In a boost converter operating in continuous conduction mode (CCM) with duty cycle D=0.6, input voltage 24V, and output voltage 60V, what is the efficiency if losses are 5%?
Answer: A
Theoretical efficiency = 95% (ideal boost), minus 5% losses = 90% actual efficiency.
Q.242Medium
A PWM inverter produces an output frequency of 50Hz with a carrier frequency of 10kHz. What is the modulation index if the peak modulating signal is 0.8V and the peak carrier signal is 1V?
Answer: C
Modulation index m = Vm/Vc = 0.18 = 0.8, where Vm is peak modulating signal and Vc is peak carrier signal.
Q.243Medium
In a half-bridge inverter, what is the minimum dead time required to prevent shoot-through if the switching frequency is 20kHz?
Answer: C
Dead time should be 2-3 times the device switching time (~0.5-1μs for modern IGBTs), typically 1-3μs to ensure safe commutation.
Q.244Medium
Which protection method is MOST suitable for protecting a thyristor against voltage transients in industrial applications?
Answer: C
Combined MOV and RC snubber provides both transient suppression (MOV) and dV/dt protection (RC), offering comprehensive protection.
Q.245Medium
A current-controlled PWM rectifier maintains unity power factor. If the input current harmonic distortion THD is 3%, what is the approximate displacement power factor?
Answer: C
With THD=3%, the fundamental current component is significant. Displacement PF ≈ 0.98 for low THD with controlled rectification.
Advertisement
Q.246Medium
Which semiconductor device combination is used in a cycloconverter to convert AC-to-AC with frequency reduction?
Answer: B
Cycloconverters use back-to-back (antiparallel) thyristor pairs to enable bidirectional power flow for frequency conversion.
Q.247Medium
In a matrix converter (direct AC-to-AC), how many bi-directional switches are required for three-phase input and three-phase output?
Answer: B
Matrix converter requires m×n switches where m=output phases and n=input phases. For 3×3: 9 bi-directional switches.
Q.248Medium
A three-phase PWM rectifier with active front-end reduces grid distortion to THD < 5%. This is primarily achieved by:
Answer: B
Active front-end rectifiers use closed-loop current control with PWM modulation to inject harmonics that cancel grid distortion.
Q.249Medium
In a boost converter operating in continuous conduction mode (CCM), if the input voltage is 24V, output voltage is 48V, and switching frequency is 100kHz, what is the duty cycle of the converter?
Answer: B
For a boost converter, Vo/Vin = 1/(1-D), where D is duty cycle. 2448 = 1/(1-D) → 2 = 1/(1-D) → 1-D = 0.5 → D = 0.5 or 50%
Q.250Medium
A moving coil galvanometer has a coil resistance of 10 Ω and gives full-scale deflection for 5 mA current. What shunt resistance is required to convert it into an ammeter of range 0-5 A?
Answer: A
Using shunt formula: S = (Ig × Rg) / (I - Ig) = (5×10⁻³ × 10) / (5 - 5×10⁻³) ≈ 0.01 Ω
Q.251Medium
Which bridge circuit is most suitable for measuring an unknown capacitance?
Answer: C
Owen bridge is specifically designed for accurate measurement of unknown inductance and capacitance values in AC circuits.
Q.252Medium
In a Wheat stone bridge, the null deflection is obtained when the ratio of resistances satisfies which condition?
Answer: B
At balance condition in Wheatstone bridge: R₁/R₂ = R₃/R₄, which can be rearranged to R₁ × R₄ = R₂ × R₃
Q.253Medium
A moving iron instrument reads the same for both AC and DC because it:
Answer: A
Moving iron instruments deflect based on the magnitude of current/voltage regardless of direction. The iron core is attracted by the magnetic field produced by the coil in both AC and DC.
Q.254Medium
A thermocouple is used to measure temperature because it:
Answer: D
Thermocouples generate EMF (Seebeck effect) proportional to the temperature difference between junctions and don't require external power.
Q.255Medium
The mutual inductance between two coils is independent of:
Answer: C
Mutual inductance depends on geometry, coil configuration, and medium properties, but NOT on the current flowing through them.
Q.256Medium
A platinum resistance thermometer (RTD) follows which relationship between resistance and temperature?
Answer: B
RTDs exhibit a nearly linear relationship between resistance and temperature: Rt = R₀(1 + αt + βt²), where the linear term dominates.
Q.257Medium
A Hall effect sensor is primarily used to measure:
Answer: B
Hall effect sensors detect the magnetic field strength by measuring the Hall voltage generated when a current-carrying conductor is placed in a perpendicular magnetic field.
Q.258Medium
The power factor of a circuit can be improved by adding:
Answer: C
Power factor (lagging) is improved by adding shunt capacitance, which creates a leading current that offsets the lagging component.
Q.259Medium
The null deflection method in bridge circuits is preferred over deflection method because it:
Answer: B
Null deflection (or balance) method is preferred because it doesn't depend on meter calibration or linearity. The measurement relies on the balance condition formula only.
Q.260Medium
A moving coil galvanometer has an internal resistance of 10 Ω and a full-scale deflection current of 5 mA. What shunt resistance is required to convert it into an ammeter with a range of 0-5 A?
