Electrical Machines - Special Purpose Machines

Special Purpose Machines - IAS previous year questions......

1.  which one of the following can be obtained by the equivalent circuit of an electrical machine?
[IAS-1993]
a. Temperature rise of the machine.
b. Type of protection used in the machine
c.  Design parameters of the windings
d. Type of protection used in the machine.

2.  the voltage across the open -circuited field terminals of a synchronous machine under slip test is [IAS-1994]
a. DC
b. Ac of slip frequency
c. a modulated supply frequency ac voltage with slip frequency envelope
d. Ac of supply frequency

3.  In a line printer used to print the output of a computer the paper is required to be advanced by a line spacing as soon as printing of the line is over. which one of the following motors is best suited for this application ?
[IAS-1995]
a. Dc motor
b. Synchronous motor
c. Induction motor
d. Steper motor

4.  Consider the following statements about single phase reluctance motor:
1. The starting torque is a function of rotor position.
2. The torque developed at sub synchronous speeds varies sinusoidally.
3. Starting is asynchronous but running is synchronous.
4. It pulls synchronism on switching on dc excitation.
Of these statements correct : [IAS-1996]
a.1,2&3
b.2,3&4
c.1,3&4
d. 1&4

5. For proper synchronisation of a large synchronous machine to a bus, the frequency of the-incoming machine [IAS-1997]
a. Should be equal to
b. Should be higher than
c. Should be lower than
d. Any value

6. In the slew range of a stepper motor, it [IAS-2001]
a. can start, stop and reverse as desired
b. cannot start but can stop and reverse on command
c. cannot start, stop and reverse on command
d. cannot start and synchronize but can reverse or command

Electrical Machines - Special Purpose Machines

Special Purpose Machines - IES Previous year questions.....

1. Hysteresis motors are used for Hi-Q record players because of their [IAS-1998]
a. Constant synchronous speed
b. insensitivity to supply voltage fluctuations
c. Extreamly study torque
d. non-dependence on centrifugal switch requirement

2. A two-phase servomotor develops maximum torque at [IAS-1999]
a.  Forward speed  half of the synchronous speed
b. Reverse speed  half of the synchronous speed
c. Synchronous speed
d. Twice the synchronous speed

3. In a shaded-pole induction motor, the rotor runs from the [IAS-2000]
a. shaded portion to the unshaded portion of the pole while the flux in the former leads that of the latter
b. shaded portion to the unshaded portion of the pole while the flux in the former lags that of the latter
c. unshaded portion to the shaded portion of the pole while the flux in the former leads that of the latter
d. Unshaded portion to the shaded portion of the pole while the flux in the former lags that of the latter

4.  Stepper motor are widely used because of [IAS-2001]
a. Wide speed range
b. no need for field control
c. Compatibility with digital system
d. large rating

5. Consider the following types of single phase motors:
1. Capacitor starts induction motor 2. Capacitor start and run induction motor
3. Permanent split capacitor motor 4. Shaded pole motor
The correct sequence of these in the ascending order of magnitude of starting torque is [IAS-2003]
a. 4,3,2,1
b. 2,3,4,1
c. 3,4,2,1
d. 1,2,3,4

6. For a reluctance type motor, if the stator magnetic field angular velocity is 'W' and the actual rotor angular velocity is 'Wr' then which one of the following equation is satisfied if the average electromagnetic torque is NOT zero? [IAS-2004]
a. W=Wr/2
b. W= Wr
c. W=2*Wr
d. W=4*Wr

7. In hand tool applications, which one of the following single phase motor is used? [IAS-2005]
a. Shaded pole motor
b. Capacitor run motor
c. Capacitor start motor
d. Ac series motor

8. In which one of the following is reluctance power developed? [IAS-2006]
a. Salient pole motor
b. Non-sailent pole motor
c. Squirrel cage induction motor
d. Transformer.

9. ) Match List -I with List -II and select the correct answer using the code given below the lists :
List - I
A. General purpose split phase FHP(Fractional HP) motor
B. General purpose capacitor start FHP motor
C. Permanent split capacitor start FHP motor
D. Shaded pole FHP motor
List-II
1. Refrigerator
2. Hair dryers
3. Unit Heaters
4. Fans, Blowers [IAS-2007]
a. A-1, B-2, C-4, D-3
b. A-4, B-1, C-2, D-3
c. A-4, B-1, C-3, D-2
d. A-4, B-3, C-2, D-1

10. The emf induced in a conductor of machine driven at 600 rpm, the peak value of flux density is 1.0 Wb/m 2 ., diameter of machine 2.0 meter and length of machine 0.30 m is [IAS-2010]
a. 41.83
b. 29.58
c. 9.42
d. 18.84

1. Statement (I) : The rotor of a servomotor is built with resistance so that its X/R ratio becomes small. Statement (II): The servomotor has good accelerating characteristics. [IAS-2012]
a. Both are true and Statement (II) correct explanation for Statement(I)
b. Both are true and Statement (II) not  correct explanation for Statement(I)
c. Statement(I) is correct and Statement (II) is false
d. Statement(I) is false and Statement (II) is true

Electrical Machines - Special purpose machines

Special purpose machines - GATE previous year questions....

1. The developed electromagnetic force and/or torque in electro-mechanical energy conversion systems act in a direction that tends [GATE-1992]
a. to increase the stored energy at constant mmf
b. to decrease the co-energy at constant mmf
c. to increase the co-energy at constant mmf
d. to decrease the stored energy at constant mmf

2. Which material is used in controlling chain reaction in a nuclear reactor? [GATE-1996]
a. Thorium
b. Boron
c. Heavy water
d. Beryllium

3. Ratio of the rotor reluctance X to the rotor resistance R for a two-phase servomotor [GATE-2000]
a. Equal to the induction motor
b. Less than to the induction motor
c. More than to the induction motor
d. More than or Less than to the induction motor

4. List-I lists different applications and List-II lists the motors for these applications . Match the application with the most suitable motor and choose the right combination among the choices given these after
List - I 
A. Food mixer 
B.Cassette tape recorder
C.Domestic water pump
D.Escalator
List - II
1 . Permanent magnet dc motor
2. Single -phase induction on motor
3. Universal motor
4.Three-phase induction motor
5.Dc series motor
6.
[GATE-2003]
a.  A-3,B-6,C-4,D-5
b. A-1,B-3,C-2,D-4
c.  A-3,B-1,C-2,D-4
d. A-3,B-2,C-1,D-4

5. The type of single –phase induction motor having the highest power factor at full load is [GATE-2004]
a. Shaded pole
b. Split phase
c. Capacitor run
d Capacitor start

6. In a single phase induction motor driving a fan load , the reason for having a high resistance rotor is to achieve  [GATE-2005]
a. low starting torque
b. High efficiency
c. Quick acceleration
d. Reduced size

7. For a single phase capacitor start induction motor which of the following statements is valid ? [GATE-2006]
a. the direction of rotation can be changed by interchanging the supply terminals
b. Direction of rotation can not be changed
c. Capacitor is used for Power factor improvment
d.  the direction of rotation can be changed by interchanging the supply terminals

8.  A 3-φ, 3 stack ,variable reluctance step motor has 20 poles on each rotor andstator stack .The step angle of this step motor is  [GATE-2007]
a. 3 degrees
b. 6 degrees
c. 9 degrees
d. 12 degrees

9. Distributed winding and short chording employed in AC machines will result in  [GATE-2008]
a. Increase in EMF and reduction in harmonics
b. Decrease in EMF and induction in harmonics
c. Decrase in EMF and harmonics
d. Increase in EMF and harmonics

Electrical Machines - Synchronous Machines

Synchronous Machines - IAS previous year questions.....

1. A synchronous machine working in the motor mode, is fed from an infinite bus and is delivering half full-load. If an increase in field current causes an increase in the armature current,then the motor will [IAS-1994]
a. Absorbs active power and reactive power from the bus.
b. Delivers active power and reactive power to the bus
c. Absorbs active power from the bus and delivers reactive power to the bus
d. Absorbs reactive power from the bus and delivers active power to the bus

2. In a synchronous generator operating at zero pf lagging, the effect of armature reaction is
[IAS-1995]
a. Magnetising
b. Demagnetising
c. Cross magnetising
d. Both magnetising and demagnetising

3. Consider the following statements about a three phase synchronous generator synchronized to an infinite bus when its mechanical input is increased gradually with field current held constant:
1. The power factor of the current supplied becomes more lagging.
2. The power factor of the current supplied improves.
3. The power factor remains unity. 4. The load angle is increased Of these statements [IAS-1996]
a. 1 alone is correct
b. 2 alone is correct
c. 2 and 4 is correct
d. 3 and 4 is correct

4. A 3-phase alternator delivers power to a balanced 3-phase load of power factor 0.707 lagging. It is observed that the open-circuit emf phasor leads the corresponding terminal voltage phasor by Neglecting the effect of harmonics, the angle between the axis of the main field mmf and the axis of the armature mmf will be [IAS-1997]
a. 30 degrees electrical
b. 60 degrees electrical
c. 90 degrees electrical
d. 150 degrees electrical

5. A 3-phase alternator delivers power to a balanced 3-phase load of power factor 0.707 lagging. It is observed that the open-circuit emf phasor leads the corresponding terminal voltage phasor by Neglecting the effect of harmonics, the angle between the axis of the main field mmf and the axis of the armature mmf will be  [IAS-1998]
a. 240 V  b. 220 V  c. 200 V  d. 180 V

6. In a synchronous machine, the electric circuits are armature circuit, field circuit and damper circuit and X is d-axis synchronous reactance; X' is d-axis transient synchronous reactance and X" is d-axis sub-transient synchronous reactance. If there is no damper winding, then [IAS-1999]
a. X = X'
b. X' = X"
c. X = X"
d. X = X' = X"

7. A 3-phase 20-pole,synchronous generator has 180 stator slots with single-layer full pitch coils.There are 6 conductors per slot and all coils per phase are committed in have series. The rotor is driven at 300 rpm and the flux per pole (sinusoidally distributed) is 25 mWb. The voltage induced per phase is nearest to
[ IAS-2000]
a. 1000   b. 500   c. 1500   d. 2000

8. An alternator is operating on infinite bus. It will develop maximum synchronizing power when it is operating at [ IAS-2001]
a. No load
b. Full load
c. Steady state stability limit
d. Any load

9. Which of the following information is obtained, by Potier's method using open circuit characteristic and zero power factor characteristics ? [ IAS-2003]
a.field current equivalent to armature reactance.
b. Synchronous reactance
c. Leakage reactance
d. Both a and b

10. A 3-phase cylindrical rotor alternator is delivering rated power to a grid at its rated power factor On account of some fault, steam supply to the turbine of the alternator is cut-off. The alternator would [ IAS-2004]
a. Stop
b. continue running as an alternator at synchronous speed Ns and deliver active power to the grid
c. continue running as a synchronous motor at Ns and deliver reactive power to the grid.
d. continue running as a synchronous motor at and absorbs reactive power from the grid.

11. Consider the following statements for a single -phase hysteresis motor :
1.Torque is constant from standstill up to synchronous speed .
2.Rotor is provided with narrow slots embedded with bars.
3. Torque at starting is somewhat more than that at synchronous speed
4. As the name suggests. only hysteresis torque is produced from zero speed right up to
5. Rotor material possesses very wide hysteresis loop
which of the statements given above [ IAS-2005]
a. 1,2 and 5 correct.
b. 3 and 5 correct.
c. 1 and 4 correct.
d. 2,3 and 4 correct.

12. An under-excited synchronous motor is operating at no load on an infinite bus of voltage 7.o p.u and drawing a current of 0.8 p.i. Its synchronous reactance is 0.5 p.u. What is the p.u. value of its excitation emf ? [ IAS-2006]
a. 1.4       b. 1.0      c. 0.6      d 0.4

Electrical Machines - Synchronous Machines

Synchronous Machines - IES Previous year wise questions......

1. If the dimensions of all the parts of a synchronous generator, and the number of field and armature turns are doubled, then the generated voltage will change by a factor of  [IES-1997]
a. 1         b. 2        c. 4       d. 8

2. The phasor diagram of a synchronous machine connected to an infinite bus is shown i above figure The machine is acting as a (on graph x-axis terminal voltage 'v' and at some angle induced voltage 'E' & angle is approxmatly at 45) [IES-1998]
a. Generator and operating at leading power factor
b. Generator and operating at lagging power factor
c. Motor and operating at leading power factor
d. Motor and operating at lagging power factor

3. 1) Match List I (Power factor) with list II (Armature reaction of an alternator) and select the correct answer :
List I
A. UPF
B. ZPF lagging
C. ZPF leading
D. Intermediate Lagging
List II
1. Fully demagnetising
2. Fully magnetising
3. Cross - magnetising
4. Partly cross-magnetising and
5. partly demagnetising [IES-2000]

        A.       B.        C.       D
a.     3.        1.        4.       2
b.     3.        1.        2.       4
c.     1.        3.        2.       4
d.     1.        3.        4.       2

4. Match List - I (equivalent circuit parameter) with List -II(Values) for a 50 MVA three phase alternator and select the correct answer
List - I
A. Armature resistance
B. Synchronous reactance
C. Leakage reactance
List - II
1. 1pu
2. 0.1 pu
3. 0.01 pu [IES-2001]
a. A-1,B-2,C-3
b. A-3,B-1,C-2
c. A-3,B-2,C-1
d. A-1,B-3,C-2

5. When the excitation of normally operating unloaded salient-pole synchronous motor suddenly gets disconnected. it continues to run as a [IES-2002]
a. S charge motor
b. Spherical motor
c switched reluctance motor
d variable reluctance motor

6. If the field of a synchronous motor is under-excited, the power factor will be [IES-2003]
a. Leading
b. Lagging
c. Unity
d. More than unity

7.  Which one of the following is the primary reason for placing field on rotor in an alternator? [IES-2004]
a. Low current in field circuits
b. Insulation of high voltage is made easy on stator than on rotor
c. Large power in stator
d. Large current in stator

8. A synchronous motor is operated from a bus zero power factor lading current. Its synchronous reactance is 0.5u. What is the excitation emf of the motor? [IES-2005]
a. 2.0    b. 1.5.    c. 1.0    d. 0.5

9. A loss-less cylindrical rotor synchronous generator is floating on the system bus bar after being properly synchronized and is running at no-load. Now, if the prime mover is acting as a mechanical load on the machine, then [IES-2006]
a. the torque developed by the machine will act in the same direction as the prime-mover torque
b. the torque developed by the machine will act in the opposite direction to that of the prime mover torque
c. the machine rotor would fall-back and the induced emf would lag behind the terminal voltage
d.  the machine rotor would fall-back and the induced emf would lead the terminal voltage

10. A 10 pole, 25 Hz alternator is directly coupled to and is driven by 60Hz synchronous motor. What is the number of poles for the synchronous motor [IES-2007]
a. 48     b. 12       c. 24      d. 16

11.  Assertion (A) : AC armature winding are short chorded by selecting value of coil span more than the pole pitch.
Reason (R) : Short chording is done to eliminate harmonics in the induced e.m.f  [IES-2008]
a. Both are true and reason is correct explanation for assertion
b. Both are true and reason is not correct explanation for assertion
c. Assertion is correct and reason is false
d. Assertion is false and  reason is true

12. The resultant flux density in the air gap of a synchronous generator is the lowest during
a. Open circuit
b. Solid Short circuit
c. At half-load
d. At full-load

13. A three phase 50 Hz, 11KV and 37.5 MW at 0.9 p.f. capacity synchronous generator has its stator bore diameter of 765 cm and an axial core length of 80 cm. For which power plants this generator is suitable? [IES-2009]
a. Thermal coal fired plant
b. Hydroelectric power plant
c. Nuclear power plant
d. pumped Storage power plant

14. Consider the following :
1.EMF
2. Reversal
3. MMF
4.Direct
which method among these are for the determination of voltage regulation of an alternator?  [IES-2010]
a. 1,2,3 and 4
b. 1 and 3
c.  1 and 2
d.  2 and 3

15. Assertion (A) : While running on light load a salient pole synchronous machine can continue to operate on infinite bus - bars even if its excitation fails.
Reason (R) : A Salient pole machine develops reluctance power due to the variable reluctance offered by the salient poles  [IES-2011]
a. Both are true and reason is correct explanation for assertion
b. Both are true and reason is not correct explanation for assertion
c. Assertion is correct and reason is false
d. Assertion is false and  reason is true

16.  A synchronous generator has its effective internal impedance Zs = 10 ohm and resistance Ra = 1.0 ohm . Its generated voltage Ef and terminal voltage Vt are both 500 V. The maximum power out put is [IES-2012]
a. 5000w
b. 4550W
c. 3000 W
d. 2250 W

17. The maximum power delivered by 1500 kW, 3-phase, star-connected, 4 kV, 48 pole 50 Hz synchronous motor, with synchronous reactance of 4n/phase and unity power factor [IES-2013]
a. 4271.2 kw
b. 35.5 kw
c. 120.61 kw
d. 2078 kw

Electrical Machines - Synchronous Machines

Synchronous Machines - GATE previous year questions......

1. The torque angle of a synchronous machine operating from a constant voltage bus, is usually defined as the space angle between [GATE- 1992]
a. Rotor mmf wave and stator mmf wave
b. Rotor mmf wave and resultant flux density wave
c.  Stator mmf wave and resultant flux density wave
d. Stator mmf wave and resultant wave

2. A synchronous motor operates at 0.8 p.f lagging. If the field current of the motor is continuously increased [GATE-1993]
a. the power factor decreases upto a certain value of field current and there after it increases
b. the armature current increases upto a certain value of field current and thereafter it decreases
c. the power factor increases upto a certain value of field current and there after it decreases
d. the armature current decreases upto a certain value of field current and thereafter it increases

3. A synchronous motor on load draws a current at a leading power factor angle 'x'.If the internal power factor angle – which is the phase angle between the excitation e.m.f. and the current in the time phasor diagram is 'y', then the air gap excitation m.m.f lags the armature m.m.f by [GATE- 1995]
a. A
b. Pi/2 + A
c. Pi/2 - A
d. Pi/2+x
( where A is )

4. A cylindrical rotor synchronous motor is switched on the supply with its field windings shorted on themselves. It will [GATE- 1996]
a. It will not start
b. Start but not run at synchronous speed
c. Start as induction motor and run as synchronous motor
d. Start and run as synchronous motor

5. The armature of a single phase alternator is completely wound with T single turn coils distributed uniformly. The induced voltage in each turn is 2V (rms). The emf of the whole winding is [GATE- 1998]
a. 2 T volts
b. 1.11 T volts
c. 1.414 T volts
d. 1.273 T volts

6. A three phase alternator is wound with a 60 degree phase-spread armature windings and develops 300 kVA. If the armature is reconnected utilizing all the coils for single phase operation with a phase spread of 180 degrees, the new rating of the machine is
[GATE- 1999]
a. 100 kva             b. 200 kva
c. 250 kva              d. 300 kva

7. It is desirable to eliminate harmonic voltage from the phase voltage of an alternator. The coils should be short-pitched by an electrical angle of [GATE-2000]
a. 30 degrees        b. 36 degrees
c. 72 degrees        d. 18 degrees

8. X,X',X"are steady state d-axis synchronous reactance, transient d-axis reactance and sub-transient d-axis reactance of a synchronous machine respectively. Which of the following statements is true? [GATE-2001]
a.  X>X'>X"
b. X">X'>X
c.  X'>X">X
d. X>X">X

9. The flux per pole in a synchronous motor with the field circuit ON and the stator disconnected from the supply is found to be 25 mWb. When the stator is connected to the rated supply with the field excitation unchanged, the flux per pole in the machine is found to be 20 mWb while the motor is running on no load. Assuming no load losses to be zero, the no load current down by the motor from the supply [GATE-2002]
a. Lags the supply voltage
b. Leads the supply voltage
c. In phase with the supply voltage
d.is Zeo

10. A 400 V, 50 kVA, 0.8 pf leading ∆-connected ,50 Hz synchronous machine has asynchronous reactance of 2 Ω and negligible armature resistance . The friction and windage losses are 2 kW and the core loss is 0.8 kW. The shaft is supplying9 kW load at a power factor of 0.8 leading . The line current drawn is  [GATE-2004]
a. 12.29A                b.16.24 A
c. 21.29 A                d. 36.88A

11.  A 3-phase ,400 V, 5 kW, star connected synchronous motor having an internal reactance of 10 Ω is operating at 50 % load, unity power factor . Now, the excitation is increased by 1%. What will be the new load in percent ,if the power factor is to be kept same ? Neglect all losses and consider linear magnetic circuit [GATE-2006]
a. 67.9 %                  b. 56.9 %
c. 51 %                      d. 50 %

12. A 100 kVA, 415(line) ,star-connected synchronous machine generates rated open circuit voltage of 415 V at a field current of 15 A. The short circuit armature current at a field current of 10A is equal to the rated armature current . The per unit saturated synchronous reactance is [GATE-2007]
a. 1.731 b. 1.5   c. 0.666  d. 0.577

13. A synchronous motor is connected to an infinite bus at 1.0 pu voltage and draws 0.6 pu current at unity power factor . Its synchronous reactance is 1.0 pu and resistance is negligible
The excitation voltage (E) and load angle (δ) will respectively be  [GATE-2008]
a. 0.8 pu and 36.86 degrees lead
b. 0.8 pu and 36.86 degrees lag
c. 1.76 pu and 30.96 degrees lag
d. 1.76 pu and 30.96 degrees lead

14. A 500MW, 21kV,, 50Hz, 3-phase, 2-pole synchronous generator having a rated p.f=0.9, has a moment of inertia of
27.5* 10^3 kg-m^2 . The inertia constant (H) will be  [GATE-2009]
a. 2.44 s.                    b. 2.71 s
c. 4.88 s.                     d. 5.42 s

15. The direct axis and quadrature axis reactance’s of a salient pole alternator are 1.2p.u and 1.0p.u respectively. The armature resistance is negligible. If this alternator is delivering rated kVA at upf and at rated voltage then its power angle is [GATE-2011]
a. 30 degrees
b. 45 degrees
c. 60 degrees
d. 90 degrees

Electrical Machines - Induction Machines

Induction Machine -IAS previous year questions.....

1. Consider the following statements regarding the equivalent circuit parameters of an induction motor :
1. Leakage reactance is dependent on supply current
2. Magnetising reactance is dependent on the air-gap flux
3. Core loss is dependent on the input voltage and frequency.
4. Rotor resistance is dependent on the speed Of these statements [IAS-1993]
a. 2 & 4 correct
b. 1,3 & 4 correct
c. 1,2,3 & 4 correct
d. 1,2 & 4 correct

2. A 3-phase induction machine operates on 3-phase fixed frequency ac mains at a per unit slip of 1.5 Consider the following statements regarding the operating condition of the machine.
1. It draws electric power from the mains
2. It draws mechanical power through the shaft
3. It delivers electrical power to the mains
4. It delivers mechanical power through the shaft . Of these statements : [IAS-1995]
a. 1 and 2 correct
b. 1 and 4 correct
c. 2 and 3 correct
d. 3 and 4 correct

3.  A 3-phase induction motor draws 1000 kVA at a pf of 0.8 lag. A synchronous condenser is connected in parallel to draw an additional 750 kVA ot 0.6 pf lead" The pf of the total load supplied by the mains is [IAS-1996]
a. unity                    b. 0.707 lead
b. 0.707 lag             d. Zero

4. Consider the following statements about the operation of a three-phase induction motor running at full load : If one of the supply line fuse blows off, the motor will
1. stall
2. continue to run with increased slip,
3. continue to run with normal supply current
4. continue to run with excessive supply current
Of these statements [IAS-1997]
a. 1 alone correct
b. 3 alone correct
c. 2 and 3 correct
d. 2 and 4 correct

5.  Consider the following statements: As a 3-phase induction motor is loaded from no load to rated load,
1.there is an improvement in the power factor.
2. the torque increases almost in proportion to slip.
3. the air-gap flux falls sharply. Of these statements [IAS-1998]
a. 1, 2 and 3 correct
b. 1 and 3 correct
c. 1 and 2 correct
d. 2 and 3 correct

6.  A 3-phase induction motor has a full-load slip of 3 per cent at normal voltage. Which one of the following will be the value of the slip of the motor if it develops the same torque theoretically while operating at 110 per cent of its normal voltage? [IAS-1999]
a. 2.48 %               b. 0.248%
c. 0.483%               d. 4.83%

7. Two separate induction motors having 6 poles and 4 poles are cascaded 3-phase, 400 V, 60Hz is the supply to the motors. The synchronous speeds (in rpm) of cascaded set will be [IAS-1999]
a. 720, 1200, 1800 and 2400
b. 600, 1000, 1500 and 3000
c. 720, 1200, 1800 and 3600
d. 600, 1200, 1800 and 2400

8. In a 3-phase induction motor,the rotor impedance angle at standstill is e. The load angle will be [IAS-2001]
a. Pi + thita
b. Pi - thita
c. Pi/2 + thita
d. Pi/2

9. The power input to an induction motor is 50 kW when it is running at 4% slip. If the stator resistance and core losses are neglected, the rotor copper loss is equal to [IAS-2002]
a. 38Kw    b. 20 Kw    c. 4Kw   d. 2Kw

10. Under variable frequency operation of 3-phase induction motor air gap flux is kept constant by adjusting the motor voltage.For constant air gap flux machine develops the same torque when the [IAS-2003]
a. Slip is kept constant
b. Slip speed is kept constant
c. power flow across the air gap is kept constant
d. Power input to stator is kept constant

11. 1) Match List -I (Motor) with List -II (Application) and select the correct answer using the code given below the Lists :
List-I (Motor)    
A. Hysteresis motor                                 B. Shaded-pole motor                                C. Reluctance motor 
D. Universal motor    
List-II (Application)
1. Photocopying machine
2. Phonograph turntable
3. Electric Shaver
4. Record player  [IAS-2006]

CODES.     A           B          C         D
          a.    4          3            2         1
          b.    2          1            4         3
          c.     4          1            2         3
          d.     2          3           4         1

12. In an induction motor, what will happen if air-gap is increased ?
[IAS-2008]
a. Speed will reduces
b. Efficiency will improves
c. Power factor will lowered
d break-down torque will reduces

Electrical Machines - Induction Machines

Induction Machines - IES previous year questions....

1. The rotor power output of a 3-phase induction motor is 15kWand the corresponding slip is 4%. The rotor copper loss will be [IES-1997]
a. 600w  b. 625w  c. 650w  d. 700w

2. A 3-phase induction motor fed a 3-phase voltage controller is suitable for driving loads whose torque is [IES-1998]
a. Irrespective of the speed
b. Invertially proportional to square of the speed
c. Constantly proportional to the square of speed
d. Invertially proportional to the speed

3. Which one of following statements regarding a capacitor start, capacitor run single-phase induction motor is correct? [IES-1999]
a. One of the capacitors is in the circuit during starting and both are in the circuit during running
b. Both the capacitors are in the circuit during starting and one of them is in the circuit during running
c. Both the capacitors are in the circuit during starting and running
d. One of the capacitors is in the circuit during starting and the other is in the circuit during running.

4. A 3-phase slip-ring induction motor is wound for 4 poles on stator and 6 poles on rotor. When 3-phase balanced voltage source at 50 Hz is applied to the motor, it will run at : [IES-2000]
a. 1500 rpm              b. 1000 rpm
c. 750 rpm                 d. Zero

5. An induction motor when started on load does not accelerate up to full speed but runs at 1/7th of the rated speed. The motor is said to be [IES-2001]
a. Locking                b. Plugging
c. Crawling              d. Coggling

6. Assertion(A): A single -phase induction motor is not self starting. Reason(R): A three-phase induction motor is self starting [IES-2002]
a. Both A and R are true and R is the correct explanation of A
b. Both A and R are true but R is not the correct explanation of A
c.  A is true but R is false
d.  A is false but R is true

7. Match List-I(Machines) with List -II(Tests) and select the correct answer:
List-I
A. Transformer
B. DC Motor
C. Alternator
D. Induction Motor
List-II
1. Slip Test
2. Blocked rotor test
3. Sumpner's Test
4. Swinburne's Test [IES-2003]
a. A-3, B-4, C-1, D-2
b. A-3, B-4, C-2, D-1
c.  A-4, B-3, C-1, D-2
d.  A-4, B-3, C-2, D-1

8. Which one of the following statement is correct?In an induction motor, if the air gap is increased, [IES-2004]
a. Speed reduces
b. Efficiency increases
c. Break down talk reduces
d. Power factor reduces

9. A starting torque of 80Nm is developed in an induction motor by an auto- transformer starter with a tapping of 30%. If the tapping of auto-transformer starter is 60%, then what is the starting torque? [IES-2005]
a. 40 Nm             b. 160 Nm
c. 240 Nm           d. 320 Nm

10. If the load on an induction motor is increased from no load to full load, its slip and the power factor will, respectively  [IES-2006]
a. Decrease, Decrease
b. Decrease, Increase
c. Increase, Decrease
d. Increase, Increase

11. Which one of the following is the correct statement?In a 3-phase induction motor, the resultant flux of a constant nature and is  [IES-2007]
a. equal to (flux)m ,where (flux)m is maximum flux due to an phase
b. 1.5 time maximum value of flux due to any phase
c. Square root of 3/2 time maximum value of flux due to any phase
d. 3 times of (flux)m

12. Why is a centrifugal switch used in a single-phase induction motor [IES-2008]
a. To protect the motor from over loading
b. To improve starting performance of the motor
c. To cut off the starting winding at an appropriate instant
d. To cut in the capacitor during runing of the motor

13.  A three phase 6 pole 50 Hz induction motor is running at 5% slip. What is the speed of the motor? [IES-2009]
a.800 rpm            b. 850 rpm
c.950 rpm            d. 1000 rpm

14. The MMF produced by the rotor currents of a 3-phase induction motor [IES-2010]
a. Rotates at the speed of rotor in the air gap
b. is at stand with respect to stator MMF
c. Rotates at slip speed with respect to stator MMF
d. Rotates at synchronous speed with respect to rotar 

15.  Assertion (A) : A squirrel cage induction motor is preferred to the slip ring induction motor. Reason (R): Higher starting torque is the main consideration. [IES-2011]
a. Both A and R are true and R is the correct explanation of A
b. Both A and R are true but R is not the correct explanation of A
c.  A is true but R is false
d.  A is false but R is true

16. A 15 kW ,400 V, 4-pole ,50 Hz, star -connected 3 - phase induction motor has full-load slip of 4 %. The output torque of the machine at full load is  [IES-2012[
a. 1.66 Nm                b. 95.5 Nm
c. 99.47 Nm              d. 624.73 Nm

17. In a synchronous machine the rotor speed becomes more than the synchronous speed during hunting, the damping bars develop [IES-2013]
a. Synchronous motor torque
b. DC motor torque
c. Induction motor torque
d. Induction generator torque

Electrical Machines- Induction Machines

Induction machines - gate previous year questions ......

1.Skew is used in induction motors in order to reduce torque due to[GATE-1994]
a. Time Harmonics
b. Space Harmonics
c. Slot Harmonics
d. reverse rotating fields

2. An inverter capable of supplying a balanced three-phase variable voltage variable frequency output is feeding a three-phase induction motor rated for 50 Hz and 440V. The stator winding resistances of the motor are negligible small. During starting, the current inrush can be avoided without sacrificing the starting torque by suitably applying: GATE-1995]
a. low voltage at rated frequency
b. low voltage keeping the V/f ratio constant
c. Rated voltage at low frequency
d. Rated  voltage at rated frequency

3. When the supply voltage to an induction motor is reduced by 10%, the maximum torque will decrease by approximately [GATE-1996]
a. 5%   b. 10%   c. 20%   d. 40%

4. If an induction machine is run at above synchronous speed, it acts as [GATE-1997]
a. Synchronous motor
b. Induction motor
c. Induction generator
d. None of the above

5. A 3 phase squirrel cage induction motor has a full load efficiency of 0.8 and a maximum efficiency of 0.9. It is operated at a slip of 0.6 by applying a reduced voltage. The efficiency of the motor at this operating point is: [GATE-1998]
a. less than 0.4
b. Greater than 0.6
c. in the range of 0.8(+or-)0
d. None of this

6. The following starting method for an induction motor is inferior view of the poor starting torque per ampere of the line current drawn:  [GATE-1999]
a. Direct lining method
b. Auto transformer method of starting
c. Star-delta method of starting
d. series-induction method of starting

7. The power input to a 415V, 50 Hz, 6 pole, 3-phase induction motor running at 975 rpm is 40 kW. The stator losses are 1kW and friction and windage losses total 2 kW. The efficiency of the motor is
[GATE-2000]
a. 92.5%  b. 90%  c.91%  d. 88%

8. The rotor of a three phase, 5 kW, 400V, 50 Hz, slip ring induction motor is wound for 6 poles while its stator is wound for 4 poles. The approximate average no load steady state speed when this motor is connected to 400V, 50 Hz supply is [GATE-2002]
a.1500rpm     b.500rpm. c.0. d.1000rpm

9. A 3-phase Induction Motor is driving a constant torque load at rated voltage andfrequency . If both voltage and frequency are halved ,following statementsrelate to the new condition if stator resistance, leakage reactance and coreloss are ignored [GATE-2003]
1.The differencebetween synchronous speed and actual speed remain same
2.their air gap flux remains same
3. Thyristor current remains same
4. Thep.u slip remains same
Choose correct among above:

a. All
b. 1,2 & 3
c. 2,3 & 4
d. 1 & 4

10. The synchronous speed for the seventh space harmonic mmf wave of a 3-phase ,8-pole ,50 Hz induction machine [GATE-2004]
a. 107.14 rpm in forward direction
b. 107.14 rpm in reverse direction
c. 5250 rpm in forward direction
d. 5250 rpm in reverse direction

11. Under no load condition, if the applied voltage to an induction motor is reduced from the rated voltage to half the rated value, [GATE-2005]
a. the speed decreases and the stator current increases
b. both the speed and the stator current decrease
c. the speed and the stator current remain practically constant.
d. there is negligible change in the speed but the stator current decreases

12. A 3-phase ,4-pole ,400 V ,50 Hz ,star connected induction motor has the following circuit parameters
r1=1ohm, r′2=0.5 ohm, x1=x′2=1.2 ohm, Xm=35 ohm.The starting torque when the motor is started direct-on-line is (use approximate equivalent circuit model)
[GATE-2006]
a. 63.6 Nm          b.74.3 Nm
c. 190.8 Nm        d.222.9 Nm

13.  A three phase squirrel cage induction motor has a starting current of seven times the full load current and full load slip of 5%
If an auto transformer is used for reduced voltage starting to provide 1.5 per unit starting torque ,the auto transformer ratio(%) should be
[GATE-2007]
a. 57.77%                b. 72.56%
c. 78.25%                d. 81.33%

14. A 400 V, 50 Hz, 30 hp, three-phase induction motor is drawing 50 A current at 0.8 power factor lagging . The stator and rotor copper losses are 1.5 kW and 900 W respectively . The friction and windage losses are 1050 W and the core losses are 1200 W. The air-gap power of the motor will be [GATE-2008]
a. 23.06kw        b.24.11kw
c. 25.01kw        d. 26.21kw

15. A three –phase 440V, 6 pole, 50Hz, squirrel cage induction motor is running at a slip of 5%. The speed of stator magnetic field to rotor magnetic field and speed of rotor with respect to stator magnetic field are [GATE-2011]
a. 0,50 rpm       c. 1000, -5 rpm
b. 0,950 rpm     d. 1000,950 rpm

16. The locked rotor current in a 3-phase star connected 15 kW 4-pole 230 V 50 Hz induction motor at rated conditions is 50 A. Neglecting losses and magnetizing current the approximate locked rotor line current drawn when the motor is connected to a 236 V 57 Hz supply is [GATE-2012]
a. 58.5   b. 45.0    c. 45.7    d. 55.6

17. A 4-pole induction motor, supplied by a slightly unbalanced three-phase 50Hz source, is rotating at 1440 rpm. The electrical frequency in Hz of the induced negative sequence current in the rotor is [GATE-2013]
a. 100     b. 98      c. 52     d. 48

Electrical Machines- Transformers

This are some IAS previous year  questions.......

1. When a short-circuit test is conducted on a single-phase transformer, 30% of the rated voltage is required to allow full-load current The short-circuit power factor is found to be 0.2. The percentage regulation at UPF is [IAS-1993]
a.57.7% b. 100% c. 50% d.75%

2. transformer designed for operation on 60 Hz supply is worked on 50 Hz supply system without changing its voltage and current ratings. When compared with full load efficiency at 60 Hz,the transformer efficiency on full load at 50 Hz would be [IAS-1994]
a. increase marginally
b. Increases by a factor 1.2
c. Remains unaltered
d. Decreases marginally

3. The series equivalent impedance of a single phase transformer referred to its secondary side is Z= R+jX, The voltage regulation of this transformer will be zero when the power factor of the load is [IAS-1995]
a. Unity                   b. [Z/X] leading c.[Z/X] lagging         d. [R/Z] leading

4.If the height to width ratio of the window of core type transformer is increased, then[IAS-1997]
a. its leakage reactance and percentage voltage regulation will decrease
b. its leakage reactance and percentage voltage regulation will increase
c.its leakage reactance will be unaffected but total space in the window will increase to accommodate the windings better d.efficiency will increase and temperature rise will decrease

5. Consider the following statements :
The use of Delta-connected tertiary windings in star-star connected power transformers
1.makes available supply for single-phase loads.
2.suppresses harmonic voltages. 3.allows flow of earth fault current for operation of protective devices 4.provides low-reactance paths for zero- sequence currents. Of the statements : [IAS-1998]
a. 1,2 and 3 correct
b. 1,3 and 4 correct
c. 2 and 4 correct
d. All are correct

6. A six -pulse non-reversible thyritorised 4I5 V ac converter Is controlling a 440 V dc motor. If total drop In the circuit Is limited to 10%, then the converter must be locked at an angle of [IAS-1999]
a. 30 b. 60 c. 45 d. 90

7. Two transformers X and Y with identical ratings and dimensions have 0.8 mm and 1.2 mm thick laminations respectively. If Rm and Xm are the magnetizing branch parameters in the equivalent circuit, then [IAS-2000]
a. Rm values in both are likely to be equal, but Xm of X is likely to be higher than of Xm of y.
b. Xm values in both are likely to be equal, but Rm of X is likely to be higher than of Rm of y.
c. Xm values in both are likely to be equal, but Rm of X is likely to be lesser than of Rm of y.
d. Rm values in both are likely to be equal, but Xm of X is likely to be lesser than of Xm of y.

8. Three units of 1 : 5 transformers are connected in to supply a 3-phase load from a 400 V 3-phase source. The line voltage on the load side is [IAS-2001]
a.1000  b. 80.    c. 3464    d. 803

9. Consider the following statements concerning the ferromagnetic materials :
1. There are easy and hard magnetization directions in the ferromagnetic Crystal.
2. Eddy current losses are smaller for materials having low electrical conductivity values.
3. All ferromagnetic materials can be used at high frequencies.
Which of the statements given above are correct ? [IAS-2004]
a. 1,2&3 b. 1&3 c. 2&3 d. 1&2

10. a transformer has a reactance of 0.1 p.u. and negligible resistance. What is its voltage regulation at full load, and 0.8 power factor lagging ? [IAS-2006]
a. 8%       b.6%     c.- 6%       d. -8%

11. An open circuit test was done on a transformer with rated voltage at rated frequency. If the open circuit test is now done on the transformer with double the rated voltage with double the rated frequency, then which one of the following is correct ? [IAS-2008]
a. Both current and power factor increase
b.  Both current and power factor decrease
c. Current decreases but power factor increases
d. Current increases but power factor decreases

12. The main purpose of the conservator in a (Driving system) transformer is to
a. store extra oil to compensate for the loss of oil due to leakage
b. achieve better cooling of the transformer
c. take up the expansion of oil due to heating
d. have the bucholtz relay fitted

Electrical Machines-Transformers

IES previous year question papers

1. If the applied voltage of a certain transformer is increased by 50% and the frequency is reduced to 50% (assuming) that the magnetic circuit remains unsaturated), the maximum core /flux density will
[IES-1997]
a. Change to three, times the original value
b.  Change to 1.5, times the original value
c. Change to 0.5, times the original value
d. Remains as same value

2. In a 3-phase transformer shown in the figure, the phase displacement of secondary line voltages with corresponding primary line voltages will be [IES-1998]
a. 0 degrees          b. 30 degrees lag
b. 30 degrees lead    d. 180 degrees

3. If an ideal centre-tapped 1: 4 transformer is loaded as shown in the figure, the impedance measured across the terminals AA would be (figure contains primary side AA terminal and secondary side centertapped with loads 2z & z ) [IES-1999]
a. 3z/64   b. 3z/32   c. 2z/3   d. z/6

4. Consider the following statements:
1. Interpole windings are connected in series with armature winding
2. Polarity of interpole must be the same as that of the main pole in advance
3. Distortion of the main field under the pole shoes is not affected by the use of interpoles Which of these statements regarding interpole used in dc motors is/are correct? [IES-2000]
a. 1&2. b. 2&3  c. 1&3  d. 1 alone

5. In case of auto transformers ,which of the following statements are correct?
1. An auto transformers requires less copper as compared to a conventional, 2-winding transformers of the same capacity. 2. An auto transformer provides isolation between the primary and secondary windings.
3. An auto transformer has less leakage reactance as compared to the conventional, 2-winding transformer of the same capacity.

Select the correct answer using the codes given below: [IES-2001]
a. 1,2&3  b. 1&2  c. 1&3   d. 2&3

6. match List-I(Transformer) with List-II(Voltage ratio)and select the correct answer [IES-2002]
List-I                               List -II
A. Power transformer 1. 230V/230V B. Auto Transformer   2.220V/240V
C. Welding T/F.           3.400V/100V
D. Isolation T/F.         4 .132KV/11KV

a. A-4, B-2, C-3, D-1
b. A-4, B-2, C-1, D-3
c. A-2, B-4, C-1, D-3
d.  A-2, B-4, C-3, D-1

6. The function of oil in a transformer is to provide [IES-2003]
a. Cooling & Insulation
b. Protection against lightening
c. Protection against short circuit
d. Lubrication

7. Assertion (A): Delta-connected tertiary windings tend to act as an automatic feed back control system. Reason(R) : In the event of unbalance secondary load currents, both secondary and primary load voltages are restored to their normal phase magnitude and angle. [IES-2004]
a. Both A and R are true and R is the correct explanation of A
b. Both A and R are true but R is not the correct explanation of A
c.  A is true but R is false
d.  A is false but R is true

8. Match List-1 (Type of coil) with List-II (Use of coil) and select the correct answer using the codes given below the lists: [IES-2005]
List-I                              List-II
A. Sandwitch coils     1. Low voltage coils for currents above 100A
B. Disc coils                2. High voltage windings of small transformers
C. Cross -over coils     3. Cooling oils is contact with each turn of the winding
D.Spiral type               4. Shell-type transformer core
a. A-2, B-3, C-4, D-1
b. A-4, B-1, C-2, D-3
c. A-2, B-1, C-4, D-3
d A-4, B-3, C-2, D-1

9. What is the efficiency of an auto transformer in comparison to that of a two-winding transformer of the same rating? [IES-2006]
a. Slightly less than that of a two-winding transformer
b. Same as that of a two winding transformer
c. More than that of a two winding transformer
d. As low as 1/5 th of the efficiency of a two winding transformer

10. A 500 KVA transformer has constant losses of 500 W and copper losses at full load are 2000 W. at what load, is the efficiency maximum? [IES-2007]
a. 250    b.500   c. 1000    d. 125

11. Core of large power transformers are made from which one of the following? [IES-2008]
a. Hot rolled steel
b. Cold rolled steel
c. Cold-rolled grain oriented steel
d. Ferrite

12. What is the power transferred conductively from primary to secondary of an auto-transformer having trans-formation ratio of 0.8 supplying a load of 3 KW? [IES-2009]
a. 0.6  b. 2.4  c. 1.5  d. 2.7

13. A delta/delta connected transformer is connected to Y/Y connected transformer. The ratio of VA rating of Y/Y connected transformer and delta/delta connected transformer [IES-2010]
a. 57.7% b.100% c. 50% d. 75%

14. The full load copper and iron loss of a transformer are 6400 W and 500 W, respectively. The above copper loss and iron loss at Half load will be  [IES-2011]
a. 3200 W & 500 W
b. 3200 W & 250 W
c. 1600 W & 125 W
d. 1600 W & 500 W

15. Match List-I with List-II and select the correct answer using the code given below the lists: [IES-2012]
List-I                            List-II
A. Magnetic Flux       1. Resistance
B. Magneto motive force 2. Electric current
C. Reluctance         3. Conductivity
D. Permeability      4. Electromotive force
a. A-2, B-1, C-4, D-3
b. A-3, B-1, C-4, D-2
c.  A-2, B-4, C-1, D-3
d. A-3, B-4, C-1, D-2

16. For power transformers of larger ratings, the tapings are located in the middle portion of the winding to  [IES-2013]
a. Increase the break-down strength of the winding
b. Enable better cooling of windings
c. Enable better distribution inter turn voltage
d. Reduce the mechanical forces affecting the windings during short circuit

Electrical Machines- Transformers

Hey,below questions are gate previous years.........

1. A 220/440 V ,SO Hz, 5 kVA single phase transformer operates on 220 V,40 Hz supply with secondary winding. Then [GATE-1993]
a. the eddy cu rrent loss and hysteresis loss of the transformer decreases
b.  the eddy cu rrent loss and hysteresis loss of the transformer increases
c. the hysteresis loss of the transformer increases while eddy current loss remains the same
d. The hysteresis loss remains the same whereas eddy current loss decreases.

2. Two transformers of identical voltages but of different capacities are operating in parallel. For satisfactory load sharing [GATE-1994]
a.  impedences must be equal
b. pu impedences must be equal
c. pu impdences and X/R ratio must be equal
d. impdences and X/R ratio must be equal

3. Supply to one terminal of a connected three-phase core type transformer which is on no-load, fails. Assuming magnetic circuit symmetry, voltages on the secondary side will be [GATE-1995]
a. 230,230,115
b. 230,115,115
c. 345,115,115
d.345,0,345

3. Auto-transformer is used in transmission and distribution [GATE-1996]
a. when operator is not available
b. When iron losses are to be reduced
c. when efficiency considerations can be ignored
d. When transformation ratio is small

4. The low voltage winding ofa 400/230V, 1-phase, 50 Hz transformer is to be connected to a 25 Hz supply at 25 Hz, the supply voltage should be [GATE-1997]
a. 230    b. 460    c. 115    d. 65

5. The magnetizing current in a transformer is rich in [GATE-1998]
a. 3rd harmonic.    b. 5th harmonic
c. 7th harmonic.    d. 13thHarmonic

6. A 10 kVA, 400 V/200V single-phase transformers with 10% impedance draws a steady short circuit line current of [GATE-1999]
a. 50A.  b.150A. c. 250A  d. 350A

7. 1) A 3-phase delta/star transformer is supplied at 6000 V on the delta-connected side. The terminal voltage on the secondary side when supplying full load as 0.8 lagging power-factor is 415 V. The equivalent resistance and reactance drops for the transformer are 1% and 5% respectively. The turn’s ratio of the transformer is: [GATE-2000]
a.14.     b. 24.    c.42.     d.20

8. The core flux of a practical transformer with a resistive load [GATE-2001]
a. is strictly constant with load changes.
b. increases linearly with load.
c.  increases as the square root of the load.
d.  decreases with increase

9. For a linear electromagnetic circuit ,the following statement is true [GATE-2004]
a.  Field energy is equal to the co-energy
b. Field energy is greater than the co-energy
c.  Field energy is less than the co-energy
d. Coenergy is zero

10. Which three –phase connection can be used in a transformer to introduce a phasedifference of between its output and corresponding input line voltages [GATE-2005]
a. star-star         b.delta-star
c. delta-delta.    d. zig-zag

9. Two transformers are to be operated in parallel such that they share load in proportion to their kVA ratings . The rating of the first transformer is 500 kvA and its pu leakage impedance is 0.05 pu. If the rating of second transformer is 250 kVA, its pu leakage impedance is [GATE-2006]
a. 0.20   b. 0.10   c. 0.005. d. 0.025

10. It is desired to measure parameters of 230V/115 V, 2 kVA, single-phase transformer . The following wattmeters are available in a laboratory :
W1 : 250 V, 10 A, low power factor
W2 : 250 V, 05 A, low power factor
W3 : 150 V, 10 A, high power factor
W4 : 150 V, 05 A, high power factor
The wattmeters used in open circuit and short circuit test of the transformer will be respectively be [GATE-2008]
a. W1 & W2.     b. W2 & W4
c.  W1 & W4.     c. W2 & W3

11. A single phase 10 kVA 50 Hz transformer with 1kV primary winding draws 0.5 A and 55 W at rated voltage and frequency on no load. A second transformer has a core with all its linear dimensions times the corresponding dimensions of the first transformer. The core material and lamination thickness are the same in both transformers. The primary windings of both the transformers have the same number of turns. If a rated voltage of 2 kV at 50 Hz is applied to the primary of the second transformer then the no load current and power rgespectively are [GATE-2012]
a. 0.7 A,77.8 w
b. 0.7 A, 155.6 w
c. 1A, 110 w
d. 1A, 220 w

12. A single-phase transformer has no-load loss of 64 W, as obtained from an open- circuit test. When a short-circuit test is performed on it with 90% of the rated currents flowing in its both LV and HV windings, he measured loss is 81 W. The transformer has maximum efficiency when operated at [GATE-2013]
a. 50.0% Rated current
b. 64.0%Rated current
c. 80.0% Rated current
d. 88.8%Rated current

Electrical Machines - DC Machines

India's finest entrance exam for Post-Graduation is GATE.Below questions are previous year gate questions  of EE stream.

1. A 220V, 15 kW, 1000 rpm shunt motor with armature resistance of has a rated line current of 68 A and a rated field current of 2.2 A. The change in field flux required to obtain a speed of 1600 rpm while drawing a line current of 52.8 A and a field current is 1.8 A [GATE-2012]
a.  18.18% increase
b. 18.18% decrease
c. 36.36% increase
d. 36.36% decrease

2. Neglecting all losses, the developed torque (T) of a d.c. separately excited motor, operating under constant terminal voltage, is related to its output power (P) as under [GATE-1992]
a. T proportional to square root of P
b. T proportional to P
c. T^2 proportional to P^3
d. T independent of P

3. A differentially compounded d.c. motor with interpoles and with brushes on the neutral axis is to be driven as a generator in the same direction with the same polarity of the terminal voltage. It will [GATE-1995]
a. be a cumulatively compound generator but the interpole coil connections are reversed.
b. be a cumulatively compound generator without reversing  interpole coil connections
c. be a differently compound generator without reversing  interpole coil connections
d. be a differently compound generator but the interpole coil connections are reversed.

4. A cumulative compounded long shunt motor is driving a load at rated torque and rated speed If the series field is shunted by a resistance at to the resistance of the series field, keeping the torque constant [GATE-1993]
a. Armature current increases
b. Motor speed increases
c. Armature current decreases
d. Motor speed decreases

5. The torque speed characteristic of a repulsion motor resembles which of the following dc motor characteristic ? [GATE-1996]
a. Separately
b. Shunt
c. Series
d. Cumulative

6. A 240 V d.c. shunt motor with an armature resistance of 0.5ohm has a full load current of 40 A. Find the ratio of the stalling torque to the full load torque when a resistance of 1ohm is connected in series with the armature [GATE-1998]
a. 4          b. 12            c. 6         d. None of the above

7. A DC shunt motor is running at 1200 rmp, when excited with 220 V DC. Neglecting the losses and saturation, the speed of the motor when connected to a 175 V DC supply [GATE-1999]
a. 750 rpm.      b. 900 rpm
c. 1050 rpm      d. 1200 rpm

8. A 240 V dc series motor takes 40A when giving its rated output at 1500 rpm. Its resistance is 0.3 ohms. The value of resistance which must be added to obtain rated torque at 1000 rpm is [GATE-2000 ]
a. 6 ohms             b. 5.7 ohms
c. 2.2 ohms          d. 1.9 ohms

9. An electric motor with ‘constant output power will have a torque speed characteristic in the form of
[GATE-2001]
a. straight line through the origin.
b. straight line parallel to the speed axis.
c. circle about the origin.
d. rectangular hyperbola.

10. A 200 V, 2000 rpm ,10A, separately excited dc motor has an armature resistance of 2Ω. Rated dc voltage is applied to both the armature and field winding of the motor. If the armature drawn 5A from the source, the torque developed by the motor is [GATE-2002]
a. 4.30 Nm            b. 4.77 Nm
c. 0.45 Nm             d. 0.50 Nm

11. A 8-pole ,DC generator has a simplex wave-wound armature containing 32 coils of 6 turns each . Its flux per pole is 0.06 Wb. The machine is running at 250 rpm. The induced armature voltage is
[GATE-2004]
a. 96 b. 192 c. 384 d. 768

12. A 50 kW dc shunt motor is loaded to draw rated armature current at any given speed, When driven
(i) at half the rated speed by armature voltage control
(ii) at 1.5 times the rated speed by field current ,the respective output powers delivered by the motors are approximately  [GATE-2005]
a. 25 Kw & 75 Kw
b. 25 Kw & 50 Kw
c.  50 Kw & 75 Kw
d. 50 Kw & 50 Kw

13. A 220 V DC machine supplies 20 A at 200 V as a generator . The armature resistance is 0.2 ohm. If the machine is now operated as a motor at same terminal voltage and current but with the flux increased by 10%, the ratio of motor speed to generator speed is
[GATE-2006]
a. 0.87  b. 0.95  c. 0.97  d. 1.06

14. The dc motor , which can provide zero speed regulation at full load without any controller is [GATE-2007]
a. series    c. cumulative compound
b. shunt.   d. differential compound

15. A 240 V, dc shunt motor draws 15 A while supplying the rated load at a speed of 80 rad/s. The armature resistance is 0.5 Ω and the field winding resistance is 80 Ω
The net voltage across the armature resistance at the time of plugging will be [GATE-2008]
a. 6V b. 236V c. 240V  d. 474V

16. A separately excited DC motor runs at 1500 rpm under no-load with 200 V applied to the armature. The field voltage is maintained at its rated value. The speed of the motor, when it delivers a torque of 5 Nm,is 1400 rpm as shown in the figure. The rotational losses and armature reaction are neglected.
The armature resistance of the motor is, [GATE-2010]
a. 2v   b. 3.4v   c. 4.4v   d. 7.7v

17. A 220 V, DC shunt motor is operating at a speed of 1440 rpm. The armature resistance is 1 ohm and armature current is 10A. of the excitation of the machine is reduced by 10%, the extra resistance to be put in the armature circuit to maintain the same speed and torque will  [GATE-2011]
a. 1.79 ohm.        b. 2.1 ohm
c. 1.89 ohm.         c. 3.1 ohm

Answers:
1.D                        11.           
2.                           12.
3.                           13.
4.                           14.
5.                           15.
6.                           16.
7.                           17.A
8.
9.
10.

Explanations :

1.   N1/N2 = (Eb1/Eb2)*(flux2/flux1)
      we know that,
      Eb = V-(Ia*Ra)
      Ia =line current - field current.
     
By putting values above one..
We get ,(flux2/flux1) = 0.6364

-It is clearly indicating that, flux has to reduce.
So,the change in flux in percentage is 36.36%
- [1-(flux2/flux1)]*100

17. Armature current is 10 A.
Let, flux is 'x1'.now,flux is reduced by 10%.now,flux is 'x2'
  So,   x2 = 0.9*x1

N1/N2 = (Eb1/Eb2)*(flux2/flux1)
      we know that,
             Eb1 = V-(Ia*Ra) &
             Eb2 = V-[Ia*(Ra+R)]
By putting values & solving.we get
           R = 1.79 ohms
     

Electrical Machines- DC Machines

Here is my collection of IES Previous year questions.

1.  Match List-I (Parts of electrical machines) with List - II (The approximate nature of the air-gap mmf pattern produced by them) and select the correct answer
List-I                              List-II
A. DC machine,stator 1. Sinusoidal B. DC machine,rotor  2. Trapezoidal C. Salient-pole synchronous machine, stator         3. Triangular D. Squerrel-cage rotor of induction motor [IES-1992]

Codes :     A      B     C       D
     a.         1       2     1        3
     b.         3       1      3       1
     c.          2       3      1       3
     d.          2       3     1       1

2. The interpoles in dc machines have a tapering shape in order to
[ IES-1998]
a.  reduce the overall weight
b.  reduce the saturation in the interpole
c.  economise on the material required for interpoles and their windings.
d. increase the acceleration of commutation.

3. Consider the following statements : For a level compounded dc generator to run at constant speed, the series field mmf must effectively compensate 1. armature reaction mmf
2. armature resistance voltage drop
3. brush contact voltage drop Which of these statements is/are correct ? [IES-2000 ]
a. 2 alone
b. 1 and 2
c.  1 and 3
d. 1,2 and 3

4. The current drawn by a 220V dc motor of armature resistance 0.5 and back emf 200V is [IES-2001]
a. 40      b. 44      c. 400      d.440

5. Match List-I ( Type of electrical loads) with List-II (Torque speed characteristics) and select the correct answer : [IES-2002]
List-I
A. Hoist
B. Fans
C. Machine Tools( Lathe, Milling machine etc)
D. Loads with fluid friction
List II
1. Torque proportional to speed^2
2. Torque proportional to speed^3
3. Constant Torque
4.Torque intentionally proportional to speed
a. A-1, B-3, C-2, D-4
b. A-1, B-3, C-4, D-2
c. A-4, B-1, C-3, D-2
d. A-3, B-1, C-2, D-4

6. The current drawn by a 120V d.c. motor with back e.m.f of 110V and armature resistance of 0.4 ohm is [IES-2003]
a. 4A  b. 25A c. 274A d. 300A

7. Assertion (A):In the "3-point" type of starter of a d.c. series motor, the "holding coil" for holding the starter handle in the "ON" stud is connected in such a manner that is short-circuited when the "over load" relay picks up.
Reason (R):In a d.c series motor starter, to guard against :racing due to sudden large reduction of shaft-load, the "holding coil" is connected in series with the armature and the series field winding. [IES-2004]
a. Both A and R are true and R is the correct explanation of A
b. Both A and R are true but R is not the correct explanation of A
c.  A is true but R is false
d.  A is false but R is true

8. In a d.c compound characteristic, required for certain applications, may be obtained by connecting a variable resistance: [ IES-2005]
a. Across the series field
b. In series with the series field
c. In parallel with the shunt field
d. In series with shunt field

9. For constant supply voltage, what are the effects of inserting a series resistance in the field circuit of a. d.c shunt motor, on its speed and torque [IES-2006]
a. Speed will decrease and the torque will decrease
b. Speed will increase and the torque will increase 
c.  Speed will increase and the torque will decrease
d. Speed will decrease and the torque will increase 

10. A dc shunt generator is supplying a load of 1.8KW at 200V. Its armature and field resistance are 0.4ohm and 200ohm respectively. What is the generated emf? [IES-2007]
a. 190v   b. 196v   c. 204v    d. 210v

11. When is the mechanical power developed by a dc motor maximum [IES-2008]
a. Back emf is equal to applied voltage
b. Back emf is equal to zero
c.  Back emf is equal to half the applied voltage
d. None of the above

12. match List-I (Machine components) with List-II (Type of machine) and select the correct answer using the code given below the lists: [IES-2009]
List-I                               List -II
A. Amortisseuer winding 1. Squirrel          cage induction motor
B. Breather               2. D.C motor
C. End-Rings             3. Transformer
D. Commutator        4. Synchronous

a.  A-2, B-3, C-1, D-4
b.  A-4, B-3, C-1, D-2
c.   A-2, B-1, C-3, D-4
d.  A-4, B-1, C-3, D-2

13. A separately excited dc motor is started using a 3-phase ac/dc controlled rectifier using 'Soft starting'. For limiting the starting current,it is required that firing angle should [IES-2010]
a. Gradually increased from 0 degrees to 180 degrees.
b. Fixed at 30 degrees.
c. Gradually decreased from 180 degrees to 30 degrees.
d. Gradually increased from 30 degrees to 180 degrees.

14. How many segments will be there for the commutator of a 6 pole d.c machine having a simple wave wound armature with 72 slots?  [IES-2011]
a. 73    b. 72     c. 71     d. 70

15. In a dc machine running with a heavy load, and with the brushes located on the geometrical neutral axis sparking occurs at the brushes during commutation, because of
[IES -2012 ]
a. the high current density at the coil ends
b. the centrifugal force exerted on the brushes by the commutator
c.  the shifting of the magnetic neutral axis.
d. the reduced main field flux -density in the inter-pole regions.

16. At 1200 rpm the induced emf of a dc machine is 200V. For an armature current of 15 A the electromagnetic torque produced would be  [IES -2013]
a. 23.8 N-m                b. 238N-m 
c. 2000 N-m               d. 3000 N-m

Answers:
1.D                        11.           
2.                           12.
3.                           13.
4.A                          14.
5.                           15.
6.B                          16.A
7.                          
8.
9.
10.B

Explanations :

4. V = 220, Eb = 200, Ra = 0.5
    Eb = V - (Ia*Ra)

6. Eb = V - (Ia*Ra)

10.line current = output/voltage
Line current = 1800/200 = 9 A
Shunt current = voltage/shunt 'R'
Shunt current ( Ish) =200/200=1 A
Ia = Ish + line current
Ia = 10 A
Eg = V-(Ia*Ra)=200-(0.4*10)=196 V

17. Eb = [N*flux*Z*(P/A)]/60...(1)
Ta = 0.159*flux*Z* (P/A)*Ia.....(2)

From 1 & 2
Ta = 0.159*60*Ia*(Eb/N) equal to
Ta = 9.55*Ia*(Eb/N)
By equating the values....
We get,torque of  23.8 N-m.

Electrical Machines - DC Machines

This are  some questions which appeared in IAS Public exam.

1. Assertion (A) : Hopkinson's test is conducted on dc machines. Reason (R) : It is a load test. [IAS-1993]
a. Both A and R are true and R is the correct explanation of A
b. Both A and R are true but R is not the correct explanation of A
c. A is true but R is false
d. A is false but R is true

2. A separately excited dc machine, having an armature resistance of 2 ohms was working on a 220 V supply and drawing 10 A armature current from the source when the supply voltage suddenly dropped to 200 V. Assuming that the field circuit source voltage remained unaffected how will the armature current of the machine react to the change? [IAS-1994]
a. It will initially rise to 11A and then settle down to 10A
b. It will fall momentarily to 9.09A and then slowly attain 10A
c. it will reduce to zero first and then settle back to 10A
d. It will remain unaffected by the change and continue to 10A

3. A dc shunt generator builds up to a voltage of 220 V at no-load while running at 18 rated speed If the speed of the generator is raised by 25% keeping the circuit conditions unaltered, then the voltage to which the machine would build up will [IAS-1995]
a. not change and remain at 220V b. Increases 1.25 times of 220V
c. increase to a value lying between 220 V and 1.25 times
d. Increases a value greater then 1.25 times of 220V

4. In a dc generator operating on load with its brushes on the geometrical neutral axis (GNA), the magnetic neutral axis (MNA) is shifted in the direction of rotation Now, if the brushes are given a lead of (electrical), then the MNA  will [IAS-1996]
a. shift forward by 90 degrees
b. shift forward by more than 90 degrees
c. shift forward by less than 90 degrees
d. coincide with the GNA

5. A synchronous machine connected to infinite bus bars is initially delivering active power to and absorbing lagging reactive power from the bus bars. As increase in the power input to the synchronous machine will cause [IAS-1997]
a. an increase in both armature current and power factor
b. an increase in armature current and a decrease in power factor
c.  a decrease in both armature current and power factor
d. a decrease in armature current and an increase in power factor

6. A shunt motor is running at its rated speed on rated load. If the field circuit gets suddenly opened.
[IAS-1998]
a. torque developed by the motor will be almost zero and the motor will stop without causing any damage to the motor
b. the motor will draw very high armature current but it will continue to drive the load at a reduced speed
c. the motor would tend to race to a high speed while driving the load
d. the motor would draw very high armature current and will stall with consequent damage to the armature. 

7. A dc shunt motor is required to drive a constant power load at rated speed while drawing rated armature current. Neglecting saturation and all machine losses, if both the terminal voltage and the field current of the machine are halved, then  [IAS-1999]
a. the speed becomes 2 pu (per unit) but armature current remains at 1 pu
b. the speed remains at 1pu but armature current becomes 2 pu
c. both speed and armature current becomes 2 pu
d. both speed and armature current remain at 1 pu

8. Which one of the following statements for a dc machine which is provided with interpole windings (IW) as well as compensating winding (CW) is correct ? [IAS-2000]
a. Both IW and CW are connected in series with the armature winding
b. Both IW and CW are connected in parallel with the armature winding
c.  IW is connected in series but CW is connected in parallel with the armature winding
d.  IW is connected in parallel but CW is connected in series with the armature winding

9. Field winding of a 4-pole separately excited d.c. machine is excited. from a 50 Hz supply. Armature is rotated in at 1500 rpm and the brush-rocker is also rotated in the same direction at the same speed. Voltage across the brushes is  [IAS-2002]
a 50 HZ a.c
b. 100 HZ a.c
c. 50 HZ a.c with d.c component
d. Zero

10. Assertion (A) : A series-generator is used as a constant-current source by making use of its drooping part of the characteristic. Reason (R) : For special applications a series DC generator is designed to have a low value of armature reaction which produces the dropping part of its external characteristic. [IAS-2004]
a. Both A and R are true and R is the correct explanation of A
b. Both A and R are true but R is not the correct explanation of A
c. A is true but R is false
d. A is false but R is true

11. To present the shifting of the magnetic neutral axis, caused by the armature reaction in a d.c machine, the most effective method. to neutralize the armature-flux is to [IAS-2005]
a. shift the 'brush-axis'
b. provide high-reluctance main pole tips
c. cut horizontal slots in the main poles
d. place compensating windings on the main-pole faces

Answers:
1.D                        11.           
2.                           12.
3.                           13.
4.                           14.
5.                           15.
6.                           16.
7.                           17.A
8.
9.
10.

Explanations :