Electrical Engineering questions for GATE, PSU recruitment and SSC JE draw from network theory, electrical machines, power systems, control systems, measurements and instrumentation, analog and digital electronics, and electromagnetic fields. Numerical answers include the formula used and the unit at each stage, which is where marks are commonly lost even when the approach is correct.
What is the purpose of a freewheeling diode in a converter circuit?
Answer: B
A freewheeling diode provides a discharge path for inductor current when the main switch opens, preventing voltage spikes and ensuring continuous current flow in inductive circuits.
Q.462Medium
In a three-phase VSI (Voltage Source Inverter), if the modulation index (Ma) is 0.8 and DC link voltage is 400V, what is the peak phase voltage?
Answer: B
Peak phase voltage = (Ma × Vdc)/2 × √3 for three-phase. Vpeak = 0.8 × 2400 × √3/√3 ≈ 184.8V (considering line-to-neutral relationship)
Q.463Medium
Which PWM technique provides the lowest THD in the output voltage of an inverter?
Answer: C
Space Vector Modulation provides superior harmonic performance with THD approximately 30% lower than SPWM and enables higher DC utilization (15.5% improvement in voltage utilization).
Q.464Medium
In a step-down DC-DC converter (buck), the inductor current ripple is ΔI = 2A at 100 kHz switching frequency. If D = 0.5 and Vi = 24V, what is the approximate inductance value?
Answer: A
For buck converter: L = (Vi × D × (1-D))/(f × ΔI) = (24 × 0.5 × 0.5)/(100×10³ × 2) = 120 μH
Q.465Easy
What does the snubber circuit in a power converter primarily protect against?
Answer: B
Snubber circuits limit the rate of change of voltage (dv/dt) across semiconductor switches during switching transients, preventing false triggering and overvoltage damage.
Q.466Hard
In a controlled rectifier circuit, the commutation overlap angle (μ) depends on:
Answer: B
Commutation overlap occurs due to finite source impedance. The overlap angle μ = sin⁻¹(ωLₛI/√2×Vline), depending on source inductance and current.
Q.467Medium
Which cooling method is most effective for high-power semiconductor modules operating above 5 kW?
Answer: C
Liquid cooling provides superior heat transfer coefficient (500-5000 W/m²K) compared to forced air (50-500 W/m²K), essential for high-power applications with stringent thermal requirements.
Q.468Easy
In a full-bridge DC-DC converter, when both upper switches conduct simultaneously, what occurs?
Answer: B
Simultaneous conduction of upper switches creates a direct short across the DC link, causing massive current spike and potential device failure. This is prevented using dead-time insertion.
Q.469Medium
The power factor of a thyristor-based rectifier with firing angle 60° is approximately:
Answer: C
Power factor = cos(α) = cos(60°) = 0.5 for fully controlled single-phase rectifier. For 3-phase, it's slightly higher at approximately 0.67 considering the waveform.
Q.470Medium
In a current-source inverter (CSI), the DC link component is typically:
Answer: B
CSI uses a large inductor in series with the DC source to maintain nearly constant current. This differs from VSI which uses a capacitor for constant voltage.
Q.471Hard
Which parameter determines the minimum OFF-time in a PWM converter?
Answer: B
Minimum OFF-time must account for semiconductor recovery characteristics: tOFF(min) = ts + tf, allowing complete device turn-off before the next cycle.
Q.472Hard
For an AC voltage controller with RMS output voltage of 230V from a 230V, 50Hz source at 30° firing angle, what is the power factor?
Answer: A
For AC voltage controller: PF = (1/π)√(π² - 4α²) × sin(2α)/(2α) where α = 30° = π/6. This yields PF ≈ 0.87 with leading reactive current.
Q.473Easy
In modern 2024-25 EV charging systems, what is the typical efficiency target for on-board DC-DC converters?
Answer: B
Modern automotive DC-DC converters target 90-95% efficiency using advanced topologies, SiC MOSFETs, and optimized gate drives to minimize losses in 12V auxiliary systems.
Q.474Medium
A thyristor has a forward breakover voltage of 800V and a holding current of 200mA. What is the minimum gate current required to trigger the device if the gate sensitivity is 40mA/V?
Answer: B
Gate current = Gate sensitivity × Gate voltage. With 800V forward breakover and 40mA/V sensitivity, minimum gate current ≈ 20mA for reliable triggering.
Q.475Easy
In a three-phase bridge rectifier, if the input AC voltage is 400V (line), what is the approximate average DC output voltage?
Answer: A
For a three-phase bridge rectifier: Vdc = (3√2/π) × Vline = 1.35 × 400 = 540V approximately.
Q.476Easy
Which of the following devices has the highest switching frequency capability among modern power semiconductors?
Answer: A
MOSFETs have switching frequencies up to several MHz, while IGBTs typically reach 20-50kHz, thyristors <10kHz, and GTOs ~5kHz.
Q.477Medium
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.478Medium
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.479Medium
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.480Medium
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.
