Saturday, 30 April 2016

Measurements Instrumentation Objective Type Questions Answers: Part-10

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Measurements Instrumentation Objective Type Questions Answers: Part-10

[1] A 12 bit counter type A/D converter uses a 1 MHz clock. Its maximum conversion time is
A) 1 �s
B) 12�s
C) 4096�s
D) 4�s


[2] A 12 bit counter type A/D converter uses a 1 MHz clock. Its maximum conversion rate is
A) 106 conversions/sec
B) 244 conversions/sec
C) 488 conversions/sec
D) 83 kilo conversions/sec


[3] An 8 bit converter type A/D converter makes at least 8000conversions/sec. The clock frequency used is
A) 2MHz
B)  8MHz
C)  1MHz
D)  4MHz


[4] The conversion time of a 12 bit successive approximation A/D converter using a 1 MHz clock is 
A) 1 �s
B) 12�s
C) 4096�s
D) 4095�s


[5] The resolution of a 12 bit D/A converter using a binary ladder is 
A) 4096
B) 0.02%
C) 2.4%
D) 4%


[6] A 12 bit counter type A/D converter uses a 1 MHz clock. If the full scale output is +10V, its resolution output is
A) 2.44mV
B) 2.4mV
C) 0.02V
D) 0.02mV


[7] A 10 bit resistive divider is constructed such that the current through the LSB resistor is 100�A. The maximum current that will flow through the MSB resistor is  
A) 200�A.
B) 1mA
C) 51.2mA 
D) 102.4mA


[8] For a certain 4 bit successive approximation A/D converter, the maximum ladder output is +8V. if a constant +6V is applied to the analog input the sequence of binary states for the SA register is 
A) 1111
B) 1010
C) 0110
D) 1011


[9] In the case of power measurement by two wattmeter method in a balanced 3F system with a pure inductive load 
A) both the  Wattmeter will indicate the same but of opposite sign.
B) both the Wattmeters will indicate zero.
C) both the Wattmeters will indicate same value and of the same sign.
D) one Wattmeter will indicate zero and the other non-zero value.


[10] The sensitivity of an instrument is 
A) smallest increment in the output the can be detected with certainity.
B) largest input change to which the instrument fails to respond.
C) ratio of the change in the magnitude of the output to the corresponding change in the magnitude of the input.
D) closeness of the output values for repeated applications of a constant input.

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Tuesday, 9 February 2016

Electrical Engineering Basics Objective Questions with Answers: Part-17

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Electrical Engineering Basics Objective Questions with Answers

[1] Electrostatics is a branch of electricity concerned with
(a) Energy flowing across a gap between conductors
(b) Charges at rest
(c) Charges in motion
(d) Energy in the form of charges


[2] Four 2 �F capacitors are connected in series. The equivalent capacitance is
(a) 8 �F
(b) 0.5 �F
(c) 2 �F
(d) 6 �F


[3] State which of the following is false.
The capacitance of a capacitor
(a) Is proportional to the cross-sectional area of the plates
(b) Is proportional to the distance between the plates
(c) Depends on the number of plates
(d) Is proportional to the relative permittivity of the dielectric


[4] The capacitance of a capacitor is the ratio
(a) Charge to potential difference between plates
(b) Potential difference between plates to plate spacing
(c) Potential difference between plates to thickness of dielectric
(d) Potential difference between plates to charge


[5] Which of the following statement is false?
(a) An air capacitor is normally a variable type
(b) A paper capacitor generally has a shorter service life than most other types of
capacitor
(c) An electrolytic capacitor must be used only on a.c. supplies
(d) Plastic capacitors generally operate satisfactorily under conditions of high temperature


[6] The potential difference across a 10 �F capacitor to charge it with 10mC is
(a) 10V
(b) 1 kV
(c) 1V
(d) 10V


[7] The energy stored in a 10�F capacitor when charged to 500V is
(a) 1.25 mJ
(b) 0.025 �J
(c) 1.25 J
(d) 1.25 C


[8] The capacitance of a variable air capacitor is at maximum when
(a) The movable plates half overlap the fixed plates
(b) The movable plates are most widely separated from the fixed plates
(c) Both sets of plates are exactly meshed
(d) The movable plates are closer to one side of the fixed plate than to the other


[9] The unit of magnetic flux density is the:
(a) Weber
(b) Weber per metre
(c) Ampere per metre
(d) Tesla


[10] The charge on a 10 pF capacitor when the voltage applied to it is 10 kV is
(a) 100 �C
(b) 0.1 C
(c) 0.1 �C
(d) 0.01 �C


[11] Four 2 �F capacitors are connected in parallel. The equivalent capacitance is
(a) 8 �F
(b) 0.5 �F
(c) 2 �F
(d) 6 �F


[12] In a series a.c. circuit the voltage across a pure inductance is 12V and the voltage across a pure resistance is 5V. The supply voltage is
(a) 13V
(b) 17V
(c) 7V
(d) 2.4V


[13] Inductive reactance results in a current that
(a) Leads the voltage by 90deg
(b) Is in phase with the voltage
(c) Leads the voltage by p rad
(d) Lags the voltage by p/2 rad


[14] A 10O resistor is connected in parallel with a 15 O resistor and the combination in series with a 12 O resistor. The equivalent resistance of the circuit is:
(a) 37O
(b) 18 O
(c) 27 O
(d) 4 O


[15] The equivalent resistance when a resistor of (1/3)O is connected in parallel with a (1/4)O resistance is:
(a) 1/7 O
(b) 7O
(c) 1/12 O
(d) 3/4 O

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Friday, 25 September 2015

DC Machine Multiple Choice Questions with Solutions: Part-12

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DC Motor DC Generator Multiple Choice Questions with Solutions:

[1] Which of the following statements is false?
(a) A DC motor converts electrical energy to mechanical energy
(b) The efficiency of a DC motor is the ratio input power to output power
(c) A DC generator converts mechanical power to electrical power
(d) The efficiency of a DC generator is the ratio output power to input power
Options:
1. C only
2. C and D only
3. B only
4. None of the above



[2] If the speed of a DC machine is doubled and the flux remains constant, the generated e.m.f.
(a) remains the same
(b) is doubled
(c) is halved
(d) None of the above



[3] If the flux per pole of a shunt-wound DC generator is increased, and all other variables are kept the same, the speed
(a) decreases
(b) stays the same
(c) increases
(d) None of the above



[4] Which of the following statements is false?
(a) A commutator is necessary as part of a DC motor to keep the armature rotating in the same direction
(b) A commutator is necessary as part of a DC generator to produce unidirectional voltage at the terminals of the generator
(c) The field winding of a DC machine is housed in slots on the armature
(d) The brushes of a DC machine are usually made of carbon and do not rotate with the armature
Options:
1. D only
2. C and D only
3. None of the above
4. C only



[5] If the flux per pole of a shunt-wound DC generator is halved, the generated e.m.f. at constant speed
(a) is doubled
(b) is halved
(c) remains the same
(d) None of the above



[6] In a series-wound generator running at constant speed, as the load current increases, the terminal voltage
(a) increases
(b) decreases
(c) stays the same
(d) None of the above



[7] Which of the following statements is false for a series-wound DC motor?
(a) The speed decreases with increase of resistance in the armature circuit
(b) The speed increases as the flux decreases
(c) The speed can be controlled by a diverter
(d) The speed can be controlled by a shunt field regulator



[8] The armature resistance of a DC motor is 0.5 , the supply voltage is 200V and the back e.m.f. is 196V at full speed. The armature current is:
(a) 4A
(b) 8A
(c) 400A
(d) 392A



[9] In DC generators iron losses are made up of:
(a) hysteresis and friction losses
(b) hysteresis, eddy current and brush contact losses
(c) hysteresis and eddy current losses
(d) hysteresis, eddy current and copper losses



[10] The effect of inserting a resistance in series with the field winding of a shunt motor is to:
(a) increase the magnetic field
(b) increase the speed of the motor
(c) decrease the armature current
(d) reduce the speed of the motor



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Friday, 21 August 2015

3 Phase Induction Motors Objective Questions with Answers: Part-2

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3 Phase Induction Motors Related ( MCQ ) Objective Questions with Answers

[1] Which of the following statements about a three-phase squirrel-cage induction motor are false?
(a) It has no external electrical connections to its rotor
(b) A three-phase supply is connected to its stator
(c) A magnetic flux which alternates is produced
(d) It is cheap, robust and requires little or no skilled maintenance

(1) A,B,C only
(2) C and D only
(3) C only
(4) None of the above
(5) B only



[2] Which of the following statements about a three-phase induction motor are false?
(a) The speed of rotation of the magnetic field is called the synchronous speed
(b) A three-phase supply connected to the rotor produces a rotating magnetic field
(c) The rotating magnetic field has a constant speed and constant magnitude
(d) It is essentially a constant speed type machine

(1) C only
(2) B only
(3) A only
(4) All of the above
(5) A and C only



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[3] Which of the following statements is false when referring to a three-phase induction motor?
(a) The synchronous speed is half the supply frequency when it has four poles
(b) In a 2-pole machine, the synchronous speed is equal to the supply frequency
(c) If the number of poles is increased, the synchronous speed is reduced
(d) The synchronous speed is inversely proportional to the number of poles

(1) All of the above
(2) D only
(3) B only
(4) None of the above
(5) A and C only



[4] A 4-pole three-phase induction motor has a synchronous speed of 25 rev/s. The frequency of the supply to the stator is:
(a) 50 Hz
(b) 100 Hz
(c) 25 Hz
(d) 12.5 Hz



[5] In a three-phase induction motor. Which of the following statements are false?
(a) The slip speed is the synchronous speed minus the rotor speed
(b) As the rotor is loaded, the slip decreases
(c) The frequency of induced rotor e.m.f.�s increases with load on the rotor
(d) The torque on the rotor is due to the interaction of magnetic fields

(1) All of the above
(2) C only
(3) B only
(4) A and C only
(5) B and D only



[6] In a three-phase induction motor. Which of the following statements are false?
(a) If the rotor is running at synchronous speed, there is no torque on the rotor
(b) If the number of poles on the stator is doubled, the synchronous speed is halved
(c) At no-load, the rotor speed is very nearly equal to the synchronous speed
(d) The direction of rotation of the rotor is opposite to the direction of rotation of the magnetic field to give maximum current induced in the rotor bars

(1) A, B , C
(2) C only
(3) B only
(4) A and C only
(5) D only



[7] The slip speed of an induction motor may be defined as the:
(a) Number of pairs of poles � frequency
(b) Rotor speed - synchronous speed
(c) Rotor speed + synchronous speed
(d) Synchronous speed  - rotor speed



[8] The slip speed of an induction motor depends upon:
(a) Armature current
(b) Supply voltage
(c) Mechanical load
(d) Eddy currents



[9] The starting torque of a simple squirrel-cage motor is:
(a) Low
(b) Increases as rotor current rises
(c) Decreases as rotor current rises
(d) High



[10] The slip speed of an induction motor:
(a) is zero until the rotor moves and then rises slightly
(b) is 100 per cent until the rotor moves and then decreases slightly
(c) is 100 per cent until the rotor moves and then falls to a low value
(d) is zero until the rotor moves and then rises to 100 per cent


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Wednesday, 8 July 2015

Multiple Choice Questions [MCQ] on Basic Electrical Engineering -Part16

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Objective Type Questions on Basic Electrical Engineering -Part16

[1]  A 240V, 60W lamp has a working resistance of:
(a) 1400O
(b) 60O
(c) 960O
(d) 325O


[2] The symbol for the unit of temperature coefficient of resistance is:
(a) O /�C
(b)O
(c) �C
(d) O /O�C


[3] The length of a certain conductor of resistance 100O is doubled and its cross-sectional area is halved. Its new resistance is:
(a) 100O
(b) 200O
(c) 50O
(d) 400O


[4] Voltage drop is the:
(a) Maximum potential
(b) Difference in potential between two points
(c) Voltage produced by a source
(d) Voltage at the end of a circuit


[5] The largest number of 100W electric light bulbs which can be operated from a 240V supply fitted with a 13A fuse is:
(a) 2
(b) 7
(c) 31
(d) 18


[6] When an atom loses an electron, the atom:
(a) Becomes positively charged
(b) Disintegrates
(c) Experiences no effect at all
(d) Becomes negatively charged


[7] The energy used by a 1.5kW heater in 5 minutes is:
(a) 5 J
(b) 450 J
(c) 7500 J
(d) 450 000 J


[8] The unit of resistivity is:
(a) ohms
(b) ohm millimetre
(c) ohm metre
(d) ohm/metre


[9] A resistor marked as4K7G indicates a value of:
(a) 47O �20%
(b) 4.7O k �20%
(c) 0.47O �10%
(d) 4.7 kO �2


[10] The resistance of a 2 km length of cable of cross-sectional area 2mm2 and resistivity of 2�10-8O m is:
(a) 0.02O
(b) 20O
(c) 0.02mO
(d) 200O


[11] A piece of graphite has a cross-sectional area of 10mm2. If its resistance is 0.1O and its resistivity 10�108 Om, its length is:
(a) 10 km
(b) 10 cm
(c) 10 mm
(d) 10 m


[12] A nickel coil has a resistance of 13O at 50�C. If the temperature coefficient of resistance at 0�C is 0.006/ �C, the resistance at 0�C is:
(a) 16.9O
(b) 10O
(c) 43.3O
(d) 0.1O


[13] A coil of wire has a resistance of 10 at 0�C. If the temperature coefficient of resistance for the wire is 0.004/ �C, its resistance at 100�C is:
(a) 0.4O
(b) 1.4O
(c) 14O
(d) 10O


[14] A colour coding of red-violet-black on a resistor indicates a value of:
(a) 27O �20%
(b) 270O
(c) 270O �20%
(d) 27O �10%


[15] An inductance of 10mH connected across a 100V, 50 Hz supply has an inductive reactance of
(a) 10p
(b) 1000p
(c) p
(d) p H
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Friday, 10 April 2015

Multiple Choice Questions with Answers on Power Electronics and Drives: Part15

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Multiple Choice Questions with Answers on Power Electronics and Drives

[Q-1] Silicon based rectifiers are preferred than germanium based rectifiers because
(a) Si is available easily compared to Ge
(b) Only Si has a stable off state
(c) Ge is very temperature sensitive
(d) Si only has the characteristics a1 + a2 < 1 at low collector currents and reaches 1 at high currents
Which of the above statements are true?
Options:
1. A, B, D
2. B, D
3. B Only
4. D Only
5. None of the above options

[Q-2] In a triac,
(a) The triggering pulse to main terminal 1 should be of the same polarity as the anode potential between MT1 and MT2
(b) The triggering pulse should be of opposite polarity to that of anode potential
(c) When the triggering pulse is positive and anode is positive, it is operating in the first quadrant
(d) When the triggering pulse is negative and anode is negative its sensitivity is highest.
Which of the above statements are true?
Options:
1. A, D
2. A, C
3. C, D
4. A, C, D
5. None of the above options


[Q-3] In a single phase full wave SCR circuit with R, L load
(a) Power is delivered to the source for firing angle of less than 90�
(b) The scr changes from inverter to converter at a = 90�
(c) The negative dc voltage is maximum at a = 180�
(d) To turn off the scr, the maximum delay angle must be less than 180�
Which of the above statements are true?
Options:
1. C, D
2. C Only
3. D Only
4. A , B
5. None of the above options


[Q-4] While comparing triac and scr,
(a) Both are unidirectional devices
(b) Triac requires more current for turn on than scr at a particular voltages
(c) A triac has less time for turn off than SCR
(d) Both are available with comparable voltage and current ratings
Which of the above statements are true?
Options:
1. A, C
2. B, C
3. A, B
4. D Only
5. None of the above options


[Q-5] Which of the following statements are true
(a) If the SCR, even with proper gate excitation and anode-cathode voltage does not conduct for a particular load resistance, then it would be necessary to decrease the load resistance to turn ON the SCR
(b) The SCR would be turned OFF by voltage reversal of the applied anode-cathode ac supply of frequencies up to 30kHz
(c) If the gate current of the SCR is increased, then the forward breakdown voltage will decrease
Options
1. A, B, C
2. B, C
3. A , C
4. A, B
5. None of the above options


[Q-6] Which of the following statements are true
When gate triggering is employed, a SCR can withstand higher values of di/dt, if the
1. Gate current is increased
2. Rate of rise of gate current is increased
3. Gate current is increased
4. Rate of rise of gate current is decreased
OPTIONS:
(a) 3, 4
(b) 1, 4
(c) 2, 3
(d) 1, 2
(e) None of the above options


[Q-7] In a SCR based converter, the free wheeling diode is used to
(a) Add to the conduction current of thyristors
(b) Oppose the SCR conduction
(c) Conduct current during the OFF period of the SCR
(d) Protect the SCR by providing a shunt path

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Wednesday, 25 March 2015

Power Electronics Objective Type Questions with Answers: Part-14

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Power Electronics Objective Questions and Answers

[Q-1] When an inductance is inserted in the load circuit of SCR
(a) The turn on time of SCR is increased
(b) Output voltage is reduced for the same firing angle
(c) Conduction continues even after reversal of phase of input voltage
(d) A free wheeling diode is connected in such circuits
Which of the above statements are true?
(1) a, d
(2) b, c, d
(3) a, b, c,d
(4) c, d


[Q-2] Snubber circuit is used to limit the rate of
(a) Rise of current
(b) Conduction period
(c) Rise of voltage across SCR
(d) None of the above


[Q-3] The provision of a free wheeling diode across an inductive load is
(a) To restore conduction angle on phase
(b) To avoid negative reversal voltage drop
(c) To reduce the PRV
(d) None of the above


[Q-4] While working in series operation, equalising circuits are added across each SCR to provide uniform
(a) Current distribution
(b) Firing of SCRs
(c) Voltage distribution
(d) None of the above


[Q-5] When the SCR conducts, the forward voltage drop
(a) Is 0.7V
(b) Is 1 to 1.5V
(c) Increases slightly with load current
(d) Remains constant with load current
Which of the above statements are true?
(1) a only
(2) b, c
(3) d only
(4) a, c


[Q-6] The latching current of a SCR is 18mA. Its holding current will be
(a) 6mA
(b) 18mA
(c) 54mA
(d) 12mA
Exp: Latching Current = 3 x Holding Current


[Q-7] The turn off time is longer than turn on time because
(a) The anode and cathode junctions get reverse biased while gate junction is still forward biased
(b) There is flow of reverse current
(c) The gate pulse has been removed
(d) The forward break over voltage is high.
Which of the above statements are true?
(1) b, c
(2) c, d
(3) a, b
(4) a, b, c, d


[Q-8] The thyristor will turn on faster with
(a) Pulse signal applied to the gate terminal of the SCR
(b) Continuous signal applied to the gate terminal of the SCR
(c) Both are same
(d) Pulse signal but with minimum duration
Which of the above statements are true?
(1) a only
(2) b only
(3) a, d
(4) none of the above


[Q-9] In an SCR,
(a) The holding current is less than latching current
(b) The holding current is greater than latching current
(c) The two currents are equal
(d) The latching current is about 3 times the holding current
Which of the above statements are true?
(1) a only
(2) d only
(3) a, c, d
(4) a, d
(5) b only


[Q-10] When a positive voltage is applied to the gate of a reverse biased SCR
(a) It injects more electrons into junction J1
(b) It increases reverse leakage current into anode
(c) Heating of junction is unaffected
(d) Failure of junctions occurs due to thermal runaway
Which of the above statements are true?
(1) b only
(2) a, b ,d
(3) b, d
(4) d only


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