Q301. The Nyquist plot of a stable open-loop transfer function encircles the critical point (-1,0) once in the clockwise direction. The closed-loop system is:
A) Stable
B) Marginally stable
C) Unstable
D) Conditionally stable
✅ Answer: C) Unstable
Explanation:
In Nyquist criterion, a clockwise encirclement of the -1 point indicates instability. Each encirclement corresponds to one unstable closed-loop pole.
Q302. The Bode plot gives information about:
A) Stability margin
B) Damping ratio
C) Root locations
D) System order only
✅ Answer: A) Stability margin
Explanation:
Bode plot provides phase margin and gain margin, both of which are direct indicators of system stability.
Q303. The phase margin of a system is measured at:
A) Frequency where phase is 0°
B) Frequency where gain is 0 dB
C) Frequency where phase is -180°
D) Natural frequency
✅ Answer: B) Frequency where gain is 0 dB
Explanation:
Phase margin is the additional phase required to reach -180° when the magnitude is unity (0 dB). It indicates relative stability.
Q304. A Type 1 system will have zero steady-state error for:
A) Step input
B) Ramp input
C) Parabolic input
D) All inputs
✅ Answer: A) Step input
Explanation:
A Type 1 system (one pole at origin) can track a step input perfectly with zero steady-state error but not a ramp or higher order input.
Q305. In a lead compensator, the maximum phase lead occurs at:
A) Low frequency
B) High frequency
C) Mid frequency
D) Zero frequency
✅ Answer: C) Mid frequency
Explanation:
A lead compensator adds positive phase in the mid-frequency range, improving stability and transient response.
Q306. The bandwidth of a control system is the range of frequencies:
A) Over which gain is unity
B) Over which system responds effectively
C) Where phase is constant
D) Beyond resonant frequency
✅ Answer: B) Over which system responds effectively
Explanation:
Bandwidth defines the frequency range where the system output follows input with acceptable accuracy—directly related to speed of response.
Q307. A PID controller consists of:
A) Proportional + Derivative
B) Proportional + Integral + Derivative
C) Proportional + Integral
D) Integral + Derivative
✅ Answer: B) Proportional + Integral + Derivative
Explanation:
PID combines proportional control (for stability), integral (for zero steady-state error), and derivative (for improved transient response).
Q308. The derivative component of a PID controller primarily improves:
A) Steady-state error
B) System speed
C) Damping and transient response
D) DC gain
✅ Answer: C) Damping and transient response
Explanation:
Derivative control adds a predictive action that reduces overshoot and improves damping.
Q309. The Nyquist stability criterion is based on:
A) Open-loop poles and zeros
B) Closed-loop transfer function directly
C) Open-loop frequency response
D) Step response characteristics
✅ Answer: C) Open-loop frequency response
Explanation:
Nyquist uses open-loop frequency response to predict closed-loop stability through encirclement of the critical point.
Q310. A system is said to be marginally stable if:
A) All poles have negative real parts
B) Poles lie on imaginary axis and no repeated poles
C) One pole is positive real
D) Poles have zero imaginary part
✅ Answer: B) Poles lie on imaginary axis and no repeated poles
Explanation:
Marginal stability means oscillatory behavior that neither decays nor diverges.
Q311. The Routh–Hurwitz criterion is used to:
A) Find frequency response
B) Check stability without finding roots
C) Locate exact poles
D) Calculate time constants
✅ Answer: B) Check stability without finding roots
Explanation:
Routh’s criterion provides a method to determine the number of poles with positive real parts without solving for roots.
Q312. If the first column of the Routh array contains all positive terms, then the system is:
A) Unstable
B) Stable
C) Marginally stable
D) Conditionally stable
✅ Answer: B) Stable
Explanation:
All positive elements in the first column indicate all poles are in the left half of the s-plane → stable system.
Q313. For a system to be critically damped, damping ratio (ζ) must be:
A) 0
B) < 1
C) = 1
D) > 1
✅ Answer: C) = 1
Explanation:
Critical damping ensures fastest settling without oscillations; ζ = 1 defines this boundary.
Q314. The settling time (Ts) of a second-order system is inversely proportional to:
A) Natural frequency
B) Damping ratio
C) Gain
D) Overshoot
✅ Answer: A) Natural frequency
Explanation:
Higher natural frequency → faster system response → shorter settling time.
Q315. The root locus starts from:
A) Zeros and ends at poles
B) Poles and ends at zeros
C) Origin and infinity
D) Imaginary axis
✅ Answer: B) Poles and ends at zeros
Explanation:
Root locus shows how closed-loop poles move from open-loop poles to zeros as gain varies.
Q316. In a second-order system, increasing damping ratio (ζ):
A) Increases overshoot
B) Decreases settling time
C) Reduces overshoot
D) Makes system oscillatory
✅ Answer: C) Reduces overshoot
Explanation:
Higher damping reduces oscillations and overshoot but can slightly slow response.
Q317. The open-loop transfer function of a unity feedback system is . The system type is:
A) Type 0
B) Type 1
C) Type 2
D) Type 3
✅ Answer: B) Type 1
Explanation:
Number of poles at origin = 1 → Type 1 system.
Q318. The steady-state error of a Type 1 system for a ramp input is:
A) 0
B) Finite
C) Infinite
D) Zero for all inputs
✅ Answer: B) Finite
Explanation:
Type 1 systems track step perfectly but have finite error for ramp and infinite for parabolic inputs.
Q319. In a feedback system, if loop gain is very large, the overall system becomes:
A) Sensitive to parameter variations
B) Less sensitive to parameter variations
C) More oscillatory
D) Unstable always
✅ Answer: B) Less sensitive to parameter variations
Explanation:
High loop gain improves accuracy and reduces sensitivity, but too high gain can reduce stability margin.
Q320. The dominant poles of a system are those which:
A) Have smallest real part
B) Have largest real part
C) Lie farthest from imaginary axis
D) Lie on the imaginary axis
✅ Answer: B) Have largest real part
Explanation:
Dominant poles (closest to imaginary axis) govern system’s transient response because they decay slowest.
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