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.422Medium
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.423Medium
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.424Medium
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.425Hard
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.
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Q.426Medium
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.427Hard
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.428Hard
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.429Medium
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.430Easy
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.431Medium
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.432Medium
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.433Medium
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.434Easy
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
Q.435Easy
Which of the following devices is a unidirectional switch in power electronics?
Answer: A
A diode conducts only in one direction (forward bias), making it a unidirectional switch
Q.436Medium
The switching frequency of a PWM inverter is 10 kHz. What is the fundamental frequency of the output if the modulation index is 0.8?
Answer: D
The output fundamental frequency is typically half the switching frequency in standard PWM, or determined by reference signal. With modulation, output frequency follows the reference signal, typically much lower than switching frequency.
Q.437Easy
In a half-bridge inverter, what is the minimum number of switches required to generate AC output from DC source?
Answer: A
A half-bridge inverter requires 2 switches (typically with 2 diodes) to generate AC output from a DC source
Q.438Medium
Which modulation technique provides the best harmonic performance in high-power applications?
Answer: B
SVPWM provides superior harmonic performance, higher DC link utilization (~15% more), and better efficiency in high-power applications compared to SPWM
Q.439Easy
What is the primary purpose of a freewheeling diode in a buck converter?
Answer: B
The freewheeling diode provides a continuous path for the inductor current during the switch OFF period, preventing voltage spikes
Q.440Easy
In a three-phase full-bridge inverter, the total number of thyristors required is:
Answer: C
A three-phase full-bridge inverter requires 6 thyristors (2 per phase) to generate three-phase AC output
