In a synchronous motor, the pull-out torque depends on:
Answer: C
Pull-out torque τmax = 1.5(PΦIf)/(ωs), depends on pole flux and field current (excitation) and supply voltage.
Q.208Hard
A transformer is operating at 110% of rated voltage. The core loss will be approximately:
Answer: C
Core loss Pc ∝ B² ∝ V². At 1.1V, loss = (1.1)² × rated loss = 1.21 × rated ≈ 21% increase
Q.209Medium
Which starting method is suitable for a 3-phase induction motor with locked rotor current limit?
Answer: D
Both star-delta and autotransformer starting reduce starting current. Star-delta reduces current by 31 of DOL current.
Q.210Hard
The efficiency of a DC generator is maximum when:
Answer: C
Maximum efficiency occurs when variable losses (copper) equal constant losses (friction, windage, and core losses).
Q.211Hard
A 100 kW, 3-phase induction motor has a full-load slip of 0.04. The rotor copper loss at full load is:
Answer: B
Rotor copper loss = s × Pg, where Pg is air gap power. At full load, assuming 96% efficiency, losses ≈ 4.17 kW ≈ 4 kW
Q.212Medium
In a DC motor, the starting resistance is required because:
Answer: B
At starting, back EMF is zero. Starting resistance limits initial armature current to safe values: Ia = V/(Ra + Rstart)
Q.213Medium
Which transformer connection is preferred for step-up operations in power generation?
Answer: C
Star-delta connection reduces neutral point problems on primary (generator side) and utilizes delta on secondary for better voltage utilization.
Q.214Hard
The reluctance torque in a synchronous motor is maximum when the load angle is:
Answer: B
Reluctance torque τrel ∝ sin(2δ), which is maximum when sin(2δ) = 1, i.e., when δ = 45°
Q.215Easy
In a DC motor, the back EMF is proportional to which of the following?
Answer: A
Back EMF in DC motor: Eb = PΦN/60, which depends on flux (Φ) and speed (N). It is independent of armature current and load torque.
Q.216Easy
What is the synchronous speed of a 6-pole, 3-phase induction motor operating at 50 Hz frequency?
Answer: B
Synchronous speed Ns = 120f/P = (120 × 50)/6 = 1000 RPM, where f = frequency and P = number of poles.
Q.217Easy
The slip of an induction motor at rated load is typically in the range of:
Answer: A
For standard induction motors, slip at full load is typically 0.5% to 2%, ensuring minimal speed variation and high efficiency.
Q.218Medium
Which type of DC motor provides excellent speed control characteristics and is suitable for cranes and hoists?
Answer: C
Compound DC motors combine advantages of series (high starting torque) and shunt (stable speed) motors, making them ideal for cranes requiring both high starting torque and controlled speed.
Q.219Medium
In a 3-phase transformer with Delta-Star connection, the secondary voltage phase angle leads the primary by:
Answer: B
Delta-Star (Dy) connection introduces a 30° phase shift. Secondary voltage leads primary by 30°, which is standard for Dy11 configuration.
Q.220Easy
The copper loss in a transformer is minimum when:
Answer: A
Copper loss PCu = I²R is proportional to current squared. It is minimum when load current is zero (no-load condition).
