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Thursday, 6 February 2014

EVERYTHING YOU NEED TO KNOW ABOUT ELECTRICAL TRANSFORMER, TYPES AND HOW IT WORKS


Article is written by Mbata Blessing J 

TRANSFORMER
A transformer is an apparatus for reducing or increasing the voltage of an alternating current. A transformer is an electrical device which, by the principles of electromagnetic induction, transfer electrical energy from one electric circuit to another, without changing the frequency. The energy transfer usually takes place with a change of voltage and current. Transformers either increases or decrease AC voltage. Transformers are used to meet a wide variety of needs. some transformers can be several stories high, like the type found at a generating station or small enough to hold in your hand, which might be used with the charging cradle for a video camera. No matter what the shape or size, a transformers purpose remains the same; transforming electrical power from one type to another. There are many different types of transformers in use today.
TYPES OF TRANSFORMERS
1.     Auto Transformers
2.     Distribution Transformers
3.   
  Instrument Transformers
4.     Isolation Transformers
5.     Potential Transformers
6.     Current Transformers
1.         AUTO TRANSFORMERS:  The autotransformer is a special type of power transformer. It consists of a single, continuous winding that is taped on one side to provide either a step-up or a step-down functions. This is different from a conventional two-winding transformer, which has the primary and secondary completely isolated from each other, but magnetically linked by a common core.
The auto transformers winding are both electrically and magnetically interconnected. An auto transformer is initially cheaper than a similarly rated two winding transformer. It also has better regulation (smaller voltage drops), and greater efficiency. Furthermore, it can be used to obtain the neutral wire of a three-wire 240/120V services, just like the secondary of a two 0 winding transformer. The auto transformer is considered unsafe for use on ordinary distribution circuits. This is because the high voltage primary circuits are connected directly to the low-voltage secondary circuit.
2.         DISTRIBUTION TRANSFORMERS:     A pole-type distribution transformers is used to supply relatively small amounts of power to residences. It is used at the end of the electrical utility’s delivery system.

3.         ISOLATION TRANSFORMERS:   An isolation transformer is a very unique transformer. It has a 1:1 turns’ ratio. Therefore, it does not step voltage up or down. Instead, it serves as a safely device. It is used to isolate the grounded conductor of a power line from the chassis or any portion of a circuit load. Using an isolation transformer does not reduce the charger or shock if contacts it made across the transformers’ secondary winding. Technically, any true transformer, whether used to transfer signals or power, it isolating, as the primary and secondary are not connected by conductors but only but induction. However, only transformers whose primary purpose is to isolate circuits (opposed to the more common transformer function to voltage conversion), are routinely described as isolation transformers.
4.         INSTRUMENT TRANSFORMERS:     For measuring high values of current or voltage, it is desirable to use standard low-range measuring instruments together with specially-constructed instrument transformers, also called accurate ratio transformers. An accurate ratio transformer does just as the name suggest. It transforms at an accurate ratio to allow an attached instrument to gauge the current or voltage without actually running full power through the instrument. It is required to transform relatively small amounts of power because its’ only load, called a burden, is the delicate moving elements of an ammeter, voltmeter or wattmeter.
There are two types of instrument transformer
1.     Current used with an ammeter to measure current in AC voltages.
2.     Potential used with a voltmeter to measure voltage (potential difference) in AC.
5.         CURRENT TRANSFORMERS:  Current transformer are a type of instrument transformers. They are used for the measurement of electric currents. A current transformer has a primary coil of one or more turns of heavy wire. It is always connected in series in the circuit in which current is to be measured. The secondary coil is made up of many turns of fine wire, which must always be connected across the ammeter terminals. The secondary of a current transformer must never be open-circuited. This is because the primary is not connected to a constant source. There is a wide range of possible primary voltages, because the device can be connected to many types of conductors. The secondary must always be available (closed-circuited) to react with the primary, to prevent the core from becoming completely magnetized. If this happens, the instruments will no longer reach accurately.
A clamp on ammeter works in a similar way. By opening  the clamp and placing it around a current carrying conductor, the conductor itself acts as a single turn primary. The secondary and the ammeter are conveniently mounted in the handle of the device.  the dial allows a number of current ranges to be gauged accurately.

6.         POTENTIAL TRANSFORMERS:    A potential transformer is a carefully designed, extremely accurate step-down transformer. It is normally used with a standard 120V  voltmeter (called the  deflections) by the ratio of transformation, the user can determine the voltage on the high side. Common transformation ratios are 10:1, 20:1, 80:1, 100:1, 120:1 and even higher. In general, a potential transformer is very similar to a standard two winding transformers, except that it handles a very small amount of power. Transformer for this service are always the shell type, because the construction has been proven to provide better accuracy.
HOW TRANSFORMER WORKS
When an electric current passes through a long, hollow coil of wire there will be a strong magnetic field inside the coil and a weaker field outside it. The lines of the magnetic field pattern run through the coil, spread out from the end, and go round the outside and in at the other end. With a hollow coil the lines form complete rings. If there is an iron core in the coil it becomes magnetized, and seems to make the field becomes much stronger while the current is on, the iron core of a transformer is normally a complete ring with two coils wound on it. One is connected to a source of electrical power and is called the primary coil, the other supplies the power to a load and is called the secondary coil.
The magnetization due to the current in the primary coil runs all the way round the ring. The primary and secondary coils can be wound anywhere on the ring, because the iron carries the changes in magnetization from one coil to the other. There is no electrical connection between the two coils. However, they are connected by the magnetic field in the iron core. When there is a steady current in the primary there is no effect in the secondary, but there is an effect in the secondary if the current in the primary is changing. A changing current in the primary induces an e.m.f in the secondary if the secondary is connected to a circuit then there is a current flow. A step-down transformer of 1,200 turns on the primary coil connected to 240V A.C will produce 2V a.c across a 10-turn second. (produced the energy losses are minimal) and so light a 2V lamp. A step-up transformer with 1,000 turns on the primary fed by 200V a.c and a 10,000 turn secondary will give a voltage of 2,000V a.c.
The iron core it itself a crude secondary (like a coil of one turn) and changes of primary current induce little circular voltages in the core. Iron is a conductor and if the iron core were solid, the induced voltages would drive wasteful secondary currents’ in it (called eddy currents’). So the core is made of very thin sheets clamped together, with the face of each sheet coated to make it a poor conductor. The edges of the sheet can be seen by looking at the edges of a transformer core.
A.C GENERATOR
In electricity generation, an electric generator is a device that converts mechanical energy to electrical energy. The reverse conversion.
A.C generators or alternator (as they are usually called), operates on the same fundamental principle of electromagnetic induction as D.C generators. Alternating voltage may be generated by rotating a coil in the magnetic field or by rotating a magnetic field within a stationary coil. The value of the voltage generate depends on the number of turns in the coil. Strength of the field. The speed at which the coil or magnetic field rotates.
An electric generator is a device that produces an electromotive force (emf) by changing the number of magnetic flux lines (lines of force)      , passing through a wire coil. When the coil is rotated between the poles of the magnet by cranking the handle, , an A.C voltage waveform is produced. Operation principle of a generator is based on electromagnetic induction, which is defined by faraday’s law which states;
Eemf  = -N d     
                 dt
The electromotive force, Eemf  induced in a coil is proportional to the number of turns, N, in the coil and the rate of change, d      /dt, of the number of magnetic flu lines,      , passing through the surface (A) enclosed by the coil. An induced effect is always such as to oppose the cause that produced it.
In the generator, the coil is under a stationary magnetic field. The magnetic flux density. (B) is constant and       = B x Aeff, so       is proportional to the effective areas, Aeff, of the loop. As the loop rotates at different angles. The rate of change of        , d      /dt, is the largest at the zero points of the waveform and is the smallest at the peaks of the waveform, therefore the induced, Eemf  is maximum at the zero points and minimum at the peaks. The induced , Eemf  output by the generators  is an AC voltage and its waveform.

HOW DOES AN AC GENERATOR GENERATES ELECTRICITY
It all has to do with magnetism. A coil of wire passed through a magnetic field will induce on electric currents in the wire. So, the generator has two basic parts. A magnet for the magnetic field and a coil wound around a shaft. The shaft with the coil wound around it is within the magnetic field. No electricity is produced until the shaft with the coil starts to turn. The turning of the shaft moves the coil through the magnetic field and produced electricity; it is also possible to have the coil be stationary (fixed in place) and to turn the magnet instead. This will still have the effect of moving a magnetic field over a coil of wire and produce electricity.
In  a generator, the electrical power that is produced constantly changes. At first, the generated electric current moves in one direction (as from left to right). Then, when the coil reaches a position where it is parallel to the magnetic lines of forces, no current at all is produced. As the coil continues to rotate, it cuts through magnetic lines of force in the opposite direction, and the electrical current generated travels in the opposite direction (as from right to left). The ends of the coil are attached to the metal slip rings that collect the electrical current. Each slip ring, in turn, is attached to a metal brush, which transfers the currents to an external circuit.
Thus, a spinning coil in a fixed magnetic field will produced an alternating current, one that travels first in one, direction and then in the opposite. The rate at which the current switches back and front is known as its frequency. Ordinary household current alternates at a frequency of 60 times per sec. (or 60 hertz). The efficiency of an AC generator can be increased by substituting an armature for the wire coil. An armature consists of a cylinder-shaped iron core with a long piece of wire wrapped around it. The longer the piece of wire, the greater the electrical current that can be generated by the armature.
Commercial generators: One of the most important uses of generator is the production of large amount of electrical energy for use in industry and homes. The two most common energy source used in operating ac generator are water and steam. Both of this energy sources have the ability to drive generator at which they operate most efficiently, usually not less than 1,500 revolution per minute.
In order to generate hydroelectric (water)powers a turbine is needed. A turbine consists of a large central shaft on which amounted a series of fan like vanes. As moving water strike, the vanes, it causes the central shaft to rotate. If the central shaft is then attached to a very large magnet, it causes the magnet to rotate around a central armature, generating electricity.
Steam power is commonly used to run electrical generating plants, coal, oil, or natural gas is burned-or the energy from a nuclear reactor is harnessed to boil water to create steam. The steam is then used to drive a turbine which in turn, spins a generator.



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