BEE Electrical Machines 1 — Questions and Answers
Question 1: A transformer has 200 primary turns and 400 secondary turns. If the primary voltage is 120V, what is the secondary voltage?
- 60V
- 240V (Correct answer)
- 120V
- 480V
Correct answer: 240V
By the turns ratio formula V2/V1 = N2/N1, so V2 = 120 × (400/200) = 240V.
Question 2: The efficiency of a transformer reaches its maximum when:
- Iron losses equal copper losses (Correct answer)
- Copper losses are zero
- Iron losses are zero
- The load is at rated full capacity
Correct answer: Iron losses equal copper losses
Maximum efficiency occurs at the load where variable copper losses equal the constant iron (core) losses.
Question 3: In a DC motor, the back EMF (Eb) is related to supply voltage (V), armature current (Ia), and armature resistance (Ra) by:
- Eb = V + IaRa
- Eb = V - IaRa (Correct answer)
- Eb = IaRa
- Eb = V / IaRa
Correct answer: Eb = V - IaRa
Applying KVL around the armature circuit gives V = Eb + IaRa, so Eb = V - IaRa.
Question 4: The synchronous speed of a 4-pole induction motor operating on a 60 Hz supply is:
- 900 RPM
- 1200 RPM
- 1500 RPM
- 1800 RPM (Correct answer)
Correct answer: 1800 RPM
Synchronous speed Ns = 120f/P = 120 × 60 / 4 = 1800 RPM.
Question 5: At synchronous speed, the slip of a three-phase induction motor is:
- 1
- 0.5
- 0 (Correct answer)
- -1
Correct answer: 0
Slip s = (Ns - Nr)/Ns; when rotor speed Nr equals synchronous speed Ns, slip equals zero.
Question 6: Which type of DC motor provides the highest starting torque and is most suitable for traction (railway) applications?
- DC shunt motor
- DC series motor (Correct answer)
- DC compound motor
- DC permanent magnet motor
Correct answer: DC series motor
A DC series motor produces torque proportional to the square of armature current, giving very high starting torque at low speeds.
Question 7: The speed regulation of a DC shunt motor compared to a DC series motor is:
- Much poorer (higher regulation)
- About the same
- Much better (lower regulation) (Correct answer)
- Negative for shunt, positive for series
Correct answer: Much better (lower regulation)
The DC shunt motor maintains nearly constant speed from no-load to full-load, giving it excellent (low) speed regulation compared to the series motor.
BEE Electrical Machines 1
A transformer has 200 primary turns and 400 secondary turns.
If the primary voltage is 120V, what is the secondary voltage?