The open-circuit test on a transformer primarily helps to determine:
Answer: A
OC test measures core loss (hysteresis + eddy current) and magnetizing component of no-load current
Q.242Medium
In a permanent magnet synchronous motor (PMSM), the torque is primarily developed due to:
Answer: A
PMSM torque = Pm × Is × sin(δ), where Pm is permanent magnet flux and Is is stator current
Q.243Hard
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.244Hard
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.245Hard
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
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Q.246Medium
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.247Medium
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.248Easy
A transformer has primary turns = 1000, secondary turns = 100. If primary current is 2 A, what is the secondary current (neglecting losses)?
Answer: C
Using transformer equation: Np/Ns = Is/Ip; 1001000 = Is/2; Is = 20 A
Q.249Medium
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.250Medium
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.251Hard
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
Q.252Hard
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.253Medium
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
Q.254Medium
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.255Medium
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.256Medium
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.257Easy
The rotating magnetic field in a 3-phase induction motor rotates at:
Answer: A
The rotating magnetic field produced by the 3-phase stator winding rotates at synchronous speed (Ns = 120f/P), independent of load or rotor speed.
Q.258Hard
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.259Medium
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.260Medium
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.
