Transformer mcq part-4

1. Buchholz's relay gives warning and protection against
(a) electrical fault inside the transformer itself
(b) electrical fault outside the transformer in outgoing feeder
(c) for both outside and inside faults
(d) none of the above

Ans: a

2. The magnetising current of a transformer is usually small because it has
(a) small air gap
(b) large leakage flux
(c) laminated silicon steel core
(d) fewer rotating parts

Ans: a

3. Which of the following does not change in an ordinary transformer ?
(a) Frequency
(b) Voltage
(c) Current
(d) Any of the above

Ans: a

4. Which of the following properties is not necessarily desirable for the material for transformer core ?
(a) Low hysteresis loss
(b) High permeability
(c) High thermal conductivity
(d) Adequate mechanical strength

Ans: c

5. The leakage flux in a transformer depends upon
(a) load current
(b) load current and voltage
(c) load current, voltage and frequency
(d) load current, voltage, frequency and power factor

Ans: a

6. The path of the magnetic flux in transformer should have
(a) high reluctance
(b) low reactance
(c) high resistance
(d) low resistance

Ans: b

7. Noise level test in a transformer is a
(a) special test
(b) routine test
(c) type test
(d) none of the above

Ans: c

8. Which of the foJIowing is not a routine test on transformers ?
(a) Core insulation voltage test
(b) Impedance test
(c) Radio interference test
(d) Polarity test

Ans: c

9. A transformer can have zero voltage regulation at
(a) leading power factor
(b) lagging power factor
(c) unity power factor
(d) zero power factor

Ans: a

10. Helical coils can be used on
(a) low voltage side of high kVA trans¬formers
(b) high frequency transformers
(c) high voltage side of small capacity transformers
(d) high voltage side of high kVA rating transformers

Ans: a

11. Harmonics in transformer result in
(a) increased core losses
(b) increased I2R losses
(c) magnetic interference with communication circuits
(d) all of the above

Ans: d

12. The core used in high frequency transformer is usually
(a) copper core
(b) cost iron core
(c) air core
(d) mild steel core

Ans: c

13. The full-load copper loss of a trans¬former is 1600 W. At half-load, the copper loss will be
(a) 6400 W
(b) 1600 W
(c) 800 W
(d) 400 W

Ans: d

14. The value of flux involved m the e.m.f. equation of a transformer is
(a) average value
(b) r.m.s. value
(c) maximum value
(d) instantaneous value

Ans: c

15. Silicon steel used in laminations mainly reduces
(a) hysteresis loss
(b) eddy current losses
(c) copper losses
(d) all of the above

Ans: a

16. Which winding of the transformer has less cross-sectional area ?
(a) Primary winding
(b) Secondary winding
(c) Low voltage winding
(d) High voltage winding

Ans: d

17. Power transformers are generally designed to have maximum efficiency around
(a) no-load
(b) half-load
(c) near full-load
(d) 10% overload

Ans: c
.

18. Which of the following is the main advantage of an auto-transformer over a two winding transformer ?
(a) Hysteresis losses are reduced
(b) Saving in winding material
(c) Copper losses are negligible
(d) Eddy losses are totally eliminated

Ans: b

19. During short circuit test iron losses are negligible because
(a) the current on secondary side is negligible
(b) the voltage on secondary side does not vary
(c) the voltage applied on primary side is low
(d) full-load current is not supplied to the transformer

Ans: c

20. Two transformers are connected in parallel. These transformers do not have equal percentage impedance. This is likely to result in
(a) short-circuiting of the secondaries
(b) power factor of one of the trans¬formers is leading while that of the other lagging
(c) transformers having higher copper losses will have negligible core losses
(d) loading of the transformers not in proportion to their kVA ratings

Ans: d

DC MOTOR MCQ PART -4

1. Which of the following motor has the poorest speed regulation ?
(a) Shunt motor
(b) Series motor
(c) Differential compound motor
(d) Cumulative compound motor

Ans: b

2. Buses, trains, trolleys, hoists, cranes require high starting torque and therefore make use of
(a) D.C. series motor
(b) D.C. shunt motor
(c) induction motor
(d) all of above motors

Ans: a
3. As -the load is increased the speed of D.C. shunt motor will
(a) reduce slightly
(b) increase slightly
(c) increase proportionately
(d) remains unchanged

Ans: a

4. The armature torque of the D.C. shunt motor is proportional to
(a) field flux only
(b) armature current only
(c) both (a) and (b)
(d) none of the above

Ans: b

5. Which of the following method of speed control of D.C. machine will offer minimum efficiency ?
(a) Voltage control method
(b) Field control method
(c) Armature control method
(d) All above methods

Ans: c

6. Usually wide and sensitive speed control is desired in case of
(a) centrifugal pumps
(b) elevators
(c) steel rolling mills
(d) colliery winders

Ans: d

7. The speed of a motor falls from 1100 r.p.m. at no-load to 1050 r.p.m. at rated load. The speed regulation of the motor is
(a) 2.36%
(6) 4.76%
(c) 6.77%
(d) 8.84%

Ans: b

8. The armature voltage control of D.C. motor provides
(a) constant torque drive
(b) constant voltage drive
(c) constant current drive
(d) none of the above

Ans: a

9. As there is no back e.m.f. at the instant of starting a D.C. motor, in order to prevent a heavy current from flowing though the armature circuit
(a) a resistance is connected in series with armature
(b) a resistance is connected parallel to the armature
(c) armature is temporarily open circuited
(d) a high value resistor is connected across the field winding

Ans: a

10. The speed of a D.C. shunt motor can be increased by
(a) increasing the resistance in armature circuit
(b) increasing the resistance in field circuit
(c) reducing the resistance in the field circuit
(d) reducing the resistance in the armature circuit

Ans: b

11. If I2 be the armature current, then speed of a D.C. shunt motor is
(a) independent of Ia
(b) proportional to la
(c) varies as (Ia)
(d) varies as la

Ans: a

12. In case the back e.m.f. and the speed of a D.C. motor are doubled, the torque developed by the motor will
(a) remain unchanged
(6) reduce to one-fourth value
(c) increase four folds
(d) be doubled

Ans: a

13. At the instant of starting when a D.C. motor is put on supply, it behaves like
(a) a highly resistive circuit
(6) a low resistance circuit
(c) a capacitive circuit
(d) none of the above

Ans: b

14. The speed of a D.C. motor can be varied by varying
(a) field current
(b) applied voltage
(c) resistance in series with armature
(d) any of the above

Ans: d

15. Which one of the following is not necessarily the advantage of D.C. motors over A.C. motors ?
(a) Low cost
(b) Wide speed range
(c) Stability
(d) High starting torque

. Ans: a

16. For a D.C. shunt motor if the excitation is changed
(a) torque will remain constant
(b) torque will change but power will remain constant
(c) torque and power both will change
(d) torque, power and speed, all will change

Ans: b

17. Which motor has the poorest speed control?
(a) Differentially compounded motor
(b) Cumulatively compounded motor
(c) Shunt motor
(d) Series motor

Ans: d

18. The plugging gives the
(a) zero torque braking
(b) smallest torque braking
(c) highest torque braking
(d) none of the above

Ans: c

19. The armature voltage control of D.C. motor provides
(a) constant voltage drive
(b) constant current drive
(c) constant torque drive
(d) none of the above

Ans: c

20. If a D.C. motor designed for 40°C ambient temperature is to be used for 50°C ambient temperature, then the motor
(a) of lower H.P. should be selected
(6) of higher H.P. should be selected
(c) can be used for 50°C ambient temperature also
(d) is to be derated by a factor recom-mended by manufacturer and select the next higher
H.P. moto

Ans: d

DC GENERATOR MCQ PART-4

1. In a D.C. generator the number of mechanical degrees and electrical degrees will be the same when
(a) r.p.m. is more than 300
(b) r.p.m. is less than 300
(c) number of poles is 4
(d) number of poles is 2

Ans: d

2. Permeance is the reciprocal of
(a) flux density
(b) reluctance
(c) ampere-turns
(d) resistance

Ans: b

3. In D.C. generators the polarity of the interpoles
(a) is the same as that of the main pole ahead
(b) is the same as that of the immediately preceding pole
(c) is opposite to that of the main pole ahead
(d) is neutral as these poles do not play part in generating e.m.f

. Ans: a

4. The e.m.f. generated in a D.C. generator is directly proportional to
(a) flux/pole
(b) speed of armature
(c) number of poles
(d) all of the above

Ans: b

5. In a D.C. generator the magnetic neutral axis coincides with the geometrical neutral axis, when
(a) there is no load on|he generator
(b) the generator runs on full load
(c) the generator runs on overload
(d) the generator runs on designed speed

Ans: a

6. In a D.C. generator in order to reduce sparking at brushes, the self-induced e.m.f. in the coil is neutralised by all of the following except
(a) interpoles
(b) dummy coils
(c) compensating winding
(d) shifting of axis of brushes

Ans: b

7. In D.C. generators on no-load, the air gap flux distribution in space is
(a) sinusoidal
(b) triangular
(c) pulsating
(d) flat topped

Ans: d

8. A shunt generator running at 1000 r.p.m. has generated e.m.f. as 200 V. If the speed increases to 1200 r.p.m., the generated e.m.f. will be nearly
(a) 150 V
(b) 175 V
(c) 240 V
(d) 290 V

Ans: c

9. The purpose of providing dummy coils in a generator is
(a) to reduce eddy current losses
(b) to enhance flux density
(c) to amplify voltage
(d) to provide mechanical balance for the rotor

Ans: d

10. In a shunt generator the voltage build up is generally restricted by
(a) speed limitation
(b) armature heating
(c) insulation restrictions
(d) saturation of iron

Ans:d

11. If a D.C. generator fails to build up the probable cause could not be
(a) imperfect brush contact
(b) field resistance less than the critical resistance
(c) no residual magnetism in the generator
(d) faulty shunt connections tending to reduce the residual magnetism

Ans: b

12. Flashing the field of D.C. generator means
(a) neutralising residual magnetism
(b) creating residual magnetism by a D.C. source
(c) making the magnetic losses of forces parallel
(d) increasing flux density by adding extra turns of windings on poles

Ans: b

13. The e.m.f. induced in the armature of a shunt generator is 600 V. The armature resistance is 0.1 ohm. If the armature current is 200 A, the terminal voltage will be
(a) 640 V
(b) 620 V
(c) 600 V
(d) 580 V

Ans: d

14. In a D.C. generator the critical resistance refers to the resistance of
(a) brushes
(b) field
(c) armature
(d) load

Ans: b

15. To achieve sparkless commutation brushes of a D.C. generator are rockedm ahead so as to bring them
(a) just ahead of magnetic neutral axis
(b) in magnetic neutral axis
(c) just behind the magnetic neutral axis

Ans: a

16. Armature coil is short circuited by brushes when it lies
(a) along neutral axis
(b) along field axis
(c) in any of the above positions
(d) in none of the above positions

Ans: a

17. A cumulatively compounded long shunt generator when operating as a motor would be
(a) cumulatively compounded long shunt
(b) differentially compounded long shunt
(c) cumulatively compounded short shunt
(d) differentially compounded short shunt

Ans: b

18. To avoid formation of grooves in the commutator of a D.C. machine
(a) the brushes of opposite polarity should track each other
(b) the brushes of same polarity should track each other
(c) brush position has no effect on the commutator grooving

Ans: a

19. The following constitute short-circuit in the armature winding.
(a) Insulation failure between two com-mutator bars
(6) Insulation failure between two turns of a coil
(c) Two of more turns of the same coil getting grounded
(d) All of the above

Ans: d

20. The rapid wear of brushes takes place due to
(a) abrasion from dust
(b) excessive spring pressure
(c) rough commutator bars
(d) high mica insulation between com-mutation bars
(e) all of the above factors

Ans: e

Measurement Part Three

Measurement Part Three

1.A torque is developed in an electro mechanical energy conversion device, the value of which depends upon
(a) stator field strength and torque angle
(b) stator field and rotor field strength
(c) stator field and rotor field strengths and the torque angle
(d) stator field strength only

Answer-C

2………………instrument is from hysteresis and eddy current errors.
(a) Electrostatics
(b) Moving Iron
(c) Moving coil-Permanent Magnet type
(d) Moving coil dynamometer type

Answer-A

3. Moving iron instruments can be used
(a) To measure dc currents only
(b) To measure ac currents only
(c) To measure a.c. voltage only
(d) To measure a.c. and d.c. currents and voltage

Answer-D

4.Moving coil instrument, having coil resistance of 4.5Ω gives a full scale deflection of 30 mA. The resistance connected in parallel with this instrument to read up to 2.5A is
(a) 6.46Ω
(b) 0.0346Ω
(c) 0.0546Ω
(d) 0.0625Ω

Answer-C

5. In a particular meter, the operating torque is directly proportional to the current passing through it, then the type of meter is
(a) Moving iron
(b) Moving coil
(c) Electrostatic
(d) Induction

Answer-B

6.In inducting instruments the springs are mainly used to
(a) Conduct the current to the coils
(b) Hold the pivot in position
(c) Control the pointer movement
(d) Reduce the vibration of the pointer

Answer-C

7. To maximize the driving torque in an induction type instrument, flux produced by shunt coil and series coil should be
(a) In phase with each other
(b) In quadrature with each other
(c) Displaced by 45º with respect to each other
(d) Out of phase with respect to each other

Answer-B

8.What is a Weston frequency meter?
(a) Moving coil instrument
(b) Moving iron instrument
(c) Dynamometer type instrument
(d) none of these

Answer-B

9. Indicating instruments should be
(a) Undamped
(b) Critical damped
(c) Over damped
(d) Under damped

Answer-B

10. The effect of stray magnetic fields on the actuating torque of a portable instrument is the maximum when the operating field of the instrument and the stray fields are
(a) Perpendicular
(b) Parallel
(c) Inclined at 60º
(d) Inclined at 30º

Answer-B

11. A moving iron instrument can be used for current and voltage measurements?
(a) In AC circuit only
(b) In DC circuits only
(c) In both AC and DC circuit
(d) None of these

Answer-C

12.The controlling torque in power factor meter is given by
(a) Spring control
(b) Sensor control
(c) Gravity control
(d) None of these

Answer-D

13. A 0-1 mA PMMC ammeter reads 4 mA in circuits. Its bottom control spring snaps suddenly. The meter will now read nearly
(a) Zero
(b) 10 mA
(c) 2 mA
(d) 8 mA

Answer-A

14. Following is not an advantage of a permanent magnet moving coil instrument
(a) It has a uniform scale
(b) It can be used without error for a long period of time
(c) It has good damping torque
(d) Its range can be easily extended

Answer-B

15. Energy savings potential of variable torque application compared to constant torque application is
(a) Higher
(b) Minor
(c) Equivalent
(d) None of the above

Answer-B

16. Determine the absolute error of measurement if measured value of a resistance = 20.65Ω and true value = 20.55Ω.
(a) 0.1Ω
(b) 0.2Ω
(c) 0.3Ω
(d) 0.4Ω

Answer-A

17. The most efficient form of damping employed in electric instruments is
(a) Air friction damping
(b) Fluid friction damping                               
(c) Eddy current daming
(d) None of the above

Answer-C

18. The moving system of an indicating type of electrical instrument is subjected to
(a) a deflecting torque
(b) a controlling torque
(c) a damping torque
(d) All of the above

Answer-D

19. The type of instruments used mainly for standardizing instruments in laboratories is
(a) Indicating instrument
(b) Integrating instrument
(c) Absolute instrument
(d) Recording instrument

Answer-C

20. Which torque is essential for indicating instruments
(a) Deflecting
(b) Controlling
(c) Damping
(d) All of the above

Answer-D

Measurement mcq part -2

1. Which of the following instruments will be used to measure the temperature above 1600 C.
(a) A simple thermometer          (b) Electrical resistance pyrometer
(c) Thermo-electric pyrometer   (d) None of the above

Answer-C

2. Wein bridge finds application in
(a) Frequency determination          (b) Resistance measurement only
(c) Harmonic distortion analyzer    (d) a and c both.

Answer-D

3. The most common method for measurement of low resistance is
(a) Wheatstone bridge                     (b) Potentiometer method
(c) Voltmeter-ammeter method     (d) Kelvin’s double bridge

Answer-D

4. Moving iron instruments can be used without much error up to a frequency of
(a) 50 Hz   (b) 100 Hz   (c) 1000 Hz   (d) 1500 Hz

Answer-D

5. Subtracting 437 ± 4 from 462 ± 4 would yield a result with percentage error of
(a) ±4% (b) ±16% (c) ±8% (d) ±32%

Answer-D

6. The internal resistance of an ammeter should be
(a) Very small (b) Medium (c) High (d) Infinity

Answer-A

7. Three resistances have the following ratings
(i) 150Ω at 5%(ii)100Ω at 5% (iii) 200 Ω at 5%.
The percentage error when all the three are connected in series will be
(a) ±(5/3) % (b) ±5% (c)15 % (d) +5%

Answer-C

8. Precision measurement of resistances is generally carried out by
(a) Potentiometer method  (b) CRO method
(c) Voltmeter-ammeter method (d) Bridge method

Answer-D

9. An ideal meter should have
(a) Infinite resistance
(b) Finite resistance
(c) Absolutely no effect on the circuit being measured
(d) Definite effect on the circuit being measured

Answer-C

10.Wagner earthing device
(a) Does not affect AC bridges
(b) Affects DC potentiometer
(c) Makes possible very high accuracy in measurement
(d) Reduces the frequency and waveform errors in AC bridges

Answer-C

11. Thermocouple instruments are also known as
(a) R.F. instruments            (b) PMMC instruments
(c) Rectifier instruments       (d) Digital instruments.

Answer-C

12. The units for the deflection sensitivity of a CRO are
(a) Meter/volt (b) mm/volt (c) mm/m-volt (d) m/m-volt.

Answer-A

13. The internal resistance of a milli-ammeter must be very low for
(a) High resolution
(b) High sensitivity
(c) Maximum voltage drop across the meter
(d) Minimum effect on the current in the circuit.

Answer-D

14. The instrument efficiency is defined as
(a) The ability of the instrument to read the smallest input changes
(b) The ratio of the measured quantity at full -scale to the power taken by the instrument
(c) The ratio of the change in output signal to the change in input signal
(d) The ability of the system to reproduce the output in the same form as the input

Answer-B

15. The time base signal in a CRO is a
(a) Rectangular waveform    (b) High frequency sinusoidal wave form
(c) High frequency sawtooth wave form (d) Square wave form

Answer-C

16. The principle of operation of Q-meter is based on
(a) Self inductance(b) Mutual inductance(c) Series resonance (d) Parallel resonance

Answer-C

17.The following error does not result in moving iron instrument with both DC and AC
(a) Stray magnetic field error (b) Hysteresis error
(c) Eddy current error (d) Temperature error

Answer-C

18. A measure of the reproducibility of measurements is known as
(a) Accuracy (b) Fidelity(c) Precision (d) Resolution

Answer-C

19.The dielectric loss can be measured by
(a) Wien bridge (b) Moving coil meter
(c) Moving iron meter (d) Electro-static meter

Answer-D

20.Resistance thermometer method of temperature measurement is normally used for temperature below
(a) 1100 0F only (b) 650 0F only(c) 300 0F only (d) 180 0F only

Answer-A

Power Electronics Part Three

Power Electronics Part Three

1.Carrier frequency gate drive is used for turn on of a thyristor to reduce
(A) di/dt (B) turn on time (C) dv/dt (D) size of pulse transformer

Answer-D

2.The correct sequence of the given devices in the decreasing order of their speed of operation is
(A) power BJT, PMOSFET, IGBT, SCR (B) IGBT, PMOSFET, power BJT, SCR
(C) SCR, PBJT, IGBT, PMOSFET (D) PMOSFET, IGBT, PBJT, SCR

Answer-D

3.Over current protection of thyristors is provided by
(A) use of saturable di/dt coils (B) use of circuit breaker and fuse
(C) use of snubber circuit (D) liberal heat sinking

Answer-B

4.A thyristor, when triggered, will change from forward blocking state to conduction state if its anode to cathode voltage is equal to
(A) peak repetitive off state forward voltage
(B) peak working off state forward voltage
(C) peak working off state reverse voltage
(D) peak non repetitive off state forward voltage

Answer-B

5.The most efficient gate triggering signal for SCR is
(A) a short duration pulse (B) a steady dc level
(C) a high frequency pulse train (D) none of the above

Answer-A

6.Ten thyristors are used in a string to withstand a dc voltage of 15 kV. The worst case steady state voltage across the thyristors is 1800 V. The derating factor is                         (A) 33.3 % (B) 15.5 % (C) 16.67 % (D) 13.3 %

Answer-C

7.The di/dt rating of an SCR specified for its
(A) decaying anode current (B) decaying gate current
(C) rising gate current (D) rising anode current

Answer-D

8.Commutation or turn off of a thyristor requires that
1. anode current is reduced below holding current 2. anode voltage is reduced to zero
3. anode current is allowed to reverse 4. anode voltage gets reversed
5. reverse voltage is applied to it
From these, the correct statements are
(A) All (B) 1, 3, 4 (C) 1, 3, 4, 5 (D) 1, 2, 4

Answer-C

9.A thyristor is triggered by a pulse train of 5 kHz. The duty ratio is 0.4. If the allowable average power is 100 W, the maximum gate power allowed is
(A) 100 2 W (B) 50 W (C) 150 W (D) 250 W

Answer-D

10.A triac operating at a voltage of 120 V rms and a frequency of 60 Hz delivers power to a resistive load. The maximum rate of change current expected is 50 A/μs. The required snubber inductor value is
(A) 2.4 μH (B) 4.4 2 μH (C) 4.4 p μH (D) 4.4π μH

Answer-B

11.Which one of the following statements regarding GTO is NOT true?
(A) The GTO retains the basic four layer structure of conventional SCR
(B) The I – V characteristics of GTO in the forward direction is different as compared to conventional SCR
(C) Due to its special structure, The GTO has limited reverse blocking capability of as compared to conventional SCR
(D) The GTO has gate controlled turn off capability unlike a conventional SCR

Answer-B

12.Which one of the following statements regarding IGBT is NOT true?
(A) The IGBT is developed by combining the characteristics of MOSFET and BJT
(B) The on state losses of an IGBT are lesser than a MOSFET
(C) The IGBT is slower than BJT (D) The IGBT contains a parasitic thyristor

Answer-C

13.Triacs are most suitable when the supply voltage is
(A) dc (B) low frequency ac
(C) high frequency ac (D) full wave rectified ac

Answer-B

14.The main advantage of IGBT over SCR in power electronics is
(A) Reduced weight (B) Self commutating capability
(C) Very high reliability (D) Self cooling property

Answer-C

15.Turn on of a thyristor (SCR) takes place
(A) Anode to cathode voltage is positive (B) Anode to cathode voltage is negative
(C) Positive current is applied to gate (D) Anode to cathode voltage is positive and positive current pulse is applied to gate

Answer-D

16.A GTO SCR
(A) requires a special turn off circuit like the commutation circuit of thyristor
(B) Can be turned off by removing the gate pulse
(C) Can be turned off by giving a negative pulse to the gate
(D) Can be turned off by giving a positive pulse to the gate

Answer-C

17.The delay time of an IGBT is defined as the time for the collector – emitter voltage to fall from
(A) VCE to 0.9 VCE (B) VCE to 0.8 VCE
(C) VCE to 0.85 VCE (D) VCE to 0.95 VCE

Answer-A

18.Turn on and turn off times of a transistor depends on
(A) Voltage gain (B) Junction temperature
(C) Junction capacitance’s (D) Current gain

Answer-C

19.Anode current through the conducting SCR is 15 A. If its gate current is made one third, then what will be the anode current?
(A) 5 A (B) 0 A (C) 7.5 A (D) 15 A

Answer-D

20.The sharing of voltage between thyristor operating in series is implemented by the
(A) dv/dt (B) junction temperature
(C) Static V – I characteristics and leakage current (D) turn on ti

Answer-C

Power electronic mcq part-2

Power Electronics Part Two

1.Once SCR starts conducting a forward current, its gate looses control over
(A) anode circuit voltage only (B) anode circuit current only
(C) anode circuit voltage and current (D) anode circuit voltage, current and time

Answer-C

2.As compared to power MOSFET, a BJT has
(A) lower switching losses but higher conduction loss
(B) higher switching losses and higher conduction loss
(C) higher switching losses and lower conduction loss
(D) lower switching losses and lower conduction loss

Answer-C

3.Secondary breakdown occurs in
(A) BJT but not in MOSFET (B) MOSFET but not in BJT
(C) both BJT and MOSFET (D) None of the above

Answer-A

4.Thermal runaway is not possible in FET because, as the temperature of FET increases
(A) The mobility of charge carriers decreases
(B) The transconductance increases
(C) The drain current increase
(D) The mobility increases

Answer-A

5.Common emitter current gain hFE of a BJT is
(A) dependent on collector current Ic
(B) dependent on collector emitter voltage, VCE
(C) dependent on base emitter voltage, VCE
(D) always constant

Answer-A

6.Which of the following device is not current triggered device?
(A) Thyristor (B) GTO (C) TRIAC (D) MOSFET

Answer-D

7.Which of the following devices should be used as a switch in a low power switched mode power supply (SMPS) where switching frequency in the range of 20 kHz to 100 kHz?
(A) GTO (B) MOSFET (C) TRIAC (D) SCR

Answer-B

8.The average current rating of a thyristor, as supplied by manufacturers, corresponds to
(A) resistive current (B) Inductive current
(C) capacitive current (D) Independent of load

Answer-A

9.A forward voltage can be applied to SCR after its
(A) Anode current reduces to zero (B) gate recovery time
(C) reverse recovery time (D) anode voltage reduces to zero

Answer-B

10.What is the average on state current for 60° conduction angle where in the average on state current for an SCR is 20 A for conduction angle of 120°
(A) 20 A (B) 25 A (C) 40 A (D) < 20 A

Answer-D

11.The average on state current for an SCR is 20 A for a resistive load. If an inductance of 5 mH is included in the load, then average on state current would be
(A) more than 20 A (B) less than 20 A (C) 15 A (D) 20 A

Answer-A

12.Data sheet for an SCR gives its maximum rms on state current as 35 A. The rms current rating for a conduction angle of 120° would be
(A) more than 35 A (B) less than 35 A (C) 35 A (D) 20 A

Answer-C

13.An SCR has specified value of dv/dt as 157 V/μs. If a sinusoidal noise signal of frequency 500 kHz appears, the maximum allowed peak value of noise signal for not triggering the SCR is
(A) 50 V (B) 25 V (C) 100 V (D) 150 V

Answer-A

14.Turn on time of an SCR can be reduced by using a
(A) rectangular pulse of high amplitude and narrow width
(B) rectangular pulse of low amplitude and wide width
(C) triangular pulse (D) trapezoidal pulse

Answer-A

15.For an SCR with turn on time of 5 μs, an ideal trigger pulse should have
(A) short rise time with pulse width = 3 μs (B) long rise time with pulse width = 6 μs
(C) short rise time with pulse width = 6 μs (D) long rise time with pulse width = 3 μs

Answer-C

16.For a pulse transformer, the material used for its core and the possible turn ratio from primary to secondary are, respectively
(A) ferrite; 20 : 1 (B) laminated iron; 1 : 1
(C) ferrite; 1 : 1 (D) powdered iron; 1 : 1

Answer-C

17.An SCR triggered by current pulse applied to gate – cathode can be turned OFF
(A) by applying pulse to cathode
(B) by applying pulse to anode
(C) by applying another pulse of opposite polarity to the gate cathode
(D) by reversing the polarity of the anode and cathode voltage

Answer-D

18.The capacitance of reverse biased junction of a thyristor is 20 pF. The charging current of this thyristor is 4 mA. The limiting value of dv/dt in V/μs is
(A) 50 (B) 100 (C) 200 (D) 500

Answer-C

19.Two identical SCRs are placed back to back in series with load. If each is fired at 90o, a dc voltmeter across the load will read
(A) Zero                         (B) 2/π*peak voltage
(C) 1/π*peak voltage   (D) 4/π*peak voltage

Answer-A

20.Which one of the following statement is correct? In order to get the best results per unit cost, the heat sinks on which thyristors are mounted, are made of
(A) copper (B) Aluminium (C) nickel (D) stainless steel

Answer-B

Control system mcq part-3

1. Which of the following is the input to a controller ?
(a) Servo signal
(b) Desired variable value
(c) Error signal
(d) Sensed signal

Ans:c

2. The on-off controller is a _____ system.
(a) digital
(b) linear
(c) non-linear
(d) discontinuous

Ans:c

3. The capacitance, in force-current analogy, is analogous to
(a) momentum
(b) velocity
(c) displacement
(d) mass
Ans: d

4. The temperature, under thermal and electrical system analogy, is considered analogous to
(a) voltage
(b) current
(c) capacitance
(d) charge
(e) none of the above

Ans: a

5. In electrical-pneumatic system analogy the current is considered analogous to
(a) velocity
(b) pressure
(c) air flow
(d) air flow rate

Ans: d

6. In liquid level and electrical system analogy, voltage is considered analogous to
(a) head
(b) liquid flow
(c) liquid flow rate
(d) none of the above

Ans: a

7. The viscous friction co-efficient, in force-voltage analogy, is analogous to
(a) charge
(b) resistance
(c) reciprocal of inductance
(d) reciprocal of conductance
(e) none of the above

Ans: b

8. In force-voltage analogy, velocity is analogous to
(a) current
(b) charge
(c) inductance
(d) capacitance

Ans: a

9. In thermal-electrical analogy charge is considered analogous to
(a) heat flow
(b) reciprocal of heat flow
(c) reciprocal of temperature
(d) temperature
(e) none of the above

Ans: d

10. Mass, in force-voltage analogy, is analogous to
(a) charge
(b) current
(c) inductance
(d) resistance

Ans: c

11. The transient response of a system is mainly due to
(a) inertia forces
(b) internal forces
(c) stored energy
(d) friction

Ans: c

12.--signal will become zero when the feedback signal and reference signs are equal.
(a) Input
(b) Actuating
(c) Feedback
(d) Reference

Ans: b

13. A signal other than the reference input that tends to affect the value of controlled variable is known as
(a) disturbance
(b) command
(c) control element
(d) reference input

Ans: a

14. The transfer function is applicable to which of the following ?
(a) Linear and time-in variant systems
(b) Linear and time-variant systems
(c) Linear systems
(d) Non-linear systems
(e) None of the above
Ans: a

15. From which of the following transfer function can be obtained ?
(a) Signal flow graph
(b) Analogous table
(c) Output-input ratio
(d) Standard block system
(e) None of the above

Ans: a

16.----is the reference input minus the primary feedback.
(a) Manipulated variable
(b) Zero sequence
(c) Actuating signal
(d) Primary feedback

Ans: c

17. The term backlash is associated with
(a) servomotors
(b) induction relays
(c) gear trains
(d) any of the above

Ans: a

18. With feedback _____ increases.
(a) system stability
(b) sensitivity
(c) gain
(d) effects of disturbing signals

Ans: a

19. By which of the following the system response can be tested better ?
(a) Ramp input signal
(b) Sinusoidal input signal
(c) Unit impulse input signal
(d) Exponentially decaying signal

Ans: c
20. In a system zero initial condition means that
(a) The system is at rest and no energy is stored in any of its components
(b) The system is working with zero stored energy
(c) The system is working with zero reference signal

Ans: a

Control system mcq part -2

1.--- increases the steady state accuracy.
(a) Integrator
(b) Differentiator
(c) Phase lead compensator
(d) Phase lag compensator

Ans: a

2. A.C. servomotor resembles
(a) two phase induction motor
(b) Three phase induction motor
(c) direct current series motor
(d) universal motor

Ans: a

3. As a result of introduction of negative feedback which of the following will not decrease ?
(a) Band width
(b) Overall gain
(c) Distortion
(d) Instability

Ans: a

4. Regenerative feedback implies feedback with
(a) oscillations
(b) step input
(c) negative sign
(d) positive sign

Ans: d

5. The output of a feedback control system must be a function of
(a) reference and output
(b) reference and input
(e) input and feedback signal
(d) output and feedback signal

Ans: a

6.--- is an open loop control system.
(a) Ward Leonard control
(b) Field controlled D.C. motor
(c) Stroboscope
(d) Metadyne

Ans: b

7. A control system with excessive noise, is likely to suffer from
(a) saturation in amplifying stages
(b) loss of gain
(c) vibrations
(d) oscillations

Ans: a

8. Zero initial condition for a system means
(a) input reference signal is zero
(b) zero stored energy
(c) ne initial movement of moving parts
(d) system is at rest and no energy is stored in any of its components

Ans: d

9. Transfer function of a system is used to calculate which of the following ?
(a) The order of the system
(b) The time constant
(c) The output for any given input
(d) The steady state gain

Ans: c

10. The band width, in a feedback amplifier.
(a) remains unaffected
(b) decreases by the same amount as the gain increase
(c) increases by the sane saaaajajt as the gain decrease
(d) decreases by the same amount as the gain decrease

Ans: c

11. On which of the following factors does the sensitivity of a closed
loop system to gain changes and
load disturbances depend ?
(a) Frequency
(b) Loop gain
(c) Forward gain
(d) All of the above

Ans: d

12. The transient response, with feedback system
(a) rises slowly
(b) rises quickly
(c) decays slowly
(d) decays quickly

Ans: d

13. The second derivative input signals modify which of the following ?
(a) The time constant of the system
(b) Damping of the system
(c) The gain of the system
(d) The time constant and suppress the oscillations
(e) None of the above
Ans: d

14. Which of the following statements is correct for any closed loop system ?
(a) All the co-efficients can have zero value
(6) All the co-efficients are always non-zero
(c) Only one of the static error co-efficients has a finite non-zero value
(d) None of the above

Ans: c

15. Which of the following statements is correct for a system with gain
margin close to unity or a phase margin close to zero ?
(a) The system is relatively stable
(b) The system is highly stable
(c) The system is highly oscillatory
(d) None of the above

Ans: c

16. Due to which of the following reasons excessive bond width in control systems should be avoided ?
(a) It leads to slow speed of response
(b) It leads to low relative stabilit
(c) Noise is proportional to band width
(d) None of the above

Ans: c

17. In a stable control system backlash can cause which of the following
?
(a) Underdamping
(b) Overdamping
(c) Poor stability at reduced values of open loop gain
(d) Low-level oscillations

Ans: d

18. In an automatic control system which of the following elements is not used ?
(a) Error detector
(b) Final control element
(c) Sensor
(d) Oscillator

Ans: d

19. In a control system the output of the controller is given to
(a) final control element
(b) amplifier
(c) comparator
(d) sensor
(e) none of the above

Ans: a

40. A controller, essentially, is a
(a) sensor
(b) clipper
(c) comparator
(d) amplifier

Ans: c