

43.What are the core losses and how can this be minimized? May-2012(R-08)

44.What are the three types of basic rotating electric machines? May-2011(R-08)

45.A conductor 80 cm long moves at right angle to its length at a constant speed of 30m/s in a

uniform magnetic field of flux density 1.2T. find the emf induced when the conductor motion is

normal to the field flux. May-2011(R-08)

46.Give the analog between electric circuit and magnetic circuit? Nov-2010(R-08)

47.Distinguish between statically and dynamically induced electromotive force? Nov-2010(R-08)

48.Define torque? May-2010(R-08)

49.How is emf induced dynamically? May-2010(R-08)

50.Distinguish between single and multiple excited systems. Nov-2013(R-07)

51.What is rotating magnetic field in rotating machines? Nov-2013(R-07)

Part –C (16 MARK)

52.Explain the methods of analyzing the magnetic circuits. AU-APRIL/MAY 2010

53.(a) Two coupled coils have self and mutual inductance of

L11=L22=2/(1+2x);L12=1/(1+2x) calculate the time average force and coil currents at

x=0.5m if: (i) both the coils are connected in parallel across a voltage source 100cos

314t. (ii) Coil 2 is shorted while coil 1 is connected across a voltage source of 100cos

314t (iii) the two coils are connected in series across a voltage source of 100cos

314t.(16) (or)

54.(b)(i) Derive an expression for the RMS value of EMF induced in a coil of N turns in the

presence of a time varying flux (8)

(ii) a 3 phase, 40kw, 4pole, 50hz induction motor has a winding (ac)designed for delta

connection. The winding has 24 conductors per slot arranged in 60 slots. The rms value

of the line current is 40a. Find the fundamental of the mmf wave of phase-A when the

current is passing through its maximum value. What is the speed and peak value of the

resultant mmf/pole.(8) AU-MAY/JUNE 2012

55.An iron magnetic circuit has a uniform cross-sectional area of 4 crn2 and a length of 25

cm. A coil of 725 turns is wound uniformly over the magnetic circuit. When the

current in the coil is 1.5 A the total flux developed is 0.3 m Web. Find the relative

permeability of the iron.AU-NOV/DEC 2010

56.Explain the concept of dynamically and statically induced emf. AU-APRIL/MAY 2011

57.A straight conductor 0.25 m long carries a current of 80 A and lies at right angles to a

magnetic field of 0.5 T. Find the mechanical force developed on the conductor. If the

force causes the conductor to move at velocity of L2 m/s, calculate the emf induced in

it. AU-APRIL/MAY 2011

58.Derive the expression for generated emf in a dc machine. AU-NOV/DEC 2011


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59.Discuss the characteristics of dc motor. AU-NOV/DEC 2011

60.(a)(i)Distinguish between statically and dynamically induced e.m.f Nov-2013(R-08)

(ii)the core loss (hysteresis + eddy current loss) for a given specimen of magnetic material is

found to be 2000W at 50HZ.Keeping the flux density constant , the frequency of the supply is

raised to 75 Hz resulting in a core loss of 3200 W. Compute separately hysteresis and eddy

current losses at losses at both the frequencies. Nov-2013(R-08)

61.A steel ring has a mean diameter of 20cm , a cross section of 25cm2 and a radial air-gap of

0.8mm cut across it. When excited by a current of 1A through a coil of 1000 turns wound on

the ring core , it produced an air gap flux of 1 mWb .Neglecting leakage and fringing.

Calculate (i) Realative permeability of steel and (ii) total reluctance of the magnetic circuit.

Nov-2013(R-08)

62. Draw and explain the typical magnetic with air gap and its equivalent electric circuit. Hence

derive the expression for air-gap flux. May-2013(R-08)

63.The magnetic circuit has dimensions: AC=4 4 cm2,Ig=0.06cm, Ic=40cm and N=600 turns.

Assume the value of r=6000 for iron, find the excited current for Bc=1.2T and the

corresponding flux and flux linkages. May-2013(R-08)

64.(i)Explain the losses in magnetic material? Nov-2012(R-08)

65.(ii)the field winding of dc electromagnetic is wound with 800 turns and has a resistance of 40

when excited voltage is 230 volt, magnetic flux around the coil is 0.004 Wb. Calculate self-

inductance and energy stored in magnetic field. Nov-2012(R-08)

66.(i)Derive the expression for self and mutual inductance of the coil. Nov-2012(R-08)

67.(ii)Two coil A and B are wound on same iron core , there are 600 turns on A and 3600 turns on

B. the current of 4 Amps through coil , a Produces a flux of 500 10-6 Wb in the core .if this

current is reversed in 0.02 second . calculate average emf induced in coils A and B. Nov-

2012(R-08)

68.Explain clearly the statically and dynamically induced EMF? May-2012(R-08)

69.(i) Discuss AC operation of magnetic circuits? May-2012(R-08)

70.(ii) A single phase 50Hz 100KVA transformer for 12000/240V ratio has a maximum flux density

of 1.2Wb/m2 and an effective core section of 300cm2, the magnetizing current (RMS) is 0.2A

Estimate the inductance of each wire an open circuit. May-2012(R-08)

71.(a)(i) Explain the principle and operation of electromechanical energy conversion Nov-2013(R-

07)

(ii)Explain with a neat diagram the multiple excited system. Nov-2013(R-07)

72.(b)(i)Explain the concept of rotating magnetic field. Nov-2013(R-07)

73.(ii)Derive the torque equation of a round rotor machine. Also clearly state what are the

assumption made. Nov-2013(R-07)


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UNIT-II TRANSFORMERS

PART-A (1 MARK)

74.A hysteresis motor

a. is not a self-starting motor.

b. is a constant speed motor.

c. needs dc excitation.

d. can not be run in reverse speed

75.The most suitable servomotor for low power applications is

a. a dc series motor.

b. a dc shunt motor.

c. an AC 2Φ induction motor.

d. an ac series motor.

76.The size of a conductor used in power cables depends on the

a. operating voltage

b. power factor.

c. current to be carried.

d. type of insulation used.

77.Out of the following methods of heating the one which is independent of supply

frequency is

a. electric arc heating

b. induction heating

c. electric resistance heating

d. dielectric heating

78.A two-winding single phase transformer has a voltage regulation of 4.5% at full-load

and unity power-factor. At full-load and 0.80 power-factor lagging load the voltage

regulation will be

a. 4.5%.

b. less than 4.5%.

c. more than 4.5%.

d. 4.5% or more than 4.5%.

79.In a dc shunt motor the terminal voltage is halved while the torque is kept constant.

a.

b.

c.

d.

80.A balanced three-phase, 50 Hz voltage is applied to a 3 phase, 4 pole, induction

motor. When the motor is delivering rated output, the slip is found to be 0.05. The

speed of the rotor m.m.f. relative to the rotor structure is

a. 1500 r.p.m.

b. 1425 r.p.m.

c. 25 r.p.m.

d. 75 r.p.m.

81.An alternator is delivering rated current at rated voltage and 0.8 power-factor lagging

case. If it is required to deliver rated current at rated voltage and 0.8 power-factor

leading, the required excitation will be

a. less.

b. more.

c. more or less.

d. the same.

82.A ceiling fan uses

a. split-phase motor.

b. capacitor start and capacitor

run motor.

c. universal motor.

d. capacitor start motor.

83.A stepper motor is


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a. a dc motor.

b. a single-phase ac motor.

c. a multi-phase motor.

d. a two phase motor.

84.The ‘sheath’ is used in cable to

a. provide strength to the cable.

b. provide proper insulation.

c. prevent the moisture from entering the cable.

d. avoid chances of rust on strands.

85.The drive motor used in a mixer-grinder is a

a. dc motor.

b. induction motor.

c. synchronous motor.

d. universal motor

86.A 1:5 step-up transformer has 120V across the primary and 600 ohms resistance

across the secondary. Assuming 100% efficiency, the primary current equals

a. 0.2 Amp.

b. 5 Amps.

c. 10 Amps.

d. 20 Amps

87.A dc shunt generator has a speed of 800 rpm when delivering 20 A to the load at the

terminal voltage of 220V. If the same machine is run as a motor it takes a line current

of 20A from 220V supply. The speed of the machine as a motor will be

a. 800 rpm.

b. more than 800 rpm.

c. less than 800 rpm.

d. both higher or lower than 800 rpm

88.A 50 Hz, 3-phase induction motor has a full load speed of 1440 r.p.m. The number of

poles of the motor are

a. 4.

b. 6.

c. 12.

d. 8.

89.In a 3-phase synchronous motor

a. the speed of stator MMF is always more than that of rotor MMF.

b. the speed of stator MMF is always less than that of rotor MMF.

c. the speed of stator MMF is synchronous speed while that of rotor MMF is zero.

d. rotor and stator MMF are stationary with respect to each other.

90.In a capacitor start single-phase induction motor, the capacitor is connected

a. in series with main winding. b. in series with auxiliary winding.

c.in series with both the windings. d.in parallel with auxiliary winding.

91.A synchro has

a. a 3-phase winding on rotor and a single-phase winding on stator.

b. a 3-phase winding on stator and a commutator winding on rotor.

c. a 3-phase winding on stator and a single-phase winding on rotor.

d. a single-phase winding on stator and a commutator winding on rotor.

92.As the voltage of transmission increases, the volume of conductor

a. Increases

b. does not change.

c. decreases.

d. increases proportionately


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93.The size of the feeder is determined primarily by

a. the current it is required to carry.

b. the percent variation of voltage in the feeder.

c. the voltage across the feeder.

d. the distance of transmission

PART-B (2 MARKS)

94.Mention the difference between core and shell type transformers. AU-APRIL/MAY 2007

In core type , the windings surround the core considerably and in shell type the core

surround the winding.

95.What is the purpose of laminating the core in a transformers ? AU-APRIL/MAY 2007

To reduce eddy current loss.

96.Give the emf equation of a transformer and define each term AU-OCT/NOV 2007

Emf induced in primary coil E1 = 4.44 fφ mN1 volt

Emf induced in secondary coil E2 = 4.44fφ mN2 volt

Where f is the frequency of AC input

N1, N2 are the number of primary and secondary turns.

97.Does the transformer draw any current when secondary is open ? AU-OCT/NOV 2007

Yes,it (primary) will draw the current from the main supply in order to magnetize the core

and to supply iron and copper losses on no load . There will not be any current in the

secondary since secondary is open.

98.Define voltage regulation of a transformer AU-APRIL/MAY 2008

When a transformer is loaded with a constant primary voltage , the secondary voltage

decreases for lagging power factor load, and increases for leading pf load because of its

internal resistance and leakage reactance .The change in secondary terminal voltage from

no load to full load expressed as a percentage of no load or full load voltage is termed as

regulation .

% regulation down = (0V2-V2) x 100/0V2

% regulation up = (0V2-V2) x 100/V2

99.Full load copper loss in a transformer is 1600 watts. What will be the loss at half load ?

AU-APRIL/MAY 2008

f x is the ratio of actual load to full load then copper loss = x2(full load copper loss)

Here Wc = (0.5)2 x 1600 = 400 watts

100.Define all day efficiency of a transformer . AU-OCT/NOV 2008, AU-MAY/JUNE 2012

74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93

b B c c c b d b d d a d a c a d b c c a


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It is the computed on the basis of energy consumed during a certain period , usually a day

of 24 hrs.

ALL DAY = OUTPUT IN KWH /INPUT IN KWH FOR 24 HRS.

101.why transformers are rated in kVA ? AU-OCT/NOV 2008

Copper loss of a transformer depends on current and iron loss on voltage . Hence total

losses depends on Volt- Ampere and not on the power factor. That is why the rating of

transformers are in kVA and not in kW.

102.What are the typical uses of auto transformer ? AU-APRIL/MAY 2009, AU-

MAY/JUNE 2012

(i)To give small boost to a distribution cable to correct for the voltage drop.

(ii)As induction motor starters.

103.What are the applications of a step-up and step-down transformers ? AU-APRIL/MAY 2009

Step-up transformers are used in generating stations. Normally the generated voltage will

be either 11 kV . This voltage(11 KV) is stepped up to 110 kV or 220 kV or 400 kV and

transmitted through transmission lines. (In short it may Be called as sending end).

Step-down transformers are used in receiving stations. The voltage are again stepped down

to 11 kV or 22 kV and transmitted through feeders.(In short it may be called as receiving

end). Further these 11 kV or 22kV are stepped down to 3 phase 400 V by means of a

distribution transformer and made available at consumer premises.The transformers used at

generating stations and receiving stations are called power transformers.

104.How transformers are classified according to their construction ? AU-OCT/NOV 2009

Transformers are classified according to their construction as ,

(i)Core type (ii)Shell type (iii)Spiracore type.

Spirakore type is a latest transformer and is used in big transformers. In “core” type, the

windings(primary and secondary)surround the core and in “shell” type, the core surround

the windings.

105.Explain on the material used for core construction. AU-OCT/NOV 2009

The core is constructed of transformer sheet steel laminations assembled to provide a

continuous magnetic path with a minimum of air gap included. The steel used is of high

silicon content sometimes heat treated to produce a high permeability and a low hysteresis

loss at the usual operating flux densities. the eddy current loss is minimized by laminating

the core, the laminations being insulated from each other by light coat of core-plate vanish

or by an oxide layer on the surface .the thickness of laminations varies from 0.35 mm for a

frequency of 50 Hz and 0.5 mm for a frequency of 25 Hz.

106.When will a Bucholz relay operate in a transformer ? AU-APRIL/MAY 2010

Bucholz rely is a protective device in a transformer. If the temperature of the coil

exceeds its limit, Bucholz relay operates and gives an alarm.

107.How does change in frequency affect the operation of a given transformer?AU-APRIL/MAY 2010

With a change in frequency, iron loss, copper loss, regulation, efficiency and heating

varies and thereby the Operation of the transformer is affected.

108.What is the angle by which no-load current will lag the ideal applied voltage?AU-OCT/NOV 2010


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In an ideal transformer, there are no copper loss and no core loss, (i.e. loss free core).

The no load current is only magnetizing current. Therefore the no-load current lags

behind by an angle of 90°. However the windings possess resistance and leakage reactance

and therefore the no-load current lags the applied voltage slightly less than 90°.

109.List the advantages of stepped core arrangement in a transformer.AU-OCT/NOV 2010

(A) To reduce the space effectively.

(B) To obtain reduced length of mean turn of the windings.

(C) To reduce I2R loss.

110.Why are breathers used in transformers ? AU-APRIL/MAY 2011

Breathers are used to entrap the atmospheric moisture and thereby not allowing it to

pass on to the transformer oil. Also to permit the oil inside the tank to expand and contract

as its temperature increases and decreases.Also to avoid sledging of oil i.e. decomposition of

oil. Addition of 8 parts of water in 1000000 reduces the insulations quantity of oil. Normally

silica gel is filled in the breather havingpink colour. This clour will be changed to white due

to continuous use, which is an indication of bad silica gel, it is normally heated and reused.

111.What is the function of transformer oil in a transformer ? AU-APRIL/MAY 2011

Nowadays instead of natural mineral oil, synthetic oils known as ASKRELS (trade

name ) are used. They are noninflammable, under an electric arc do not decompose to produce

inflammable gases. PYROCOLOR oil possess high dielectric strength. Hence it can be said that

transformer oil provides , (i)good insulation and (ii)cooling

112.A 1100/400 V, 50 Hz single phase transformer has 100 turns on the secondary winding.

Calculate the number of turns on its primary. AU-OCT/NOV 2011

i. We know V1 / V2 = k = N2 / N1

ii. Substituting 400/1100 = 100/N1

iii. N1 = 100/400 x 1100

iv. = 275 turns.

113.What are the functions of no-load current in a transformer ? AU-OCT/NOV 2011

No-load current produces flux and supplies iron loss and copper loss on no-load.

114.How will you transfer the quantities from one circuit to another circuit in a

transformer? AU-OCT/NOV 2008

1.Secondary to primary 2.Primary to secondary

115.Define regulation of a transformer? Nov-2013(R-08)

116.State the advantage and application of auto transformer? Nov-2013(R-08)

117.What are the losses in a transformer? May-2013(R-08)

118.List out any four three phase transformer connections. May-2013(R-08)

119.What is meant by all day efficiency in transformer ? Nov-2012(R-08)

120.State the advantage of auto transformer . Nov-2012(R-08)

121.What happed if DC supply is applied to transformer? May-2012(R-08)

122.Why all day efficiency is lower than commercial efficiency? May-2012(R-08)

123.Which equivalent circuit parameter can be determined from the open circuit test on a

transformer? May-2011(R-08)


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124.The EMF per turns for a single phase 2200/220 V , 50 Hz transformer is 11V calculate the

number of primary and secondary test. May-2011(R-08)

125.What are the no load losses in a two winding transformer and state the reason for such losses?

Nov-2010(R-08)

126.Mention the conditions to be satisfied for parallel operation of two winding transformer. Nov-

2010(R-08)

127.Give the principle of transformer? May-2010(R-08)

128.What are the conditions for parallel operation of transformer? May-2010(R-08)

129.What is armature reaction and how can it be reduced? Nov-2013(R-07)

130.What are the arrangements to be done for satisfactory parallel operation of DC shunt

generators? Nov-2013(R-07)

PART-C (16 MARKS)

131.(a)(i)Describe the construction and principle of operation of single phase transformer. Nov-

2013(R-08)

(ii)Derive an expression for maximum efficiency of a transformer Nov-2013(R-08)

132.A 500 kVA transformer has 95% efficiency at full load and also at 60 % full load both at UPF.

(i)Separately out the transformer losses. (ii)Determine the transformer efficiency at 75 % full

load ,upf Nov-2013(R-08)

133.Explain the constructional details and working of core type and shell type transformer with

neat sketches. May-2013(R-08)

134.Obtain the equivalent circuit of a 200/400V , 50Hz, 1-Phase transformer from the following

test data :O.C. test : 200V,0.7A, 70W-on L.V side, :S.C. test :15V, 10A, 85W- on H.V Side.

Calculate the secondary voltage when delivering 5kW at 0.8p.f lagging the primary voltage

being 200V May-2013(R-08)

135.(i)Derive the EMF equation of a transformer Nov-2012(R-08)

(ii)The voltage per turns of a single phase transformer is 1.1volt, when the primary winding is

connected to a 220 Volts,50Hz AC supply the secondary voltage is found to be 550 volt. Find

the primary and secondary turns and core area if maximum flux density is 1.1 tesla. Nov-

2012(R-08)

136.(i) Explain in detail step by step the procedure to draw the equivalent circuit of transformer.

Nov-2012(R-08)

137.(ii)In a 25 KVA , 2000/200V transformer the constant and variable losses are 350watts and

respectively .calculate the efficiency on unity power factor at full load and half the full load.

Nov-2012(R-08)

138.Describe the method of calculating the regulation and efficiency of a single phase transformer

by OC and SC test. May-2012(R-08)

139.(i)Derive an expression for the emf of an ideal transformer . May-2012(R-08)

140.(ii)Calculate the efficiency at half full load of a 100 KVA transformer for PF of unity and 0.8.

the copper loss is 1000W at full load and iron loss is 1000W. May-2012(R-08)

141.(i)Explain with a neat diagram the construction and principle of operation of transformers.

Nov-2013(R-07)

(ii) Derive the EMF equation of a transformer. Nov-2013(R-07)


13

142.(i).Draw the phasor diagram of a transformer on No load and with load conditions. Nov-

2013(R-07)

(ii)Draw the equivalent circuit of a transformer and drive the components with respects to

primary side. Nov-2013(R-07)

143.(i)Derive the expression for obtaining the maximum condition of a transformer Nov-2013(R-07)

(ii)Explain with a neat diagram the procedure to predetermine the efficiency of a transformer

using sumpner`s test. Nov-2013(R-07)

144.(i)Explain with a neat diagram the open circuit and short circuit test of a transformer. Nov-

2013(R-07)

(ii) Explain with a neat the working of an auto transformer Nov-2013(R-07)

145. Derive expressions for the current shared by two transformers operating in parallel,

with unequal no load voltages. (8) AU-APRIL/MAY 2008

146.12.(a) (i) explain the effect of armature reaction in DC generators and also determine

the expression for demagnetizing and cross-magnetizing ampereturns perpole.(12)

(ii) a wave connected 8pole, 60w,300v dc generator has 540 conductors and

delivers full load current. If the brush shift is 4 degree (mechanical) calculate

demagnetizing and cross-magnetising AT/pole. (4) (or)

(b)(i) draw and explain the no-load and load characteristics of DC compound

generators.(8)

(ii) Two DC shunt generators with emfs 120v and 115v, armature resistance of 0.05

and 0.04 and field resistance of 20 and 25 respectively are in parallel supplying a

total load of 25kw. Determine the load shared by the generators.(8) AU-MAY/JUNE

2012

147. Derive the emf equation of single phase transformer. (8) AU-APRIL/MAY 2008

148. A 120kVA, 6000/400V, Y/Y, 3-phase, 50Hz transformer has an iron loss of 1800W.

The maximum efficiency occurs at ¾ full loads. Find the efficiency of the transformer

at (I) Full load and 0.8 p f (ii) the maximum efficiency at unity pf. (8) AU-APRIL/MAY 2008

149. A 100 kVA, 6.6kV/415V, single phase transformer has an effective impedance of

(3+8j) _ referred to HV side. Estimate the full load voltage regulation at 0.8 p f lagging

and 0.8 leading pf. (16) AU-APRIL/MAY 2008

150. Explain the working of auto transformer and prove that when transformation ratio

approaches unity, the amount copper used approaches smaller value. (8) AU-OCT/NOV 2011

151.Draw and explain the no load phasor diagram of a single phase transformer (8)

152.The emf per turn of a single phase, 6.6kV/440V, and 50 Hz transformer is

approximately 12V. Calculate the number of turns in the HV and LV windings and the

net cross sectional area of the core for a maximum flux density of 1.5T.(8) AU-

APRIL/MAY 2009

153. Give the constructional details and classification of transformers. AU-APRIL/MAY 2009

154. Obtain the equivalent circuit of a 200/400V, 50Hz, single phase transformer from the

following test data: AU-OCT/NOV 2009

OC test: 200V, 0.7A, 70W on LV side SC test: 15V, 10A, 85W on HV side (16).

Find the all day efficiency of a 500kVA, distribution transformer whose iron loss and

full load copper loss are 1.5kW and 6kW respectively. In a day it is loaded as follows:

AU-OCTL/NOV 2009

i. Duration (Hi) Output (Po) in kW Power factor (cosØ2)


14

ii. 6 400 0.8

iii. 10 300 0.75

iv. 4 100 0.8

v. 4 0 -

155.Draw the circuit diagrams for conducting OC and SC tests on a single phase

transformer. Also explain how the efficiency and voltage regulation can be estimated

by these tests. AU-APRIL/MAY 2010

156.What is the Sumner’s test? Draw the circuit diagram to conduct this test and explain

its principle. AU-NOV/DEC 2010

157.Explain in detail about parallel operation of single phase transformers. AU-NOV/DEC 2010

158.Explain how the efficiency of the transformer may be estimated from the open circuit

and short circuit test. AU-APRIL/MAY 2011

159.Deduce the equivalent circuit of an auto transformer.AU-APRIL/MAY 2011

160.Draw and explain the phasor diagram of a single phase transformer supplying

b. (1) A UPF load (2) a Lagging load. AU-NOV/DEC 2011

161.A 100 kvA, 6.6 kVl4L' V single-phase Transformer has an effective impedance of (3 +

i8) o referred to the Hv side. Estimate the full load voltage regulation at (1) 0.8 p f

lagging and (2) 0.8 p f leading AU-NOV/DEC 2011

UNIT-III ELECTROMECHANICAL ENERGY CONVERSION

Part –A (1 MARK)

162.The boundary of the protective zone is determined by the

a. Location of CT

b. sensitivity of relay used

c. Location of PT

d. None of these

163.In a three phase transformer, if the primary side is connected in star and secondary

side is connected in delta, what is the angle difference between phase voltage in the two

cases.

a. delta side lags by -

b. star side lags by -

c. delta side lea

d. star side leads by -

164.To achieve low PT error, the burden value should be ____________.

a. low

b. high

c. medium

d. none

165.Slip of the induction machine is 0.02 and the stator supply frequency is 50 Hz.

What will be the frequency of the rotor induced emf?

a. 10 Hz.

b. 50 Hz.

c. 1 Hz.

d. 2500 Hz

166.A 4 pole lap wound dc shunt motor rotates at the speed of 1500 rpm, has a flux of 0.4

mWb and the total number of conductors are 1000. What is the value of emf?

a. 100 Volts.

b. 0.1 Volts.

c. 1 Volts.

d. 10 Volts.

167.The synchronous reactance of the synchronous machine is ______________.


15

a. Ratio between open circuit voltage and short circuit current at constant field current

b. Ratio between short circuit voltage and open circuit current at constant field current

c. Ratio between open circuit voltage and short circuit current at different field current

d. Ratio between short circuit voltage and open circuit current at different field current

168.A 3 stack stepper motor with 12 numbers of rotor teeth has a step angle of

a. 120 (b)80 (c)240 (d)100

169.In case of a universal motor, torque pulsation is minimized by _________.

a. load inertia

b. rotor inertia

c. both rotor and load inertia

d. none of the above

170.Oil-filled cable has a working stress of __________ kV/mm

a. 10

b. 12

c. 13

d. 15

171.Inverse definite minimum time lag relay is also called ___________

a. pilot relay.

b. differential relay.

c. over current relay.

d. directional overcurrent relay.

172.Specific heat of nickel –chrome is _____________

a. 0.112

b. 0.106.

c. 0.108.

d. 0.110.

173.The polarity test is not necessary for the single-phase transformer shown in Fig. 1 so as

to correctly determine _____________of the transformer.

a. shunt branch parameters.

b. transformation ratio.

c. series parameters.

d. any of the above.

174.The speed-torque characteristics of a DC series motor are approximately similar to

those of the _________motor.

a. universal

b. synchronous

c. DC shunt

d. two-phase

175.The rotor frequency for a 3 phase 1000 RPM 6 pole induction motor with a slip of 0.04

is________Hz

a. 8

b. 4

c. 6

d. 2

176.The torque-speed characteristics of an a.c. operated universal motor has a

______characteristic and it______ be started under no-load condition.

a. inverse, can

b. nearly inverse, can

c. inverse, cannot

d. nearly inverse, cannot

177.In the heating process of the ________type a simple method of temperature control is

possible by means of a special alloy which loses its magnetic properties at a particular

high temperature and regains them when cooled to a temperature below this value.

a. Indirect induction over

b. core type induction furnace

c. coreless induction furnace

d. high frequency eddy current


16

178.In order to reduce the harmful effects of harmonics on the A.C. side of a high voltage

D.C. transmission system ______are provided.

a. synchronous condensers

b. shunt capacitors

c. shunt filters

d. static compensators

179.An a.c. tachometer is just a ________with one phase excited from the carrier

frequency.

a. two-phase A.C. servomotor

b. two-phase induction motor

c. A.C. operated universal motor

d. hybrid stepper motor.

180.The torque, in a _____________is proportional to the square of the armature current

a. DC shunt motor

b. stepper motor

c. 2-phase servomotor

d. DC series motor

181.In electrical machines the material preferred for pole shoes of electro-magnets is

a. Pure iron

b. Aluminium

c. Copper

d. Lead.

Answers

162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181

B b C a C d a d c d B e d b a d c d c d

PART-B (2 MARKS)

182.What is electromechanical energy conversion?AU-APRIL/MAY 2008

A Electromechanical energy conversion device which converts mechanical energy into

electrical energy, it is a generator and electrical energy conversion into mechanical

energy, it as a motor.

183.State three types of electromechanical energy conversion. AU-NOV/DEC 2008

1. The various transducers such as microphones, loudspeakers and thermocouples.

2. The device which produce the mechanical force or torque based on translatory motion

such as electromagnet and relays.

3. The devices used for continuous energy conversion using rotational motion such as

generators and motors.

184.Write energy balance equation. AU-MAY/JUNE 2012

dWe = dWm+dWf+dWloss

dWm = dWe+dWf+dWloss

185.Define Fleming right hand rule. AU-NOV/DEC 2009

F= BIl N

186.What are the types of magnetic system?AU-APRIL/MAY 2010

a) Single excited system

b) Multiple excited system

187.State concept of Co-energy. AU-NOV/DEC 2010


17

When armature is held open then almost entire mmf is requiredto drive the flux

through air gap and hence magnetic saturation may not occur.

188.What is current excited system?AU-NOV/DEC 2010

This is the expression for system in which i is independent variable. This means

input current constant, such a system is current excited system.

189.How the direction of mechanical force or torque developed in any physical system?

AU-APRIL/MAY 2009

c) decrease the magnetic stored energy at constant λ

d) increase the stored energy and co-energy at constant i

e) decrease the reluctance

f) increase the inductance

190.What are the advantages of field energy method? AU-APRIL/MAY 2011

i) it is applicable to all types of devices such as translatory, rotational, vibratory and linear.

ii) both steady state as well as transient behaviour of devices can be analyzed.

iii) the approach forms the basis of generalized theory of electrical machine.

191.Define statically induced emf. AU-APRIL/MAY 2011

According to Lenz’s law the induced emf setup a current in such a direction so as to oppose

the cause, producing it.

192.Define dynamically induced emf. AU-NOV/DEC 2011

If there is a relative motion between conductor and flux, emf gets induced in the conductor.

The relative motion is due to physical movement of conductor or magnetic field.

193.Draw a schematic diagram indicating flow of energy in the conversion of mechanical energy to

electrical energy form? Nov-2013(R-08)

194.Why do all practical energy conversion devices make use of the magnetic field as a coupling

medium rather than an electric field? Nov-2013(R-08)

195.What do you mean by co-energy? May-2013(R-08)

196.Give examples for multiple excitation systems. May-2013(R-08)

197.Draw the diagram indicating the flow of energy in electromechanical energy conversion via

coupling medium. Nov-2012(R-08)

198.Give the expression for energy stored in the magnetic field . Nov-2012(R-08)

199.What is you meant by co-energy? May-2012(R-08)

200.What are the requirement of the excitation system? May-2012(R-08)

201.Based on the principle of conservation of energy, write an energy balance equation for a

motor. May-2011(R-08)

202.What are the three phase basic principle for the electromechanical energy conversion? May-

2011(R-08)

203.Draw the power low diagram for motor and generator operation ? Nov-2010(R-08)

204.In a magnetic circuit with a small air gap , in which part the maximum energy is stored and

why? Nov-2010(R-08)

205.In a linear system prove that field energy and co-energy are equal ? May-2010(R-08)

206.Write are expression for the stored energy in the magnetic field? May-2010(R-08)

207.Draw the electrical and mechanical characteristics of series motor. Nov-2013(R-07)


18

208.What is the purpose of conducting Swinburne’s test and Hopkinsons test? Nov-2013(R-07)

PART –C (16 MARKS)

209.Derive an expression for co-energy density of an electromechanical energy conversion device.

Nov-2013(R-08)

210.The double- excited magnetic field has coil self and mutual inductance of

L11=L22=2,L12=L21=cos where is the angle between the axes of the coils. The coils are

connected in parallel to a voltage source V=Vmsint. Derive an expression of the instantaneous

torque as a function of the angular position . Find the time –average torque. Evaluate for

=300, =100 sin314t. Nov-2013(R-08)

211.Derive an expression for the magnetic force developed in a multiply excited magnetic systems.

May-2013(R-08)

212.Find an expression for the force per unit area between the plates of a parallel plate condenser

in terms of the electric field intensity. Use both the energy and Co-energy methods. Find the

value of the force per unit area when E=3106V/m, the breakdown strength of air. May-

2013(R-08)

213.Derive the expression for energy and force in a doubly excited magnetic field system. Nov-

2012(R-08)

214.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 of x. the first coil is excited by a constant current of

20 Amp and the second by constant current of -10amp .find mechanical work done if x change

from 0.5 to 1 m and also the energy supplied by each electrical source. Nov-2012(R-08)

215.Deduce an expression for the magnetic force of field origin in a typical attraction armature

relay. May-2012(R-08)

216.Find an expression for the magnetic force developer in a multiply excited magnetic system.

May-2012(R-08)

217.(a)(i) Explain the principle and operation of electromechanical energy conversion Nov-2013(R-

07)

(ii)Explain with a neat diagram the multiple excited system. Nov-2013(R-07)

218.(b)(i)Explain the concept of rotating magnetic field. Nov-2013(R-07)

(ii)Derive the torque equation of a round rotor machine. Also clearly state what are

the assumption made. Nov-2013(R-07)

219.Derive the expression for generated emf in a dc machine. AU-APRIL/MAY 2009

220.Discuss the characteristics of dc motors. AU-NOV/DEC 2009

221.Give the construction detail of dc machines. AU-NOV/DEC 2009

222.Explain about AC operation of magnetic circuits. AU-APRIL/MAY 2010

223.Explain in detail about energy in magnetic system. AU-NOV/DEC 2010

224.Write one example derives the co-energy of a multiply-exited magnetic field system.

AU-APRIL/MAY 2011

225.What are the various types of machines? Explain them. AU-APRIL/MAY 2011

226.Explain the function of multiply-excited magnetic system. AU-NOV/DEC 2011

227.Derive the expression for the field energy and mechanical force of electrical system

and explain them., AU-NOV/DEC 2011

228.(a) (i) explain with neat diagram the function of no volt release and overload release in

3 point DC motor starter.(8)


19

(ii) a DC machine running at 700rpmis put to a retardation test. The time taken for the

speed to fall from 730rpm to 670rpm is (1) 27seconds with no excitation, (2)10 seconds

with full excitation and (3) 6 seconds with full excitation and armature supplying a

extra load of 7A at 200v, calculate the moment of inertia of the armature and iron

losses at mean speed of 700rpm. (8) (Or)

229.(b) (i) explain shunted armature speed control method.(8)

(ii) A 240v DC shunt motor has armature resistance of 0.25 and draws a current of

40A from the supply mains on half of full-load. The speed is to be increased twice half

full-load speed. Determine the percentage change in flux if the torque of the motor

remains constant. (8) AU-MAY/JUNE 2012

UNIT-IV BASIC CONCEPTS OF ROTATING MACHINES

PART-A (1 MARK)

230.The synchronous speed for a 3 phase 6-pole induction motor is 1200 rpm. If the

number of poles is now reduced to 4 with the frequency remaining constant, the rotor

speed with a slip of 5% will be _________.

1. 1690 rpm

2. 1750 rpm

3. 1500 rpm

4. 1710 rpm

231.The eddy current loss in an ac electric motor is 100 W at 50 Hz.Its loss at 100Hz willbe

1. 25 watts

2. 59 watts

3. 100 watts

4. 400 watts

232.The maximum power for a given excitation in a synchronous motor is developed when

the power angle is equal to

1. 0o

2. 45o

3. 60o

4. 90o

233.A commutator in a d.c. machine

1. Reduces power loss in armature.

2. Reduces power loss in field circuit.

3. Converts the induced a.c armature voltage into direct voltage.

4. Is not necessary.

234.The speed of a d.c. shunt motor at no-load is

1. 5 to 10%

2. 15 to 20%

3. 25 to 30%

4. 35 to 40%

235.The efficiency of a transformer is mainly dependent on

1. core losses.

2. copper losses.

3. stray losses.

4. dielectric

losses.

236.When two transformers are operating in parallel, they will share the load as under:

1. proportional to their impedances. 2.inversely proportional to their impedances.

3.50% - 50% 4. 25%-75%


20

237.The percentage of silicon in transformer stampings is usually limited to

1. 0.4%

2. 1.4%

3. 4%

4. 14%

238.A 3-phase, 400 votts, 50 Hz, 100 KW, 4 pole squirrel cage induction motor with a rated

slip of 2% will have a rotor speed of

1. 1500 rpm

2. 1470 rpm

3. 1530 rpm

4. 1570 rpm

239.Hysteresis loss varies with maximum flux density (B) as

a. B

b. B1.6

c. B2

d. B2.6.

240.The voltage at the two ends of a transmission line are 132 KV and its reactance is 40

ohm. The Capacity of the line is

a. 435.6 MW

b. 217.8 MW

c. 251.5 MW

d. 500 MW

241.A 220/440 V, 50 Hz, 5 KVA, single phase transformer operates on 220V, 40Hz supply

with secondary winding open circuited. Then

1. Both eddy current and hysteresis losses decreases.

2. Both eddy current and hysteresis losses increases.

3. Eddy current loss remains the same but hysteresis loss increases.

4. Eddy current loss increases but hysteresis loss remains the same.

242.A synchronous motor is operating on no-load at unity power factor. If the field current

is increased, power factor will become

1. Leading & current will decrease 2.Lagging & current will increase.

3.Lagging & current will decrease. 4.Leading & current will increase.

243.A d.c. shunt motor runs at no load speed of 1140 r.p.m. At full load, armature reaction

weakens the main flux by 5% whereas the armature circuit voltage drops by 10%. The

motor full load speed in r.p.m. is

a. 1080

b. 1203

c. 1000

d. 1200

244.The introduction of interpoles in between the main pole improves the performance of

d.c. machines, because

1. The interpole produces additional flux to augment the developed torque.

2. The flux waveform is improved with reduction in harmonics.

3. The inequality of air flux on the top and bottom halves of armature is removed.

4. A counter e.m.f. is induced in the coil undergoing commutation.

245.The rotor power output of a 3-phase induction motor is 15 KW and corresponding slip

is 4%. The rotor copper loss will be

1. 600 W.

2. 625 W

3. 650 W

4. 700 W

246.The direction of rotation of hysteresis motor is reversed by


21

1. Shift shaded pole with respect to main pole

2. Reversing supply lead

3. Either A or B

4. Neither A nor B

247.A 1.8°step, 4-phase stepper motor has a total of 40 teeth on 8 pole of stator. The

number of rotor teeth for their rotor will be

1. 40

2. 50

3. 100

4. 80

248. Low head plants generally use

1. Pelton Turbines

2. Francis Turbine

3. Pelton or

Francis Turbine

4. Kaplan

Turbines

249.The charging reactance of 50 Km length of line is 1500_. The charging reactance for

100Km length of line will be

1. 1500

2. 3000

3. 750

4. 600

230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249

d d A c a a a b b c a a a a D B A B a b

PART B (2 MARKS)

250.What are the major parts of rotating machines? AU-APRIL/MAY 2008

a. Stator b.rotor c.shaft d.slip rings

251.What is the function of brush? AU-NOV/DEC 2008

Brushes are used which are stationary and rotating against slip rings. The brushes behave as

stationary terminals of rotating winding. The power can be fed in or taken out from the brushes.

252.What is the function of commutator? AU-APRIL/MAY 2009

To convert alternating emf in to dc.

253.What is alternators? AU-NOV/DEC 2009

The synchronous machine are the ac machines. The ac producing synchronous machines

are called alternating generators or alternators.

254.What are the advantages of rotating field system? AU-APRIL/MAY 2010

a. Alternators are used to produce high voltage in the range of KV

b.It is better to keep high voltage winding stationary and away from centrifugal forces caused

due to rotation.

c. It is easy to collect large currents from the stationary member.

255.What are the types of rotor? AU-APRIL/MAY 2010

a. Salient pole

b. Smooth Cylindrical or non-salient pole

256.Define synchronous speed. AU-NOV/DEC 2010

Ns=120 f / p


22

257.Write the equation of generated emf in dc machine. AU-NOV/DEC 2010

Eg= ФNPZ / 60A volts

258.Define pole pitch.AU-NOV/DEC 2009

One pole is responsible for 180 deg electrical of induced emf, so 180 deg is called pole pitch.

259.Define slot angle. AU-APRIL/MAY 2011

The phase difference contributed by 1 slot in degrees electrical is called slot angle (β).

260.What are the types of three phase winding? AU-APRIL/MAY 2011

single layer & double layer

full pitch & short pitch

concentrated & distributed

261.Write the equation of generated emf in dc machine. AU-NOV/DEC 2011

Ephase= 4.44 Ф f Tph volts

262.Define rotating magnetic field. AU-NOV/DEC 2011

A magnetic field having constant amplitude but whose axis continuously rotates in a plane with

a certain speed is called rotating magnetic field.

263.What is meant by SPP? What is its significance ? Nov-2013(R-08)

264.Enumerate the advantages of using short-pitched winding in a synchronous machine. Nov-

2013(R-08)

265.Write down the expression for torque in round rotor machine. May-2013(R-08)

266.Why fractional pitched winding is preferred over full pitched winding? May-2013(R-08)

267.Define pitch factor and winding factor. Nov-2012(R-08)

268.What is meant by mechanical angle? Explain. Nov-2012(R-08)

269.What is meant by reactance voltage? May-2012(R-08)

270.Why frictional pitch windings preferred over full pitched winding? May-2012(R-08)

271.What is magnetic leakage flux? May-2011(R-08)

272.Why is the efficiency of a three phase induction motor less than that of a three phase

transformer? May-2011(R-08)

273.Explain the concept of electrical degree . How is the electrical angle of the voltage in a rotor

conductor related to the mechanical angle of the machines shaft? Nov-2010(R-08)

274.Why does curving the pole faces in a D.C machine contributed to a smoother D.C output

voltage from it? Nov-2010(R-08)

275.What are the basic magnetic field effects that result in the production of mechanical forces?

May-2010(R-08)

276.What are the assumption made to determine the distribution of coil mmf? May-2010(R-08)

277.What is the EMF equation of a transformer? Nov-2013(R-07)

278.Define voltage regulation of a transformer. Nov-2013(R-07)

Part-C 16 Marks

279.How is torque developed in round rotor machine? Derive an expression for the same . state the

assumption made. Nov-2013(R-08)

280.Find the number of series turns required for each phase of a 3-phase, 50Hz,10 pole alternate

with 90 slots. The winding is to be star connected to give a line voltage of 11Kv. The flux/pole is

0.16Wb. Nov-2013(R-08)

281.(i)Derive an expression for the generated voltage of DC machine. May-2013(R-08)


23

(ii)Calculate the fundamental, third and fifth harmonic breadth factors for a stator with 36 slots

wound for3-phase, 4-pole. May-2013(R-08)

282.A 3-phase, 50Hz,Star-connected alternator with 2 layer winding is running at 600rpm.It has

12 turns/coil, If the flux/pole is 0.035 Wb sinusoidally distributed, find the phase and line emfs

induced assume that the total turns/phase are series connected. May-2013(R-08)

283.Explain the concept of rotating MMF waves in AC machine. Nov-2012(R-08)

284.A 3 Phase, 50 Hz star connected alternator with 2 layer winding is running at 600rpm.It has

12 pole/coil,4 slot/pole/phase and a coil pitch of 10 slots. if the flux per pole is 0.035Wb

sinusoidal distributed, find the phase and line emf induced. Assume that the total turns/Phase

are series connected. Nov-2012(R-08)

285.Explain the construction and principle of operation of synchronous machines. May-2012(R-08)

286.A 2000V, three phase star connected synchronous reactance of 0.2 and 2.2 per

phase respectively. The input is 800Kw at normal voltage and induced line emf is

2500V,calculate the line current and power factor. May-2012(R-08)

287.a)Explain why distributed field winding is employed in cylindrical rotor

synchronous machine (6)

b) With neat sketch explain the multiple excited magnetic field system in electro

mechanical energy conversion systems. Also obtain the expression for field

energy in the system,(10) AU-APRIL/MAY 2008

288.a)Explain clearly how a rotating magnetic field is set up around the three phase

AC winding having 120º (electrical) phase displacement each when three phase

balanced supply is given to it.(8)

b) Obtain the torque equation for round rotor machine having P number of

poles. State the assumptions made.(8) AU-APRIL/MAY 2008

289.a)Derive an expression for the RMS value of EMF induced in a coil of N turns

in the presence of time varying flux.(8)

b)Draw and explain the general block diagram of an electromechanical energy

conversion device(8) AU-NOV/DEC 2008

290.Two coupled coils have self and mutual inductance of L11=2+1/(2x);

(A) 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 20A and the second by a

constant current of -10A. AU-NOV/DEC 2009

(B) (i)mechanical work done if x changes from 0.5to1m

(C) (ii)energy supplied by each electrical source in part 1

(D) (iii)change in field energy in part1

291.Hence verify that the energy supplied by the sources is equal to the increase in energy

in motor AU-NOV/DEC 2009

292.Explain why distributed field winding is employed in cylindrical rotor synchronous

machine . AU-APRIL/MAY 2009

293.With neat sketch explain the multiple excited magnetic field system in

electromechanical energy conversion systems. Also obtain the expression for field

energy in the system,(10)

294.Explain clearly how a rotating magnetic field is set up around the three phase AC

winding having 120º (electrical) phase displacement each when three phase balanced

supply is given to it.(8) AU-NOV/DEC 2010


24

295.Obtain the torque equation for round rotor machine having P number of poles. State

the assumptions made. (8)

296.Derive an expression for the RMS value of EMF induced in a coil of N turns in the

presence of time varying flux.(8) AU-APRIL/MAY 2010

297.Draw and explain the general block diagram of an electromechanical energy

conversion device (8)

298.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 20A and the second by a constant current of -10A.

(A) Mechanical work done if x changes from 0.5to1m

(B) Energy supplied by each electrical source in part 1

c.Change in field energy in part1 AU-NOV/DEC 2010

299.Hence verify that the energy supplied by the sources is equal to the increase in field

energy plus the mechanical work done (16) AU-APRIL/MAY 2011

300.Derive the torque equation for round rotor machine. (8)

301.A 3-phase, 16-po1es tar-connected Alternator as 240 stator slots with 8 conductors per

slot and the conductor of each phase are connected in series. The coil-span is 744

electrical degrees. AU-APRIL/MAY 2011

302.Determine the phase and line emfs if the machine speed is at 375 rpm and the flux per

pole is 6.1 mega lines distributed sinusoidally in the air-gap.

303.Derive the EMF equation of an alternator. Discuss the effect of winding factor on the

induced EMF. (8) AU-NOV/DEC 2011

304.Prove that the resultant MMF wave of 3-phase AC winding rotates in space with speed

a but its magnitude is constant. AU-NOV/DEC 2011

305.(a) draw and explain the phasor diagrams for transformer under no load and under

full load for unity and lagging power factor loads(16) (or)

(b) draw the equivalent circuit of a transformer and derive the formula for the percentage

regulation at leading and lagging power factor loads with relevant phasor diagrams(16) AU-

MAY/JUNE 2012

UNIT-V DC MACHNES

PART A (1 MARKS)

306.Electric ovens using heating elements of _______ can produce temperature upto

3000°C.

1. Nickel

2. Graphite

3. Chromium

4. Iron

307.In DC generators, armature reaction is produced actually by

1. Its field current.

2. Armature

conductors.

3. Field pole

winding.

4. Load current in

armature.

308.Two transformers operating in parallel will share the load depending upon their

1. Rating.


25

2. Leakage

reactance.

3. Efficiency

4. Per-unit

impedance.

309.As compared to shunt and compound DC motors, the series DC motor will have the

highest torque because of its comparatively ____________ at the start.

1. Lower armature resistance.

2. Stronger series field.

3. Fewer series turns.

4. Larger armature current.

310.A 400kW, 3-phase, 440V, 50Hz induction motor has a speed of 950 r.p.m. on fullload.

The machine has 6 poles. The slip of the machine will be _______________.

1. 0.06

2. 0.10

3. 0.04

4. 0.05

311.Reduction in the capacitance of a capacitor-start motor, results in reduced

1. Noise.

2. Speed.

3. Starting torque.

4. Armature

reaction.

312.Regenerative braking

1. Can be used for stopping a motor.

2. Cannot be easily applied to DC series motors.

3. Can be easily applied to DC shunt motors

4. Cannot be used when motor load has overhauling characteristics.

313.At present level of technology, which of the following method of generating electric

power from sea is most advantageous?

1. Tidal power.

2. Ocean thermal

energy

conversion

3. Ocean currents.

4. Wave power

314.If the field circuits of an unloaded salient pole synchronous motor gets suddenly open

circuited, then

1. The motor stops.

2. It continues to run at the same speed.

3. Its runs at the slower speed.

4. It runs at a very high speed.

315.Electric resistance seam welding uses __________ electrodes.

1. Pointed

2. Disc.

3. Flat

4. Domed

316.For LV applications (below 1 kV), ______________ cables are used.

1. Paper insulated.

2. Plastic.

3. Single core

cables.


26

4. Oil filled.

317.No load current in a transformer:

1.

2.

3.

4.

318.A transformer operates most efficiently at 3/4th full load. Its iron (PI) and copper

loss (PCu) are related as:

1. Cu I P

P

2. Cu I P P

3. Cu I P P

319.In lap windings, the equalizer rings arc used to save

1. armature winding from carrying circulating currents

2. commutator from carrying circulating currents

3. brushes from carrying circulating currents

4. armature core from eddy currents.

320.The armature of a dc machine is laminated to reduce:

1. Eddy current

loss

2. Hysteresis loss

3. copper losses

4. friction and

windage losses

321.The number of commutator segments in a dc machine is equal to the no. of

1. coil-sides

2. turns

3. coils

4. slots.

322.A single phase Hysteresis motor

1. can run at synchronous speed only

2. can run at sub synchronous speed only

3. can run at synchronous and super synchronous speed

4. can run at synchronous and sub synchronous speed

323.The temperature of resistance furnaces can be controlled by changing the:

1. applied voltage

2. number of

heating

elements

3. circuit

configuration

4. All of the above

324.The line trap unit employed in carrier current relaying:

1. offers high impedance to 50 Hz power frequency signal

2. offers high impedance to carrier frequency signal

3. offers low impedance to carrier frequency signal

4. Both (A) & (C)

325.For a line voltage V and regulation of a transmission line R

1. R

2. V/ R

3. V2 R

4. V2 R

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306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325

C d a d d c b a b B c b d c a a a b b B

PART –B (2 MARKS)

326.Write down the emf equation for d.c generator.AU-APRIL/ MAY 2007

E = (NZ / 60)(P/A) V

Where P= number of poles

Z= Total number of conductors

A= number of parallel paths

N= speed in rpm

327.Why the armature core in d.c machines is constructed with laminated steel sheets

instead of solid steel steel? AU-APRIL/ MAY 2007

Steel sheets offer low relutance path for the magnetic field , laminated sheets reduce eddy

current loss.

328.Why is commutator employed in d.c machines?AU-NOV/DEC 2007

city between armature and fixed brushes

2.Converts altenating emf into unidirectional emf and vice versa

329.Distinguish between shunt and series field coil constructions. AU-NOV/DEC 2007

Shunt field coils are wound with wires of small cross section and have more number of.

Series field coils are wound with wires of larger cross section and have less number of turns.

330.How does a d.c motor differ from d.c generator in construction?AU-APRIL/MAY

2008

Generators are normally placed in closed room , accessible only to skilled operators.

Therefore on ventilation point of view they may be constructed with large opening in the

frame.

Motors on the other hand , have to be installed right in the place of use which may have

dust, dampness,inflammable gases, chemical fumes etc . To protect the motors against these

elements , the motor frames are made either partly closed or totally closed or flame proof

etc.

331.How will you change the direction of rotation of a d.c motor? AU-APRIL/MAY

2008

Either the direction of the main field or the direction of current through the armature

conductors is to be reserved.

332.What is back emf in d.c motors ?AU-NOV/DEC 2008

As the motor armature rotates , the system of conductor come across alternate North and

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South pole magnetic fields causing an emf induced in the conductors. The direction of the

emf induced in the conductors . The direction of the emf induced is in the direction opposite

to the current .As this emf always opposes the flow of current in motor operation it is called

back emf.

333.What is the function of a no-voltage release coil provided in a dc motor starter? AU-

NOV/DEC 2008

As long as the supply voltage is on healthy condition the current through the NVR coil

produce enough magnetic force of attraction and retain the starter handle in the ON

position against spring force. When the supply voltage fails or becomes lower than a

prescribed value the electromagnet may not have enough force and the handle will

come back to OFF position due to spring force automatically. Thus a no-voltage or

under voltage protections given to the motor.

334.Under what circumstances does a dc shunt generator fail togenerate?

AU-APRIL/MAY 2009

flux

335.How can one differentiate between long shunt compound generator and short shunt

compound generator? AU-APRIL/MAY 2009

In a short shunt compound generator the shunt field circuit is shorter i.e. across the

armature terminals. In a long shunt compound generator the shunt field circuit is

connected across the load terminals.

336.Why is the emf not zero when the field current is reduced to zero in a dc generator

AU- NOV/DEC 2009

Even after the field current/magnetizing force is reduced to zero the machine is left out

with some flux as residue.Emf due to this residual flux is available when field current is

zero.

337.On what occasions dc generators may not have residual flux? AU-NOV/DEC 2009

1.The generator may be put for its first operation after its construction.

2.In previous operation the generator would have been fully demagnetized.

338.What are the conditions to be fulfilled for a dc shunt generator to build up emf? AU-APRIL/MAY 2010

The generator should have residual flux

The field winding should be connected in such a manner that the flux set up by the field

winding should be in the same direction as that of residual flux

The field circuit resistance should be less than critical field resistance

Load circuit resistance should be above its critical load resistance

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339.How the critical field resistance of a dc shunt generator is estimated from its OCC?

AU-APRIL/MAY 2010

Critical field resistance can be obtained from OCC by drawing a straight line passing

through the origin and tangent to the initial straight line portion of OCC. The slope of

this line gives the value of critical field resistance for the given speed at which OCC is

obtained.

340.Define the term armature reaction in dc machines. AU-NOV/DEC 2010

The interaction between the flux set up by the current carrying armature conductors

with

the main field flux is defined as armature reaction.

341.What are the two unwanted effects of armature reaction? AU-NOV/DEC 2010

o Cross magnetizing effect / Distorting effect

o Demagnetizing effect

342.Define critical field resistance in dc shunt generator AU-APRIL/MAY 2011

Critical field resistance is defined as the resistance of the field circuit which will cause

the shunt generator just to build up its emf at a specified field.

343.What is the function of carbon brush used in D.C generator?AU-APRIL/MAY 2011

The function of carbon brush is to collect current from the commutator and supply to

the

external load circuit and to the field circuit.

344.Write the number of parallel paths in a lap and wave connected windings AU-

MAY/JUNE 2012

In a lap wound machine, the number of parallel paths is equal to the number of poles.

But in wave wound machine, the number of parallel paths is always two irrespective of

number of poles.

345.What is the basic difference between dc generator and dc motorAU-NOV/DEC 2011

Generator converts mechanical energy into electrical energy. Motor converts electrical

energy into mechanical energy. But there is no constructional difference between the

two.

346.Write the emf equation of a DC machine? Nov-2013(R-08)

347.List the merits and demerits of Swinburne’s test. Nov-2013(R-08)

348.Write the application of dc series motor. May-2013(R-08)

349.What is meant by armature reaction? May-2013(R-08)

350.Define commutation. Nov-2012(R-08)

351.State the methods of speed control in dc series motor? Nov-2012(R-08)

352.Define Commutation. May-2012(R-08)

353.Why DC series motor is not suitable for belt driven loads? May-2012(R-08)

354.Draw the circuit model of DC shunt motor? May-2011(R-08)

355.What is the function of no volt release in a three point starter? May-2011(R-08)

356.State the condition under which a D.C shunt generator fails to excite? Nov-2010(R-08)

357.What is the precaution to be taken during starting of a D.C series motor? Nov-2010(R-08)

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358.What is armature reaction? May-2010(R-08)

359.What are the method of speed control in DC motor? May-2010(R-08)

360.What is the condition for obtaining maximum efficiency of a transformer? Nov-2013(R-07)

361.Define all day efficiency of a transformer? Nov-2013(R-07)

Part-C 16 Marks

362.(i)Explain the various methods of commutation. (ii) Draw a neat sketch of 3-point starter

and explain its working. Nov-2013(R-08)

363.A 100kW dc shunt generator driven by a belt from an engine runs at 750 rpm and is

connected to 230V dc main. When the belt breaks, it continues to run as a motor drawing

9kW from the mains. At what speed would it run? Given : armature resistance =0.018 and

field resistance=115 Nov-2013(R-08)

364.With schematic diagram explain the working principle of different types of dc generator

based on its excitation . May-2013(R-08)

365.Explain the different methods of speed control of DC shunt motor with neat circuit

diagrams. May-2013(R-08)

366.With neat sketch explain the working of 4 point starter. Nov-2012(R-08)

367.Explain the process of commutation and method of commutation . Nov-2012(R-08)

368.Explain the different methods of excitation and characteristics of a DC generators with

suitable diagram. May-2012(R-08)

369.What are the methods of speed control of a dc shunt motor? and briefly explain them with

help of neat diagram.May-2012(R-08)

370.(i)Explain clearly the process of commutation in a dc machine. What causes sparking at the

commutator surface? Nov-2013(R-07)

(ii)Explain with a neat diagram the parallel operation of DC shunt generator. Nov-2013(R-

07)

371.(i)Draw the different characteristics of different types of DC motors. Nov-2013(R-07)

372.(ii) Describe with the help of neat diagram, the working of a three point starter for a

DC motor using Hopkinson’s test. Nov-2013(R-07)

373.Describe with sketches the construction of a DC machine. (8)

374.Derive the EMF equation of DC generator. (8)AU-APRIL/MAY 2008

375.Draw and explain the no-load and load characteristics of DC shunt, series and

compound generators. (16) AU-APRIL/MAY 2008

376.Explain the effect of armature reaction in a DC shunt generator. How is its

demagnetizing and cross-magnetizing ampere turns calculated? (16) AU-NOV/DEC

2008

377.Explain the process of commutation in a DC machine. (16) AU-NOV/DEC 2008

378.With an aid of a circuit diagram, describe the procedure for paralleling two DC

shunt generators and for transferring the load from one machine to the other.

(16)AU-APRIL/MAY 2009

379.A 4-pole, 50 kW, 250 V, wave wound shunt generator has 400 armature conductors.

Brushes are given a lead of 4 commutator segments. AU-APRIL/MAY 2009

380.A 4-pole, lap connected DC machine has 540 armature conductors. If the flux per

pole is .03 webs and runs at 1500 RPM, determine the emf generated. If this

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machine is driven as a shunt generator with same field flux and speed, calculate the

line current if the terminal voltage is 400V.Given the RSH=450_ and RA=2.

(16).AU-NOV/DEC 2009

381.Two separately excited DC generators are connected in parallel and supply a load

of 200A. The machines have armature circuit resistances of 0.05 _ and 0.1 _ and

induced emfs of 425V and 440V respectively. Determine the terminal voltage,

current and power output of each machine. The effect of armature reaction is to be

neglected. (16) AU-NOV/DEC 2009

382.Explain the principle of operation of a DC motor. (8) AU-APRIL/MAY 2010

383.A shunt machine connected to a 200V main has an armature resistance of 0.15 _

and field resistance is 100 _. Find the ratio of its speed as a generator to its speed as

a motor, line current in each case being 75 A.(8) AU-APRIL/MAY 2010

384.Draw and explain the mechanical characteristics of DC series and shunt motors. (8)

385.A 230V, DC shunt motor, takes an armature current at 3.33A at rated voltage and

at a no load speed of 1000RPM. The resistances of the armature circuit and field

circuit are 0.3 _ and 160 _ respectively. The line current at full load and rated

voltage is 40A. Calculate, at full load, the speed and the developed torque in case the

armature reaction weakens the no load flux by 4%. AU-NOV/DEC 2010

386. Derive an expression for the torque developed in a DC machine. (8) AU-APRIL/MAY

2011

387.A 220V, Dc shunt motor with an armature resistance of 0.4 _ and a field resistance

of 110 drives a load, the torque of which remains constant. The motor draws from

the supply, a line current of 32A when the speed is 450RPM. If the speed is to be

raised to 700RPM, what change must be effected in the value of the shunt field

circuit resistance? Assume that the magnetization characteristic of the motor is a

straight line. (8) AU-NOV/DEC 2011

388.Calculate the demagnetization ampere-turns per pole if shunt field resistance is 50

ohm. Also calculate extra shunt field turns per pole to neutralize the

demagnetization. (16)

389. (a) (i) with a suitable circuit diagram explain how hopkinson’s test is conducted.

(ii) What are the merits and demerits of this test? (12+4) (or)

(b) (i) derive the condition for maximum efficiency in transformer.(8)

(ii) A 11000/230v, 150kva 1-phase,50hz transformer has core loss of 1.4 kw and full

load copper loss of 1.6kw. determine (1) the KVA load for the maximum efficiency

and the value of maximum efficiency at unity p.f.

(2) The efficiency at the half of the full load at 0.8 pf leading.(8). AU-MAY/JUNE

2012