Question 1

Q.1. (a) Illustrate the basic layered structure, construction and transistor …

Accepted Answer

The SCR (Silicon Controlled Rectifier) is a four-layer PNPN device with three terminals (anode, cathode, gate), analyzed via a two-transistor (PNP + NPN) analogy showing regenerative feedback; forward blocking mode occurs with anode positive but gate un-triggered (junction J2 reverse-biased, blocking), while reverse blocking mode occurs with cathode positive (junctions J1 and J3 reverse-biased), both preventing conduction until the appropriate breakover/triggering condition is met.

Question 2

Q.1. (a) Briefly explain various triggering methods used for thyristor. [8]

Accepted Answer

Thyristors can be triggered by forward voltage triggering (breakover), thermal triggering, light triggering (LASCR), dv/dt triggering, and gate triggering (the standard, controlled method); IGBT, FET, and MOSFET differ in operation mode, input/output impedance, amplification, and switching property, with MOSFETs and IGBTs offering high input impedance voltage-controlled operation and fast switching, while IGBTs additionally combine bipolar-like low on-state conduction loss suited to higher-power applications.

Question 3

Q.2. A single phase bridge rectifier is connected to 240 Vac, 60Hz source. Th…

Accepted Answer

For the given single-phase bridge rectifier (240V, 60Hz, R=20 ohm, Vo,avg=40% of Vo,max), the calculated results are: delay angle alpha=66.42 degrees, average output current Io,avg=4.32A, rms output current Irms=10.38A, average thyristor current=2.16A, rms thyristor current=7.34A, and input power factor=0.865.

Question 4

Q.2. What is an Inverter? List a few industrial applications of inverters. Ex…

Accepted Answer

An inverter is a power-electronic converter that converts DC input power into AC output power of a desired voltage and frequency, widely used in industrial applications such as variable-frequency motor drives, uninterruptible power supplies (UPS), induction heating, HVDC transmission, and renewable-energy grid interfacing; its basic principle of operation involves alternately switching two pairs of controlled semiconductor switches to reverse the polarity of the DC supply applied across the load at the desired output frequency.

Question 5

Q.3. (a) Distinguish between Buck-Boost and Boost converters. Give typical ap…

Accepted Answer

Buck-boost and boost converters differ in that a boost converter always steps up the input DC voltage (output greater than input, same polarity), while a buck-boost converter can step the output either above or below the input magnitude, with inverted output polarity; a dual converter combines two full converters back-to-back to allow four-quadrant (bidirectional voltage and current) DC output, with the two converters' firing angles related for circulating-current-mode operation by alpha1+alpha2=180 degrees.

Question 6

Q.3. (a) Describe the principle of step-down chopper with different waveforms…

Accepted Answer

A step-down (buck) chopper operates by periodically switching a series semiconductor device to connect and disconnect the DC source from the load, with a freewheeling diode maintaining load current continuity during the off-interval, producing an average output voltage Vo=D*Vd, where D is the duty cycle - derived directly from the on-time to total-period ratio and independent of load current for continuous conduction.

Question 7

Q.4. (a) Discuss the speed-torque curves for a chopper controlled induction m…

Accepted Answer

Speed-torque curves for a chopper-controlled induction motor drive show a family of parallel torque-speed characteristics, each shifted according to the chopper's applied voltage/frequency setting, allowing the drive to operate at any desired speed within its rated range while maintaining approximately constant maximum (breakdown) torque capability across the controlled speed range, in contrast to the single fixed characteristic of a motor operated directly from a fixed-voltage, fixed-frequency supply.

Question 8

Q.4. (a) Draw and explain the operation of a speed control of d.c. series mot…

Accepted Answer

A single-phase full converter provides continuous speed control of a DC series motor by varying the firing angle of its four thyristors, producing a controllable average DC output voltage applied to the series-connected armature and field winding, with characteristic chopped voltage and continuous current waveforms; four-quadrant operation of a DC-DC chopper is achieved using a full H-bridge (Class E) chopper configuration, allowing independent control of both forward/reverse motoring and forward/reverse regenerative braking operation.

Question 9

Q.5. (a) Identify the major constructive difference between a permanent magne…

Accepted Answer

The major constructive difference between a permanent magnet (PM) and a variable reluctance (VR) stepper motor is that the PM rotor contains permanently-magnetized material providing its own magnetic field, giving detent torque and bidirectional excitation response, whereas the VR rotor is made of plain, unmagnetized soft-iron/steel with salient teeth, relying entirely on the stator's electromagnetically-created field and the reluctance-minimization principle for torque production, with no detent torque and no permanent magnetic field of its own; for the given step motor problem (130 steps/rev, 10.5 rev/s), the required input pulse rate is 1365 pulses per second.

Question 10

Q.5. (a) Distinguish between the terms full stepping and half stepping. [8]

Accepted Answer

Full stepping energizes windings such that the rotor advances by its full, nominal step angle per input pulse (using either one-phase-on or two-phase-on excitation), while half stepping alternates between one-phase-on and two-phase-on excitation sequences, causing the rotor to advance by only half the nominal full-step angle per pulse, doubling angular resolution; stepper motors are driven using either open-loop control schemes (pulses applied without position feedback, relying on the motor not missing steps) or closed-loop control schemes (incorporating a position/speed feedback sensor to verify and correct actual rotor position, avoiding step loss under demanding load conditions).

Industrial Electronics