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.
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.22Medium
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.23Medium
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.24Easy
The maximum frequency of a thyristor is typically limited by:
Answer: B
Turn-off time (tq) determines the minimum period required between successive gate pulses, thus limiting the maximum operating frequency of thyristors.
Q.25Medium
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.26Hard
The dv/dt rating of a power semiconductor is important because it:
Answer: B
High dv/dt can cause displacement currents through parasitic capacitances, potentially triggering devices unintentionally. dv/dt rating specifies the maximum safe rate of voltage change.
Q.27Easy
A PWM inverter with sinusoidal modulation produces output voltage with:
Answer: B
Sinusoidal PWM produces a voltage waveform containing the fundamental frequency plus high-frequency harmonics that can be filtered out.
Q.28Medium
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.29Medium
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.30Medium
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.31Hard
In a three-level diode-clamped inverter (Neutral Point Clamped), the advantage over two-level inverter is:
Answer: B
Three-level inverter generates three voltage levels, reducing the step size and dv/dt compared to two-level inverter, thus reducing EMI and stress on motor windings.
Q.32Medium
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.33Hard
Which control strategy for a DC-DC converter provides the tightest output voltage regulation under varying input voltage and load conditions?
Answer: B
Closed-loop feedback control with error amplifier and compensation networks provides superior regulation by continuously adjusting duty cycle based on output voltage error.
Q.34Hard
In a synchronous buck converter, the advantages of using a low-side MOSFET instead of a diode are:
Answer: D
Synchronous MOSFETs have lower on-state resistance than diode forward voltage drop, reducing losses. Active switching also allows higher frequency operation with better control.
Q.35Medium
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.36Easy
The presence of freewheeling diode in a DC motor drive is essential because it:
Answer: A
The motor inductance tends to maintain current flow. The freewheeling diode provides a path for this current when the main switch is off, preventing voltage spikes.
Q.37Medium
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.38Medium
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.39Medium
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.
Q.40Easy
In a boost converter operating in continuous conduction mode (CCM), if the duty cycle is 0.5 and input voltage is 12V, what is the output voltage?
Answer: A
For a boost converter, Vo/Vin = 1/(1-D). With D=0.5, Vo = 12/(1-0.5) = 24V
