All Free Electrical Engineering MCQs with Answers

• A. Continuous signal
• B. Large isolating pulse transformer
• C. A train of pulses
• D. None of these

Explanation: With an R-L load, the thyristor current may continue after the supply voltage changes sign, so a train of gate pulses gives reliable…

• A. Both harmonic distortion and quality of input current increases
• B. Harmonic distortion increases and quality of input current decreases
• C. Harmonic distortion decreases and quality of input current increases
• D. Both harmonic distortion and quality of input current decreases

Explanation: Increasing the firing angle chops more of the supply waveform, increasing harmonic distortion.

• A. Current controlled unipolar device
• B. Voltage controlled unipolar device
• C. Current controlled bipolar device
• D. Voltage controlled bipolar device

Explanation: A power MOSFET is controlled by gate-to-source voltage and conducts through one type of majority carrier, making it both…

• A. More than that of MOSFET
• B. Lower than that of MOSFET
• C. Equal to that of MOSFET
• D. Equal to that of BJT

Explanation: An IGBT generally has lower conduction loss than a high-voltage MOSFET because its on-state voltage drop is comparatively low at high…

• A. A parallel combination of controllable switch and a diode is used
• B. A parallel combination of controllable switch and capacitor is used
• C. A series combination of controllable switch and a diode is used
• D. A series combination of controllable switch and a capacitor is used

Explanation: Bidirectional current flow is obtained by placing a controllable switch and a diode in parallel with opposite current directions.

• A. Unidirectional
• B. Bidirectional
• C. Either (a) or (b)
• D. Non directional

Explanation: A conventional rectifier converts AC to DC, so its output or load current is normally constrained to one direction by the rectifying…

• A. Negatively affected by firing angle
• B. Negatively affected by distortion of the input current
• C. Positively affected by both firing angle and distortion of the input current
• D. Both (a) and (b)

Explanation: Rectifier power factor is the product of displacement factor and distortion factor.

• A. Power supplies
• B. Ups
• C. Multilevel configuration
• D. All of these

Explanation: Single-phase voltage-source inverters are widely used in power supplies and UPS systems, and they can also form single-phase multilevel…

• A. Square AC output voltage
• B. Sine AC output voltage
• C. Triangular AC output voltage
• D. Trapezoidal AC output voltage

Explanation: A very high modulation index drives the inverter into overmodulation, where PWM pulses disappear over portions of the cycle and the…

• A. 2nd
• B. 3rd
• C. 5th
• D. 7th

Explanation: A single-phase VSI with the usual half-wave-symmetric square-wave output contains odd harmonics such as the 3rd, 5th and 7th, but its even…

• A. Two dimensional functions
• B. Three dimensional functions
• C. One dimensional functions
• D. All of these

Explanation: PWM depends on both the carrier variable and the modulating-signal variable, so its double Fourier series represents a two-dimensional…

• A. AC - DC
• B. DC - DC
• C. DC - AC
• D. All of these

Explanation: Commutation is required in SCR converters used for AC-DC, DC-DC and DC-AC conversion, whether the current transfer is naturally or…

• A. One
• B. two
• C. Three
• D. Four

Explanation: The switching states 000 and 111 connect all three inverter legs to the same DC bus rail, producing zero line-to-line voltage.

• A. Directly proportional to their harmonic order
• B. Inversely proportional to their harmonic order
• C. Not related to their harmonic order
• D. None of these

Explanation: For a square wave, the magnitude of each present harmonic is proportional to 1 divided by its harmonic order, so higher-order harmonics…

• A. Only amplitude of voltage is controllable
• B. Only phase is controllable
• C. Both amplitude and phase is controllable
• D. Amplitude, phase and frequency of voltages should always be controllable

Explanation: A three-phase VSI can regulate the output voltage magnitude and establish its frequency and phase through the switching reference.

• A. Sine wave
• B. Square wave
• C. Triangular wave
• D. Trapezoidal wave

Explanation: With a sinusoidal reference, double-edge PWM produces symmetrical switching pulses, causing selected harmonic components to cancel.

• A. Sawtooth carrier
• B. Inverted sawtooth carrier
• C. Triangle carrier
• D. All of these

Explanation: Constant-frequency PWM can compare the reference with a triangular, sawtooth, or inverted-sawtooth carrier.

• A. 5
• B. 7
• C. 11
• D. All of these

Explanation: In a three-phase CSI, properly selected chopping angles can eliminate targeted low-order harmonics such as the 5th, 7th, and 11th.

• A. Small current spikes and can be reduced by using an inductive filter
• B. Large current spikes and can be increased by using an inductive filter
• C. Small current spikes and can be increased by using an inductive filter
• D. Large current spikes and can be reduced by using an inductive filter

Explanation: A capacitive load can draw a large impulsive current when the inverter voltage changes rapidly.

• A. One
• B. Three
• C. Six
• D. Nine

Explanation: A three-phase six-pulse converter has six commutations, or notches, in each supply-voltage cycle.