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
I1= V1 /R1 = 120/600 = 0.2 (η = 100%, losses are zero ∴V1 = VR = I1R1)
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=1440
Q. 36 In a 3-phase synchronous motor
(A) the speed of stator MMF is always more than that of rotor MMF.
(B) the speed of stator MMF is always less than that of rotor MMF.
(C) the speed of stator MMF is synchronous speed while that of rotor MMF is zero. (D) rotor and stator MMF are stationary with respect to each other.
Ans: D
Because, Motor is magnetically locked into position with stator, the rotor poles are engaged with stator poles and both run synchronously in same direction Therefore, rotor & stator mmf are stationary w.r.t each other.
Q.37 In a capacitor start single-phase induction motor, the capacitor is connected
(A) in series with main winding. (B) in series with auxiliary winding.
(C) in series with both the windings. (D) in parallel with auxiliary winding.
Ans: B
To make single phase motor self start. We split the phases at 90 degree. Hence, motor behaves like a two phase motor.
Q.38 A synchro has
(A) a 3-phase winding on rotor and a single-phase winding on stator.
(B) a 3-phase winding on stator and a commutator winding on rotor.
(C) a 3-phase winding on stator and a single-phase winding on rotor.
(D) a single-phase winding on stator and a commutator winding on rotor.
Ans: C
Synchros : The basic synchro unit called a synchro transmitter. It’s construction similar to that of a Three phase alternator.
Q.39 As the voltage of transmission increases, the volume of conductor
(A) increases. (B) does not change.
(C) decreases. (D) increases proportionately.
Ans: C
Decreases due to skin effect.
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 0.4 mWb and the total number of conductors are 1000. What is the value of emf?
(A) 100 Volts. (B) 0.1 Volts. (C) 1 Volts. (D) 10 Volts.
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°
Q.48 In case of a universal motor, torque pulsation is minimized by _________. (A) load inertia (B) rotor inertia (C) both rotor and load inertia (D) none of the above Ans: C In a universal motor, torque pulsation is minimized by rotor and load inertia.
Q.49 Oil-filled cable has a working stress of __________ kV/mm (A) 10 (B) 12 (C) 13 (D) 15 Ans: D This is defined by dielectric strength of mineral oil i.e. 15 kV/mm.
Q.50 Inverse definite minimum time lag relay is also called ___________ (A) pilot relay. (B) differential relay. (C) over current relay. (D) directional overcurrent relay. Ans: B Inverse definite minimum time lag relay characteristic is inverse but minimum time is fixed. The operating time is inversely proportional to the magnitude of actuating quantity.
Q.51 Specific heat of nickel –chrome is _____________ (A) 0.112 (B) 0.106. (C) 0.108. (D) 0.110. Ans: None of these Specific heat of Nickel-Chrome is 440 J/kg°C to 450 J/kg°C
Q.52 The polarity test is not necessary for the single-phase transformer shown in Fig. 1 so as to correctly determine _____________of the transformer.
(A) shunt branch parameters. (B) transformation ratio.
(C) series parameters. (D) any of the above characteristics.
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 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. 54 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 = N
P/ 120 = 1000
6/120 = 50 Hz Rotor frequency fr=s f = 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 = i2R and i α f2
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.60 The torque, in a _____________is proportional to the square of the armature current
(A) DC shunt motor (B) stepper motor (C) 2-phase servomotor (D) DC series motor
Ans: D Ta α Φ.Ia and Φ α Ia ; therefore Ta α Ia2
Q.61 The synchronous speed for a 3 phase 6-pole induction motor is 1200 rpm. If the number of poles is now reduced to 4 with the frequency remaining constant, the rotor speed with a slip of 5% will be _________.
(A) 1690 rpm (B) 1750 rpm (C) 1500 rpm (D) 1710 rpm
Ans: D Given : Ns1 =1200 , P1= 6, P2 = 4, s = 0.05, Frequency f = Ns
P/120 = 120
6/120 = 60 Hz rotor frequency f/ = s.f = 0.05
60 = 3.0 Hz
Now, Ns2 = 120
60 /4 = 1800 and Ns – N = 120 f / P2 Therefore, N=Ns- 120 f / P2 = 1800-120
0.05
60/4 = 1800-90 = 1710
Q.62 The eddy current loss in an a-c electric motor is 100 watts at 50 Hz. Its loss at 100 Hz will be
(A) 25 watts (B) 59 watts (C) 100 watts (D) 400 watts
Ans: D Eddy current losses α f2 New loss α (2f)2 New loss α 4f2 ∴ 4 times
Q.63 The maximum power for a given excitation in a synchronous motor is developed when the power angle is equal to (A) 0o (B) 45o (C) 60o (D) 90o Ans: A P = VI cosΦ Pmax = VI ∴Φ = 00
(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
I1= V1 /R1 = 120/600 = 0.2 (η = 100%, losses are zero ∴V1 = VR = I1R1)
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=1440
Q. 36 In a 3-phase synchronous motor
(A) the speed of stator MMF is always more than that of rotor MMF.
(B) the speed of stator MMF is always less than that of rotor MMF.
(C) the speed of stator MMF is synchronous speed while that of rotor MMF is zero. (D) rotor and stator MMF are stationary with respect to each other.
Ans: D
Because, Motor is magnetically locked into position with stator, the rotor poles are engaged with stator poles and both run synchronously in same direction Therefore, rotor & stator mmf are stationary w.r.t each other.
Q.37 In a capacitor start single-phase induction motor, the capacitor is connected
(A) in series with main winding. (B) in series with auxiliary winding.
(C) in series with both the windings. (D) in parallel with auxiliary winding.
Ans: B
To make single phase motor self start. We split the phases at 90 degree. Hence, motor behaves like a two phase motor.
Q.38 A synchro has
(A) a 3-phase winding on rotor and a single-phase winding on stator.
(B) a 3-phase winding on stator and a commutator winding on rotor.
(C) a 3-phase winding on stator and a single-phase winding on rotor.
(D) a single-phase winding on stator and a commutator winding on rotor.
Ans: C
Synchros : The basic synchro unit called a synchro transmitter. It’s construction similar to that of a Three phase alternator.
Q.39 As the voltage of transmission increases, the volume of conductor
(A) increases. (B) does not change.
(C) decreases. (D) increases proportionately.
Ans: C
Decreases due to skin effect.
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 0.4 mWb and the total number of conductors are 1000. What is the value of emf?
(A) 100 Volts. (B) 0.1 Volts. (C) 1 Volts. (D) 10 Volts.
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°
Q.48 In case of a universal motor, torque pulsation is minimized by _________. (A) load inertia (B) rotor inertia (C) both rotor and load inertia (D) none of the above Ans: C In a universal motor, torque pulsation is minimized by rotor and load inertia.
Q.49 Oil-filled cable has a working stress of __________ kV/mm (A) 10 (B) 12 (C) 13 (D) 15 Ans: D This is defined by dielectric strength of mineral oil i.e. 15 kV/mm.
Q.50 Inverse definite minimum time lag relay is also called ___________ (A) pilot relay. (B) differential relay. (C) over current relay. (D) directional overcurrent relay. Ans: B Inverse definite minimum time lag relay characteristic is inverse but minimum time is fixed. The operating time is inversely proportional to the magnitude of actuating quantity.
Q.51 Specific heat of nickel –chrome is _____________ (A) 0.112 (B) 0.106. (C) 0.108. (D) 0.110. Ans: None of these Specific heat of Nickel-Chrome is 440 J/kg°C to 450 J/kg°C
Q.52 The polarity test is not necessary for the single-phase transformer shown in Fig. 1 so as to correctly determine _____________of the transformer.
(A) shunt branch parameters. (B) transformation ratio.
(C) series parameters. (D) any of the above characteristics.
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 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. 54 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 = N
P/ 120 = 1000
6/120 = 50 Hz Rotor frequency fr=s f = 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 = i2R and i α f2
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.60 The torque, in a _____________is proportional to the square of the armature current
(A) DC shunt motor (B) stepper motor (C) 2-phase servomotor (D) DC series motor
Ans: D Ta α Φ.Ia and Φ α Ia ; therefore Ta α Ia2
Q.61 The synchronous speed for a 3 phase 6-pole induction motor is 1200 rpm. If the number of poles is now reduced to 4 with the frequency remaining constant, the rotor speed with a slip of 5% will be _________.
(A) 1690 rpm (B) 1750 rpm (C) 1500 rpm (D) 1710 rpm
Ans: D Given : Ns1 =1200 , P1= 6, P2 = 4, s = 0.05, Frequency f = Ns
P/120 = 120
6/120 = 60 Hz rotor frequency f/ = s.f = 0.05
60 = 3.0 Hz
Now, Ns2 = 120
60 /4 = 1800 and Ns – N = 120 f / P2 Therefore, N=Ns- 120 f / P2 = 1800-120
0.05
60/4 = 1800-90 = 1710
Q.62 The eddy current loss in an a-c electric motor is 100 watts at 50 Hz. Its loss at 100 Hz will be
(A) 25 watts (B) 59 watts (C) 100 watts (D) 400 watts
Ans: D Eddy current losses α f2 New loss α (2f)2 New loss α 4f2 ∴ 4 times
Q.63 The maximum power for a given excitation in a synchronous motor is developed when the power angle is equal to (A) 0o (B) 45o (C) 60o (D) 90o Ans: A P = VI cosΦ Pmax = VI ∴Φ = 00
Dout is solved
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