Monday, 4 April 2022

Definition Of Energy Transformations - Energy Transfer Law Examples

Definition Of Energy Transformations | The Law of Energy Conversion | Energy Transfer Law Examples 

ENERGY CONVERSION

Electric energy is obtained by conversion from other forms of energy stored in naturally occurring materials or from energy being continuously received from the sun in its primary form or in its secondary manifestations, for example, rain, snow at high altitude, wind, plants etc.

Energy is stored in natural materials (coal, oil and gas and in the atom) in a chemical form. Coal, oil and gas (methane) were formed by natural processes over enormous periods of time aeons ago. These are available near the earth surface or underground, mostly at great depths, particularly, oil and gas. These are limited resources provided by nature and are non replenish able. Their extraction leaves gorges, which, when near earth surface, render vast tracts of land unfit for use. It is not clearly known what effect is caused by voids deep under the earth caused by oil extraction.

Energy from the atom (nuclear energy) can be obtained from certain materials with a high atomic number like uranium/thorium. Their resources are also limited, though they contain a great amount of energy.

Energy directly received from the sun (during day time) can be used directly (to be elaborated later in Next Post), but the surface density of solar energy is quite low and is variable during day, cloudy weather and different seasons. It is the solar energy which is responsible for rain/snow and winds. Rain collected at high altitude has potential energy and winds possess kinetic energy. 

Further, trees, plants and vegetation absorb solar energy, which is stored there in a chemical form. Pull of the moon on earth imparts energy to sea water in the form of tidal waves. High winds cause energy to be imparted to sea waves. Energy in the forms enumerated here are replenish able (renewable) and further they are nonpolluting, when used for conversion to the electric energy form.

The aim of this section is to describe various means and processes for converting the above listed energy forms to electric energy. With certain exceptions this conversion requires the first step of conversion to rotational mechanical energy, which then is used to run a generator for conversion to electric form. As only very small quantities of electric energy can be stored and that also mainly by chemical means, it has to be continuously generated and transported to use points.

A panoramic view of energy conversion to electric form is presented As below:-

Definition Of Energy Transformations | The Law of Energy Conversion | Energy Transfer Law Examples

which at a glance brings into focus all aspects of electric energy including conservation. A similar pictorial view of all the related issues in electric energy-generation, efficiency, nature, environmental impact and use classification is given above will be discussed in fair detail in various sections of this chapter.

Energy is converted to a mechanical rotational form by means of the following turbines.

  • Steam turbine
  • Gas turbine
  • Hydraulic turbine

Steam is raised in a boiler by heat released by combustion of coal/oil or by atomic fission in a suitable vessel called the reactor. 

Combustion and steam raising is combined into a single boiler unit for coal-oil-based operation. However, in fission process, steam is raised by heat exchange processes from the reactor to boiler (or directly in the reactor). 

A gas turbine directly extracts energy from the products of combustion. In a hydraulic turbine, water's potential energy is directly converted to a rotational form.


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Saturday, 12 March 2022

Transformers - Simple Transformer - Core Type Transformer - Shell Type Transformer

What Is Transformers? Definition Of A Simple Transformer | Core Type Transformer | Shell Type Transformer

INTRODUCTION:-

Economical and technologically feasible voltage levels at which large chunks of electric power can be generated are typically 11-37 kV, while the most convenient utilization voltages are 230/400 V for industrial, commercial and domestic purposes.

Large industrial motors may be run at 3.3, 6.6 or 11 kV. It is impossible to transmit directly, even over modest distances, the electric power as it is generated (11-37 kV). Unacceptably large power losses and voltage drops would result. As a rule of thumb economical transmission voltage is 0.625 kV/km line-to-line, e.g. 400 kV for a line of about 640 km. It is therefore essential to step-up voltages at the sending (generating) end and to step-down at the receiving end. Usually more than one step of Step-down may be necessary. Step-up and step-down of voltage levels is accomplished by means of static electromagnetic devices called Transformers.


It was seen in alternating flux is set up in a core by a coil excited with ac voltage, which in turn induces coil emf (The magnitude of flux is determined by the fact that the coil emf must equal the excitation voltage (KVL) ) of excitation frequency proportional to the number of coil turns. 

If another coil is wound on the same core, the mutual flux (alternating) would induce emf in it also of the same frequency and of magnitude proportional to its coil turns. The ratio of the voltage of the two coils can be easily adjusted by means of their turn-ratio. 

Such a device, which indeed is a mutually coupled circuit, is called a Transformer and is exhibited diagrammatically in Fig. Below:-

SIMPLE TRANSFORMER

PIC:- 1 -  "A SIMPLE TRANSFORMER"

The coil excited from the ac source is called the primary and receives electric power from the source. The other coil is called the secondary and the voltage induced in it could be used to feed a load. The subscript '1' will be associated with the primary and '2' with the secondary. Primary and secondary roles in a transformer are easily reversed by the prevailing electrical conditions at the two ports. To avoid confusion in practice the two transformer coils are known as HV (high-voltage) and LV (low-voltage) windings.

Also shown in  are the mutual and leakage flux paths. Since a significant part of the leakage flux paths is through air, leakage fluxes Φ11 and Φ12 are less than the mutual flux Φ .

The dots indicate on the two coils (windings) are the polarity marks. As the mutual flux alternates, these coil ends simultaneously acquire the same polarity. Also current into the dot in one coil and out of the dot in the other coil would tend to produce core flux in the opposite direction.

The transformer shown in PIC:- 1 is an iron-core transformer. Transformers operated at 25-400 Hz are invariably of iron-core construction. 

In special cases (particularly at high frequencies), the core may be made of nonmagnetic material in which case it is called an air-core transformer. Application range for air-core transformers are radio devices and certain types of measuring and testing instruments.

Since the transformer core carries alternating flux, it is made of laminated steel (0.35 mm thickness for 50 Hz transformers). The transformer core is constructed of rectangular sheet steel strips. Two types of core constructions are adapted for single-phase transformers-core and shell type as below:-

(a) Core Type Transformer (b) Shell Type Transformer

              PIC:- 2 - (a) Core Type Transformer  (b) Shell Type Transformer

The core type construction has a longer mean length of flux path and a shorter mean length of coil turn.

Flux linking only one winding of the transformer (leakage flux) is detrimental to transformer performance in terms of voltage drop. To reduce leakage flux half-LV and half-HV are wound on each limb of the core type transformer as shown in PIC:- 2 - (a). 

For economical insulation, the LV coils are placed inside (next to core) and HV coils are placed on the outside. In a shell type transformer reduced leakage flux is achieved by sandwiching HV and LV coil packets.

To prevent ingress of moisture and deterioration of winding insulation, the built-in core and windings are placed in a steel tank filled with transformer oil. 

Oval or circular tubes are provided on the outside surfaces of the transformer tank, aiding in natural circulations of oil, which removes the heat of core and winding (I2R) losses and transports it to the tank surfaces for cooling purpose. Oil circulations also removes the heat generated by iron losses in the core. 

To prevent the coil from absorbing moisture from air and from being oxidized, the tank must be sealed and connected to the atmosphere through a narrow passage for breathing purposes. Inside this passage is placed silica gel for drying the air that the transformer breathes in.

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