Explain each of the following components of a vapour compression refrigeration system: compressor, condenser, expansion valve, evaporator.

Draw and label diagram of speed-time curve for main service line of a train. A speed-time curve for a train typically consists of three main sections: acceleration, free running (or constant speed), and braking (or retardation). ` ^ Speed | | /------------------\ | / \ | / \ | / \ | / \ | / \ |/ \ +-------------------------------------> Time 0 A B C D ` OA (Acceleration Period): The train starts from rest and accelerates. This phase can be further divided into: Constant acceleration:* Initial period where acceleration is constant. Speed curve:* As speed increases, back EMF rises, and acceleration decreases. AB (Free Running / Constant Speed Period): The train runs at a constant speed, where the tractive effort balances the resistance to motion. BC (Coasting Period): Power is cut off, and the train slows down due to resistance, but without active braking. This phase may or may not be present depending on the service. CD (Braking / Retardation Period): Brakes are applied, and the train decelerates to a stop. --- (i) State two properties of refrigerants; (ii) Explain each of the following components of a vapour compression refrigeration system: (I) compressor; (II) condensor; (III) expansion valve; (IV) evaporator. (i) Two properties of refrigerants: Low boiling point: Allows the refrigerant to vaporize at low temperatures, absorbing heat from the refrigerated space. High latent heat of vaporization: Enables the refrigerant to absorb a large amount of heat per unit mass during evaporation. (Other valid properties include: non-corrosive, non-toxic, non-flammable, chemically stable, high critical temperature, low freezing point, easily detectable leaks). (ii) Explanation of components of a vapour compression refrigeration system: (I) Compressor: This component increases the pressure and temperature of the low-pressure, low-temperature refrigerant vapor from the evaporator. It consumes mechanical energy to circulate the refrigerant and prepare it for heat rejection in the condenser. (II) Condenser: In the condenser, the high-pressure, high-temperature refrigerant vapor releases its latent heat to the surroundings (air or water), causing it to condense into a high-pressure, high-temperature liquid. This is where heat is rejected from the system. (III) Expansion valve (or throttling device): This valve reduces the pressure and temperature of the high-pressure liquid refrigerant from the condenser. As the refrigerant passes through the valve, it undergoes an isenthalpic expansion, causing a portion of it to flash into vapor and significantly lowering its temperature. (IV) Evaporator: The low-pressure, low-temperature liquid-vapor mixture from the expansion valve enters the evaporator. Here, it absorbs heat from the refrigerated space (e.g., food compartment, room air), causing the liquid refrigerant to completely vaporize into a low-pressure, low-temperature vapor, thus producing the cooling effect. --- SECTION B: POWER ELECTRONICS Answer TWO questions from this section. State two: (i) merits of the pulse gate triggering of silicon-controlled rectifier (SCR); (ii) applications of insulated gate bipolar transistor. (i) Two merits of the pulse gate triggering of silicon-controlled rectifier (SCR): Reduced gate power loss: Short duration pulses deliver sufficient energy to turn on the SCR, minimizing continuous power dissipation in the gate circuit. Electrical isolation: Pulse transformers or optocouplers can be used for triggering, providing excellent electrical isolation between the control circuit and the high-power SCR circuit. (ii) Two applications of insulated gate bipolar transistor (IGBT): Motor drives: Used in variable frequency drives (VFDs) for AC motors and choppers for DC motors due to their high power handling and fast switching capabilities. Uninterruptible Power Supplies (UPS): Employed in inverters to convert DC battery power to AC for critical loads during power outages. (Other valid applications include: induction heating, power factor correction, welding equipment, electric vehicles). --- Using the emitter characteristic curve, describe the operation of a unijunction transistor. The emitter characteristic curve of a unijunction transistor (UJT) plots the emitter voltage (V_E) against the emitter current (I_E) for a constant interbase voltage (V_BB). The curve typically exhibits three distinct regions: 1. Cutoff Region: When V_E is less than the peak point voltage (V_P), the emitter diode is reverse-biased or slightly forward-biased but not enough to cause significant current flow. In this region, I_E is very small (leakage current), and the UJT is in its OFF state. 2. Negative Resistance Region: As V_E increases and reaches the peak point voltage (V_P), the emitter diode becomes sufficiently forward-biased, and the UJT "fires." This causes the resistance between the emitter and base 1 (R_B1) to decrease rapidly due to conductivity modulation. Consequently, I_E increases significantly, while V_E drops, exhibiting a negative resistance characteristic. This region is crucial for oscillator applications. 3. Saturation Region: After the negative resistance region, as I_E continues to increase, the voltage drop across the internal resistance of the UJT becomes dominant. The emitter diode is fully forward-biased, and V_E increases slightly with increasing I_E, behaving like a conventional forward-biased diode. The UJT is now in its ON state. The UJT remains in the ON state until I_E falls below a certain minimum value called the valley point current (I_V), at which point it returns to the cutoff region. This switching behavior makes the UJT suitable for relaxation oscillators and timing circuits. --- (i) State three functions of a snubber circuit in switching semi-conductor device; (ii) A single-phase 240 V 2 kW heater is connected across a single-phase (i) Three functions of a snubber circuit in switching semi-conductor device: dv/dt protection: Limits the rate of rise of voltage across the switching device during turn-off, preventing false turn-on or damage due to excessive voltage stress. di/dt protection: Limits the rate of rise of current through the switching device during turn-on, preventing localized hot spots and potential damage. Damping oscillations: Suppresses voltage and current oscillations that can occur due to parasitic inductances and capacitances in the circuit during switching transitions, reducing electromagnetic interference (EMI) and improving device reliability. (ii) A single-phase 240 V 2 kW heater is connected across a single-phase The question is incomplete.