POWER ELECTRONICS
POWER ELECTRONICS MCQs
1. Which of these amplifiers is used in a digital to analog converter?
(A) Noninverter (B) Voltage follower (C) Summer (D) Difference amplifier
Ans: C
2. A system can be completely described by a transfer function if is is
(A) Non linear and continuous (B) Linear and time-varying (C) Non linear and time invariant (D) Linear and time invariant
Ans: D
3. Photoflash unit consists of which type of circuit?
(A) RLC (B) LC (C) RL (D) RC
Ans: D
4. Which type of power is measured by a wattmeter?
(A) Instantaneous (B) RMS (C) Average (D) Instantaneous, RMS and average
Ans: C
5. The point on the Bode plot, where two asymptotic lines meet is known as
(A) Corner frequency (B) Break frequency (C) Either corner or break frequency (D) Neither corner not break frequency
Ans: C
6. The emitter of a transistor is generally doped the heaviest because it
(A) has to dissipate maximum power (B) has to supply more number of charge carriers. (C) is the first region of the transistor (D) must possess low resistance
Ans: B
7. When induction motor drive from a source of fixed frequency then regenerative braking is possible only for
(A) speed less than synchronous speed, (B) speed greater than synchronous speed,
(C) speed may be greater or less than synchronous speed, (D) None of the above.
Ans: B
8. Which of the following is the preferred triggering pattern of thyristor?
(A) Thermal triggering (B) High forward voltage triggering (C) Gate triggering (D) dv/dt triggering
Ans: C
9. What is the resolution of a 12-bit data converter?
(A) 0.00024% (B) 0.0041% (C) 0.024% (D) 0.41%
Ans: C
10. Consider the following statements:
(i) BJT is a current controlled devices with a high input impedance and high gain band width.
(ii) FET is a voltage controlled devices with high input impedance and low gain band width.
(iii) UJT is a negative resistance devices and can be used as an oscillator.
(iv) BJT, UJT and FET can all be used for amplification.
Which of the statements given above are correct?
(A) i and ii (B) ii and iii (C) iii and iv (D) i and iv
Ans: B
11. What kind of filter can be used to select a signal of one particular radio station?
(A) Low pass (B) High pass (C) Band pass (D) Band stop
Ans: C
12. The most commonly used amplifier in sample and hold circuit is
(A) a unity gain inverting amplifier (B) a unity gain non-inverting amplifier
(C) an inverting amplifier with a gain of 10 (D) an inverting amplifier with a gain of 100
Ans: B
13. Which layer is present in Controller Area Network (CAN)
(A) Physical layer (B) Transfer layer (C) Object layer (D) Invisible layer
Ans: D
14. Which is not an interfacing device used to connect processor to the peripheral device?
(A) SPI (B) RS422 (C) Ethernet (D) Wi Fi
Ans: D
15. A 10 kHz even-symmetric square wave is passed through a bandpass filter with centre frequency at 30 kHz and 3 dB passband of 6 kHz. The filter output is
(A) a highly attenuated square wave at 10 kHz. (B) nearly zero.
(C) a nearly perfect cosine wave at 30 kHz. (D) a nearly perfect sine wave at 30 kHz
Ans: C
16. Which is not a part of transformer design steps?
(A) Load power estimation (B) Area product calculation (C) Window area check (D) None of the above
Ans: D
17. The undesirable property of an electrical insulating material is
(A) high dielectric strength (B) high relative permittivity (C) high thermal conductivity (D) high insulation resistivity
Ans: B
18. The heating element to be used in a furnace employed for heating around 1,600° C should be of material
(A) nichrome (B) eureka (C) molybdenum (D) silicon-carbide
Ans: C
19. The basic elements of an electric drive are:
(A) electric motor and the transmission system (B) electric motor, the transmission and control system
(C) the transmission and control system (D) electric motor and conversion equipment
Ans: B
20. Which of the following statement is correct?
(A) Ladder logic is a PLC graphical programming technique introduced in the last 10 years.
(B) A ladder logic program is hard to analyze because it is totally different when compared with the equivalent relay logic solution.
(C) The number of ladder logic virtual relays and input and output
instructions is limited only by memory size.
(D) The number of contacts for a mechanical relay is limited to number of coils on the relay.
Ans: C
21. Which one of the following statements is true for all real symmetric matrices?
(A) All the eigenvalues are real. (B) All the eigenvalues are positive,
(C) All the eigenvalues are distinct. (D) Sum of all the eigenvalues is zero.
Ans: A
22. For a specified input voltage and frequency, if the equivalent radius of the core of a transformer is reduced by half, the factor by which the number of turns in the primary should change to maintain the same no load current is
(A) 1/4 (B) 1/2 (C) 2 (D) 4
Ans: C
23. In a transmission line, reflection will be zero if it is terminated by an impedance
(A) equal to twice the characteristic impedance (B) equal to the characteristic impedance
(C) less than characteristic impedance (D) equal to one fourth the characteristic impedance
Ans: A
24. A rotating electrical machine having its self inductance of both the stator and the rotor windings, independent of rotor position will definitely not develop
(A) starting torque (B) synchronizing torque (C) hysteresis torque (D) reluctance torque
Ans: B
25. A differentiator and an oscillator using Op-amp are examples of:
(A) zero and first order respectively (B) first and zero order respectively
(C) zero and second order respectively (D) first and second order respectively
Ans: D
26. The material of the core of the current transformer (C.T. ) should be of
(A) High reluctance and high core loss (B) Low reluctance and low core loss
(C) High reluctance and low core loss (D) Low reluctance and high core loss
Ans: C
27. If the receiving end voltage is 220 kV then the no load sending end voltage and the voltage rise from sending end to receiving end for a 50 Hz, 300 km long line will be
(A) 209.14 kV, 6268 volts/phase (B) 418.28 kV, 8152 volts/phase
(C) 418.28 kV, 6268 volts/phase (D) 209.14 kV, 8152 volts/phase
Ans: A
28. Corona loss is ______
(A) inversely proportional to system frequency (B) directly proportional to system frequency
(C) independent to system frequency (D) directly proportional to square root of system frequency
Ans: B
29. A 3-phase 500 V motor load has a power factor of 0.4. Two wattmeters are connected to measure the input. They show the input to be 30 kW. What will be the reading of each wattmeter?
(A) 34.85 kW, -4.85 kW (B) -34.85 kW, 4.85 kW (C) 34.85 kW, 34.85 kW (D) 34.85 kW, 4.85 kW
Ans: A
30. The energy stored in the magnetic field of a solenoid 30 cm long and 3 cm diameter wound with 1000 turns of wire carrying a current of 10A is
(A) 0.015 joule (B) 0.15 joule (C) 0.5 joule (D) 1.15 joule
Ans: B
31. The short pitch winding for an alternator gives
(A) improved waveform of the generated emf per phase (B) reduced value of self-inductance of the winding
(C) increased rating of the machine (D) reduced tooth ripples
Ans: B
32. Which of the following represents the range of temperatures for thermocouples?
(A) 0°C to 750°C (B) -200°C to 350°C (C) -200°C to 1250°C (D) -200°C to 2000°C
Ans: D
33. DAD H instruction can be used to shift ______ by one bit the ______ bit contents of HL register pair.
(A) Left, 16 (B) right, 8 (C) right, 16 (D) none of the above
Ans: A
34. A set of simultaneous non-linear algebraic equation can be solved by
(A) Fourier series method
(B) Gauss-seidel method
(C) Iterative techniques
(D) (B) & (C) both
Ans: D
35. Maximum power loss occurs during
(A) Delay time (B) Rise time (C) Spread time (D) Turn off time
Ans: B
36. In a dc machine, which of the following statement is true?
(A) Compensating winding is used for neutralizing armature reaction while interpole winding is used for producing residual flux
(B) Compensating winding is used for neutralizing armature reaction while interpole winding is used for improving commutation
(C) Compensating winding is used for improving commutation while interpole winding is used for neutralizing armature reaction
(D) Compensating winding is used for improving commutation while interpole winding is used for producing residual flux
Ans: B
37. For a fully transposed transmission line
(A) positive, negative and zero sequence impedances are equal (B) positive and negative sequence impedances are equal
(C) zero and positive sequence impedances are equal (D) negative and zero sequence impedances are equal
Ans: B
38. The conduction loss versus device current characteristic of a power MOSFET is best approximated by
(A) a straight line (B) a rectangular hyperbole
(C) a parabola (D) an exponentially decaying function
Ans: C
39. Corona loss is more in
(A) Unstranded conductor (B) ACSR (C) Stranded conductor (D) Transposed conductor
Ans: A
40. A bulb in a staircase has two switches, one switch being at the ground floor and the other one at the first floor. The bulb can be turned ON and also can be turned OFF by any one of the switches irrespective of the state of the other switch. The logic of switching of the bulb resembles
(A) an AND gate (B) an OR gate (C) an XOR gate (D) a NAND gate
Ans: A
41. Resonant mode power supplies in comparison to square mode
(A) have smaller component count (B) have negligible power loss
(C) do not cause overvoltage's (D) slower in control action
Ans: B
42. A 50 Hz generator of reactance 1 pu is connected to an infinite bus through a line of reactance 0.5 pu. The generator e.m.f (E) is 1.1 pu and voltage of infinite bus is V=l pu. The inertia constant is 5 MW-sec/MVA. The generator is loaded to 50% of the maximum power limit. What will be the frequency of natural oscillations?
(A) 0.49 Hz (B) 0.71 Hz (C) 0.89 Hz (D) 0.57 Hz
Ans: B
43. The converter used in static synchronous compensator (STATCOM) can be______
(A) voltage- sourced (B) current-sourced (C) voltage-sourced or current-sourced (D) Independent-sourced
Ans: C
44. Which is not a 8085 interrupts?
(A) RST7.5 (B) ADD (C) RST 5.5 (D) TRAP
Ans: B
45. The direction of rotation of 3-phase induction motor is clockwise when it is supplied with 3-phase sinusoidal voltage having phase sequence A-B-C. For counter clockwise rotation of the motor, the phase sequence of the power supply should be
(A) B-C-A
(B) C-A-B
(C) A-C-B
(D) B-C-A and C-A-B.
Ans: C
46. The interpolation formula to get the discrete time fourier transform (DTFT) from discrete fourier transform (DFT) coefficients is called (A) Synchronizing function (B) ratio of two sine functions
(C) Lagrange interpolation (D) ratio of two synchronizing functions
Ans: C
47. DFT coefficients are Z transform evaluated at N equally spaced points in Z domain on
(A) unit circle (B) half circle (C) circle with radius 2 (D) circle with radius 1/2
Ans: A
48. The plates of VF display are coated with a ______ fluorescent material that glows when bombarded by electrons
(A) barium oxide (B) zinc chloride (C) zinc sulfate (D) barium chloride
Ans: A
49. A 4-pole machine has 60 slots and 8 conductors per slot. The total flux per pole is 20mWb. The machine is a DC machine with lap connected winding and a relative speed of 1500 rpm between field flux and armature windings. What will be the generated armature voltage?
(A) 200 V (B) 180 V (C) 280 V (D) 240 V
Ans: D
50. A converter which can operate in both 6-pulse and 3-pulsc modes is a
(A) 1-phase full-wave converter (B) 3-phase half-wave converter
(C) 3-phase semi converter (D) 3-phase full wave-converter
Ans: C
(A) Noninverter (B) Voltage follower (C) Summer (D) Difference amplifier
Ans: C
2. A system can be completely described by a transfer function if is is
(A) Non linear and continuous (B) Linear and time-varying (C) Non linear and time invariant (D) Linear and time invariant
Ans: D
3. Photoflash unit consists of which type of circuit?
(A) RLC (B) LC (C) RL (D) RC
Ans: D
4. Which type of power is measured by a wattmeter?
(A) Instantaneous (B) RMS (C) Average (D) Instantaneous, RMS and average
Ans: C
5. The point on the Bode plot, where two asymptotic lines meet is known as
(A) Corner frequency (B) Break frequency (C) Either corner or break frequency (D) Neither corner not break frequency
Ans: C
6. The emitter of a transistor is generally doped the heaviest because it
(A) has to dissipate maximum power (B) has to supply more number of charge carriers. (C) is the first region of the transistor (D) must possess low resistance
Ans: B
7. When induction motor drive from a source of fixed frequency then regenerative braking is possible only for
(A) speed less than synchronous speed, (B) speed greater than synchronous speed,
(C) speed may be greater or less than synchronous speed, (D) None of the above.
Ans: B
8. Which of the following is the preferred triggering pattern of thyristor?
(A) Thermal triggering (B) High forward voltage triggering (C) Gate triggering (D) dv/dt triggering
Ans: C
9. What is the resolution of a 12-bit data converter?
(A) 0.00024% (B) 0.0041% (C) 0.024% (D) 0.41%
Ans: C
10. Consider the following statements:
(i) BJT is a current controlled devices with a high input impedance and high gain band width.
(ii) FET is a voltage controlled devices with high input impedance and low gain band width.
(iii) UJT is a negative resistance devices and can be used as an oscillator.
(iv) BJT, UJT and FET can all be used for amplification.
Which of the statements given above are correct?
(A) i and ii (B) ii and iii (C) iii and iv (D) i and iv
Ans: B
11. What kind of filter can be used to select a signal of one particular radio station?
(A) Low pass (B) High pass (C) Band pass (D) Band stop
Ans: C
12. The most commonly used amplifier in sample and hold circuit is
(A) a unity gain inverting amplifier (B) a unity gain non-inverting amplifier
(C) an inverting amplifier with a gain of 10 (D) an inverting amplifier with a gain of 100
Ans: B
13. Which layer is present in Controller Area Network (CAN)
(A) Physical layer (B) Transfer layer (C) Object layer (D) Invisible layer
Ans: D
14. Which is not an interfacing device used to connect processor to the peripheral device?
(A) SPI (B) RS422 (C) Ethernet (D) Wi Fi
Ans: D
15. A 10 kHz even-symmetric square wave is passed through a bandpass filter with centre frequency at 30 kHz and 3 dB passband of 6 kHz. The filter output is
(A) a highly attenuated square wave at 10 kHz. (B) nearly zero.
(C) a nearly perfect cosine wave at 30 kHz. (D) a nearly perfect sine wave at 30 kHz
Ans: C
16. Which is not a part of transformer design steps?
(A) Load power estimation (B) Area product calculation (C) Window area check (D) None of the above
Ans: D
17. The undesirable property of an electrical insulating material is
(A) high dielectric strength (B) high relative permittivity (C) high thermal conductivity (D) high insulation resistivity
Ans: B
18. The heating element to be used in a furnace employed for heating around 1,600° C should be of material
(A) nichrome (B) eureka (C) molybdenum (D) silicon-carbide
Ans: C
19. The basic elements of an electric drive are:
(A) electric motor and the transmission system (B) electric motor, the transmission and control system
(C) the transmission and control system (D) electric motor and conversion equipment
Ans: B
20. Which of the following statement is correct?
(A) Ladder logic is a PLC graphical programming technique introduced in the last 10 years.
(B) A ladder logic program is hard to analyze because it is totally different when compared with the equivalent relay logic solution.
(C) The number of ladder logic virtual relays and input and output
instructions is limited only by memory size.
(D) The number of contacts for a mechanical relay is limited to number of coils on the relay.
Ans: C
21. Which one of the following statements is true for all real symmetric matrices?
(A) All the eigenvalues are real. (B) All the eigenvalues are positive,
(C) All the eigenvalues are distinct. (D) Sum of all the eigenvalues is zero.
Ans: A
22. For a specified input voltage and frequency, if the equivalent radius of the core of a transformer is reduced by half, the factor by which the number of turns in the primary should change to maintain the same no load current is
(A) 1/4 (B) 1/2 (C) 2 (D) 4
Ans: C
23. In a transmission line, reflection will be zero if it is terminated by an impedance
(A) equal to twice the characteristic impedance (B) equal to the characteristic impedance
(C) less than characteristic impedance (D) equal to one fourth the characteristic impedance
Ans: A
24. A rotating electrical machine having its self inductance of both the stator and the rotor windings, independent of rotor position will definitely not develop
(A) starting torque (B) synchronizing torque (C) hysteresis torque (D) reluctance torque
Ans: B
25. A differentiator and an oscillator using Op-amp are examples of:
(A) zero and first order respectively (B) first and zero order respectively
(C) zero and second order respectively (D) first and second order respectively
Ans: D
26. The material of the core of the current transformer (C.T. ) should be of
(A) High reluctance and high core loss (B) Low reluctance and low core loss
(C) High reluctance and low core loss (D) Low reluctance and high core loss
Ans: C
27. If the receiving end voltage is 220 kV then the no load sending end voltage and the voltage rise from sending end to receiving end for a 50 Hz, 300 km long line will be
(A) 209.14 kV, 6268 volts/phase (B) 418.28 kV, 8152 volts/phase
(C) 418.28 kV, 6268 volts/phase (D) 209.14 kV, 8152 volts/phase
Ans: A
28. Corona loss is ______
(A) inversely proportional to system frequency (B) directly proportional to system frequency
(C) independent to system frequency (D) directly proportional to square root of system frequency
Ans: B
29. A 3-phase 500 V motor load has a power factor of 0.4. Two wattmeters are connected to measure the input. They show the input to be 30 kW. What will be the reading of each wattmeter?
(A) 34.85 kW, -4.85 kW (B) -34.85 kW, 4.85 kW (C) 34.85 kW, 34.85 kW (D) 34.85 kW, 4.85 kW
Ans: A
30. The energy stored in the magnetic field of a solenoid 30 cm long and 3 cm diameter wound with 1000 turns of wire carrying a current of 10A is
(A) 0.015 joule (B) 0.15 joule (C) 0.5 joule (D) 1.15 joule
Ans: B
31. The short pitch winding for an alternator gives
(A) improved waveform of the generated emf per phase (B) reduced value of self-inductance of the winding
(C) increased rating of the machine (D) reduced tooth ripples
Ans: B
32. Which of the following represents the range of temperatures for thermocouples?
(A) 0°C to 750°C (B) -200°C to 350°C (C) -200°C to 1250°C (D) -200°C to 2000°C
Ans: D
33. DAD H instruction can be used to shift ______ by one bit the ______ bit contents of HL register pair.
(A) Left, 16 (B) right, 8 (C) right, 16 (D) none of the above
Ans: A
34. A set of simultaneous non-linear algebraic equation can be solved by
(A) Fourier series method
(B) Gauss-seidel method
(C) Iterative techniques
(D) (B) & (C) both
Ans: D
35. Maximum power loss occurs during
(A) Delay time (B) Rise time (C) Spread time (D) Turn off time
Ans: B
36. In a dc machine, which of the following statement is true?
(A) Compensating winding is used for neutralizing armature reaction while interpole winding is used for producing residual flux
(B) Compensating winding is used for neutralizing armature reaction while interpole winding is used for improving commutation
(C) Compensating winding is used for improving commutation while interpole winding is used for neutralizing armature reaction
(D) Compensating winding is used for improving commutation while interpole winding is used for producing residual flux
Ans: B
37. For a fully transposed transmission line
(A) positive, negative and zero sequence impedances are equal (B) positive and negative sequence impedances are equal
(C) zero and positive sequence impedances are equal (D) negative and zero sequence impedances are equal
Ans: B
38. The conduction loss versus device current characteristic of a power MOSFET is best approximated by
(A) a straight line (B) a rectangular hyperbole
(C) a parabola (D) an exponentially decaying function
Ans: C
39. Corona loss is more in
(A) Unstranded conductor (B) ACSR (C) Stranded conductor (D) Transposed conductor
Ans: A
40. A bulb in a staircase has two switches, one switch being at the ground floor and the other one at the first floor. The bulb can be turned ON and also can be turned OFF by any one of the switches irrespective of the state of the other switch. The logic of switching of the bulb resembles
(A) an AND gate (B) an OR gate (C) an XOR gate (D) a NAND gate
Ans: A
41. Resonant mode power supplies in comparison to square mode
(A) have smaller component count (B) have negligible power loss
(C) do not cause overvoltage's (D) slower in control action
Ans: B
42. A 50 Hz generator of reactance 1 pu is connected to an infinite bus through a line of reactance 0.5 pu. The generator e.m.f (E) is 1.1 pu and voltage of infinite bus is V=l pu. The inertia constant is 5 MW-sec/MVA. The generator is loaded to 50% of the maximum power limit. What will be the frequency of natural oscillations?
(A) 0.49 Hz (B) 0.71 Hz (C) 0.89 Hz (D) 0.57 Hz
Ans: B
43. The converter used in static synchronous compensator (STATCOM) can be______
(A) voltage- sourced (B) current-sourced (C) voltage-sourced or current-sourced (D) Independent-sourced
Ans: C
44. Which is not a 8085 interrupts?
(A) RST7.5 (B) ADD (C) RST 5.5 (D) TRAP
Ans: B
45. The direction of rotation of 3-phase induction motor is clockwise when it is supplied with 3-phase sinusoidal voltage having phase sequence A-B-C. For counter clockwise rotation of the motor, the phase sequence of the power supply should be
(A) B-C-A
(B) C-A-B
(C) A-C-B
(D) B-C-A and C-A-B.
Ans: C
46. The interpolation formula to get the discrete time fourier transform (DTFT) from discrete fourier transform (DFT) coefficients is called (A) Synchronizing function (B) ratio of two sine functions
(C) Lagrange interpolation (D) ratio of two synchronizing functions
Ans: C
47. DFT coefficients are Z transform evaluated at N equally spaced points in Z domain on
(A) unit circle (B) half circle (C) circle with radius 2 (D) circle with radius 1/2
Ans: A
48. The plates of VF display are coated with a ______ fluorescent material that glows when bombarded by electrons
(A) barium oxide (B) zinc chloride (C) zinc sulfate (D) barium chloride
Ans: A
49. A 4-pole machine has 60 slots and 8 conductors per slot. The total flux per pole is 20mWb. The machine is a DC machine with lap connected winding and a relative speed of 1500 rpm between field flux and armature windings. What will be the generated armature voltage?
(A) 200 V (B) 180 V (C) 280 V (D) 240 V
Ans: D
50. A converter which can operate in both 6-pulse and 3-pulsc modes is a
(A) 1-phase full-wave converter (B) 3-phase half-wave converter
(C) 3-phase semi converter (D) 3-phase full wave-converter
Ans: C
51. For enhancing the power transmission in along EHV transmission line, the most preferred method is to connect a
(A) Series inductive compensator in the line (B) Shunt inductive compensator at the receiving end
(C) Series capacitive compensator in the line (D) Shunt capacitive compensator at the sending end
Ans: C
52. A negative sequence relay is commonly used to protect
(A) an alternator (B) an transformer (C) a transmission line (D) a bus bar
Ans: A
53. The binary equivalent of hexadecimal number 4FAD is
(A) 0101 1111 0010 1100 (B) 0100 1111 0010 1100 (C) 0100 1111 1010 1101 (D) 0100 1110 0010 1101
Ans: C
54. The current in a 25 mH inductor changes at a constant rate of 7 A/s. What voltage is induced across this coil?
(A) 3.57 mV (B) 175 mV (C) 350 mV (D) 1.75 V
Ans: B
55. Which is not a gate driver IC?
(A) MCT2E (B) TLP250 (C) LM741 (D) TLP300
Ans: C
56. Which one of the following statements is NOT TRUE for a continuous time causal and stable LTI system?
(A) All the poles of the system must lie on the left side of the jw axis. (B) Zeros of the system can lie anywhere in the s-plane.
(C) All the poles must lie within 1 sec. (D) All the roots of the characteristic equation must be located on the left side of the jw axis.
Ans: C
57. If the frequency of input voltage of a transformer is increased keeping the magnitude of voltage unchanged, then
(A) both hysteresis loss and eddy current loss in the core will increase
(B) hysteresis loss will increase but eddy current loss will decrease
(C) hysteresis loss will decrease but eddy current loss will increase
(D) hysteresis loss will decrease but eddy current loss will remain unchanged
Ans: D
58. The fulfload copper loss and iron loss of a transformer are 6400 W and 5000 W respectively. The copper loss and iron loss at half load will be, respectively
(A) 3200 W and 2500 W (B) 3200 W and 5200 W
(C) 1600 W and 1250 W (D) 1600 W and 5000 W
Ans: D
59. An 8-pole, 3-phase, 50 Hz induction motor is operating at a speed of 700 rpm. The frequency of the rotor current of the motor in Hz is (A) 1.5 Hz (B) 3.56 Hz
(C) 48.5 Hz (D) 51.5 Hz
Ans: B
60. Regarding Ward-Leonard system of speed control, which statement is false?
(A) it is usually used where wide and very sensitive speed control is required
(B) it is used for motors having ratings from 750 kw to 4000 kw
(C) capital outlay involved in the system is right since it uses two extra machines
(D) it gives a speed range of 10:1 but in one direction only
Ans: D
61. A three-phase fully controlled thyristor bridge converter is used as line commutated inverter to feed 50 kW power 420 V dc to a three phase, 415 V (line), 50 Hz ac mains. Consider dc link current to be constant. The rms current of the thyristor is
(A) 119.05 A (B) 79.37 A (C) 68.73 A (D) 39.68 A
Ans: C
62. A step down chopper is operated in the continuous conduction mode in steady state with a constant duty ratio D. If V0 is the magnitude of the DC output voltage and if Vs is the magnitude of the input voltage, the ratio V0/Vs is given by
(A) D
(B) 1-D
(C) 1/(1-D)
(D) D/(1-D)
Ans: A
(A) Series inductive compensator in the line (B) Shunt inductive compensator at the receiving end
(C) Series capacitive compensator in the line (D) Shunt capacitive compensator at the sending end
Ans: C
52. A negative sequence relay is commonly used to protect
(A) an alternator (B) an transformer (C) a transmission line (D) a bus bar
Ans: A
53. The binary equivalent of hexadecimal number 4FAD is
(A) 0101 1111 0010 1100 (B) 0100 1111 0010 1100 (C) 0100 1111 1010 1101 (D) 0100 1110 0010 1101
Ans: C
54. The current in a 25 mH inductor changes at a constant rate of 7 A/s. What voltage is induced across this coil?
(A) 3.57 mV (B) 175 mV (C) 350 mV (D) 1.75 V
Ans: B
55. Which is not a gate driver IC?
(A) MCT2E (B) TLP250 (C) LM741 (D) TLP300
Ans: C
56. Which one of the following statements is NOT TRUE for a continuous time causal and stable LTI system?
(A) All the poles of the system must lie on the left side of the jw axis. (B) Zeros of the system can lie anywhere in the s-plane.
(C) All the poles must lie within 1 sec. (D) All the roots of the characteristic equation must be located on the left side of the jw axis.
Ans: C
57. If the frequency of input voltage of a transformer is increased keeping the magnitude of voltage unchanged, then
(A) both hysteresis loss and eddy current loss in the core will increase
(B) hysteresis loss will increase but eddy current loss will decrease
(C) hysteresis loss will decrease but eddy current loss will increase
(D) hysteresis loss will decrease but eddy current loss will remain unchanged
Ans: D
58. The fulfload copper loss and iron loss of a transformer are 6400 W and 5000 W respectively. The copper loss and iron loss at half load will be, respectively
(A) 3200 W and 2500 W (B) 3200 W and 5200 W
(C) 1600 W and 1250 W (D) 1600 W and 5000 W
Ans: D
59. An 8-pole, 3-phase, 50 Hz induction motor is operating at a speed of 700 rpm. The frequency of the rotor current of the motor in Hz is (A) 1.5 Hz (B) 3.56 Hz
(C) 48.5 Hz (D) 51.5 Hz
Ans: B
60. Regarding Ward-Leonard system of speed control, which statement is false?
(A) it is usually used where wide and very sensitive speed control is required
(B) it is used for motors having ratings from 750 kw to 4000 kw
(C) capital outlay involved in the system is right since it uses two extra machines
(D) it gives a speed range of 10:1 but in one direction only
Ans: D
61. A three-phase fully controlled thyristor bridge converter is used as line commutated inverter to feed 50 kW power 420 V dc to a three phase, 415 V (line), 50 Hz ac mains. Consider dc link current to be constant. The rms current of the thyristor is
(A) 119.05 A (B) 79.37 A (C) 68.73 A (D) 39.68 A
Ans: C
62. A step down chopper is operated in the continuous conduction mode in steady state with a constant duty ratio D. If V0 is the magnitude of the DC output voltage and if Vs is the magnitude of the input voltage, the ratio V0/Vs is given by
(A) D
(B) 1-D
(C) 1/(1-D)
(D) D/(1-D)
Ans: A