Junior Engineer (Electrical/Electronics) MCQs – Series 7 (Q81-90)
Q81. The transfer function of a linear system is defined as the ratio of:
- A) The Laplace transform of the output to the Laplace transform of the input, with zero initial conditions
- B) The output to the input in the frequency domain only, and is always unstable
- C) The input to the output in the time domain
- D) The error signal to the input signal
Answer: A
Transfer function G(s) = Y(s)/X(s) is defined under zero initial conditions using Laplace transforms.
Q82. A linear time-invariant system is said to be stable if all its poles lie:
- A) In the left half of the s-plane
- B) Exactly on the imaginary axis
- C) At the origin of the s-plane
- D) In the right half of the s-plane
Answer: A
For BIBO stability, all closed-loop poles must have negative real parts, i.e., lie strictly in the left half of the s-plane.
Q83. The purpose of negative feedback in a control system is to:
- A) Always cause instability
- B) Reduce sensitivity to parameter variations and generally improve stability
- C) Increase system gain without limit
- D) Eliminate the need for any controller
Answer: B
Negative feedback reduces the effect of disturbances and parameter changes on system output and typically improves stability and accuracy.
Q84. In the Routh-Hurwitz stability criterion, a sign change in the first column of the Routh array indicates:
- A) That the system has zero poles
- B) That the system is unconditionally stable
- C) The number of poles located in the right half of the s-plane
- D) That the characteristic equation has no roots
Answer: C
Each sign change in the first column corresponds to one closed-loop pole in the right-half s-plane, indicating instability.
Q85. The ‘type’ of a control system is determined by:
- A) The system’s damping ratio
- B) The number of poles located at the origin in the open-loop transfer function
- C) The number of zeros in the transfer function
- D) The order of the overall characteristic equation
Answer: B
System type (0, 1, 2, …) equals the number of integrators (poles at s=0) in the open-loop transfer function, and determines steady-state error behavior.
Q86. A PID controller combines which three control actions?
- A) Position, Integral and Damping
- B) Proportional and Integral only
- C) Derivative action only
- D) Proportional, Integral and Derivative
Answer: D
A PID controller sums proportional, integral, and derivative terms of the error signal to generate the control action.
Q87. The damping ratio (ζ) of a critically damped second-order system is:
- A) Greater than 1
- B) Equal to 1
- C) Between 0 and 1
- D) Equal to 0
Answer: B
Critical damping corresponds to ζ = 1, the boundary between the oscillatory (underdamped) and non-oscillatory (overdamped) responses.
Q88. On a Bode plot, the gain margin is measured at the frequency where the:
- A) Frequency equals zero
- B) Magnitude is 0 dB
- C) Phase angle is −180°
- D) Phase angle is 0°
Answer: C
Gain margin is the amount of gain increase (in dB) required at the phase crossover frequency (where phase = −180°) to bring the system to the verge of instability.
Q89. The steady-state error of a type-0 control system for a unit step input is:
- A) Undefined
- B) Zero
- C) Infinite
- D) A finite, non-zero value
Answer: D
A type-0 system has a finite position error constant, resulting in a finite non-zero steady-state error for a step input.
Q90. The root locus technique is primarily used to study the effect of:
- A) Load variation on machine efficiency
- B) Varying the system’s open-loop gain on closed-loop pole locations
- C) Ambient temperature on system response
- D) Input frequency on output amplitude only
Answer: B
Root locus plots the paths traced by the closed-loop poles in the s-plane as a system parameter, typically gain K, is varied from 0 to infinity.
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