In a synchronous generator, the voltage regulation is negative when:
Answer: A
Leading power factor means reactive component opposes armature reaction, reducing terminal voltage drop
Q.42Hard
For a 3-phase synchronous motor connected to infinite bus, increasing field excitation beyond the critical value will:
Answer: B
Over-excitation reduces armature current by creating leading reactive component that cancels lagging current
Q.43Hard
The cogging torque in a DC motor is primarily due to:
Answer: A
Cogging torque results from interaction between poles and armature teeth, causing reluctance variation
Q.44Hard
A 3-phase induction motor develops maximum torque at a slip of 0.4. The motor is operating at slip 0.1. What is the ratio of starting torque to running torque?
Answer: D
Using slip relationship, Ts/T = (smax/s)² = (0.04.1)² = 16 for normal induction motor characteristics
Q.45Hard
A synchronous generator operating at leading power factor supplies current to an infinite bus. This condition represents:
Answer: A
Over-excitation produces leading current (capacitive), supplying reactive power to the bus
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Q.46Hard
The crawling phenomenon in induction motors occurs due to:
Answer: B
Crawling results from 5th, 7th harmonics creating torques at different slip values, causing low-speed oscillations
Q.47Hard
A synchronous motor is running at no-load with normal excitation. If excitation is increased, the armature current will:
Answer: A
At no-load with increased excitation, the motor operates at leading power factor. Armature current decreases because the motor absorbs less reactive power from the supply.
Q.48Hard
The phenomenon of cogging in induction motors occurs due to:
Answer: B
Cogging (or coggling) occurs when the number of stator and rotor slots are equal or have common factors, causing reluctance torque variations due to slot permeance interaction.
Q.49Hard
The double-field revolving theory is used to analyze:
Answer: C
The double-field revolving theory represents a single-phase AC winding's pulsating magnetic field as two counter-rotating fields, used to analyze single-phase induction motor behavior.
Q.50Hard
If the frequency of supply to an induction motor is reduced while maintaining the same voltage, the motor torque will:
Answer: B
At constant V and reduced frequency, the air gap flux increases (Φ = V/f), but the rotor reactance decreases. However, the overall effect results in reduced starting and running torque.
Q.51Hard
In a DC series motor, the back EMF at rated load is 200V when the supply voltage is 230V. If the motor is suddenly unloaded, which of the following will occur?
Answer: A
In a DC series motor, when unloaded: (1) Load torque decreases, so armature current reduces significantly since field current equals armature current in series connection. (2) With reduced current, voltage drop (Ia×Ra) decreases. (3) Back EMF increases (Eb = V - Ia×Ra). (4) Since N ∝ Eb/Φ and both Eb increases while Φ decreases (less field current), speed increases substantially. This is why series motors are unsuitable for no-load operation.
Q.52Hard
The swing equation of a synchronous machine is given by: 2H(d²δ/dt²) = Pm - Pe - D(dδ/dt). What does H represent?
Answer: A
H is the inertia constant (M/2×SB) representing the kinetic energy relative to the system base power.
Q.53Hard
The critical clearing angle (δcr) in transient stability analysis represents:
Answer: A
δcr is the limiting rotor angle; if exceeded during a fault, the machine cannot return to synchronism even after fault clearance.
Q.54Hard
The fault level (short-circuit capacity) at a bus is determined by:
Answer: A
Fault level = MVA base / (Zth in p.u.), where Zth is the equivalent impedance of the entire network.
Q.55Hard
The maximum power transfer in an AC transmission system occurs when the sending and receiving end voltages are:
Answer: C
Maximum power transfer (Pmax = V²/X) occurs when power angle δ = 90°, regardless of absolute voltage magnitudes.
Q.56Hard
For a system with open-loop transfer function G(s)H(s) = K/[s(s+1)(s+2)], the number of asymptotes in root locus is:
Answer: B
Number of asymptotes = n - m = 3 - 1 = 2, where n=3 (poles) and m=1 (zeros).
Q.57Hard
A proportional-integral (PI) controller transfer function is Gc(s) = Kp + Ki/s. Its effect is:
Answer: B
PI controller adds a pole at origin (integral term), increasing system type by 1 and eliminating steady-state error for step and ramp inputs.
Q.58Hard
The centroid of asymptotes in root locus is located at:
Answer: A
Centroid σ = (∑poles - ∑zeros)/(n-m), where n and m are number of poles and zeros respectively.
Q.59Hard
In phase-lead compensation, the zero is placed:
Answer: A
In lead compensation, zero is placed to the left of pole (closer to origin), providing phase lead to improve transient response and stability margin.
Q.60Hard
For a second-order system with natural frequency ωn = 5 rad/s and ζ = 0.7, the peak time tp is approximately:
Answer: B
tp = π/(ωn√(1-ζ²)) = π/(5√(1-0.49)) = π/(5×0.714) ≈ 0.88 seconds ≈ 0.89 seconds. Closest answer is B.
