The power factor of a transformer at no-load is low because:
Answer: B
At no-load, the transformer draws mainly magnetizing current which lags the applied voltage by approximately 90°, resulting in low power factor (typically 0.1-0.2).
Q.262Medium
Which of the following losses in an induction motor is proportional to slip?
Answer: C
Rotor copper loss = sP_gap, where s is slip and P_gap is the air gap power. It is directly proportional to slip.
Q.263Easy
In an autotransformer, if primary turns = 100 and secondary turns = 400, the voltage transformation ratio is:
Answer: B
In an autotransformer, the turns ratio is N2/N1 = 100400 = 14, so voltage transformation ratio is Vs/Vp = 1:4 for a step-up autotransformer.
Q.264Easy
A 3-phase induction motor has 4 poles and operates from a 50 Hz supply. The synchronous speed is:
Answer: A
Synchronous speed Ns = 120f/P = (120 × 50)/4 = 1500 rpm for a 4-pole motor at 50 Hz.
Q.265Hard
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.
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Q.266Medium
In a separately excited DC motor, if flux is reduced to half while keeping the supply voltage constant, the armature current will:
Answer: C
Back EMF = KΦω. When Φ is halved, back EMF reduces, so armature current (Ia = (V - Eb)/Ra) increases approximately to double for the same load.
Q.267Hard
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.268Easy
In a transformer, the no-load current lags the applied voltage by approximately:
Answer: C
At no-load, the transformer draws mainly reactive (magnetizing) current, which lags the applied voltage by approximately 90°, giving a power factor of about 0.1-0.2.
Q.269Medium
The regulation of a DC shunt generator is poor because:
Answer: B
In a DC shunt generator, as load increases, armature current increases, causing armature reaction that demagnetizes the field, reducing terminal voltage significantly.
Q.270Medium
In a 3-phase synchronous generator, the pitch factor (or coil span factor) accounts for:
Answer: B
Pitch factor (Kp) accounts for the reduction in EMF when the coil span is less than the pole pitch (full pitch), representing incomplete flux linkage.
Q.271Easy
A shaded pole induction motor uses ________ for starting torque generation.
Answer: B
Shaded pole motors use a copper ring or shaded pole around part of the pole face to create a phase shift in the magnetic field, producing starting torque without additional components.
Q.272Hard
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.273Medium
In a transformer connected to a load with lagging power factor, the secondary current will:
Answer: B
If the load has lagging power factor, the secondary current lags the secondary voltage. The phase relationship between primary and secondary depends on the load power factor.
Q.274Medium
A 3-phase squirrel cage induction motor is running at 960 rpm when connected to a 50 Hz supply. If the supply frequency is suddenly changed to 60 Hz, what will be the approximate new speed of the motor, assuming the slip remains constant?
Answer: A
Synchronous speed Ns = 120f/P. When frequency changes from 50 Hz to 60 Hz with constant slip, new speed will be proportional to frequency. New speed = 960 × (5060) = 1152 rpm. This is because slip s = (Ns - N)/Ns remains approximately constant for the same load.
Q.275Hard
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.276Easy
What is the primary function of a circuit breaker in a power system?
Answer: A
Circuit breakers detect abnormal currents and automatically disconnect faulty sections to prevent equipment damage and cascading failures.
Q.277Medium
The stability of a power system depends on which of the following factors?
Answer: B
Power system stability is determined by the rotor dynamics characterized by inertia (M), damping (D), and synchronizing coefficient (Ks).
Q.278Easy
Which of the following is NOT a renewable energy source integrated into the Indian grid?
Answer: C
As of 2024-25, tidal energy has not been commercially integrated into the Indian grid on a significant scale, unlike solar, wind, and biomass.
Q.279Easy
What is the standard frequency of the Indian power grid?
Answer: A
India follows the 50 Hz standard frequency for AC power systems, as per IEC standards.
Q.280Easy
In a power system, the angle between voltage and current phasors is known as:
Answer: B
The phase angle (φ) between voltage and current phasors determines the power factor of the system.
