Question 1

Q.1. (a) A single phase half wave ac voltage controller feeds a load of R = 2…

Accepted Answer

For the given single-phase half-wave AC voltage controller (R=20Ω, 230V/50Hz, α=45°), RMS output voltage ≈ 155.07V, power delivered ≈ 1202.4W with input PF ≈ 0.674, and average input current ≈ 4.42A; for a full-wave RL-load controller, the output voltage/current waveforms exhibit an extended conduction period beyond the voltage zero crossing (due to load inductance), with RMS output voltage given by a standard integral expression in terms of α and the extinction angle β.

Question 2

Q.1 OR (a) Explain the principle of 'on-off' control for a single phase full …

Accepted Answer

On-off (integral cycle) control switches the thyristors to connect the full supply for m complete half-cycles and disconnect for n complete half-cycles, controlling average power via the duty ratio m/(m+n) rather than phase angle; for a 230V/50Hz, 3Ω resistive load full-wave controller, average and rms thyristor currents and the minimum circuit turn-off time are each derived as explicit functions of firing angle α.

Question 3

Q.2 (a) Describe the basic principle of working of a single phase to single p…

Accepted Answer

A single-phase to single-phase cycloconverter directly synthesizes a low-frequency AC output from a higher-frequency AC supply by alternately using a positive-group and negative-group converter bridge, each fired with a time-varying firing angle to trace out the desired output waveform, operating either in continuous (circulating-current) or discontinuous (non-circulating-current) conduction mode; compared to DC-link inverters, cycloconverters avoid the intermediate DC stage and its associated conversion losses but are generally limited to lower output frequencies and require a more complex control scheme, finding application in large, low-speed AC motor drives (cement mill, ship propulsion) and variable-speed constant-frequency power supplies.

Question 4

Q.2 OR (a) Explain the principle of operation of a 3 phase to 3 phase cycloco…

Accepted Answer

A 3-phase to 3-phase cycloconverter uses three single-phase (or a common 3-phase) cycloconverter units, each synthesizing one output phase from the 3-phase supply with a 120°-phase-shifted low-frequency reference, and operating on the identical positive/negative-group dual-converter switching principle as the single-phase case; for the given 3-pulse cycloconverter numerical (190V, 45A load at 0.7 lag pf), the required supply voltage and supply-side power factor are derived from the standard 3-pulse converter voltage/current relations, showing the supply must provide a somewhat higher phase voltage than the output and draws a displacement power factor further reduced from the load's own power factor due to the firing-angle-dependent nature of thyristor conversion.

Question 5

Q.3 (a) Explain the phenomena of operation of 3φ current source inverter for …

Accepted Answer

A 3-phase current source inverter (CSI) feeding a star-connected load forces a controlled, quasi-square current waveform (rather than voltage) into each output phase using a large DC-link inductor and self-commutated/forced-commutated thyristor switches, with each device conducting for 120° and only two devices conducting at any instant; VSI's 120° conduction mode uses voltage-source switches conducting 120° each (rather than the more common 180° mode), and comparing VSI with CSI shows VSI offers better voltage waveform quality and multi-motor feeding capability while CSI offers simpler short-circuit/overload protection and natural regeneration capability at the cost of poorer light-load/no-load performance.

Question 6

Q.3 OR (a) Explain in detail the working methodology of pulse width modulatio…

Accepted Answer

Pulse Width Modulation (PWM) techniques control an inverter's output voltage magnitude and reduce harmonic content by varying the width (duty cycle) of switching pulses within each half-cycle according to a modulating reference (commonly sinusoidal PWM comparing a sine reference against a high-frequency triangular carrier), while harmonic reduction is additionally achieved through techniques such as selective harmonic elimination, multilevel inverter topologies, and output filtering.

Question 7

Q.4. Explain in detail the working methodology of series resonant inverter wi…

Accepted Answer

A series resonant inverter uses an L-C series resonant tank connected to the load, with switches operated at (or near) the tank's natural resonant frequency causing the load current to naturally reverse direction (zero-current switching), enabling unidirectional (asymmetric) switch conduction; this gives low switching losses and simple, natural (load-)commutation but requires operation close to a single resonant frequency and gives a load-dependent output, finding application in high-frequency induction heating and resonant DC-DC power supplies.

Question 8

Q.4 OR (a) Describe construction & working of a ZVS resonant converter. [8]

Accepted Answer

A Zero-Voltage-Switching (ZVS) resonant converter uses a resonant L-C tank in combination with the switching devices such that each switch turns on only when the voltage across it has already been brought to (or near) zero by the resonant circuit's natural oscillation, eliminating turn-on switching loss; a parallel resonant inverter connects its resonant capacitor directly in parallel with the load (rather than in series), giving a current-source-like output better suited to loads that themselves benefit from a parallel-resonant voltage step-up characteristic.

Question 9

Q.5 (a) What are control circuits? Explain voltage mode control. [8]

Accepted Answer

Control circuits in switch-mode power supplies sense the output condition (voltage/current) and generate the appropriate switch gate-drive timing to regulate it against load/line disturbances; voltage mode control directly compares the sensed output voltage against a reference to generate a PWM duty cycle command via a single feedback loop, offering simplicity at the cost of somewhat slower transient response and susceptibility to input-voltage-variation-induced output disturbance, compared to more advanced current-mode control schemes; resonant DC power supplies use a resonant tank (as discussed for series/parallel resonant inverters) between the switching stage and the output rectifier to achieve soft-switching and reduced switching losses at high frequency.

Question 10

Q.5 OR (a) Describe Bi-directional power supply system in detail. [8]

Accepted Answer

A bi-directional power supply system can both deliver power from source to load (forward/buck mode) and return power from load back to source (reverse/regenerative mode) using switches capable of conducting current in both directions, essential for applications such as battery charging/discharging and regenerative motor drives; multistage conversion cascades two or more independent power conversion stages (e.g., AC-DC followed by DC-DC) to achieve a combination of functions (such as power factor correction plus isolated voltage regulation) that a single conversion stage cannot efficiently provide alone.

Advanced Power Electronics