In a transformer, the hysteresis loss depends on which of the following factors?
Answer: B
Hysteresis loss = η × B^1.6 × f × V, where η is Steinmetz constant, material property is crucial
Q.62Easy
A 3-phase induction motor has 4 poles and operates at 50 Hz. What is the synchronous speed?
Answer: B
Ns = 120f/P = 120×450 = 1500 rpm
Q.63Easy
What is the typical slip value for a squirrel-cage induction motor operating at full load?
Answer: B
Full load slip for squirrel-cage motors typically ranges from 3-5% for good efficiency
Q.64Hard
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.65Medium
A series DC motor is suitable for which of the following applications?
Answer: B
Series motors develop very high starting torque, ideal for electric vehicles and heavy-duty applications
Advertisement
Q.66Medium
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.67Medium
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.68Hard
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.69Hard
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.70Hard
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.71Medium
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.72Medium
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.73Easy
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.74Medium
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.75Medium
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.76Hard
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.77Hard
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.78Medium
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.79Medium
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.80Medium
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.
