(SEM VII) THEORY EXAMINATION 2024-25 UTILIZATION OF ELECTRICAL ENERGY & ELECTRIC TRACTION

SECTION A

(2 × 10 = 20 Marks)

a) List the properties of heating element material

A good heating element material should have high electrical resistivity so that sufficient heat is produced with reasonable dimensions. It must possess a high melting point to withstand elevated temperatures, good mechanical strength, low temperature coefficient of resistance, high resistance to oxidation, and long service life under repeated heating and cooling cycles.

b) State any two disadvantages of core type induction heating

Core type induction heating systems are bulky and heavy due to the presence of magnetic cores. They are also limited to applications where the workpiece can be placed inside the core, making them unsuitable for large or irregularly shaped materials.

c) List the different types of welding

The different types of welding include arc welding, resistance welding, gas welding, thermit welding, spot welding, seam welding, and butt welding.

d) Define quenching

Quenching is a heat treatment process in which a heated metal is rapidly cooled by immersing it in a liquid medium such as water or oil. This process increases the hardness and strength of the metal by altering its microstructure.

e) Define luminous flux and write its unit

Luminous flux is defined as the total quantity of visible light emitted per second by a light source. It represents the light energy perceived by the human eye. The unit of luminous flux is the lumen.

f) Define candle power

Candle power is the measure of luminous intensity of a light source in a particular direction. It indicates the ability of the source to emit light and is expressed in candelas.

g) Write a short note on schedule speed

Schedule speed is the average speed of a train over a given route including the time spent at intermediate stops. It is an important parameter in traction systems as it reflects the efficiency of transport service.

h) What is crest speed?

Crest speed is the maximum speed attained by a train during its run between two stations. It depends on acceleration, distance between stations, and permissible speed limits.

i) Advantages of electric traction compared with diesel locomotives

Electric traction offers higher efficiency, better acceleration, lower operating cost, reduced pollution, and improved reliability when compared with diesel locomotives. It also enables regenerative braking and quieter operation.

j) Factors affecting specific energy consumption

Specific energy consumption is affected by factors such as track gradient, train speed, acceleration and braking characteristics, loading conditions, station spacing, and efficiency of traction equipment.

 SECTION B

(Attempt any three – answers provided for all)

2(a) Types of heating and induction heating

Electrical heating methods include resistance heating, arc heating, induction heating, and dielectric heating. In induction heating, alternating current in a coil produces a magnetic field that induces eddy currents in the workpiece. These currents generate heat due to the resistance of the material, making induction heating suitable for rapid and localized heating.

2(b) Butt welding

Butt welding is a resistance welding process in which two metal pieces are joined end to end. The surfaces are pressed together, and electric current is passed through the joint, producing heat due to contact resistance. When sufficient temperature is reached, pressure is applied to form a strong weld.

2(c) Utilization factor, maintenance factor, and depreciation factor

The utilization factor represents the ratio of actual luminous flux reaching the working plane to the total flux emitted by the lamps. The maintenance factor accounts for reduction in illumination due to dust accumulation and lamp ageing. The depreciation factor considers the gradual decrease in light output over time.

2(d) Linear induction motor for traction

A linear induction motor operates on the same principle as a rotary induction motor but produces linear motion instead of rotational motion. It consists of a primary winding and a secondary conductive reaction plate. These motors provide smooth acceleration and are used in modern traction systems such as metro railways.

2(e) Mechanical and electrical characteristics of traction motors

Traction motors must have high starting torque, good speed control, and the ability to withstand frequent acceleration and braking. Electrically, they should have high overload capacity, good commutation, and efficient performance over a wide speed range.

SECTION C

3(a) Types of electrodes used in arc furnace

Arc furnaces use electrodes made of carbon, graphite, or a combination of both. Graphite electrodes are preferred due to their high conductivity, low thermal expansion, and ability to withstand high temperatures.

3(b) Working of Ajax Wyatt furnace

The Ajax Wyatt furnace is an induction furnace used for melting non-ferrous metals. It consists of a primary coil surrounding a crucible containing the metal. When alternating current flows through the coil, induced currents heat and melt the metal efficiently.

4(a) Laws governing electro-deposition

Electro-deposition is governed by Faraday’s laws of electrolysis. The first law states that the mass deposited is proportional to the quantity of electricity passed, while the second law states that the mass deposited is proportional to the equivalent weight of the substance.

4(b) Applications of electrolysis

Electrolysis is used for metal extraction, electroplating, refining of metals, chemical manufacturing, and surface treatment processes.

5(a) Objectives and principles of street lighting

The main objectives of street lighting are to ensure safety, improve visibility, reduce accidents, and enhance public security. The principles include uniform illumination, glare reduction, proper spacing of lamps, and efficient energy utilization.

5(b) Air-conditioning plant and its components

An air-conditioning plant consists of a compressor, condenser, expansion valve, and evaporator. The compressor circulates refrigerant, the condenser rejects heat, the expansion valve reduces pressure, and the evaporator absorbs heat from the air.

6(a) Choice of supply system for main line traction

The choice of traction supply depends on factors such as traffic density, route length, power requirements, and economics. Indian Railways uses a 25 kV, 50 Hz single-phase AC supply for main line traction due to its efficiency and reduced transmission losses.

6(b) Speed-time curve of a train

The speed-time curve represents the variation of train speed with time during a run. It consists of acceleration, constant speed, coasting, and braking periods. For main line services, longer constant speed periods are observed, while suburban services involve frequent acceleration and braking.

7(a) Comparison of DC series motor and AC series motor

DC series motors provide excellent starting torque and smooth control, making them suitable for traction. AC series motors are robust and suitable for AC supply systems but have more complex control characteristics.

7(b) Diesel engine driven alternator feeding induction motor

In this system, a diesel engine drives an alternator that supplies electrical power to induction motors. This arrangement offers flexibility, better control, and reduced mechanical transmission losses.