In a wound-rotor induction motor, adding external resistance to the rotor circuit:
Answer: A
External rotor resistance shifts the point of maximum torque to higher slip values
Q.122Medium
The commutation in a DC machine is said to be 'overcommutated' when:
Answer: B
Over-commutation occurs when induced EMF > required EMF, causing sparking at trailing edge of brushes
Q.123Medium
In an induction motor, the torque under running conditions is proportional to:
Answer: C
T ∝ Ir × cos(φr) × r/s, where torque depends on rotor current magnitude and its power factor
Q.124Medium
Which of the following represents the condition for maximum power transfer in a DC generator connected to external load?
Answer: A
Maximum power transfer theorem: Load resistance equals source resistance for maximum power output
Q.125Medium
In a 3-phase transformer with Y-Δ connection, the secondary voltage lags the primary by:
Answer: B
Y-Δ connection introduces 30° phase shift with secondary lagging primary voltage phasor
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Q.126Medium
For a DC shunt motor operating at rated conditions, if the load is suddenly increased, the motor will:
Answer: B
Increased load increases Ia, causing larger voltage drop (IaRa), reducing back EMF and speed until new equilibrium
Q.127Medium
A DC series motor is used in electric trains. If the load torque decreases suddenly, what happens to the motor speed?
Answer: A
In a DC series motor, as load decreases, back EMF increases relative to applied voltage, causing field current to reduce. This results in a significant increase in motor speed.
Q.128Medium
In a 3-phase squirrel cage induction motor, which parameter can be increased to improve the starting torque?
Answer: C
Starting torque in an induction motor is proportional to rotor current and rotor resistance. Increasing rotor resistance improves starting torque at the cost of full-load efficiency.
Q.129Medium
The slot leakage reactance in an induction motor depends on:
Answer: B
Slot leakage reactance is determined by the magnetic flux confined within the stator and rotor slots, hence depends on dimensions of both stator and rotor slots.
Q.130Medium
In a DC generator, armature reaction causes:
Answer: C
Armature reaction in DC generators creates a demagnetizing effect and shifts the neutral plane forward (in direction of rotation), both effects occurring simultaneously.
Q.131Medium
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.132Medium
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.133Medium
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.134Medium
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.135Medium
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.136Medium
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.137Medium
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.138Medium
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.139Medium
The per-unit (p.u.) system in power systems is used primarily to:
Answer: A
Per-unit system normalizes all quantities relative to chosen base values, simplifying analysis and comparison across different voltage levels and equipment ratings.
Q.140Medium
Which protective relay is used for phase-to-phase fault detection in transmission lines?
Answer: B
Differential relays compare currents at both ends of a line and trip if there is a mismatch indicating an internal fault.
