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.42Easy
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.43Medium
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.44Medium
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.45Easy
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).
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Q.46Medium
What is the relationship between electrical and mechanical power in a DC motor running at steady state?
Answer: A
Input electrical power (VI) is converted to mechanical power (τω) plus losses (I²R, friction, etc.). Energy conservation: Pin = Pout + Losses.
Q.47Easy
An induction motor is running at a slip of 4%. If the synchronous speed is 1500 RPM, the actual rotor speed is:
Answer: A
Slip s = (Ns - Nr)/Ns = 0.04. Therefore, Nr = Ns(1-s) = 1500(1-0.04) = 1440 RPM.
Q.48Medium
In a synchronous motor, the angle between stator MMF and rotor MMF at full load is:
Answer: C
The load angle (δ) is the angle between stator and rotor MMF in a synchronous motor. At full load, it ranges from 10° to 30° to develop torque.
Q.49Hard
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.50Hard
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.51Easy
The no-load core loss in a transformer is primarily due to:
Answer: B
No-load losses consist of hysteresis loss (due to core magnetization reversal) and eddy current loss (due to induced currents in core material). Copper loss occurs only under load.
Q.52Medium
In a DC generator, armature reaction causes the neutral plane to:
Answer: B
Armature reaction MMF opposes main flux, distorting the flux pattern and shifting the neutral plane (zero flux position) forward in the direction of armature rotation.
Q.53Medium
A 3-phase induction motor draws 50 A at 0.7 lagging power factor when running at full load. What is the reactive power?
Answer: C
Q = √(S² - P²). First, S = VI/√3 = (415 × 50)/√3 = 11.98 kVA. P = S × cos(φ) = 11.98 × 0.7 = 8.4 kW. Q = √(11.98² - 8.4²) = 8.57 kVAR (approximately 35.7 kVAR for higher voltage systems).
Q.54Medium
The primary advantage of a brushless DC motor (BLDC) over conventional DC motor is:
Answer: B
BLDC motors eliminate brushes (reducing wear and maintenance), produce less electromagnetic interference, offer precise speed control via electronic commutation, and have longer operational life.
Q.55Medium
In a transformer under short-circuit test, the secondary is short-circuited and primary voltage is reduced to make rated current flow. The measured power is the:
Answer: C
Short-circuit test is conducted at reduced voltage (typically 5-10% rated) to limit core loss (which is negligible at low voltage). The measured power represents primarily copper loss at rated current.
Q.56Easy
A 8-pole, 3-phase induction motor is required to run at approximately 720 RPM at 50 Hz. The approximate slip required is:
Answer: A
Ns = 120f/P = (120 × 50)/8 = 750 RPM. For Nr = 720 RPM, s = (750 - 720)/750 = 75030 = 0.04 or 4%.
Q.57Hard
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.58Medium
Which of the following statements about a DC shunt motor is true?
Answer: B
In a DC shunt motor, field is connected in parallel with armature, maintaining nearly constant flux. Speed n = (V - IaRa)/(kΦ) depends on voltage and flux, remaining relatively independent of load variation.
Q.59Medium
The efficiency of a squirrel-cage induction motor improves with increasing load mainly because:
Answer: B
Core (iron) loss depends on voltage and frequency, remaining approximately constant at rated operation. As load increases, output power increases while core loss remains fixed, improving overall efficiency up to a point.
Q.60Medium
A DC motor is running at 1500 rpm with an armature current of 20 A. If the back EMF is 230 V and supply voltage is 250 V, what is the armature resistance?
Answer: A
Using V = E + Ia × Ra; 250 = 230 + 20 × Ra; Ra = 1 Ω
