Transformer is a gadget that moves electrical energy from one circuit to another, usually with a modification in voltage. Transformers work only with a varying electric current, such as alternating existing (Air Conditioner). Transformers are necessary in the distribution of electrical power. They raise the voltage of the electrical energy produced at a power plant to the high levels needed to transfer the electricity efficiently. Other transformers reduce the voltage at the locations where the electrical energy is used. Many household gadgets contain transformers to raise or lower house-current voltage as needed.
Television and stereo devices, for instance, need high voltages; doorbells and thermostats, low voltages. A simple transformer consists essentially of 2 coils of insulated wire. In many transformers, the wires are wound around an iron-containing structure called the core. One coil, called the primary, is linked to a source of alternating existing that produces a constantly varying electromagnetic field around the coil. The differing magnetic field, in turn, produces a rotating current in the other coil.
This coil, called the secondary, is linked to a separate electrical circuit.
The ratio of the variety of turns in the main coil to the variety of turns in the secondary coil– the turns ratio– identifies the ratio of the voltages in the 2 coils. For instance, if there is one turn in the primary and 10 turns in the secondary coil, the voltage in the secondary coil will be 10 times that in the main. Such a transformer is called a step-up transformer. If there are 10 turns in the primary coil and one turn in the secondary the voltage in the secondary will be one-tenth that in the primary.
This sort of transformer is called a step-down transformer. The ratio of the electric current strength, or amperage, in the two coils remains in inverse proportion to the ratio of the voltages; thus the electrical power (voltage increased by amperage) is the exact same in both coils. The impedance (resistance to the flow of a rotating present) of the primary coil depends on the impedance of the secondary circuit and the turn’s ratio.
With the proper turn’s ratio, the transformer can, in effect, match the impedances of the two circuits. Matched impedances are important in stereo systems and other electronic systems because they permit the maximum amount of electric power to be delivered from one component to another. In an autotransformer, there is only one coil and both circuits are connected to it. They are connected at different points, so that one circuit contains a larger portion of the coil (that is, has more turns) than the other. The name itself offers a simple definition. Electrical transformers are used to transform electrical energy. How electrical transformers do so is by altering voltage, generally from high to low. Voltage is simply the measurement of electrons, how many or how strong, in the flow. Electricity can then be transported more easily and efficiently over long distances. While power line electrical transformers are commonly recognized, there are other various types and sizes as well.
They range from huge, multi-ton units like those at power plants, to intermediate, such as the type used on electric poles, and others can be quite small. Those used in equipment or appliances in your home or place of business are smaller electrical transformers and there are also tiny ones used in items like microphones and other electronics. Probably the most common and perhaps the most necessary use of various electrical transformers is the transportation of electricity from power plants to homes and businesses.
Because power often has to travel long distances, it is transformed first into a more manageable state. It is then transformed again and again, or “stepped down,” repeatedly as it gets closer to its destination. When the power leaves the plant, it is usually of high voltage. When it reaches the substation the voltage is lowered. When it reaches a smaller transformer, the type found on top of electric poles, it is stepped down again. It is a continuous process, which repeats until the power is at a usable level.
BACKGROUND OF STUDY:
Electrical transformers are used to “transform” voltage from one level to another, usually from a higher voltage to a lower voltage. They do this by applying the principle of magnetic induction between coils to convert voltage and/or current levels. In this way, electrical transformers are a passive device which transforms alternating current (otherwise known as “AC”) electric energy from one circuit into another through electromagnetic induction. An electrical transformer normally consists of a ferromagnetic core and two or more coils called “windings”. A changing current in the primary winding creates an alternating magnetic field in the core.
In turn, the core multiplies this field and couples the most of the flux through the secondary transformer windings. This in turn induces alternating voltage (or emf) in each of the secondary coils. A transformer is an electrical device that takes electricity of one voltage and changes it into another voltage. You’ll see transformers at the top of utility poles and even changing the voltage in a toy train set. Basically, a transformer changes electricity from high to low voltage using two properties of electricity. In an electric circuit, there is magnetism around it. Second, whenever a magnetic field changes (by moving or by changing strength) a voltage is made. Voltage is the measure of the electric force or “pressure” that “pushes” electrons around a circuit.
Electrical transformers can be configured as either a single-phase or a three-phase configuration. There are several important specifications to specify when searching for electrical transformers. These include: maximum secondary voltage rating, maximum secondary current rating, maximum power rating, and output type. An electrical transformer may provide more than one secondary voltage value. The Rated Power is the sum of the VA (Volts x Amps) for all of the secondary windings. Output choices include AC or DC. For Alternating Current waveform output, voltage the values are typically given in RMS values. Consult manufacturer for waveform options.
For direct current secondary voltage output, consult manufacturer for type of rectification. Cores can be constructed as either a toroidal or laminated. Toroidal units typically have copper wire wrapped around a cylindrical core so the magnetic flux, which occurs within the coil, doesn’t leak out, the coil efficiency is good, and the magnetic flux has little influence on other components. Laminated refers to the laminated-steel cores. These steel laminations are insulated with a nonconducting material, such as varnish, and then formed into a core that reduces electrical losses. There are many types. These include autotransformer, control, current, distribution, general-purpose, instrument, isolation, potential (voltage), power, step-up, and step-down. Mountings include chassis mount, dish or disk mount, enclosure or free standing, h frame, and PCB mount.
STATEMENT OF THE PROBLEM:
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SIGNIFICANCE OF THE STUDY:
Electrical Engineering Students- They will find the material needed to advance toward the level of professional. They can use to obtain a deeper understanding of many topics.
Electrical Engineers- They deeply involve with the overall subject matter of this book may smugly grin with the self-satisfying attitude of “I know all that!” This person must recognize that there are many transformer topics. There is always room to learn.
Electronics Engineering Students- To train specialists for transformers needed in industry, higher education and research.
Electronics Engineers- To immerse in one or a few very narrow specialties within the field that they also may benefit greatly from their knowledge imparted in the peripheral specialties.
Electrical Options- Ensure a high level of applied, scientific and instructional work in the transformer field through collaboration.
Researchers- They will become knowledgeable about the selected topic.
SCOPE AND LIMITATION:
This standard establishes firm capacity and short term overload ratings for distribution substation electric transformers based on temperature rise of the transformer core and coils. The electric transformers consider a limitation imposes by substation ancillary equipment such as tap changers, bushings, breakers, etc. Power capacity has already been observed, transformers must be well designed in order to achieve acceptable power coupling, tight voltage regulation, and low exciting current distortion. Also, transformers must be designed to carry the expected values of primary and secondary winding current without any trouble. This means the winding conductors must be made of the proper gauge wire to avoid any heating problems. An ideal transformer would have perfect coupling (no leakage inductance), perfect voltage regulation, perfectly sinusoidal exciting current, no hysteresis or eddy current losses, and wire thick enough to handle any amount of current.
Unfortunately, the ideal transformer would have to be infinitely large and heavy to meet these design goals. Thus, in the business of practical transformer design, compromises must be made. Additionally, winding conductor insulation is a concern where high voltages are encountered, as they often are in step-up and step-down power distribution transformers. Not only do the windings have to be well insulated from the iron core, but each winding has to be sufficiently insulated from the other in order to maintain electrical isolation between windings. Respecting these limitations, transformers are rated for certain levels of primary and secondary winding voltage and current, though the current rating is usually derived from a volt-amp (VA) rating assigned to the transformer. For example, take a step-down transformer with a primary voltage rating of 120 volts, a secondary voltage rating of 48 volts, and a VA rating of 1 kVA (1000 VA). The maximum winding currents can be determined as such:
Sometimes windings will bear current ratings in amps, but this is typically seen on small transformers. Large transformers are almost always rated in terms of winding voltage and VA or kVA. When energy losses transformers transfer power, they do so with a minimum of loss. As it was stated earlier, modern power transformer designs typically exceed 95% efficiency. It is good to know where some of this lost power goes, however, and what causes it to be lost. There is, of course, power lost due to resistance of the wire windings. Unless superconducting wires are used, there will always be power dissipated in the form of heat through the resistance of current-carrying conductors. Because transformers require such long lengths of wire, this loss can be a significant factor. Increasing the gauge of the winding wire is one way to minimize this loss, but only with substantial increases in cost, size, and weight. Resistive losses aside, the bulk of transformer power loss is due to magnetic effects in the core.
Perhaps the most significant of these “core losses” is eddy-current loss, which is resistive power dissipation due to the passage of induced currents through the iron of the core. Because iron is a conductor of electricity as well as being an excellent “conductor” of magnetic flux, there will be currents induced in the iron just as there are currents induced in the secondary windings from the alternating magnetic field. These induced currents — as described by the perpendicularity clause of Faraday’s Law — tend to circulate through the cross-section of the core perpendicularly to the primary winding turns. Their circular motion gives them their unusual name: like eddies in a stream of water that circulates rather than move in straight lines. Iron is a fair conductor of electricity, but not as good as the copper or aluminum from which wire windings are typically made. Consequently, these “eddy currents” must overcome significant electrical resistance as they circulate through the core.
In overcoming the resistance offered by the iron, they dissipate power in the form of heat. Hence, we have a source of inefficiency in the transformer that is difficult to eliminate. This phenomenon is so pronounced that it is often exploited as a means of heating ferrous (iron-containing) materials. The photograph of shows an “induction heating” unit raising the temperature of a large pipe section. Loops of wire covered by high-temperature insulation encircle the pipe’s circumference, inducing eddy currents within the pipe wall by electromagnetic induction. In order to maximize the eddy current effect, high-frequency alternating current is used rather than power line frequency (60 Hz). The box units at the right of the picture produce the high-frequency AC and control the amount of current in the wires to stabilize the pipe temperature at a pre-determined “set-point.”
Induction heating: Primary insulated winding induces current into loss iron pipe (secondary). DEFINITION OF TERMS: Alternating Current (AC)- An electrical current flow of continuously changing polarity, which rises to a maximum voltage in one direction, decreases to zero and then sinks to the maximum voltage in the other direction before changing polarity once again. This pattern is referred to as a sinusoidal wave and the number of cycles per second is equal to the frequency, which is measured in “Hertz”. Ambient Temperature- The normal surrounding temperature of the environment in which a transformer will operate. Auto Transformer- A transformer used to step voltage up or down. The primary and secondary windings share common turns and thus provide no electrical isolation.
Air cooled Transformer- A transformer which uses air as the cooling medium. This may be a forced air with the use of fans. Arc voltage- The amount of voltage present between electrodes of different potential or between an electrode and ground. The magnitude is determined by the distance between electrodes and the dielectric constant of the medium surrounding them. Breakdown voltage- The voltage at which an electrical breakdown occurs. It is also known as breakdown potential, sparking potential or sparking voltage. Core- The ferrous center part of a transformer or inductor used to increase the strength of the magnetic field. It carries the flux and forms the magnetic coupling between primary and secondary Core Saturation- Condition that occurs when an inductor or transformer core has reached maximum magnetic strength. Current Transformer (CT)- A transformer used in instrumentation to assist in measuring current.
It utilizes the strength of the magnetic field around the conductor to form an induced current that can then be applied across a resistance to form a proportional voltage. Compensated Transformer- A transformer with a turn’s ratio which provides a higher than rated voltage at no load, and, rated voltage at rated load. Core Loss- Core loss is also known as iron loss. Core loss is a form of energy loss that occurs in electrical transformers and other inductors. Core losses do not include the losses due to resistance in the conductors of the windings, which is often termed copper loss. It does not vary with load and hence also called constant losses. It mainly consists of eddy current and hysteresis losses. Double conversion- A UPS design in which the primary power path consists of a rectifier and inverter. Dropout voltage- The voltage at which a device fails to operate properly or safely. Computer systems will reboot, reset, or lose data. Delta- Delta is a three phase connection where the ends of each phase winding connection in series to form a closed loop with each phase 120 electrical degrees from the other. Delta-Delta- The connection between a delta source and a delta load. Delta-Wye- The connection between a delta source and a wye load.
Duty Cycle- The percentage of time a transformer will be supplying the Full Rated Power to the load. Percentage of time a unit is expected to perform at Full Rated power versus time spent in idle can significantly affect the physical size of a transformer. Electrostatic Shield- A grounded conductor sheet which provides a ground shield between primary and secondary windings to decrease or eliminate line to line or line to ground noise. It is also known as Faraday Shield. Effective Voltage or current- The amount of power being delivered to a DC circuit load can be calculated easily by dividing the load resistance into the applied DC voltage squared. Eddy Currents- It is induced into a metal when magnetic lines of force move across it. Efficiency- Ratio of its power output to its total power input. Excitation Current- Current required magnetizing a core..
Electrostatic Shielding- Placed between windings (usually the primary and secondary) to provide maximum isolation. Additional Electrostatic Shields can be placed between secondary windings as required. Shielding is normally connected to the transformer’s ground. Exciting Current- The current drawn by a transformer at nominal input voltage in its unloaded (open-circuit) condition. Frequency- It means the number of times an AC voltage will change from positive to negative and vice versa within a precise time, usually expressed in cycles per second and identified as Hz as in 60 Hz. Ferroresonant Transformer- A voltage-regulating transformer that depends on core saturation and output capacitance. Filter- A selective network of resistor, inductors, or capacitors which offers comparatively little opposition to certain frequencies or direct current, while blocking or attenuating other frequencies. Flux- The lines of forces of a magnetic field.
Forced Air- A method of temperature regulation that involves air from an external environment being forcibly exchanged with a transformer’s enclosed environment. Generator- A device that converts mechanical energy into electrical energy by magnetic induction. Ground- A conducting path, whether intended or unintended, between an electric circuit or equipment and the earth or some other conductor. Grounded- Connected to the earth or some other conductor.
Ground Fault- Any undesirable current flow from a current carrying conductor to ground. Hydroelectric- Electricity produced by turbines that are turned by water flow. Hertz (Hz)- Cycles per second.
Isolating Transformer- Transformer in which input windings are connected to the line and are completely isolated from those connected to the load. Insulation- Material with high electrical resistance.
Insulator- Device used for supporting or separating electrical conductors. Instrument Transformer- A transformer designed to transform the conditions of current or voltage and phase position in the primary with a specified accuracy of the secondary circuit. Inductor- A coiled conductor that opposes change in current. Inverter- A device used to change DC into AC power.
Network Transformer- Transformer which is electrically and mechanically connected to and coordinated in design with switch-gear or motor control assemblies for use on a utility network power system. Power Factor- Watts divided by volt amps, kW divided by kVA. Power factor: leading and lagging of voltage versus current caused by inductive or capacitive loads, and harmonic power factor: from nonlinear current. Primary winding- The coil winding that is directly connected to the input supply. Peak voltage- Highest voltage measured during an event. Or the maximum voltage obtained from an oscillating voltage wave. With an AC source, this occurs twice and lasts for only a fraction of the cycle. Direct current voltage is considered peak voltage at all times. Rated Power- The total output power available from all secondary windings, expressed in Volt-amperes (VA) or Kilovolt amperes (kVA). Reactance- Opposition to changes in flow of alternating current. Capacitive reactance is opposition in change from a capacitor, and inductive reactance is the opposition in change from a coil or other inductor.
Rectifier- An electrical device used to change AC power into DC power. Regulation- The percentage difference between a secondary winding’s output voltage when operating under no-load and open-circuit and full load conditions. Secondary Winding(s)- The coil winding(s) supplying the output voltage to the load(s). Short circuit- A low resistance connection, usually accidental, across part of a circuit, resulting in excessive current flow. Transformer Bank / Bank of transformers- Two or more single-phase transformers connected together to supply a three-phase load. Taps or Voltage Taps- Additional connections to winding allowing different voltages to be obtained from the same winding. Often used on the primary winding to allow the transformer to be used in different countries having different line voltages available. Transformer- An electrical device, which, by electromagnetic induction, regenerates AC power from one circuit into another. Transformers are also used to change voltage from one level to another. This is accomplished by the ratio of turns on the primary to turns on the secondary (turns ratio). |
Working Voltage- The voltage that a winding will operate at, but not necessarily the output voltage of the winding.Wye- A wye connection refers to a three-phase electrical supply where the source transformer has the conductors connected to the terminals in a physical arrangement resembling a Y. Each point of the Y represents the connection of a hot conductor. The angular displacement between each point of the Y is 120 degrees. The center point is the common return point for the neutral conductor.Watt- Unit of electrical power when the current in the circuit is one ampere and the voltage is one volt (for DC) and for AC, even the p.f. should be unity.Weather shield- When added to ventilated enclosures, allow indoor-rated units to be situated outdoors, changing the enclosure rating to NEMA 3R.