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 1 of original torque. (B) 2 1 of original torque. (C) 8 1 of original torque.
(D) 16 1 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
o 50 , the maximum
torque angle of the relay is
(A) o 130 (B) o 100 (C) o 50 (D) o 25
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 = (V2/R) or (V2/Z) when cos Φ =1
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 = kef2Bm2.k Therefore, hysteresis and eddy current losses will be decreased when frequency decreases.
Q.73 A synchronous motor is operating on no-load at unity power factor. If the field current is increased, power factor will become
(A) Leading & current will decrease (B) Lagging & current will increase.
(C) Lagging & current will decrease. (D) Leading & current will increase.
Ans: A Initially synchronous motor is operating at no load and unity power factor. When field current increases, the excitation will increase. Therefore, p.f will be leading and current will be I CosΦ < I
Q.74 A d.c. shunt motor runs at no load speed of 1140 r.p.m. At full load, armature reaction weakens the main flux by 5% whereas the armature circuit voltage drops by 10%. The motor full load speed in r.p.m. is
(A) 1080 (B) 1203 (C) 1000 (D) 1200
Ans: A N2 / N1 =Eb2 /Eb1 x Φ1 / Φ2 ; Φ2 = 0.95Φ1 ; Eb2 = 0.9Eb1 ∴ N2 /1140 = 0.9 x 1/0.95 N2 = 1080
Q.75 The introduction of interpoles in between the main pole improves the performance of d.c. machines, because
(A) The interpole produces additional flux to augment the developed torque.
(B) The flux waveform is improved with reduction in harmonics.
(C) The inequality of air flux on the top and bottom halves of armature is removed.
(D) A counter e.m.f. is induced in the coil undergoing commutation.
Ans: D Counter e.m.f is produced, it neutralizes the reactive emf.
Q.76 The rotor power output of a 3-phase induction motor is 15 KW and corresponding slip is 4%. The rotor copper loss will be
(A) 600 W. (B) 625 W (C) 650 W (D) 700 W
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 3600 ∴ 1-8 = -40 + Nr/40 Nr x 3600 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) 0.10 (C) 0.04 (D) 0.05
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 1 of original torque. (B) 2 1 of original torque. (C) 8 1 of original torque.
(D) 16 1 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
o 50 , the maximum
torque angle of the relay is
(A) o 130 (B) o 100 (C) o 50 (D) o 25
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 = (V2/R) or (V2/Z) when cos Φ =1
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 = kef2Bm2.k Therefore, hysteresis and eddy current losses will be decreased when frequency decreases.
Q.73 A synchronous motor is operating on no-load at unity power factor. If the field current is increased, power factor will become
(A) Leading & current will decrease (B) Lagging & current will increase.
(C) Lagging & current will decrease. (D) Leading & current will increase.
Ans: A Initially synchronous motor is operating at no load and unity power factor. When field current increases, the excitation will increase. Therefore, p.f will be leading and current will be I CosΦ < I
Q.74 A d.c. shunt motor runs at no load speed of 1140 r.p.m. At full load, armature reaction weakens the main flux by 5% whereas the armature circuit voltage drops by 10%. The motor full load speed in r.p.m. is
(A) 1080 (B) 1203 (C) 1000 (D) 1200
Ans: A N2 / N1 =Eb2 /Eb1 x Φ1 / Φ2 ; Φ2 = 0.95Φ1 ; Eb2 = 0.9Eb1 ∴ N2 /1140 = 0.9 x 1/0.95 N2 = 1080
Q.75 The introduction of interpoles in between the main pole improves the performance of d.c. machines, because
(A) The interpole produces additional flux to augment the developed torque.
(B) The flux waveform is improved with reduction in harmonics.
(C) The inequality of air flux on the top and bottom halves of armature is removed.
(D) A counter e.m.f. is induced in the coil undergoing commutation.
Ans: D Counter e.m.f is produced, it neutralizes the reactive emf.
Q.76 The rotor power output of a 3-phase induction motor is 15 KW and corresponding slip is 4%. The rotor copper loss will be
(A) 600 W. (B) 625 W (C) 650 W (D) 700 W
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 3600 ∴ 1-8 = -40 + Nr/40 Nr x 3600 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) 0.10 (C) 0.04 (D) 0.05
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