Unit 51: Electrical Technology P1 describe the characteristics and principles of operation of a DC electromagnetic generator. A DC electromagnetic generator produces direct power and also, electrical power (just like the AC generator). It is also produced based on same fundamental principle of Faraday’s Law of Electromagnetic induction where whenever a conductor cuts magnetic flux, dynamically induced electromotive force is produced, and this causes current to flow if the conductor/ conductors is closed.
The direction of induced current is given by Fleming’s right hand rule. A DC generator consists of these vital parts: o Yoke- It is the outer frame of a DC and it is made up of cast iron or steel. It carries the magnetic flux produced by field wind. o Poles/pole shoes. These are joined to the yoke with bolts or welding. This carries field winding and pole shoes are fastened to them. These support field coils and spread out flux. o Field winding- These are made out of copper placed on each pole and connected in series. These form alternate North and South poles. Armature core- This is the rotor of the machine which has a cylindrical shape with bits for armature winding. It is made up of thin laminated circular steel disks and some are provided with air ducts for cooling purposes. o Armature winding- This rests in armature bits and the conductors of these are insulated from each other and also from the armature core. This can be done by lap or wave winding. o Commutator and brushes- This collects the current generated in armature conductors. The commutator consists of set of copper segmentsa number of segments = number of armature.
P2 describe the characteristics and principles of operation of an AC electromagnetic generator. An AC generator produces alternating power generated by rotating a coil in the magnetic field or by rotating a magnetic field within a stationary coil. The voltage value that is generated depends on the number of the coil turns, the strength of the field and the speed at which the coil or magnetic field rotator. The AC operates on the same fundamental principles of electromagnetic induction as D. C. generators.
The main parts of the AC generator include the stator and rotator. But, in most Ac generators, field exciters are rotating and the armature coil is stationary because at high voltages, it easier to insulate stationary armature winding, which may be as high as 30 kV or more. o Stator- This acts as the armature or the field winding of a particular device. It remains stationary at all times while the rotor spins. For an AC generator it is not meant to serve path for magnetic flux whereas, the DC generator does. Instead, the stator is used for holding the armature winding.
The core of the stator is made up of lamination of steel alloys or sometimes of magnetic iron, so it can minimize the eddy current losses. o Rotator- There is two types of these salient and the cylindrical type. Salient pole type rotors are used in low and medium speed alternators only whereas the cylindrical type rotors are used in high speed alternators. P3 describe the operation and an application of a solar power source. Solar power source is used in remote location or where electricity is generally very poor (e. g. India, Africa etc. ).
The procedure and operation of this source basically consists of solar panels driving in the current from the sunlight from the sun and then it sends it to an inverter which inverts the DC voltage to AC. Solar power source is reliable as it produces no air pollution or greenhouse gases however it can be expensive but beneficial for the long-term. Some applications of solar power sources include the following: o Power plants- These are non-renewable and used to boil water and form stream so the turbines can rotate and water to produce electricity.
Homes- There is solar heaters to supply hot water in homes through photovoltaic cell in the roof. o Commercial use- There are glass PV modules or other solar panel installed in the roof that supply electricity to different parts of the building reliably. o Ventilation system- These are useful for running bath fans, floor fans and ceiling fans in buildings and also helps controls the moisture and smell inside a building. P4 describe the characteristic features of two different types of electro-chemical cells or batteries.
Electrochemical cell is when a redox reaction is used to convert chemical energy into electrical energy with the help of oxidation and reduction reaction. These consist of two vessels (connected by a salt bridge/porous pot), two electrodes (made of different materials) and the electrode on which oxidation takes place is called the anode (or – ve pole) and the electrode on which reduction takes place is called the cathode (or + ve pole), two electrolytic solutions and a salt bridge. The electrolytes are taken in by half – cells.
There are many different types of electro-chemical cells and batteries but here is an explanation on two of them: 0 Chemical cells without transference- In this type of chemical cells, the liquid junction potential is avoided and the transference number is not taken into notice. In these cells, one electrode is reversible to cations while the other is reversible to the anions. o Chemical cells with transference- In this type of chemical cells, the liquid junction potential or diffusion potential is developed across the boundary between the two solutions.
This develops due to the difference in motilities of +ve (positive) ions of the electrolytes. P5 describe the properties and a typical application of a solid and a liquid or gas electrical conductor. Solid as an electrical conductor including metal is a very good conductor. However, electric charges do not move as easily in water and have an even more difficult time with gases. In each element, there are cases where some solids are poor conductors, and some gases can become excellent conductors.
Solids and electricity is a perfect pair for conductivity. The electrical properties of copper, steel, and other metals provide the maximum chances because of how tightly close the atoms are. When electrons can pass easily between atoms, this promotes electrical conductivity. Solids like silver, copper and aluminium are popular with electrical work because very little energy is lost when the electricity travels through these metals and this is an advantage of having solid as electrical conductors.
The electrical properties of liquids depend on the material. Salt water, for example, has properties that have excellent conductivity of electricity because the ion in salt gives a free flow of electricity. Even though electricity can pass through normal water, drinking and distilled waters are known as insulators because of the poor flow of electricity. The electrical properties of gases fluctuate and known to be generally the worst electrical conductor.
In a normal state, gases like oxygen, carbon dioxide and nitrogen are such poor onductors of electricity they are actually not even conductors. If these gases are exposed to different elements, however, properties quickly change. For example, when barometric pressure drops, like with an electrical storm, gases become a better conductor of electricity. The pressure creates a denser atmosphere and makes way for electricity, and moves more freely. P6 describe the properties and a typical application of a solid and a liquid or gas electrical insulator.
Some examples of common solid electrical insulators are: o Clay (ceramic)(porcelain) – Used for high voltage and RF insulators. o Plastics – PVC, Cresyl Pthalate, DEHP and other plastics replaced rubber as an insulator for wires and other parts. PVC and nylon are common for wiring. o Glass (silica, soda ash and limestone) – Used for low voltage apparatus. O Paper/Cardboard – paper and cardboard are used as insulators in certain situations as these materials are cheap and can work in situations without high heat or high voltages.
Gas electrical insulator: Normally when you separate two high voltage conductors an arc forms in between in open air. In the utility industry they use special non-conductive gas in a compact encapsulated metal container to stop arcs from forming. Gas-insulated switchgear is designed to disconnect very high voltages safely. Liquid electrical insulator: Insulating Oil (Transformer Oil) – This petroleum product is used as an electrical insulator and also as a thermal conductor. It conducts heat away from hot transformer coils.
Some capacitors also use insulator oil. P7 describe the properties and an application of two different magnetic materials commonly used in electrical and electronic engineering. Ferrite- These types of magnets magnets are made of a sintered composite of powdered iron oxide and barium/ strontium carbonate ceramic. Because of the cheap prices of the materials and manufacturing methods, inexpensive magnets like this (or non- magnetized ferromagnetic cores, for use in electronic component such as radio antennas, for example) of ifferent shapes can be easily mass produced.
These magnets are non-corroding, but can be brittle and must be treated like other ceramics for it to be efficient. o Iron- Transformers are used widely in consumer electronic products to step down the supply voltage to a level where it will be suitable for the low voltage circuits they contain. In these kinds of applications the transformer could also act as a key safety component that electrically isolates the end user from direct contact with the extremely dangerous supply voltage.
