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.
A single-phase full-wave bridge rectifier with a 230V AC input produces an average output voltage of approximately:
Answer: C
For a full-wave rectifier: Vdc = (2√2/π) × Vrms = 0.9 × Vrms = 0.9 × 230 = 207 V approximately.
Q.202Medium
Which of the following thyristors can be turned off by a gate signal?
Answer: B
GTO can be turned on and off by gate signals. SCR can only be turned on by gate signal; turn-off requires anode current to become zero.
Q.203Medium
In a step-down DC-DC converter (buck converter), the output voltage is always:
Answer: B
A buck converter reduces voltage. Output voltage Vo = D × Vin, where D is duty cycle (0 < D < 1). For step-up, use boost converter.
Q.204Medium
The switching frequency of power electronic devices is increased to:
Answer: B
Higher switching frequency allows use of smaller inductors and capacitors, reducing converter size. Trade-off is increased switching losses.
Q.205Medium
A 3-phase diode rectifier (6-pulse) produces a ripple frequency of:
Answer: C
For a 3-phase 6-pulse rectifier: Ripple frequency = 6 × supply frequency = 6 × 50 = 300 Hz (or 6 × 60 = 360 Hz for 60 Hz supply).
Q.206Medium
The holding current of an SCR is important because:
Answer: B
Holding current (Ih) is the minimum anode current below which SCR turns off. If Ia < Ih, SCR will not remain in conducting state.
Q.207Medium
A PWM (Pulse Width Modulation) controlled DC motor drive uses which power electronic device?
Answer: C
PWM control of DC motors uses switching devices like MOSFETs or IGBTs in chopper configuration to vary average voltage and control speed.
Q.208Medium
The commutation of a thyristor in a circuit means:
Answer: C
Commutation is the process of turning off a thyristor by reducing anode current to below holding current value (natural commutation in AC circuits or forced commutation in DC).
Q.209Medium
The reverse recovery time of a diode is critical in power electronics because it:
Answer: D
Reverse recovery time affects switching frequency limits, causes switching losses, and creates EMI. Fast recovery diodes are preferred in high-frequency applications.
Q.210Medium
Which of the following devices can block voltage in both forward and reverse directions?
Answer: C
GTO can be turned on and off by gate control and blocks voltage bidirectionally when off, making it suitable for AC circuit applications.
Q.211Medium
In a flyback converter, the energy transfer occurs during:
Answer: B
In flyback converters, energy is stored in the transformer core during ON time and transferred to the output during OFF time through the secondary winding.
Q.212Medium
In a three-phase bridge inverter with 120° conduction mode, each switch conducts for:
Answer: B
In 120° conduction mode, each IGBT or switch conducts for 120° of the fundamental frequency cycle, reducing harmonics compared to 180° mode.
Q.213Medium
In an induction motor fed by a variable frequency drive (VFD), the V/f ratio is maintained constant to:
Answer: B
Maintaining constant V/f ratio ensures the magnetic flux in the motor remains constant across the speed range, preventing core saturation and maintaining torque capability.
Q.214Medium
The parasitic capacitance in a power MOSFET causes:
Answer: B
The gate-source (Cgs), gate-drain (Cgd), and drain-source (Cds) capacitances require energy to charge/discharge, causing switching losses and limiting frequency.
Q.215Medium
A series RLC filter connected at the output of an inverter serves to:
Answer: B
Output LC filters attenuate high-frequency switching harmonics and reduce voltage rise time (dv/dt), protecting connected equipment like motors.
Q.216Medium
The snubber capacitor in a power converter acts to:
Answer: B
Snubber capacitors provide a path for displacement current during switching transitions, clamping the voltage and reducing dv/dt stress on semiconductor devices.
Q.217Medium
The phenomenon of commutation in a thyristor-based rectifier circuit refers to:
Answer: A
In line-commutated converters, commutation occurs naturally due to phase voltage transitions. Thyristor turns off when the anode voltage becomes negative relative to cathode.
Q.218Medium
In a multilevel inverter topology, the total harmonic distortion (THD) compared to a two-level inverter is:
Answer: B
Multilevel inverters generate more voltage levels, creating a staircase waveform closer to sinusoid, resulting in lower THD and fewer harmonics compared to two-level inverters.
Q.219Medium
The thermal resistance (Rtheta-JA) of a power semiconductor determines:
Answer: C
Thermal resistance is the temperature difference per unit power dissipation: ΔT = Rθ × P. Lower thermal resistance means better heat transfer and lower junction temperature.
Q.220Medium
In a current-source inverter (CSI), the DC link contains:
Answer: B
CSI uses an inductor in the DC link to maintain a relatively constant current source, in contrast to VSI (Voltage Source Inverter) which uses a capacitor for constant voltage.
