PLC Engineer Interview Questions and Answers

The following are the PLC Engineer Interview Questions and Answers useful for PLC engineer, DCS engineer, Automation engineer for the interview preparation.

PLC Interview Questions

1)PLCs are _____________ designed for use in the control of a wide variety of manufacturing machines and systems.

a) special-purpose industrial computers

b) personal computers

c) electromechanical systems

d) All of the above

Ans : a

2-Which of the following statements is not correct?

a) The PLC rung output [-( )-] is a discrete output instruction or bit in memory.

b) Each rung of the ladder logic represents a logical statement executed in software - inputs on the right and outputs on the left.

c) Input and output instructions in ladder logic do not directly represent the switches and actuators.

d) PLC input instructions are logical symbols associated with voltage at the input module terminals.

Ans b

3-Which of the following statements is correct?

a) Ladder logic is a PLC graphical programming technique introduced in the last 10 years.

b) A ladder logic program is hard to analyze because it is totally different when compared with the equivalent relay logic solution.

c) The number of ladder logic virtual relays and input and output instructions is limited only by memory size.

d) The number of contacts for a mechanical relay is limited to number of coils on the relay.

Ans:c

4-Which of the following statements about a single pole double throw relay is NOT true?

a) It is called an SPDT type of relay.

b) It has one common contact.

c) It has two positions (NC and NO).

d) It has a center off position.

Ans:d

5-Which of the following statements about a single pole double throw relay is true?

a) Insulators are used in the armature to isolate the electrical switching contacts from the rest of the relay components.

b) The NC contact and the pole are in contact when the relay is off.

c) It has just one coil.

d) All of the above.

Ans:d

6-Which of the following statements about RLL is NOT true?

a) NO contact symbol has two parallel lines to indicate an open contact.

b) RLL stands for Relay Ladder Logic.

c) NC contact symbol has the same two parallel lines with a line across them to indicate closed contacts.

d) The right power rail is positive or the high side of the source, and the left power rail is the power return or ground.

Ans:d

7-The ______________ is moved toward the relay electromagnet when the relay is on.

a) Armature

b) Coil

c) NO contact

d) NC contact

Ans:a

8-When a relay is NOT energized:

a) There is an electrical path through the NO contacts

b) There is an electrical path through the NC contacts

c) Neither the NO or the NC contacts have an electrical path

d) Both the NO and the NC contacts have an electrical path

Ans:b

9-Which of the following RLL applications is not normally performed in early automation systems?

a) On/off control of field devices

b) Logical control of discrete devices

c) On/off control of motor starters

d) Proportional control of field devices

Ans:d

10-Current flows into the ___________.

a) Input terminal of a sinking DC input module

b) Input terminal of a sinking output field device

c) Output terminal of a sinking input field device

d) All of the above

Ans:d

11-Current flows into the ___________.

a) Input terminal of a sinking DC input module

b) Input terminal of a sinking output field device

c) Output terminal of a sinking input field device

d) All of the above

Ans:d

12-In a current sinking DC input module _____________.

a) The current flows out of the input field device

b) Requires that a AC sources be used with mechanical switches

c) The current flows out of the input module

d) Currents can flow in either direction at the input module

Ans:a

13-In a current sinking DC input module _____________.

a) The current flows out of the input field device

b) Requires that a AC sources be used with mechanical switches

c) The current flows out of the input module

d) Currents can flow in either direction at the input module

Ans:a

14-AC output field devices can interface to _______________.

a) AC output modules

b) Relay output modules

c) Both a and b

d) Neither a or b

Ans:c

15-What one item in the output module circuit above should be changed to make it correct.

a) The battery polarity

b) Output module should be sourcing

c) Field device should be sinking

d) Current flow direction

Ans:d

16-In an open loop control system

(a) Output is independent of control input
(b) Output is dependent on control input
© Only system parameters have effect on the control output
(d) None of the above

Ans:a

17-For open control system which of the following statements is incorrect ?

(a) Less expensive
(b) Recalibration is not required for maintaining the required quality of the output
© Construction is simple and maintenance easy
(d) Errors are caused by disturbances

Ans:b

18-For open control system which of the following statements is incorrect ?

(a) Less expensive
(b) Recalibration is not required for maintaining the required quality of the output
© Construction is simple and maintenance easy
(d) Errors are caused by disturbances

Ans:b

19-A control system in which the control action is somehow dependent on the output is known as

(a) Closed loop system
(b) Semiclosed loop system
© Open system
(d) None of the above

Ans:a

20-In closed loop control system, with positive value of feedback gain the overall gain of the system will

(a) decrease
(b) increase
© be unaffected
(d) any of the above

Ans:a

21-Which of the following is an open loop control system ?

(a) Field controlled D.C. motor
(b) Ward leonard control
© Metadyne
(d) Stroboscope

Ans:a

22-In open loop system

(a) the control action depends on the size of the system
(b) the control action depends on system variables
© the control action depends on the input signal
(d) the control action is independent of the output

Ans:d

23-One of the following has tendency to oscillate.

(a) Open loop system
(b) Closed loop system
© Both (a) and (b)
(d) Neither (a) nor (b)

Ans:b

24-A good control system has all the following features except

(a) good stability
(b) slow response
© good accuracy
(d) sufficient power handling capacity

Ans:b

25-The initial response when the output is not equal to input is called

(a) Transient response
(b) Error response
© Dynamic response
(d) Either of the above

Ans:a

26-A control system working under unknown random actions is called

(a) computer control system
(b) digital data system
© stochastic control system
(d) adaptive control system

Ans:c

27-Any externally introduced signal affecting the controlled output is called a

(a) feedback
(b) stimulus
© signal
(d) gain control

Ans:b

28-A closed loop system is distinguished from open loop system by which of the following ?

(a) Servomechanism
(b) Feedback
© Output pattern
(d) Input pattern

Ans:b

29-Which of the following should be done to make an unstable system stable ?

(a) The gain of the system should be decreased
(b) The gain of the system should be increased
© The number of poles to the loop transfer function should be increased
(d) The number of zeros to the loop transfer function should be increased

Ans:b

30-Which one of the following increases the steady state accuracy.

(a) Integrator
(b) Differentiator
© Phase lead compensator
(d) Phase lag compensator

Ans:a

31-A.C. servomotor resembles

(a) two phase induction motor
(b) Three phase induction motor
© direct current series motor
(d) universal motor

Ans:a

32-As a result of introduction of negative feedback which of the following will not decrease ?

(a) Band width
(b) Overall gain
© Distortion
(d) Instability

Ans:a

33-Regenerative feedback implies feedback with

(a) oscillations
(b) step input
© negative sign
(d) positive sign

Ans:d

34-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

35-A control system with excessive noise, is likely to suffer from

(a) saturation in amplifying stages
(b) loss of gain
© vibrations
(d) oscillations

Ans:a

36- Zero initial condition for a system means

(a) input reference signal is zero
(b) zero stored energy
© ne initial movement of moving parts
(d) system is at rest and no energy is stored in any of its components

Ans:d

37-The band width, in a feedback amplifier.

(a) remains unaffected
(b) decreases by the same amount as the gain increase
© increases by the sane amount as the gain decrease
(d) decreases by the same amount as the gain decrease

Ans:c

38-The transient response, with feedback system,

(a) rises slowly
(b) rises quickly
© decays slowly
(d) decays quickly

Ans:d

39-Due to which of the following reasons excessive band width in control systems should be avoided ?

(a) It leads to slow speed of response
(b) It leads to low relative stability
© Noise is proportional to band width
(d) None of the above

Ans:c

40-In an automatic control system which of the following elements is not used ?

(a) Error detector
(b) Final control element
© Sensor
(d) Oscillator

Ans:d

41-In a control system the output of the controller is given to

(a) final control element
(b) amplifier
© comparator
(d) sensor
(e) none of the above

Ans:a

42-A controller, essentially, is a

(a) sensor
(b) clipper
© comparator
(d) amplifier

Ans:c

43-The capacitance, in force-current analogy, is analogous to

(a) momentum
(b) velocity
© displacement
(d) mass

Ans:d

44-The temperature, under thermal and electrical system analogy, is considered analogous to

(a) voltage
(b) current
© capacitance
(d) charge
(e) none of the above

Ans:a

45-In electrical-pneumatic system analogy the current is considered analogous to

(a) velocity
(b) pressure
© air flow
(d) air flow rate

Ans:d

46-In liquid level and electrical system analogy, voltage is considered analogous to

(a) head
(b) liquid flow
© liquid flow rate
(d) none of the above

Ans:a

47-Which one of the following is the reference input minus the primary feedback.

(a) Manipulated variable
(b) Zero sequence
© Actuating signal
(d) Primary feedback

Ans:c

48-The phase lag produced by transportation relays

(a) is independent of frequency
(b) is inversely proportional to frequency
© increases linearly with frequency
(d) decreases linearly with frequency

Ans:c

49-Which one of the following is not a final control element.

(a) Control valve
(b) Potentiometer
© Electro-pneumatic converter
(d) Servomotor

Ans:b

50-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
© 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

51-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
© electric signal to pressure signal
(d) position change to pressure signal
(e) none of the above

Ans:b

52-The effect of error damping is to

(a) provide larger settling lime
(b) delay the response
© reduce steady state error
(d) any of the above
(e) none of the above

Ans:c

53-A differentiator is usually not a part of a control system because it

(a) reduces damping
(b) reduces the gain margin
© increases input noise
(d) increases error

Ans:c

54-Which of the following is the output of a thermocouple ?

(a) Alternating current
(b) Direct current
© A.C. voltage
(d) D.C. voltage
(e) None of the above

Ans:d

55-Eddy current drive can be a retrofit for ________.

a) constant speed system requirement

b) variable speed system requirement

c) dual speed system requirement only

d) none of the above

Ans:b

56-The basic functions of electronic ballast excludes one of the following:

a) to ignite the lamp

b) to stabilize the gas discharge

c) to reduce lumen output of the lamp

d) to supply power to the lamp

Ans:c

57-Electronic soft starters are used for motors to:

a) achieve variable speed

b) provide smooth start and stop

c) improve the loading

d) none of the above

Ans:b

58-A salient pole synchronous motor is running at no load. Its field current is switched off.

The motor will

a. come to stop.

b. continue to run at synchronous speed.

c. continue to run at a speed slightly more than the synchronous speed.

d. continue to run at a speed slightly less than the synchronous speed.

Ans:b

59-The d.c. series motor should always be started with load because

a. at no load, it will rotate at dangerously high speed.

b. it will fail to start.

c. it will not develop high starting torque.

d. all are true.

Ans:a

60-When a synchronous motor is running at synchronous speed, the damper winding produces

a. damping torque.

b. eddy current torque.

c. torque aiding the developed torque.

d. no torque.

Ans:d

61 - In a transformer the voltage regulation will be zero when it operates at

a. unity p.f.

b. leading p.f.

c. lagging p.f.

d. zero p.f. leading.

Ans:b

62-The size of a conductor used in power cables depends on the

a. operating voltage.

b. power factor.

c. current to be carried.

d. type of insulation used.

Ans:c

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