ME or MEng Master of Engineering Master of Electrical Engineering 2 — Questions and Answers
Question 1: In a three-phase induction motor, if the rotor resistance is doubled while the load torque remains constant, what primarily happens to the slip?
- Slip doubles (Correct answer)
- Slip halves
- Slip remains unchanged
- Slip increases by a factor of four
Correct answer: Slip doubles
In the equivalent circuit model, slip is proportional to rotor resistance for a given torque, so doubling rotor resistance doubles the slip.
Question 2: Which modulation scheme achieves the highest spectral efficiency in a bandwidth-limited wireless channel?
- BPSK
- QPSK
- 16-QAM
- 256-QAM (Correct answer)
Correct answer: 256-QAM
256-QAM encodes 8 bits per symbol, giving the highest spectral efficiency (bits/s/Hz) among these options.
Question 3: For a buck-boost DC-DC converter operating in continuous conduction mode, what is the voltage conversion ratio M = V_out/V_in as a function of duty cycle D?
- D/(1-D)
- -D/(1-D) (Correct answer)
- (1-D)/D
- 1/(1-D)
Correct answer: -D/(1-D)
The buck-boost converter inverts polarity, giving M = -D/(1-D), where the negative sign indicates output polarity reversal.
Question 4: In CMOS logic design, which phenomenon causes dynamic power dissipation?
- Leakage current through reverse-biased junctions
- Short-circuit current during transitions only
- Charging and discharging of load capacitances (Correct answer)
- Threshold voltage variation with temperature
Correct answer: Charging and discharging of load capacitances
Dynamic power P = α·C·V²·f arises from charging and discharging load capacitances at each switching event.
Question 5: A transmission line has a characteristic impedance Z₀ = 50 Ω and is terminated with Z_L = 100 Ω. What is the reflection coefficient Γ?
- 0.5
- 0.33 (Correct answer)
- -0.33
- 2.0
Correct answer: 0.33
Γ = (Z_L - Z₀)/(Z_L + Z₀) = (100-50)/(100+50) = 50/150 = 1/3 ≈ 0.33.
Question 6: In power systems, the per-unit system is used primarily to:
- Eliminate the need for Kirchhoff's laws
- Simplify calculations across different voltage levels by normalizing quantities (Correct answer)
- Convert AC systems to equivalent DC circuits
- Remove transformer magnetizing branch from the equivalent circuit
Correct answer: Simplify calculations across different voltage levels by normalizing quantities
Per-unit normalization removes the transformer turns-ratio effect, allowing uniform analysis across multiple voltage levels.
Question 7: Which control strategy is most commonly used in field-oriented control (FOC) of AC motors to decouple torque and flux?
- Scalar V/f control
- Direct torque control with hysteresis band
- Park transformation to d-q rotating reference frame (Correct answer)
- Space vector modulation without coordinate transformation
Correct answer: Park transformation to d-q rotating reference frame
FOC applies the Park (d-q) transformation so flux-producing and torque-producing current components can be controlled independently like a DC motor.
ME or MEng Master of Engineering Master of Electrical Engineering 2
In a three-phase induction motor, if the rotor resistance is doubled while the load torque remains constant, what primarily happens to the slip?