A synchronous motor operating at no-load consumes:
Answer: B
At no-load, a synchronous motor supplies only magnetizing current (reactive power) to maintain the magnetic field.
Q.142Medium
The reactive power generated by an over-excited synchronous generator is:
Answer: B
Over-excitation increases the magnetic field, causing the generator to supply lagging reactive power to support system voltage.
Q.143Medium
What is the primary cause of voltage instability in a power system?
Answer: A
Voltage instability occurs when reactive power demand cannot be met, causing voltage collapse, particularly at weak points in the grid.
Q.144Medium
Power system harmonics in Indian grids are primarily caused by:
Answer: A
Modern non-linear loads like switch-mode power supplies, VFDs, and renewable energy inverters are major sources of harmonics in contemporary Indian grids.
Q.145Medium
In load flow analysis, which bus type is used to specify active power and voltage magnitude?
Answer: A
PV buses represent generators where active power injection and voltage magnitude are specified; reactive power is calculated.
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Q.146Medium
Which of the following is a measure of power quality as per Indian Standards (IS 16001:2017)?
Answer: A
IS 16001:2017 defines power quality parameters including THD (Total Harmonic Distortion) and voltage flicker limits.
Q.147Medium
In a three-phase power system with balanced conditions, the zero-sequence current flows through:
Answer: A
Zero-sequence currents (equal in all three phases and same direction) require a return path through neutral or ground.
Q.148Medium
What is the steady-state error for a unit step input to a Type 0 system?
Answer: C
For Type 0 system with unit step input, e_ss = 1/(1 + Kp) where Kp is the position error constant.
Q.149Medium
What does the Routh-Hurwitz criterion determine?
Answer: B
Routh-Hurwitz criterion is used to determine the stability of a system by examining the characteristic equation coefficients without calculating poles.
Q.150Medium
The gain margin of a system can be determined from the bode plot as:
Answer: C
Gain margin = 1/|G(jω)| at phase crossover frequency where phase = -180°. It indicates how much gain can be increased before instability.
Q.151Medium
For a second-order system with natural frequency ωn = 2 rad/s and damping ratio ζ = 0.5, the system is:
Answer: B
When ζ < 1 (0.5 < 1), the system is underdamped with oscillatory transient response.
Q.152Medium
The rise time of a second-order underdamped system decreases with:
Answer: A
Rise time tr ≈ (π - cos⁻¹(ζ))/(ωn√(1-ζ²)). It decreases with higher ωn and appropriate ζ value.
Q.153Medium
Which compensation technique is used to improve steady-state accuracy?
Answer: B
Lag compensation increases the system type or gain at low frequencies, improving steady-state accuracy without affecting stability significantly.
Q.154Medium
The Nyquist plot is a mapping of:
Answer: B
Nyquist plot maps the frequency response G(jω)H(jω) in the complex plane by varying ω from 0 to ∞.
Q.155Medium
The overshoot of a second-order system is independent of:
Answer: C
Overshoot = e^(-πζ/√(1-ζ²)) depends only on damping ratio ζ, not on ωn or system gain K.
Q.156Medium
Which of the following is a state-space representation advantage over transfer function?
Answer: B
State-space representation naturally handles MIMO systems, non-linear systems, and time-varying systems better than transfer functions.
Q.157Medium
In a closed-loop control system, the feedback path gain is reduced from 1 to 0.5. How does this affect the system's steady-state error?
Answer: A
Reducing feedback gain reduces the effectiveness of feedback, leading to increased steady-state error for the same input command.
Q.158Medium
For a control system, the gain crossover frequency and phase crossover frequency are equal. What does this indicate about the system's stability margin?
Answer: C
When gain crossover frequency equals phase crossover frequency, the system is at the stability boundary (phase = -180°), indicating marginal stability.
Q.159Medium
A lead compensator has a transfer function Gc(s) = K(s+2)/(s+8). What is the effect of this compensator?
Answer: B
Lead compensator has zero at -2 and pole at -8 (pole further left). This increases bandwidth and adds phase lead, improving transient response.
Q.160Medium
For a system with transfer function G(s)H(s) = 10/(s²(s+2)), the number of asymptotes in the root locus is:
Answer: B
Number of asymptotes = |number of poles - number of zeros| = |3 - 0| = 3. However, for this specific configuration with two poles at origin, effective asymptotes for root locus are 2.
