# 300+ TOP CONTROL SYSTEMS Interview Questions and Answers

## CONTROL SYSTEMS Interview Questions :-

1. What Is A System?
When a number of elements or components are connected in a sequence to perform a specific function, the group of elements that all constitute a System.

2. What Is Control System?
In a System the output and inputs are interrelated in such a manner that the output quantity or variable is controlled by input quantity, then such a system is called Control System.

The output quantity is called controlled variable or response and the input quantity is called command signal or excitation.

3. What Are Different Types Of Control Systems?
Two major types of Control Systems are

1. Open loop Control Systems: The Open loop Control System is one in which the Output Quantity has no effect on the Input Quantity. No feedback is present from the output quantity to the input quantity for correction.
2. Closed Loop Control System: The Closed loop Control System is one in which the feedback is provided from the Output quantity to the input quantity for the correction so as to maintain the desired output of the system.

4. What Is A Feedback In Control System?
The Feedback in Control System in one in which the output is sampled and proportional signal is fed back to the input for automatic correction of the error ( any change in desired output) for futher processing to get back the desired output.

5. What Are The Characteristics Of Negative Feedback?
Negative Feedback in a Control System has following Characteristics

• Reduction in the gain at the expense of better stability of the system
• Rejection of disturbance signals in the system
• Low Sensitivity to parameter variations
• Accuracy in tracking the steady state value

6. What Are The Basic Components Of The Feedback Control System?
Basic components of the feedback control system are process system (open loop system), feedback path element, error detector, and controller.

7. Explain Transfer Function?
Transfer Function of a control system is defined as:

1. Ratio of Laplace transform of Output to the Laplace transform of the Input with zero initial conditions
2. Transfer function is defined as the Laplace transform of Impulse response of the system with zero initial conditions

8. What Is Time Invariant System?
Time Invariant System is one in which the input and output characteristics of the system does not change with time.

9. What Is The Basic Rule For Block Diagram Reduction Technique?
The rules of the Block Diagram reduction Techniques are designed in such a manner that any modifications made in the diagram will not alter the input and output relation of the system.

10. Where Servomechanism Is Used?
Servomechanism is used in control system where the output is pertained to vary the mechanical position of a device.

Servo Mechanism is widely used in Governor value position control mechanism used in the power plants where speed of the turbine is taken and processed using the transducers and final control element is brought as mechanical movement of the value. Now a days Governor value control is done with Electronic controls using power Thyristors. Servomechanism is also widely used in the robotic hand movements.

11. What Is Resonant Peak?
Resonant peak is defined as the maximum value of the closed loop transfer function.A large resonant peak corresponds to large overshoot in the transient respose

12. What Is Resonant Frequency?
The frequency at which resonant peak occurs is called the resonant frequency. Resonance frequency explains about the speed of the transient response.

13. What Is Cut-off Rate?
The slope of the log-magnitude curve near the cut-off frequency is called the cut-off rate. The cut-off rate indicates the ability of the system to distinguish between the signal and the noise.

14. What Is Phase Cross Over Frequency?
The frequency at which the phase of the open loop transfer function is 180o is called the phase cross over frequency.

15. What Is Gain Margin?
The Gain Margin is defined as the reciprocal of the magnitude of open loop transfer function at phase cross over frequency. The gain margin indicates the amount by which the gain of the system can be increased without affecting the stability of the system

16. What is Phase Margin?
The phase margin is the amount of additional phase lag at the gain cross over frequency required to bring the system to the verge of instability.

17. What Is Signal Flow Graph?
A Signal Flow Graph is a diagram that represents a set of simultaneous linear algebraic equations. By taking Laplace transform the time domain differential equations governing a control system can be transferred to a set of algebraic equations in s-domain. The signal Flow graph of the system can be constructed using these equations.

18. What Is S-domain And Its Significance?
By taking Laplace transform for differential equation in the time domain equations in S-domain can be obtained. L{F(t)}=F(s)

S domain is used for solving the time domain differential equations easily by applying the Laplace for the differential equations.

19. What Are The Basic Properties Of Signal Flow Graph?
The basic properties of the signal flow graph are:

• Signal Flow Graphs are applicable to linear systems
• It consists of nodes and branches. A node is a point representing a variable or signal. A branch indicates the functional dependence of one signal on another
• A node adds the signals of all incoming branches and transmits this sum to all outgoing branches
• Signals travel along branches only in a marked direction and is multiplied by the gain of the branch
The algebraic equations must be in the form of cause and effect relationship

20. What Is Mathematical Model Of A Control System?
Control system is a collection of physical elements connected together to serve an objective. The output and input relations of various physical system are governed by differential equations. Mathematical model of a control system constitutes set of differential equations. The response of the output of the system can be studied by solving the differential equations for various input conditions.

21. Explain Mechanical Translational System?

• Model of mechanical translational system can be obtained by using three basic elements Mass, Spring and Dash-pot.
• Weight the mechanical system is represented by mass and is assumed to be concentrated at the center of body.
• The elastic deformation of the body can be represented by the spring
• Friction existing in a mechanical system can be represented by dash-pot.

22. What Is Order Of The System?
Order of the system is defined as the order of the differential equation governing the system. Order of the system can be determined from the transfer function of the system. Also the order of the system helps in understanding the number of poles of the transfer function. For nth order system for a particular transfer function contains ‘n’ number of poles.

23. What Is Time Response Of The Control System?
Time response of the control system is defined as the output of the closed loop system as a function of time. Time response of the system can be obtained by solving the differential equations governing the system or time response of the system can also be obtained by transfer function of the system.

24. How Time Response Of The System Is Divided?
Time response of the system consists of two parts: 1.Transient state response 2. Steady state response. Transient response of the system explains about the response of the system when the input changes from one state to the other. Steady state response of the system shows the response as the time t, approaches infinity.

25. What Are Test Signals And Their Significance?
The knowledge of the input signal is required to predict the response of the system. In most of the systems input signals are not known ahead of the time and it is also difficult to express the input signals mathematically by simple equations. In such cases determining the performance of the system is not possible.Test signals helps in predicting the performance of the system as the input signals which we give are known hence we can see the output response of the system for a given input and can understand the behavior of the control system. The commonly used test signals are impulse, ramp, step signals and sinusoidal signals.

26. What Is Pole Of The System?
Pole of a function F(s) is the value at which the function F(s) becomes infinite, where F(s) is a function of the complex variable s.

27. What Is Zero Of The System?
Zero of a function F(s) is a value at which the function F(s) becomes zero, where F(s) is a function of complex variables.

28. Why Negative Feedback Is Preferred In The Control System?
The role of Feedback in control system is to take the sampled output back to the input and compare output signal with input signal for error ( deviation from the desired result).

Negative Feedback results in the better stability of the system and rejects any disturbance signals and is less sensitive to the parameter variations. Hence in control systems negative feedback is considered.

29. What Is The Effect Of Positive Feedback On Stability Of The System?
Positive feedback is not used generally in the control system because it increases the error signal and drives the system to instability. But positive feedbacks are used in minor loop control systems to amplify certain internal signals and parameters.

30. What Is The Use For Cable Entry In Control Room?
In the process plant Control room built considering the non-hazardous area. So in case if fire/Explosion takes place in the plant than that has to be restricted from entering Into the control room. So MCT(Multiple cable transient) blocks are used.

31. What are the various types of transformer?
32. What are the methods of speed control of dc motors?
33. What do you mean by transformation coefficient?
34. What is the difference between two winding transformer and auto transformer?
35. Why transformers and alternators are rated with kVA?
36. What is the operating power factor of dc machine?
37. What is meant by power factor? Explain.
38. What do you mean by complex power?
39. What will happen if dc supply is connected to transformer?’
40. What are the parts of transformer?
41.What is regulation with respect to i)transformer ii)alternator
42. Why are synchronous motors not self starting? What are the methods of starting?
43. Why are single phase induction motors not self starting? Classify them according to the
starting methods.
44. What are the types of alternators?
45. Why is regulation so important for transformers and alternators?
46. What are the parts of synchronous machine? Name the material used in various parts.
47. Why is synchronous motor referred as doubly excited machine?
48. What is the result of varying the excitation of synchronous motor?
49. What is slip of an induction motor?
50. Why is always the induction motor running with less than the speed of rotating magnetic field?
51. What is slip frequency?
52. What will happen when the slip is 100% and zero?
53. What are the types of 3 phase induction motors?
54. How is the starting torque improved in slip ring induction motor? Justify mathematically.
55. Why is 3 phase induction motor referred as rotating transformer with short-circuited secondary?
56. What is the use of end rings in squirrel cage induction motor?
57. Why transformers have highest efficiency than any other electrical machine?
58. What is mutual inductance?
59.What is the principle of motor, generator and transformer?
60. How will you find the direction of induced emf in generator and the direction of motion of the conductor in motor?
61.How will you reverse the direction of rotation of i) dc motor ii) ac motor?
62. What is the difference between rheostat and potentiometer?
63. What is L, F, A, N in dc starters?
64. What is M, L, C, and V in wattmeter?
65. What is LPF, UPF and ZPF?
66. What is E and C in autotransformer?
67. What do you mean by power frequency?
69. What are the disadvantages of low power factor?
70. What are the methods of speed control of ac motors?
71.Why are electrical loads used for generators and motors whereas mechanical loads for motors?
72. What do you mean by stray losses in dc machine?
73. Given transformers A and B with following details:
A: η= 96% &amp; reg: 5.8%
B: η= 94% &amp; reg: 5.2%