Institute of Engineering Studies (IES,Bangalore) Leading Institute for GATE/IES/JTO/PSUs GATE/IES/JT O/PSUs in (Branches (B ranches @ Jayanagar & Malleshwaram) Bangalore. Contact: (+91) 99003 99699 E-Mail:
[email protected] Site: www.onlineies.com
Marks: 0.00 1
User Answer Sheet
If an ideal centre-tapped centre-tap ped 1: 1 : 4 transformer transformer is is load loaded ed as shown in in the the figu figure, re, the im impedance peda nce measured measured across acr oss the termi terminals nals AA would be
Answer: A
A)
3Z /64
B)
3Z/18
C)
2Z/3
D)
Z/6 Report Question Answer Explanation :
Equivalent secondary impedance refereed to primary=
2
Rank: 7
Zero sequence currents can flow flow from a line line into into a transformer transformer bank if if the windi winding ngss are in Answer: A
A)
Grounded Grounded star/delta
B)
Delta/star Delta/star
C)
Star/grounded star
D)
Delta/delta Delta/de lta Report Question Answer Explanation :
From zero sequence network , for Ground star / delta zero sequence current can flow line into t/F bank . 3
Hysteresi Hysteresiss loss loss of a transf transform ormer er design designed ed at voltag voltagee 'V' and frequen frequency cy 'f' 'f' is is . If the the transf transform ormer er is is operated from from a voltage 'xV' and frequency 'xf' the hysteresis loss for for a Stein Ste inmetz metz constant consta nt of 2 wil will be Answer: B
A) B) C) D) Report Question Answer Explanation :
4
If the the mai main n fl flux ' ' and the the comm commutati utatin ng fl flux ' ' are consi considered dered togeth together er in in parts parts A and and B of a dc as shown shown in th the giv given figure, then the condition obtained is that
Answer: C
A)
increases ncrea ses the densi de nsity ty in A, but decre de crease asess the densi de nsity ty in B
B)
decrea dec reases ses the density in A, but incre increase asess the density in B
C)
increases ncrea ses the densi de nsity ty in both bo th A and B
D)
decrea dec reases ses the density in both bo th A and B Report Question Answer Explanation :
From the concept of the inter poles 5
In o dc machi machine, the the demagnetiz demagnetiziing effect effect of armature armature reaction is due to Answer: C
A)
compone co mponent nt of armature armature mmf mmf along field field axis
B)
non-sin non-s inusoidal usoidal nature of armature mmf mmf
C)
magnetic satura saturation tion in half of the field field pole po le
D)
uneven air gap length Report Question Answer Explanation :
Demagnetizing effect of armature reaction is mainly because of saturation of field poles at pole tip 6
The The foll following owing lists relate to speed spee d control co ntrol of dc motors. Match List List I with with Li List II, and select the correct correc t answer using using the codes given below the lists : List I List II A. Armature Armature voltage voltage control 1 Speeds Spe eds above base speed B. Field Field current control 2. Speeds Spe eds below base speed C. Use of diverter diverter resistance resistance 3. Poor Poo r motor effi efficiency ciency D. Rheostatic Rheostatic control 4. Speed Sp eed control of series motor Codes : A B C D Answer: B
A)
1
2
3
4
B)
2
1
C)
4
1
3
2
D)
2
3
1
4
4
3
Report Question Answer Explanation :
above rated speed field current control below elow base speed armatu armature re vol voltt control control Rhoestatic Rhoesta tic control co ntrol poor po or motor effi efficiency (loss component compo nent incre increase ase ) series motor control can be done by use of diverter resistance 7
The The given given figu figure re shows the the equival equivalent ent circui circuitt of a 3-phase 3- phase inducti induction on motor motor with with deepdeep - bar rotor. The sequence sequence of the the magnitu magnitude dess of the parameters para meters of resistances and reactances reac tances in the ascending order ord er is
Answer: B
A) B) C) D) Report Question Answer Explanation :
will will be greater greate r than
8
In an inducti nduction on motor, motor, if the the air air gap is is increased increased Answer: C
A)
speed spe ed will will reduce red uce
B)
efficiency efficiency will will impr improve ove
C)
power facTor facTor wil will be lowered
D)
breakdo brea kdown wn torque wil will reduce Report Question Answer Explanation :
As air-gap in induction motor increases then magnetizing current required by the induction motor to maintain as constant is increases . Hence power-factor angle increases then power factor become poor. 9
Duri During ng hunt huntiing of synch synchronou ronouss motor motor Answer: C
A)
negativ negativee phase sequence currents are
B)
harmonics harmonics are devel de velop oped ed in the armature
C)
damper bar develops torque
D)
field field excitation incre increase asess Report Question Answer Explanation :
During sudden change in load, supply, field and load containing harmonic torques, the synchronous machine starts oscillating. So as to reduce (or) suppress these oscillator, damper bar develops a torque which known as damping torque. 10 Fig shows shows the magneti magnetizati zation on curves of an alternator alternator at rated armatu
urrent, urrent, unity unity power factor and also at no load. oad . The
magnetization curve for rated armature current, 0.8 power factor leading is given by
Answer: D
A)
curve A
B)
curve B
C)
curve C
D)
curve D Report Question Answer Explanation :
For a given given rated current V
are shown
Hence curve D 11 Two Two 10KV/440V, 10 KV/440V, 1-p 1 -phase hase transf transform ormers ers of rating ratingss 600 KVA and 350K VA are connected connected in parall parallel to to share a load of 800KVA. The The reactan reactance ce of the the transf transform ormers, ers, referred referred to th the secondary secondary side side are 0.0198 and and 0.0304 respecti respectivel vely (resistance negligible). The load share by the two transformers will be , respectively Answer: A A)
484.5KVA 484.5 KVA and 315.5K VA
B)
315.5K VA and 484.5K VA
C)
533 KVA and 267KVA 267K VA
D)
267 KVA and 533 KVA Report Question Answer Explanation :
Load shared by transformer transformer L is
= 484.5 kvA Load shared by transformer transformer is
= 315 .5 kVA 12 If the secondar sec ondary y windin winding g of the ideal transformer of o f the circuit circuit shown in the the given given figu figure re has 40 turns, then for maximu maximum m power tran transf sfer er to th the 2-ohm resistor resistor,, the the nu number of turn turnss requi required in in the the prim primary win windin ding wil will be
Answer: C
A)
20
B)
40
C)
80
D)
120 Report Question Answer Explanation :
secondary no of turns =40 let 'K' be the transformer ratio :- K =second =se condary ary no no of turns/primary turns/primary no no of turns equivalent circuit of transformer when secondary is transformed to the primary side
in order to transformer max power to the load
prim rimary no of turn turnss =80 13
A two-winding transformer (200 V/120 V)has a load of 2.4 k VA as shown in Figure-I. The windings of transformers are reconnected as an auto transformer shown in Figure- II. Assuming the same current rating for both the windings as before efore and and neg negllecting ecting the the losses losses,, the the k VA rating rating of the the auto auto tran transf sform ormer er wil will be Answer: A A)
6.4 kVA
B)
4.0 kVA
C)
3.84 kVA
D)
2.4 kVA Report Question Answer Explanation :
g iven iven K=200/320= K=2 00/320=10/16 10/16 we know that
14 Assertion Asser tion (A) :The :The exciting exciting current of a singl singlee phase p hase transf tra nsformer ormer with sinu sinusoidal soidal voltage supply iiss non-sin non- sinusoidal. usoidal. Reason (R) : Eddy currents induced in the transformer core are non-sinusoidal introducing harmonics of higher order in the exciting current. Answer: C
A)
Both A and R are indiv individual idually ly true and R is the correc cor rectt explanation explanation of A.
B)
Both A and R are indiv individual idually ly true and R is not the correc cor rectt explanation explanation of A.
C)
A is true but R is false. false.
D)
A is false false but R is true. Report Question Answer Explanation :
in transformer excitation current is non- sinusoidal even though when it is excited from sinusoidal voltage source because of saturation and hysteresis non- linearity and eddy current losses has no effect on excitation current 15 Consi Co nsider der the followin ollowing g types of drives drives : 1.Slip energy recovery schemes with converter cascade in the rotor circuit. 2.Stator voltage control using ac voltage controller. 3.Co 3. Constant nstant V/f control contro l (V =appli =app lied ed voltage, f= frequency). 4.Variable 4.Variable voltage voltage and variable frequency schemes with with airair-gap gap flux flux constant. consta nt. Those suitab suitable le for blowers and centrifug centrifugal al pumps ( ) include Answer: B A)
1, 3 and 4
B)
1,2and4
C)
1. 2 and 3
D)
2, 3 and 4 Report Question Answer Explanation :
The power drives like fans,blowers,and certifugal pumps required low starting torque and load torque(T) Propotional to the square of speed ( ) Therefore Therefore dri d rives ves that are compatiable compatiable are, 1.stator voltage control 2.slip energy recovery schemes 3.variable voltage and variable schemes. 16 Match List List I (m ( motors) with with List List II (appl (app lications) and select the correct corre ct answer using using the codes given given below below the lists lists List I List List II A.1-phase A.1- phase induction nduction 1.Fast1.F ast-actin acting g motor servo drive B.Step B.S tepper per'' motor 2.Hand 2.Ha nd drill drilling ing machine C.Uni C. Universal versal motor 3.Computer 3.C omputer numerical numerical control co ntrol D.Brushless D.Brushless 4.Domestic 4. Domestic pump dc motor Answer: C
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 C- 2
D-1 Report Question
Answer Explanation :
from the applications of motors 17 Assume Assume that the magn magnetisati etisation on curve, curve, the rated rate d values values of o f voltag voltage, e, arm a rmature ature current, speed and torque torq ue are same for the following motors:
A.Series motor B.Shunt motor C.Cumulative compound motor D.Differenti D.Differential al compound co mpound motor moto r Match the type of motor with corresponding speed-current characteristics shown In the above figure and select the correct answer using the codes given below : Answer: B A)
A-II
B-III
B)
A-I
B-III
C-II C- II
D-IV
C)
A- I
B-IV
C- II
D-III
D)
A-II
C-I C- I
D-III
B- IV
C- I
D-IV
Report Question Answer Explanation :
From the speed torque characteristics. series motor has more constant cumulati cumulative ve compound co mpound motor characteristics charac teristics in between between shunt shunt and series 18 A separately excited excited dc motor isfed isfed from from a phase controll controlled converter. The The motor motor exhibits exhibits poor poo r speed regulati regulation. on. this this
is due to Answer: A
A)
inductive inductive voltage drop dro p of the conv co nverte erterr and possible po ssible discontinuous discontinuous conduction conduction
B)
harmonics harmonics in the armature current
C)
large inductance inductance of the load current
D)
poor poo r power factor of the converter Report Question Answer Explanation :
the converter produce discontinuous conduction &connecter offers more inductance due to this there will be voltage drop in converter 19 Match List-I(DC motor) with List-II(Characteristic) and select the correct answer using the code given below the lists: List- I A. Cumulatively compound motor B. Different compound motor C. Series motor D. Shunt motor List-II 1.Fairly constant speed irrespective of the load 2. It may start in reverse direction 3. Definite no-load speed 4. Is never started without load
Answer: B
A)
A-4, B-1, C-3, C- 3, D-2
B)
A-3, B-2, C-4, C- 4, D-1
C)
A-4, B-2, C-3, C- 3, D-1
D)
A-3, B-1, C-4, C- 4, D-2 Report Question Answer Explanation :
A) Cumulati Cumulativel vely y compound co mpound motor From speed torque characteristic cumulatively compound motor runs at define no load speed .
B) Different compound motor
At starting series field dominates the shunt field , as a result , motor runs in reverse direction C) Series Motor At No load , At no loa load d series runs dang da ngero erousl usly y high high speed spe ed , and it may damage machin machinee series motor always starts star ts and operate ope ratess under under load D)Shunt D)Shunt Motor Mo tor
As load oa d increases ncrea ses numera numerator tor increases ncrea ses but due to armature reaction rea ction demag de magnetiz netizee the filed filed Here speed spe ed incre increase asess with increases in load speed fall sligthly from no load speed. 20 A 6-pole, 6- pole, 50Hz 50 Hz,, 3- phase synchronous synchronous motor motor and a nd an 8-pole, 8-p ole, 50 Hz, 3- phase slipslip- ring ring inducti induction on motor motor are a re mechani mechanically cally coupled and operate on the same 3-phase, 50 Hz supply system. If they are left open- circuited, then the frequency of the voltage produced across any two slip rings would be Answer: A A)
12.5 Hz
B)
25.0 Hz
C)
37.5 Hz
D)
50.0 50. 0 Hz Report Question Answer Explanation :
when both synchronous motor and induction motor are connected to 50 Hz supply speed of rotating field produced by synchronous motor
p=8
rotating field produced by induction motor
when both the motors are mechanically coupled,then both will rotate at same speed and synchronous motor at 750 rpm induction motor slip = frequency of voltage produced across any two slip ring would be the slip frequency =sf =1/4×50=12.5 Hz 21 Assertion A: In a sali salient-pole ent-p ole synch synchronous ronous motor, motor, the angu angullar displacement displacement of rotor per unit unit change change of power is less less
than that for a similar cylindrical rotor machine. Reason R: A reluctance torque is developed in the salient- pole machine due to the effect of direct axis flux on the damper da mper winding winding.. Answer: A A)
Both A and Rare true and R is the correc cor rectt explanation explanation of A.
B)
Both A and R are true but R is N OT a correc cor rectt explanation explanation of A
C)
A is true but R is false false
D)
A is false false but R is true Report Question Answer Explanation :
Due to salient salient poles an extra e xtra power[(20 powe r[(20 to 25%) 2 5%) of total power] developed called called Reluctance Reluctance power" power "in the salient salient pole synchronous motor,which makes machine more stable. therefore For per unit change of load, angular displacement of rotor is less in salient pole motor compared to cylindrical motor 22 A 3-phase 3- phase alternator alternator delivers delivers power to a balanced 3-phase 3- phase load load of o f power factor 0.707 0.7 07 laggi lagging ng.. It is is observed obser ved that that the open-circuit emf phasor leads the corresponding terminal voltage phasor by Neglecti eglecting ng the the eff effect of harmoni armonics, cs, th the angle betwee bet ween n the axis of the main main field field mmf mmf and the the axis of the armature armat ure mmf mmf will will be Answer: D A) B)
electrical electrical
C)
electrical
D)
electrical Report Question Answer Explanation :
angle between armature mmf and main fields mmf in alternator is given by = where given load angle
:- required angle 23 The fif fifth th harmonic harmonic component of the induced induced voltage in a 3- phase ac generator genera tor coli c oli,, be b e entirely elimi eliminated nated by b y usin using ga winding pitch of Answer: B A)
2/3
B)
4/5
C)
5/6
D)
6/7 Report Question Answer Explanation :
coil span to eli eliminated n harmoni harmonicc is is
for 5
harm armoni onic
24 which which one of o f the the following following motors runnin running g unde underr the specifi sp ecified ed conditi co nditions ons will will have have a leadin ead ing g power powe r factor? facto r? Answer: C A)
Slip Slip ring ring induction induction motor at no-load no- load
B)
Under excited synchronous motor
C)
Over excited excited synchronous synchronous motor
D)
Double-cage Double- cage induction induction motor at load Report Question Answer Explanation :
slip ring induction motor at no load →lag p.f under excited synchronous motor → lag p.f double - cage induction motor at load → lag p.f but over excited synchnorous motor operates at leading p.f and deliver reactive power 25 Match Matc h List List I with List List II and select the correct corre ct answer using using the codes code s giv given en below the lists lists : List List I A. Rolling Rolling mill mill driv drivee 1. Adjustab Adj ustable le spee speed d drive B. Power drill drill drive drive 2. Variable Variable speed spee d drive C. Hoist drive drive 3.Constant 3.C onstant speed drive drive Answer: C
A)
A- 1
B-2
B)
A- 2
C)
A- 3
B-1
D)
A- 1
B-2
B-3
C-3 C- 3 C-1 C- 1 C-2 C-1 C- 1 Report Question
List List II
Answer Explanation :
option 3 current 26 Startin Sta rting g torque torq ue can be b e obta o btain ined ed in the case cas e of o f a singl singlee phase phase induction induction motor with identical identical main main and auxili auxiliary ary winding windingss by Answer: C connecting A) a capacitor capa citor across acros s the mainS ainS B)
a capacitor in series with the machine
C)
a capa ca pacitor citor in series ser ies with the auxiliary auxiliary winding winding
D)
the main and the auxiliary auxiliary windings windings in series series Report Question Answer Explanation :
In order to produce a torque,there should be a time phase displacement between auxilary winding current and main winding current. This can be obtained by putting a suitable capacitor in series with the auxilary winding Common Data for Q27 and Q28 is given below A 1000 kVA , 6.6 kV, 3-phase star connected cylindrical pole synchronous generator has a synchronous reactance of 20 O. Neglect the armature resistance and consider operation at full load and unity power factor
27 The induced emf (line-to-line) is close to
Answer: B
A) 5.5 kV
B)
7.2 kV
C)
9.6 kV
D)
12.5 kV Report Question Answer Explanation :
From Fro m Phasor diagram diagram [ if resistance neglecte neglected d]
28 The power(or torque) angle is close to Answer: C
A) B) C) D) Report Question Answer Explanation :
From Fro m phasor di d iagram ,
Since ,
Data for Lin Linke ked d answer answer Q29 and Q30 Q30 is given give n be be low
29 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 Answer: D
A)
0.8 pu and
lag
B)
0.8 pu and
lead
C)
1.17 pu and
pu lead
D)
1.17 1.1 7 pu and
pu lag Report Question
Answer Explanation :
From phasor diagram of synchronous motor.
= 1.36 E = 1.17
30 Keepin Ke eping g the the excitation excitation voltage voltage same same , the load on the motor motor is increased increased such that that the motor motor current c urrent increased increased by b y 20 %. The operating power factor will become Answer: A A)
0.99 0. 995 5 laggi lagging ng
B)
0.995 0.99 5 leading
C)
0.79 0. 791 1 laggi lagging ng
D)
0.848 0.8 48 leading Report Question Answer Explanation :
With increase ncrea se in load oa d , From cosine rule
increased ncrea sed and phasor phaso r diagram is illu illustra strated ted below and
= 1.2 1. 2 x0.6 = 0.72 0. 72
Cosine Cosine Rule Rule :
For Free Online tests/Materials Register at www.gateiespsu.com & Site: www.onlineies.com For Regul Regular udpates: www w ww .facebook.com/onli .fac ebook.com/onlinei neies es Groups: http://groups.google.c http://groups.google.com/group/onl om/group/onlin inei eies es Contact: (+91) 99003 99699 E-mail:
[email protected]
