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Code: AE10 Subject: ELECTRICAL ENGINEERING PART - I
TYPICAL QUESTIONS & ANSWERS
OBJECTIVE TYPE QUESTIONS
Q.1 The two windings of a transformer is
(A) conductively linked. (B) inductively linked. (C) not linked at all. (D) electrically linked.
Ans : B
Q.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.
Ans: B
Q.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.
Ans: A
Q.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.
Ans: C
Q.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.
Ans: A
Q.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.
Ans: A
Q.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.
Ans: A
Q.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. Ans: D
Q.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.
(C) A triangular wave. (D) A pulse wave.
Ans: A
Q.15 The maximum power in cylindrical and salient pole machines is obtained respectively at load angles of
(A) 90 o^ , 90 o. (B) < 90 o^ , 90 o.
(C) 90 o ,> 90 o. (D) 90 o ,< 90 o.
Ans: D
Q.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. Ans: C
Q.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.
Ans: C
Q.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.
Ans: all options are correct
Q.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. Ans: C
Q.20 A motor which can conveniently be operated at lagging as well as leading power factors is the
(A) squirrel cage induction motor. (B) wound rotor induction motor.
(C) synchronous motor. (D) DC shunt motor.
Ans: C
Q.21 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.
Ans: B
Q.22 The most suitable servomotor for low power applications is
(A) a dc series motor.
(B) a dc shunt motor.
(C) an ac two-phase induction motor.
(D) an ac series motor.
Ans: B
Q.23 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.
Ans: C
Q.24 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
Ans: C
Q.25 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%.
Ans: C
% R = Vr cos Φ + Vx sin Φ = Vr p.f = cos Φ =1 ∴ Φ =0^0 ∴ kVA = kW & kVAR =
Q.29 A ceiling fan uses
(A) split-phase motor.
(B) capacitor start and capacitor run motor.
(C) universal motor.
(D) capacitor start motor.
Ans: D
To give starting torque and to maintain speed.
Q.30 A stepper motor is
( A ) a dc motor. (B) a single-phase ac motor.
(C) a multi-phase motor. (D) a two phase motor.
Ans: D
Stepper motor works on 1-phase-ON or 2-phase –ON modes of operation
Q.31 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.
Ans: A
The sheath in underground cable is provided to give mechanical strength.
Q.32 The drive motor used in a mixer-grinder is a
(A) dc motor. (B) induction motor.
(C) synchronous motor. (D) universal motor.
Ans: D
The universal motor is suitable for AC & DC both supply systems.
Q.33 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.
Ans: A
I 1 = V 1 /R 1 = 120/600 = 0.2 (η = 100%, losses are zero ∴V 1 = VR = I 1 R 1 )
Q.34 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.
Ans: C
Ng= Eg (60A / Φpz) Eg = V + Ia Ra ; in generator
Nm= Eb (60A / Φpz) Eb = V - Ia Ra ; in motor
Eg > E (^) b for same terminal voltage Therefore, Ng > N (^) m
Q.35 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.
Ans: A
N= Ns (1-S) = NS –NS x S 1440 = Ns (1-S) Ns = 1440 / (1-S) Ns = (120 f/ p) = 120 x 50/p = 6000 p Ns will be closer to N i.e 1440 When P=2 ; Ns = 3000 rpm , not close to N When P=4 ; Ns = 1500 rpm , it is closer to N Therefore P =4 for N=
Q.40 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.
Ans: A Size of conductor depends upon amount of current flow.
Q. 41 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
Ans: B
The boundary of the protective zone is determined by the sensitivity of relay used. If the relay is more sensitive, the protective zone will be increased.
Q.42 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 -30°. (B) star side lags by -30°.
(C) delta side leads by 30°. (D) star side leads by -30°.
Ans: C
This is vector group and has +30° displacement. Therefore, delta side leads by +30°.
Q.43 To achieve low PT error, the burden value should be ____________.
(A) low (B) high
(C) medium (D) none of the above
Ans: A
In a Potential transformer, burden should be in permissible range to maintain errorless measurement.
Q.44 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.
Ans: C Given : s = 0.02 ; f = 50 Hz Therefore, frequency of rotor induced emf = s f = 0.02 x 50 = 1.0 Hz
Q.45 A 4 pole lap wound dc shunt motor rotates at the speed of 1500 rpm, has a flux of 0. 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.
Ans: D Given N = 1500 rpm, Φ = 0.4 mWb, Z = 1000, P = 4, & A= 4 Therefore, Eb = NΦPZ / 60 A = 1500 x 0.4 x 4 x 1000 x 10-3^ / 60 x 4 = 60/6 = 10 volts
Q.46 The synchronous reactance of the synchronous machine is ______________.
(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
Ans. A The Synchronous reactance of a synchronous machine is a total steady state reactance, presented to applied voltage, when rotor is running synchronously without excitation.
Therefore , XS = Ef / IS = Emf of OC for same If / short circuit current
Q.47 A 3 stack stepper motor with 12 numbers of rotor teeth has a step angle of ____________.
(A) 12 ° (B) 8 °
(C) 24 ° (D) 10 ° Ans. D Given m = 3, Nr = 12 Step angle = 360 / m x Nr = 360 /3 x 12 = 10°
Ans: D Polarity test is required for parallel operation of transformers to know the direction of current flow in secondary circuit w.r.t primary circuit.
Q.53 The short-circuit ratio of a typical synchronous machine is obtained from the OCC and SCC curves of Fig.2 as
(A) ob
oa
(B)
ob
o a ′
(C)
ob
oa ′
(D) ob
o c ′
Ans: B As shown in SCC curve the ratio of two field currents
Q.54 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
Ans: A Universal motor has same characteristics as DC series motor and also known as an a.c series motor.
Q. 55 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
Ans: D Given: N=1000 rpm ; P= 6; s= 0.04; and f = NP/ 120 = 10006/ = 50 Hz Rotor frequency fr=sf = 0.04 50 = 2.0 Hz
Q.56 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
Ans: C
If torque is zero then speed may exceed up to infinite, that is dangerous for machine and machine can be damaged. N α 1/ T
Q.57 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
Ans: D
Magnetic property of alloy changes with change of the temperature and Heat is produced due to eddy current = i^2 R and i α f^2
Q.58 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
Ans: C
Xc= 1/ ωc
Q.59 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.
Ans: D
This is a special purpose machine whose stator coil can be energized by electronically switched current.
Q. 64 A commutator in a d.c. machine
(A) Reduces power loss in armature. (B) Reduces power loss in field circuit. (C) Converts the induced a.c armature voltage into direct voltage. (D) Is not necessary.
Ans: C As name suggests, it commutes ac into dc.
Q.65 The speed of a d.c. shunt motor at no-load is
(A) 5 to 10% (B) 15 to 20% (C) 25 to 30% (D) 35 to 40% higher than its speed at rated load. Ans: A Ta α Φ Ia ,, Φ = constant, ∴T α Ia N α Eb / Φ or N α Eb initially Eb less , so speed is less.
Q.66 The efficiency of a transformer is mainly dependent on
(A) core losses. (B) copper losses. (C) stray losses. (D) dielectric losses. Ans: A Core loss has prominent value over other losses
Q.67 When two transformers are operating in parallel, they will share the load as under:
(A) proportional to their impedances. (B) inversely proportional to their impedances. (C) 50% - 50% (D) 25%-75% Ans: A High rating transformer has higher impedance. kVA rating α Impedance of transformer
Q.68 If the voltage is reduced to half, the torque developed by an induction motor will be reduced to
(A) 4
of original torque. (B) 2
of original torque.
(C)
of original torque. (D) 16
of original torque.
Ans: B Tg α V or Tg α Pm (rotor gross output)
Q.69 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 (A) 1500 rpm (B) 1470 rpm (C) 1530 rpm (D) 1570 rpm Ans: B N = NS (1-S) and NS =120 f / p =120 x 50 /4 = 1500 rpm ∴N= 1500 (1-0.02) =1470 rpm
Q.70 If the phase angle of the voltage coil of a directional relay is 50 , the maximumo torque angle of the relay is
(A) 130 o (B) 100 o
(C) 50 o (D) 25 o Ans: C Torque α Power Power α Voltage Therefore, It has same angle as ‘V’ has.
Q.71 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
Ans: A Line capacity is determined by power of line P = (V^2 /R) or (V^2 /Z) when cos Φ =
Q.72 A 220/440 V, 50 Hz, 5 KVA, single phase transformer operates on 220V, 40Hz supply with secondary winding open circuited. Then
(A) Both eddy current and hysteresis losses decreases.
(B) Both eddy current and hysteresis losses increases.
(C) Eddy current loss remains the same but hysteresis loss increases.
(D) Eddy current loss increases but hysteresis loss remains the same.
Ans: A Wh = khfBm1.6^ and We = kef^2 Bm^2 .k Therefore, hysteresis and eddy current losses will be decreased when frequency decreases.
Ans: B Rotor copper losses = rotor input- rotor output and output = (1-s) input ∴ Input = output/(1-s) = 15000 /1-0.04 = 15625 ∴ loss = 15625 -1500 = 625 watt
Q.77 The direction of rotation of hysteresis motor is reversed by
(A) Shift shaded pole with respect to main pole (B) Reversing supply lead (C) Either A or B (D) Neither A nor B
Ans: A This motor used single phase, 50Hz supply and stator has two windings. These are connected continuously from starting to running.
Q.78 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
(A) 40 (B) 50
(C) 100 (D) 80
Ans: B Step angle ‘β’ = NS – Nr / NS Nr x 360^0 ∴ 1-8 = -40 + Nr/40 Nr x 360^0 Nr = 50
Q.79 Low head plants generally use
(A) Pelton Turbines (B) Francis Turbine
(C) Pelton or Francis Turbine (D) Kaplan Turbines
Ans: A In the hysterisis motor, the direction of rotation can be reversed by shifting the shaded pole region with respect to main pole. But not by changing supply lead because it has ac supply.
Q.80 The charging reactance of 50 Km length of line is 1500Ω. The charging reactance for 100Km length of line will be
(A) 1500 Ω (B) 3000 Ω
(C) 750 Ω (D) 600 Ω
Ans: B Characteristic reactance per km = 1500/50 = 30 ohms ∴ Characteristic reactance per 100km = 30 x 100 = 3000 ohms
Q.81 Electric ovens using heating elements of _______________ can produce temperature upto 3000°C.
(A) Nickel (B) Graphite
(C) Chromium (D) Iron
Ans: C
Chromium has high melting point.
Q.82 In DC generators, armature reaction is produced actually by
(A) Its field current. (B) Armature conductors. (C) Field pole winding. (D) Load current in armature.
Ans: D Because load current in armature gives rise to armature mmf which react with main field mmf.
Q.83 Two transformers operating in parallel will share the load depending upon their
(A) Rating. (B) Leakage reactance. (C) Efficiency. (D) Per-unit impedance.
Ans: A Transformers having higher kVA rating will share more load.
Q.84 As compared to shunt and compound DC motors, the series DC motor will have the highest torque because of its comparatively ____________ at the start. (A) Lower armature resistance. (B) Stronger series field. (C) Fewer series turns. (D) Larger armature current.
Ans: D T α Φ Ia (before saturation) Φ α Ia T α Ia 2
Q.85 A 400kW, 3-phase, 440V, 50Hz induction motor has a speed of 950 r.p.m. on full- load. The machine has 6 poles. The slip of the machine will be _______________. (A) 0.06 (B) 0.
(C) 0.04 (D) 0.
