The torque developed in a 3-phase induction motor is maximum at a slip of:
Answer: C
Maximum torque occurs at slip sm = R2/X2. This is crucial for motor design and performance analysis.
Q.2Hard
An 8-pole induction motor running at 720 rpm with 50 Hz supply has a slip of approximately:
Answer: A
Ns = 120×850 = 750 rpm. Slip s = (750-720)/750 = 75030 = 0.04 or 4%.
Q.3Hard
In a DC machine, armature reaction causes:
Answer: B
Armature reaction is caused by armature MMF which distorts the main magnetic field, reduces net flux, and causes commutation issues. Compensation is needed through commutating poles.
Q.4Hard
The equivalent circuit of an induction motor includes a series resistance R2/s in the rotor branch. The power dissipated in this resistance represents:
Answer: B
In equivalent circuit, voltage across R2/s is rotor voltage and current through it gives mechanical power = I2² × (R2/s - R2) = mechanical power developed.
Q.5Hard
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.
Advertisement
Q.6Hard
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.7Hard
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.8Hard
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.9Hard
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.10Hard
Which statement is correct regarding the torque-slip characteristic of a squirrel-cage induction motor?
Answer: C
For induction motor, maximum torque occurs at slip s_max = R2/X2, where R2 and X2 are rotor resistance and reactance. This is derived from dT/ds = 0.
Q.11Hard
A 3-phase, 50 Hz, 4-pole synchronous motor operates at rated speed. The excitation is reduced by 20%. What happens?
Answer: B
Synchronous motor speed depends only on frequency and poles. Reducing excitation causes the motor to draw more lagging reactive current to maintain synchronism, operating at lagging power factor.
Q.12Hard
In a synchronous generator connected to an infinite bus, if the excitation is increased beyond rated value:
Answer: C
In an infinite bus system, frequency and terminal voltage are fixed by the bus. Increasing excitation beyond rated value causes the generator to draw leading reactive power (operating as a synchronous condenser).
Q.13Hard
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.14Hard
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.15Hard
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.16Hard
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.17Hard
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.18Hard
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.19Hard
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.20Hard
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.
