Junior Engineer (Electrical/Electronics) MCQs – Series 3 (Q41-50)
Q41. The turns ratio of a transformer is defined as:
- A) N1×N2 = V1×V2
- B) N1/N2 = V1/V2
- C) N1/N2 = V2/V1
- D) N1+N2 = V1+V2
Answer: B
The transformation ratio a = N1/N2 = V1/V2 relates primary and secondary turns and voltages.
Q42. In a transformer, hysteresis loss mainly depends on:
- A) Supply voltage only
- B) Load current
- C) Winding resistance
- D) Frequency and maximum flux density
Answer: D
Hysteresis loss, Ph ∝ f × Bmax^1.6 (Steinmetz’s law), depends on frequency and peak flux density, not on load current.
Q43. The slip of an induction motor running exactly at synchronous speed is:
- A) Infinite
- B) 0.5
- C) 1
- D) 0
Answer: D
Slip s = (Ns − N)/Ns; at synchronous speed N = Ns, so slip = 0 (though this is a theoretical, unattainable condition under load).
Q44. The synchronous speed of an induction motor is given by:
- A) Ns = f/120P
- B) Ns = 120f/P
- C) Ns = 60f/P
- D) Ns = P/120f
Answer: B
Synchronous speed in rpm is Ns = 120f/P, where f is supply frequency and P is number of poles.
Q45. In an induction motor, the rotor frequency at slip s and supply frequency f is:
Answer: A
Rotor (induced) frequency fr = s × f, since the rotor sees the relative speed between the rotating field and the rotor.
Q46. A synchronous motor operated at leading power factor is commonly used for:
- A) Braking applications
- B) Starting other synchronous motors
- C) Speed control of induction motors
- D) Power factor correction
Answer: D
An over-excited synchronous motor (synchronous condenser) draws leading current and is used to improve overall system power factor.
Q47. Parallel operation of alternators requires that the machines have equal:
- A) Field current only
- B) kVA rating only
- C) Voltage, frequency, and phase sequence
- D) Speed only
Answer: C
For successful synchronization, alternators must match in terminal voltage, frequency, phase sequence, and phase angle.
Q48. Which test on a transformer is used to determine the copper (load) loss?
- A) Insulation resistance test
- B) Open circuit test
- C) Short circuit test
- D) Sumpner’s test
Answer: C
The short circuit test, performed at reduced voltage with rated current flowing, gives the copper losses of the transformer.
Q49. In a squirrel cage induction motor, the rotor bars are:
- A) Left open-circuited
- B) Connected to external starting resistance
- C) Wound with copper wire and connected to slip rings
- D) Short-circuited by end rings
Answer: D
Squirrel cage rotors consist of bars short-circuited at both ends by end rings, unlike wound rotors which use slip rings.
Q50. The main advantage of a star-delta starter for an induction motor is that it:
- A) Reduces the starting current to about one-third of the direct-on-line value
- B) Increases the running speed of the motor
- C) Increases the starting torque above DOL value
- D) Improves the motor’s power factor at all loads
Answer: A
Starting in star reduces the voltage per phase to 1/√3, reducing the starting current and torque to about one-third of DOL values.
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