Power electronics imp MCQs for SSC JE RRB JE 2025

Power Electronics – Previous Year Questions (ECE)

1. In a single-phase full converter feeding an R–L load, the average output voltage is controlled by varying:
   A) Supply frequency
   B) Firing angle ✅
   C) Load resistance
   D) Load inductance
2. For a single-phase half-wave controlled rectifier with a resistive load, the average output voltage is:
   $$V_{avg} = \frac{V_m}{2\pi}(1 + \cos\alpha)$$
3. In a full-bridge inverter using 180° conduction mode, how many switches conduct simultaneously?
   A) 1
   B) 2 ✅
   C) 3
   D) 4
4. A single-phase full converter feeds a 10 Ω load. The input voltage is 230 V, 50 Hz. If firing angle is 60°, the average output voltage is:
   $$V_{avg} = \frac{2V_m}{\pi}\cos\alpha = \frac{2 \times 325}{\pi} \times \cos60° \approx 103.6V$$
5. A freewheeling diode in a converter circuit:
   A) Increases output voltage
   B) Decreases output current
   C) Provides continuity of current in inductive loads ✅
   D) Blocks reverse current
6. A step-down chopper operates from 220 V DC supply with duty ratio 0.4. Average output voltage:
   $$V_o = 0.4 \times 220 = 88V$$
7. In a step-up chopper, output voltage is:
   $$V_o = \frac{V_s}{1 - \alpha}$$
8. In a class-C chopper, power flows:
   A) From source to load
   B) From load to source
   C) In both directions ✅
   D) None
9. The control characteristic of an SCR shows that latching current is:
   A) > Holding current ✅
   B) < Holding current
   C) = Holding current
   D) Not related
10. The turn-off time of a thyristor depends on:
    A) Gate current
    B) Forward voltage
    C) Device parameters and circuit conditions ✅
    D) Frequency only
11. In a single-phase bridge inverter using square wave, the output contains:
    A) Only fundamental
    B) Odd harmonics only ✅
    C) Even harmonics only
    D) Both even and odd
12. In a cycloconverter, the frequency of output is:
    A) Equal to supply
    B) Lower than supply ✅
    C) Higher than supply
    D) Independent of supply
13. In a single-phase AC voltage controller with resistive load, RMS output voltage:
    $$V_{o,rms} = V_m \sqrt{\frac{1}{2\pi}(\pi - \alpha + \frac{\sin2\alpha}{2})}$$
14. A TRIAC can conduct in:
    A) One quadrant
    B) Two quadrants ✅
    C) Three quadrants
    D) Four quadrants
15. In a DC-DC buck converter, increasing duty cycle will:
    A) Increase output voltage ✅
    B) Decrease output voltage
    C) No effect
    D) Reverse current
16. Snubber circuits are used for:
    A) Reducing dv/dt ✅
    B) Increasing gate current
    C) Reducing conduction losses
    D) Reducing firing angle
17. In a three-phase full converter, the output DC voltage:
    $$V_{dc} = \frac{3V_m}{\pi}\cos\alpha$$
18. In PWM inverters, harmonics are reduced by:
    A) Increasing carrier frequency ✅
    B) Reducing modulation index
    C) Reducing switching frequency
    D) Using square-wave control
19. Commutation in an SCR means:
    A) Turning ON the device
    B) Turning OFF the device ✅
    C) Increasing current
    D) Limiting voltage
20. In a single-phase full converter feeding RLE load, for continuous current, the average output voltage:
    $$V_{dc} = \frac{2V_m}{\pi}\cos\alpha$$

Read also : transient analysis RL and RC