Control system mcq part -5

1. In a stable control system saturation can cause which of the following ?
(a) Low-level oscillations
(b) High-level oscillations
(c) Conditional stability
(d) Overdamping

Ans: d

2. Which of the following can be measured by the use of a tachogenerator ?
(a) Acceleration
(b) Speed
(c) Speed and acceleration
(d) Displacement
(e) None of the above

Ans: b

3.---- is not a final control element.
(a) Control valve
(b) Potentiometer
(c) Electropneumatic converter
(d) Servomotor
Ans: b

4. Which of the following is the definition of proportional band of a controller ?
(a) The range of air output as measured variable varies from maximum to minimum
(b) The range of measured variables from set value
(c) The range of measured variables through which the air output changes from
maximum to minimum
(d) Any of the above
(e) None of the above

Ans: c

5. In pneumatic control systems the control valve used as final control element converts
(a) pressure signal to electric signal
(b) pressure signal to position change
(c) electric signal to pressure signal
(d) position change to pressure signal
(e) none of the above

Ans: b

6. Pressure error can be measured by which of the following ?
(a) Differential bellows and straingauge
(b) Selsyn
(c) Strain gauge
(d) Strain gauge and potentiometer

Ans: a

7. Which of the following devices is used for conversion of co-ordinates ?
(a) Microsyn
(b) Selsyn
(c) Synchro-resolver
(d) Synchro-transformer

Ans: c

8. The effect of error damping is to
(a) provide larger settling lime
(b) delay the response
(c) reduce steady state error
(d) any of the above
(e) none of the above

Ans: c

9.---technique gives quick transient and stability response
(a) Root locus
(b) Bode
(c) Nyquist
(d) Nichols

Ans: a

10. A phase lag lead network introduces in the output
(a) lag at all frequencies
(b) lag at high frequencies and lead at low frequencies
(c) lag at low frequencies and lead at high frequencies
(d) none of the above

Ans: c

11. Which of the following is the non-linearity caused by servomotor ?
(a) Static friction
(b) Backlash
(c) Saturation
(d) None of the above

Ans: c

12.-- can be extended to systems which are time-varying ?
(a) Bode-Nyquist stability methods
(b) Transfer functions
(c) Root locus design
(d) State model representatives

Ans: d

13. When the initial conditions of a system are specified to be zero it implies that the system is
(a) at rest without any energy stored in it
(b) working normally with reference input
(c) working normally with zero reference input
(d) at rest but stores energy

Ans: d

14. Which of the following is an electromagnetically device ?
(a) Induction relay
(b) Thermocouple
(c) LVDT
(d) Any of the above
(e) None of the above

Ans: c

15. A differentiator is usually not a part of a control system because it
(a) reduces damping
(b) reduces the gain margin
(c) increases input noise
(d) increases error

Ans: c

16. If the gain of the critical damped system is increased it will behave as
(a) oscillatory
(b) critically damped
(c) overdamped
(d) underdamped
(e) none of the above

Ans: d

17. In a control system integral error compensation _______ steady state error
(a) increases
(b) minimizes
(c) does not have any effect on
(d) any of the above

Ans: b

18. With feed back _____ reduces.
(a) system stability
(6) system gain
(c) system stability and gain
(d) none of the above

Ans: b

19. An amplidyne can give which of the following characteristics ?
(a) Constant current
(b) Constant voltage
(c) Constant current as well as constant voltage
(d) Constant current, constant voltage and constant power
(e) None of the above

Ans: d

20. Which of the following can be measured by LVDT?
(a) Displacement
(b) Velocity
(c) Acceleration
(d) Any of the above

Ans: d

21.-- directly converts temperature into voltage.
(a) Thermocouple
(b) Potentiometer
(c) Gear train
(d) LVDT
(e) None of the above

Ans: a

22. The transfer function technique is considered as inadequate under which of the following conditions ?
(a) Systems having complexities and non-linearities
(b) Systems having stability problems
(c) Systems having multiple input disturbances
(d) All of the above

Ans: d

23. Which of the following is the output of a thermocouple ?
(a) Alternating current
(b) Direct current
(c) A.C. voltage
(d) D.C. voltage
(e) None of the above

Ans: d

24. A.C. servomotor is basically a
(a) universal motor
(b) single phase induction motor
(c) two phase induction motor
(d) three phase induction motor

Ans: c

25. The first order control system, which is well designed, has a
(a) small bandwidth
(b) negative time constant
(c) large negative transfer function pole
(d) none of the above

Ans: c

26. Which of the following is exhibited by Root locus diagrams ?
(a) The poles of the transfer function for a set of parameter values
(b) The bandwidth of the system
(c) The response of a system to a step input
(d) The frequency response of a system
(e) None of the above

Ans: a

Power electronic mcq part -5

Power Electronics Part Five

1.The forward voltage drop during SCR on state is 1.5 V. This voltage drop
(A) remains constant and is independent of load current
(B) increases slightly with load current
(C) decreases slightly with load current
(D) Varies linearly with load current

Answer-B

2.Turn on time of an SCR in series with RL circuit can be reduced by
(A) increasing circuit resistance R (B) decreasing R
(C) increasing circuit inductance L (D) decreasing L

Answer-D

3.In an SCR, anode current flows over a narrow region near the gate during
(A) delay time td (B) rise time tr and spread time tp
(C) td and tp (D) td and tr

Answer-D

4.Consider the following statements
1. The triac is a five layer device
2. The triac may be considered to consist of two parallel sections of p1 n1 p2 n2 and p2 n1 p1 n4
3. An additional lateral region serves as the control gate
4. The triac is double ended SCR
From above, the correct statements are
(A) all (B) 1, 2, 3 (C) 1 only (D) 1, 4

Answer-A

5.Use of a reverse conducting thyristor in place of anti parallel combination of thyristor and feedback diode in an inverter
(A) effectively minimizes the peak commutating current
(B) decreases the operating frequency of operation
(C) minimizes the effects of load inductances on the commutation performance
(D) causes deterioration in the commutation performance

Answer-C

6.Which of the following characteristic of a silicon power diode makes it suitable for use as an ideal diode
(A) It has very low saturation current (B) It has high value of forward cut in voltage
(C) It can withstand large reverse voltage
(D) When compared with germanium diodes, silicon diodes show a lower degree of temperature dependence under reverse bias conditions
Select the correct answer using the codes given below
(A) 1 and 2 (B) 1, 2, 3 and 4 (C) 2, 3 and 4 (D) 1 and 3

Answer-D

7.Current unbalance in the parallel connected SCRs is due to the non uniformity in the
(A) forward characteristics (B) reverse characteristics
(C) di/dt withstanding capability (D) dv/dt withstanding capability

Answer-A

8.The turn on time of an SCR is 30 μs. The pulse train at the gate has a frequency of 2.5 kHz with a mark/space ratio of 1.0. This SCR will
(A) turn on (B) not turn on
(C) turn on if pulse frequency increased (D) turn on if pulse frequency decreased

Answer-A

9.Which one of the following statements regarding the two transistor model of the p – n – p – n four layer device is correct
(A) It explains only the turn on portion of the device characteristics
(B) It explains only the turn off portion of the device characteristics
(C) It explains only the negative region portion of the device characteristics
(D) It explains all the regions of the device characteristics

Answer-A

10.When compared with BJTs, MOSFETs have lesser turn off time, enabling them to operate at high operating frequencies. What is the reason that can be contributed to that property
(A) High input impedance (B) positive temperature coefficient
(C) Absence of minority stored charge (D) Smaller leakage current

Answer-C

11.Satisfactory turn off of a thyristor will takes place if the reverse bias period of the device, after its current has become zero, is
(A) greater than its turn off time (B) less than its turn off time
(C) greater than circuit time constant (D) less than circuit time constant

Answer-A

12.A BJT operates as a switch
(A) in active region of transfer characteristics (B) with no signal condition
(C) under small signal condition (D) under large signal condition

Answer-D

13.n – p – n transistors are preferred in power electronic circuits over p – n – p transistors because they have
(A) high mobility of holes (B) high mobility of electrons in p – n – p
(C) low mobility of holes
(D) higher mobility of electrons than the mobility of holes in p – n – p transistors

Answer-D

14.Which one of the following device has bi directional current capability?
(A) SCR (B) MOSFET (C) TRIAC (D) IGBT

Answer-C

15.For the V – I characteristics of an SCR
1. It will trigger when the applied voltage is more than the forward break over voltage
2. Holding current is greater than latching current
3. When reverse biased, a small value of leakage current will flow
4. It can be triggered without gate
Which of the above statements are correct
(A) 1, 2 and 3 (B) 1, 3 and 4 (C) 1, 2 and 4 (D) 2, 3 and 4

Answer-B

16.Consider the following statements regarding thyristor
1. It conducts when forward biased and positive current flows through gate
2. It conducts when forward biased and negative current flows through gate
3. It commutates when reverse biased and negative current flows through the gate
4. It commutates when gate current is withdrawn

Which of these statement(s) is/are correct
(A) 1, 2 and 3 (B) 1 and 2 only (C) 2 and 3 only (D) 1 only

Answer-D

17.Consider the following statements
1. Speed of operation of MOSFET is more than the speed of operation of SCR
2. SCRs have low power loss than MOSFETs
3. The current in conducting state can be easily controlled through the gate in SCR
4. MOSFET is not a current controlled device
The correct statements are
(A) 1 and 4 only (B) 1 and 2 only (C) 2 and 3 only (D) 1, 2, 3 and 4

Answer-A

18.Match List – I with List – II and select the correct answer using the codes given below the lists
List I
P. di/dt rating limits
Q. dv/dt rating limits
R. i2t limit
S. Junction temperature limit
List II
1. Snubber circuit
2. Heat sink
3. Series reactor
4. Fuse

(A) P – 2, Q – 4, R – 1, S – 3
(B) P – 3, Q – 4, R – 1, S – 2
(C) P – 2, Q – 1, R – 4, S – 3
(D) P – 3, Q – 1, R – 4, S – 2

Answer-D

19.Match List – I (Device) with List – II (Switching times) and select the correct answer using the codes given below the lists
List I
P. TRIAC
Q. SCR
R. MOSFET
S. IGBT
List II
1. 5 – 10 μs
2. 100 – 400 μs
3. 50 – 100 μs
4. 200 – 400 μs

(A) P – 4, Q – 3, R – 2, S – 1
(B) P – 1, Q – 2, R – 3, S – 4
(C) P – 4, Q – 2, R – 3, S – 1
(D) P – 1, Q – 3, R – 2, S – 4

Answer-C

20.A thyristor has a PIV of 650 V; the voltage safety factor is 2. Then the voltage upto which the device can be operated is given by
(A) 1300 V (B) 650 V (C) 325 V (D) 230 V

Answer-B

Power electronic mcq part -4

Power Electronics Part Four

1.Circuit turn off time of an SCR is defined as the time
(A) taken by the SCR to turn off                      
(B) required for the SCR current to become zero
(C) for which the SCR reverse biased by the commutation circuit
(D) for which the SCR is reverse biased to reduce its current below the holding current

Answer-C

2.The typical ratio of latching current to holding current in a 20 A thyristor is
(A) 5.0 (B) 2.0 (C) 1.0 (D) 0.5

Answer-B

3.The thermal resistance between the body of a power semiconductor device and the ambient is expressed as
(A) Voltage across the device divided by current through the device
(B) Average power dissipated in the device divided by the square of the RMS current in the device
(C) Average power dissipated in the device divided by the temperature difference from body to ambient
(D) Temperature difference from body to ambient divided by average power dissipated in the device
Answer-D

4.Power electronic device with poor turn off gain is
(A) A symmetrical thyristor (B) A conventional thyristor
(C) power bipolar junction transistor (D) Gate turn off thyristor

Answer-D

5.Consider the following statement in respect of IGBT?
1. It combines the attributes of MOSFET and BJT                            
2. It has low forward voltage drop
3. Its switching speed is very much lower than that of MOSFET
4. It has high input impedance
Which of the statements given above are correct?
(A) 1, 2, 3 and 4 (B) 1, 2 and 4 (C) 1, 2 and 3 (D) 3 and 4

Answer-A

6.When cathode of thyristor made more positive than its anode
(A) all the junctions are forward biased
(B) Outer junctions are reverse biased and central one is forward biased
(C) Outer junctions are forward biased and central one is reverse biased
(D) All the junctions are reverse biased

Answer-B

7.When the thyristor is turned on and conducting
(A) J1 and J2 are forward biased and J3 is reverse biased
(B) J1 and J3 are forward biased and J2 is reverse biased
(C) J2 and J3 are reverse biased and J1 is forward biased
(D) J1, J2 and J3 are all forward biased

Answer-D

8.Consider the following statements
1. A thyristor circuit requires turn off circuit while transistor does not
2. The voltage drop of thyristor is less than that of a transistor
3. A thyristor requires a continuous gate current
4. A transistor draws a continuous base current
Which of these statements are correct?
(A) 1, 2, 3 and 4 (B) 1and 2 (C) 2 and 4 (D) 1 and 4

Answer-D

9.Thyristor is a semi conductor switch which is
(A) unilateral and stable (B) bilateral and astable
(C) unilateral and bistable (D) bilateral and bistable

Answer-C

10.The main reason for connecting a pulse transformer at the output stage of a thyristor triggering circuit is
(A) to amplify the power of the triggering pulse
(B) to provide electrical isolation
(C) to reduce turn on time of the thyristor
(D) to avoid spurious triggering of the thyristor due to noise

Answer-B

11.For thyristors, pulse triggering is preferred to dc triggering because
(A) gate dissipation is low (B) pulse system is simple
(C) triggering signal is required for very short duration (D) All of the above

Answer-D

12.High frequency modulation of the gate pulse is normally done in triggering an SCR in order to
(A) reduce power dissipation in the SCR
(B) minimize the saturation of the pulse transformer
(C) reduce the turn on time of SCR
(D) reduce the turn off time of SCR

Answer-B

13.An SCR has half cycle surge current rating of 3,000 A for 50 Hz supply. One cycle surge current rating is
(A) 1500 A (B) 2121.32 A (C) 4242.62 A (D) 6000 A

Answer-B

14.An SCR is in conducting state, a reverse voltage is applied between anode and cathode, but it fails to turn off. What could be the reason?
(A) Positive voltage is applied to gate (B) The reverse voltage is small
(C) Anode current is more than holding current (D) Turn off time of SCR is large

Answer-C

15.Most SCRs can be turned off by voltage reversal during negative half cycle of the ac supply for
(A) all frequencies (B) frequencies upto 300 Hz
(C) frequencies upto 30 kHz (D) frequencies upto 300 kHz

Answer-B

16.If a diode is connected in anti parallel with thyristor, then
(A) both turn off power loss and turn on time decreases
(B) turn off power loss increases and turn on time decreases
(C) turn off power loss decreases and turn on time increases
(D) both turn off power loss and turn on time increases

Answer-C

17.During turn off process in a thyristor, the current flow does not stop at the instant reaches zero but continues to flow to a peak value in the reverse direction. This is due to
(A) hole storage effect (B) commutation failure
(C) presence of reverse voltage across thyristor (D) protective inductance in series with SCR

Answer-A

18.Practical way of obtaining static voltage equalization in series connected SCRs is by the use of
(A) one resistor across the string
(B) resistor of different values across each SCR
(C) resistors of the same value across each SCR
(D) one resistor in series with the string

Answer-C

19.For series connected SCRs, dynamic equalizing circuit consists of
(A) resistor R and capacitor C in series but with a diode D across C
(B) series R and D circuit but with C across R
(C) series R and C circuit but with D across R
(D) series C and D circuit but with R across C

Answer-C

20.For dynamic equalizing circuit used for series connected SCRs, the choice of C is based on
(A) reverse recovery characteristic (B) turn on characteristics
(C) turn off characteristic (D) rise time characteristic

Answer-A

Control system mcq part -4

1. In a system low friction co-efficient facilitates
(a) reduced velocity lag error
(b) increased velocity lag error
(c) increased speed of response
(d) reduced time constant of the system

Ans: a

2. Hydraulic torque transmission system is analog of
(a) amplidyne set
(b) resistance-capacitance parallel circuit
(c) motor-generator set
(d) any of the above

Ans: a

3. Spring constant in force-voltage analogy is analogous to
(a) capacitance
(b) reciprocal of capacitance
(c) current
(d) resistance

Ans: b

4. The frequency and time domain are related through which of the following?
(a) Laplace Transform and Fourier Integral
(b) Laplace Transform
(c) Fourier Integral
(d) Either (b) or (c)

Ans: a

5. An increase in gain, in most systems, leads to
(a) smaller damping ratio
(b) larger damping ratio
(c) constant damping ratio
(d) none of the above

Ans: a

6. Static error coefficients are used as a measure of the effectiveness of closed loop systems for specified ________ input signal.
(a) acceleration
(b) velocity
(c) position
(d) all of the above

Ans: d

7. A conditionally stable system exhibits poor stability at
(a) low frequencies
(b) reduced values of open loop gain
(c) increased values of open loop gain
(d) none of the above

Ans: b

8. The type 0 system has ______ at the origin.
(a) no pole
(b) net pole
(c) simple pole
(d) two poles
(e) none of the above

Ans: a

9. The type 1 system has ______ at the origin.
(a) no pole
(b) net pole
(c) simple pole
(d) two poles

Ans: c

10. The type 2 system has ______ at the origin.
(a) no net pole
(b) net pole
(c) simple pole
(d) two poles

Ans: d

11. The position and velocity errors of a type-2 system are
(a) constant, constant
(b) constant, infinity
(c) zero, constant
(d) zero, zero

Ans: c

12. Velocity error constant of a system is measured when the input to the system is unit _______ function.
(a) parabolic
(b) ramp
(c) impulse
(d) step

Ans: b

13. In case of type-1 system steady state acceleration is
(a) unity
(b) infinity
(c) zero
(d) 10

Ans: b

14. If a step function is applied to the input of a system and the output remains below a certain level for all the time, the system is
(a) not necessarily stable
(b) stable
(c) unstable
(d) always unstable
(e) any of the above

Ans: a

15. Which of the following is the best method for determining the stability and transient response ?
(a) Root locus
(b) Bode plot
(c) Nyquist plot
(d) None of the above

Ans: a

16. Phase margin of a system is used to specify which of the following ?
(a) Frequency response
(b) Absolute stability
(c) Relative stability
(d) Time response

Ans: c

17. Addition of zeros in transfer function causes which of the following ?
(a) Lead-compensation
(b) Lag-compensation
(c) Lead-lag compensation
(d) None of the above

Ans: b

18. technique is not applicable to nonlinear system ?
(a) Nyquist Criterion
(b) Quasi linearization
(c) Functional analysis
(d) Phase-plane representation

Ans: a

19. In order to increase the damping of a badly underdamped system which of following compensators may be used ?
(a) Phase-lead
(b) Phase-lag
(c) Both (a) and (b)
(d) Either (a) and (b)
(e) None of the above
Ans: a

20. The phase lag produced by transportation relays
(a) is independent of frequency
(b) is inverseh’proportional to frequency
(c) increases linearly with frequency
(d) decreases linearly with frequency

Ans: c

Transmission and distribution mcq part -4

1. The steel used in steel cored conductors is usually
(a) alloy steel
(b) stainless steel
(c) mild steel
(d) high speed steel
(e) all of the above

Ans: c

12. Which of the following distribution systems is more reliable ?
(a) Radial system
(b) Tree system
(c) Ring main system
(d) All are equally reliable

Ans: c

3. Which of the following characteristics should the line supports for transmission lines
possess ?
(a) Low cost
(b) High mechanical strength
(c) Longer life
(d) All of the above

Ans: d

4. Transmission voltage of llkV is normally used for distances upto
(a) 20—25 km
(b) 40—50 km
(c) 60—70 km
(d) 80—100 km

Ans: a

5. Which of the following regulations is considered best?
(a) 50%
(b) 20%
(c) 10%
(d) 2%

Ans: d

6. Skin effect is proportional to
(a) (conductor diameter)/2
(b) (conductor diameter)*2
(c) (conductor diameter)/3
(d) (conductor diameter)²
(e) none of the above

Ans: d

7. A conductor, due to sag between two supports, takes the form of
(a) semi-circle
(b) triangle
(c) ellipse
(d) catenary

Ans: d

8. In AC.S.R. conductors, the insulation between aluminium and steel conductors is
(a) insulin
(b) bitumen
(c) varnish
(d) no insulation is required

Ans: d

9. Which of the followingbus-bar schemes has the lowest cost ?
(a) Ring bus-bar scheme
(b) Single bus-bar scheme
(c) Breaker and a half scheme
(d) Main and transfer scheme

Ans: b

10. Owing to skin effect
(a) current flows through the half cross-section of the conductor
(b) portion of the conductor near the surface carries more current and core of the
conductor carries less current
(c) portion of the conductor near the surface carries less current and core of the conductor
carries more current
(d) any of the above
(e) none of the above

Ans: b

1. By which of the following methods string efficiency can be improved ?
(a) Using a guard ring
(b) Grading the insulator
(c) Using long cross arm
(d) Any of the above
(e) None of the above

Ans: d

12. In aluminium conductors, steel core is provided to
(a) compensate for skin effect
(b) neutralise proximity effect
(c) reduce line inductance
(d) increase the tensile strength

Ans: d

13. By which of the following a bus-bar is rated ?
(a) Current only
(b) Current and voltage
(c) Current, voltage and frequency
(d) Current, voltage, frequency and short time current

Ans: d

14. A circuit is disconnected by isolators when
(a) line is energized
(b) there is no current in the line
(c) line is on full load
(d) circuit breaker is not open

Ans: b

15. For which of the following equipment current rating is not necessary ?
(a) Circuit breakers
(b) Isolators
(c) Load break switch
(d) Circuit breakers and load break switches

Ans: b

16. In a substation the following equipment is not installed
(a) exciters
(b) series capacitors
(c) shunt reactors
(d) voltatre transformers

Ans: a

17. jCorona usually occurs when the electrostatic stress in air around the conductor
exceeds
(a) 6.6 kV (r.m.s. value)/cm
(b) 11 kV (r.m.s. value)/cm
(c) 22 kV (maximum value)/cm
(d) 30 kV (maximum v^lue)/cm

Ans: d

18. The voltage drop, for constant voltage transmission is compensated by installing
(a) inductors
(b) capacitors
(c) synchronous motors
(d) all of above
(e) none of the above

Ans: c

19. The use of strain type insulators is made where the conductors are
(a) dead ended
(b) at intermediate anchor towers
(c) any of the above
(d) none of the above

Ans: c

20. The current drawn by the line due to corona losses is
(a) non-sinusoidal
(b) sinusoidal
(c) triangular
(d) square

Ans: a

Synchronous motor mcq part -4

1. The mechanical displacement of the rotor with respect to the stator, in polyphase multipolar synchronous motors running at full load, is of the order of
(a) zero degree
(b) two degrees
(c) five degrees
(d) ten degrees

Ans: c

2. Power factor of a synchronous motor is unity when
(a) the armature current is maximum
(b) the armature current is minimum
(c) the armature current is zero
(d) none of the above

Ans: b

3. Change of D.C. excitation of a synchronous motor changes
(a) applied voltage of the motor
(b) motor speed
(c) power factor of power drawn by the motor
(d) any of the above
(e) all of the above

Ans: c

4. While starting a synchronous motor by induction motor action, field winding is
usually
(a) connected to D.C. supply
(b) short-circuited by low resistance
(c) kept open-circuited
(d) none of the above

Ans: b

5. Which of the following motors will be used in electric clocks ?
(a) D.C. shunt motor
(b) D.C. series motor
(c) A.C. induction motor
(d) A.C. synchronous motor

Ans: d

6. If in a synchronous motor, driving mechanical load and drawing current at lagging power factor from constant voltage supply, its field excitation is increased, then its power factor
(a) become more
(b) become less
(c) remain constant
(d) none of the above

Ans: b

7. A synchronous motor installed at the receiving end substation operates with such an excitation that it takes power at lagging power factor. Now if the applied voltage of the synchronous motor goes down, the power factor of the synchronous motor will
(a) remain same
(b) go down
(c) improve
(d) none of the above

Ans: c

8. While starting a salient pole synchronous motor by induction motor action and connecting field discharge resistance across field, starting and accelerting torque is produced by
(a) induction motor torque in field winding
(b) induction motor torque in damper winding
(c) eddy current and hysteresis torque in pole faces
(d) reactance motor torque due to saliency of the rotor
(e) all of the above methods

Ans: e

9. Armature of a synchronous machine is
(a) of reducing number of slip rings on the rotor
(b) armature is associated with large power as compared to the field circuits
(c) of difficulty of providing high voltage insulation on rotor
(d) all of the above reasons

Ans: d

10. If excitation of a synchronous motor running with a constant load is decreased from its normal value, ignoring effects of armature reaction, it leads to
(a) increase in both armature current and power factor angle
(b) increase in back e.m.f. but decrease in armature current
(c) increase in both armature current and power factor which is lagging
(d) increase in torque angle but decrease in back e.m.f.

Ans: a

11. When a 3-phase synchronous generator is supplying a zero power factor lagging load, the armature field affects the main field in the following way
(a) augments it directly
(b) directly opposes it
(c) cross-magnetises it
(d) none of the above

Ans: b

12. Stability of a synchronous machine
(a) decreases with increase in its excitation
(b) increases with increase in its excitation
(c) remains unaffected with increase in excitation
(d) any of the above

Ans: b

13. The power factor of a synchronous motor is better than that of induction motor because
(a) stator supply is relieved of responsibility of producing magnetic field
(b) mechanical load on the motor can be adjusted
(c) synchronous motor runs at synchronous speed
(d) synchronous motor has large air gap

Ans: a

14. If in a synchronous motor, driving a given mechanical load and drawing current at a leading power factor from constant voltage supply its field excitation is increased, its power factor
(a) will become more
(b) will become less
(c) will remain unchanged
(d) none of the above.

Ans: b

15. A synchronous motor is running with normal excitation. When the load is increased, the armature current drawn by it increases because
(a) speed of the motor is reduced
(b) power factor is decreased
(c) Eb (back e.m.f.) becomes less than V (applied voltage)
(d) Er (net resultant voltage) in armature is increased
(e) none of the above

Ans: d

16. If one-phase of a 3-phase synchronous motor is short-circuited, motor
(a) will refuse to start
(b) will overheat in spots
(c) will not come upto speed
(d) will fail to pull into step

Ans: a

17. If the field circuit of an unloaded salientpole synchronous motor gets suddenly open- circuited, then
(a) it runs at a slower speed
(b) the motor stops
(c) it continues to run at the same speed
(d) it runs at a very high speed

Ans: b

18. In which of the following motors the stator and rotor fields rotate simultaneously ?
(a) D.C. motor
(b) Reluctance motor
(c) Universal motor
(d) Synchronous motor
(e) Induction motor

Ans: d

19. The speed of a synchronous motor
(a) increases as the load increases
(b) decreases as the load decreases
(c) always remains constant
(d) none of the above

Ans: c

20. A rotory converter can also be run as a
(a) d.c. shunt motor
(b) d.c. series motor
(c) d.c. compound motor
(d) induction motor
(e) synchronous motor

Ans: e

Singal phase induction motor part- 4

1. In a hysteresis motor, the rotor must have
(a) retentivity
(b) resistivity
(c) susceptibility
(d) none of the above

Ans: a

2. The rotor of a hysteresis motor is made of
(a) aluminium
(b) cast iron
(c) chrome steel
(d) copper

Ans: c

3. The electric motor used in portable drills is
(a) capacitor run motor
(b) hysteresis motor
(c) universal motor
(d) repulsion motor

Ans: c

4. Which of the following applications always have some load whenever switched on ?
(a) Vacuum cleaners
(b) Fan motors
(c) Pistol drills
(d) All of the above

Ans: c

5. The speed control of universal motor used for sewing machines is by
(a) friction
(b) varying the resistance
(c) tapping the field
(d) centrifugal mechanism

Ans: b

6. Torque developed by a single phase induction motor at starting is
(a) pulsating
(b) uniform
(c) none of the above
(d) nil

Ans: d

7. In split phase motor main winding is of
(a) thin wire placed at the top of the slots
(b) thin wire placed at the bottom of the slots
(c) thick wire placed at the bottom of the slots
(d) thick wire placed at the top of the slots

Ans: c

8. In repulsion motor, maximum torque is developed when
(a) brush axis is at 45° electrical to the field axis
(b) brush axis coincides with the field axis
(c) brush axis is at 90° electrical to the field axis
(d) none of the above

Ans: a

9. If the centrifugal switch does not open at 70 to 80 percent of synchronous speed of
motor, it would result in
(a) damage to the starting winding
(b) damage to the centrifugal switch
(c) overloading of running winding
(d) none of the above

Ans: a

10. Speed torque characteristic of a repulsion induction motor is similar to that of a D.C.
(a) shunt motor
(b) series motor
(c) compound motor
(d) separately excited motor

Ans: c

11. In a ceilingfan employing capacitor run motor
(a) secondary winding surrounds the primary winding
(b) primary winding surrounds the secondary winding
(c) both are usual arrangements
(d) none of the above

Ans: a

12. The shaded pole motor is used for
(a) high starting torque
(b) low starting torque
(c) medium starting torque
(d) very high starting torque

Ans: b

13. The rotor slots, in an induction motor, are usually not quite parallel to the shaft because it
(a) improves the efficiency
(b) helps the rotor teeth to remain under the stator teeth
(c) helps in reducing the tendency of the rotor teeth to remain under the stator teeth
(d) improves the power factor

Ans: c

14. The speed/load characteristics of a universal motor is same as that of
(a) A.C. motor
(b) D.C. shunt motor
(c) D.C. series motor
(d) none of the above

Ans: c

15. The purpose of stator winding in the compensated repulsion motor is to
(a) provide mechanical balance
(b) improve power factor and provide better speed regulation
(c) prevent hunting in the motor
(d) eliminate armature reaction

Ans: b

16. Which of the following motors is used for unity power factor ?
(a) Hysteresis motor
(b) Schrage motor
(c) Universal motor
(d) Reluctance motor

Ans: b

17. The motor used for the compressors is
(a) d.c. series motor
(b) shaded pole motor
(c) capacitor-start capacitor-run motor
(d) reluctance motor

Ans: c

18. Which of the following motors is used in a situation where load increases with speed ?
(a) Induction motor
(b) Three-phase series motor
(c) Schrage motor
(d) Hysteresis motor

Ans: b

19. In repulsion motor, zero torque is developed when
(a) brush axis is 45° electrical to field axis
(b) brush axis coincides with the field axis
(c) brush axis is 90° electrical to field axis
(d) both (b) and (c)

Ans: d

20. Centrifugal switch disconnects the auxiliary winding of the motor at about percent of synchronous speed
(a) 30 to 40
(b) 70 to 80
(c) 80 to 90
(d) 100

Ans: b

three-phase induction motor mcq part-4

1. In a three-phase induction motor
(a) iron losses in stator will be negligible as compared to that in rotor
(6) iron losses in motor will be neg¬ligible as compared to that in rotor
(c) iron losses in stator will be less than that in rotor
(d) iron losses in stator will be more than that in rotor

Ans: d

2. In case of 3-phase induction motors, plugging means
(a) pulling the motor directly on line without a starter
(b) locking of rotor due to harmonics
(c) starting the motor on load which is more than the rated load
(d) interchanging two supply phases for quick stopping

Ans: d

3. Which is of the following data is required to draw the circle diagram for an induction motor ?
(a) Block rotor test only
(b) No load test only
(c) Block rotor test and no-load test
(d) Block rotor test, no-load test and stator resistance test

Ans: d

4. In three-phase induction motors sometimes copper bars are placed deep in the rotor to
(a) improve starting torque
(b) reduce copper losses
(c) improve efficiency
(d) improve power factor

Ans: a

5. In a three-phase induction motor
(a) power factor at starting is high as compared to that while running
(b) power factor at starting is low as compared to that while running
(c) power factor at starting in the same as that while running

Ans: b

6. The vafcie of transformation ratio of an induction motor can be found by
(a) open-circuit test only
(b) short-circuit test only
(c) stator resistance test
(d) none of the above

Ans: b

7. The power scale of circle diagram of an induction motor can be found from
(a) stator resistance test
(6) no-load test only
(c) short-circuit test only
(d) noue of the above

Ans: c

8. The shape of the torque/slip curve of induction motor is
(a) parabola
(b) hyperbola
(c) rectangular parabola
(d) straigth line

Ans: c

9. A change of 4% of supply voltage to an induction motor will produce a change of appromimately
(a) 4% in the rotor torque
(b) 8% in the rotor torque
(c) 12% in the rotor torque
(d) 16% in the rotor torque

Ans: d

10. The stating torque of the slip ring induction motor can be increased by adding
(a) external inductance to the rotor
(b) external resistance to the rotor
(c) external capacitance to the rotor
(d) both resistance and inductance to rotor

Ans: b

11. A 500 kW, 3-phase, 440 volts, 50 Hz, A.C. induction motor has a speed of 960 r.p.m. on full load. The machine has 6 poles. The slip of the machine will be
(a) 0.01
(b) 0.02
(c) 0.03
(d) 0.04

Ans: d

12. The complete circle diagram of induetion motor can be drawn with the help of data found from
(a) noload test
(6) blocked rotor test
(c) stator resistance test
(d) all of the above

Ans: d

13. In the squirrel-cage induction motor the rotor slots are usually given slight skew
(a) to reduce the magnetic hum and locking tendency of the rotor
(b) to increase the tensile strength of the rotor bars
(c) to ensure easy fabrication
(d) none of the above

Ans: a

14. The torque of a rotor in an induction motor under running condition is maximum
(a) at the unit value of slip
(b) at the zero value of slip
(c) at the value of the slip which makes rotor reactance per phase equal to the resistance per phase
(d) at the value of the slip which makes the rotor reactance half of the rotor

Ans: c

15. What will happen if the relative speed between the rotating flux of stator and rotor of the induction motor is zero ?
(a) The slip of the motor will be 5%
(b) The rotor will not run
(c) The rotor will run at very high speed
(d) The torque produced will be very large

Ans: b

16. The circle diagram for an induction motor cannot be used to determine
(a) efficiency
(b) power factor
(c) frequency
(d) output

Ans: a

17. Blocked rotor test on induction motors is used to find out
(a) leakage reactance
(b) power factor on short circuit
(c) short-circuit current under rated voltage
(d) all of the above

Ans: d

18. Lubricant used for ball bearing is usually
(a) graphite
(b) grease
(c) mineral oil
(d) molasses

Ans: b

19. An induction motor can run at synchronous speed when
(a) it is run on load
(b) it is run in reverse direction
(c) it is run on voltage higher than the rated voltage
(d) e.m.f. is injected in the rotor circuit

Ans: d

20. Which motor is preferred for use in mines where explosive gases exist ?
(a) Air motor
(b) Induction motor
(c) D.C. shunt motor
(d) Synchronous motor

Ans: a