NV7004 New

Single Phase Transformer Lab NV7004

Operating Manual Ver 1.1

141-B, Electronic Complex, Pardeshipura, Indore- 452 010 India Tel.: 91-731- 4211500 email: [email protected] Toll free : 1800-103-5050

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Table of Contents 1. Introduction 3

2. Features 4 3. Technical Specifications 5

4. Theory 6 5. Details of components used in Single Phase Transformer lab 21

6. Experiments

• Experiment 1 23 Study of the Polarity Test.

• Experiment 2 27 Study of the Transformation ratio and Turns Ratio

• Experiment 3 33 Study of the Open Circuit Test

• Experiment 4 36 Study of the Short Circuit Test

• Experiment 5 40 Measurement of Efficiency and Regulation by Load Test

7. Warranty 43

8. List of Accessories 43



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Introduction NV7004 Single Phase Transformer Lab is a very exclusive and important product designed for students to explain very first block of any electrical supply-“The Single Phase Transformer”. With this trainer, students can study the various tests performed on a single-phase transformer. They can learn about how losses, equivalent parameters, efficiency and regulation can be calculated with simplicity by this trainer.

This lab is an elite training system for the Electrical laboratories. The product helps you to get fully acquainted with the basic concepts and functioning of a Single Phase Transformer. The product is represented in such an easy way so that each test can be studied differently in proper sequence. The Lab practically expertises you in exercises like Polarity, Turns Ratio, Transformation Ratio, Iron Loss, Copper Loss, Efficiency etc. The varied scope of learning makes the subject understanding complete.

Single Phase Transformer Lab



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Features • R-Core Transformer

• Exclusive and rugged designed panel

• Stand alone operation

• Terminals are provided in different sections

• Designed by considering all the safety precautions

• High quality meters

• Diagrammatic representation for the ease of connections

• Provided with an extensive e-manual

• Two years warranty



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Technical Specifications Mains Supply : 230V, ± 10%, 50Hz Transformer Rating : 1KVA Primary voltage : 0-125V, 0-125V Secondary voltage : 0-125V, 0-125V Meters used Voltmeter (MI) : 3 Nos. Ammeter (MI) : 3 Nos. Wattmeter (MI) : 2 Nos. Auto Transformer : 270V, 5A MCB : 5A Dimensions (in mm) : 450W × 600D × 600H



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Theory Transformer : Michael Faraday built the first (Single phase pole mounted step down) transformer in 1831, although he used it only to demonstrate the principle of electromagnetic induction and did not foresee its practical uses.

Figure 1 Figure 2

• Single phase transformer :

Step down transformer Output: Low voltage AC

Figure 3 Transformer is a static device, which is used to convert AC electricity from one voltage to another without any change in frequency. Transformer works only with AC and this is one of the reasons why mains electricity is AC. Step-up transformers increase voltage, Step-down transformers decrease voltage. Most power supplies use a step-down transformer to reduce the dangerously high mains voltage (230V) to a safer low voltage.

The input coil is called as primary and the output coil is called as Secondary. There is no electrical connection between the two coils; instead they are linked by an alternating magnetic field created in the soft-iron core of the transformer. The two lines in the middle of the circuit symbol represent the core. Transformers waste very little power so the power output is (almost) equal to the power input.



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Number of turns on each coil, called as Turn’s Ratio, determines the ratio of the voltages. A step-down transformer has a large number of turns on its primary (input) coil, which is connected to the high voltage mains supply, and a small number of turns on its secondary (output) coil to give a low output voltage. For step up transformer it is vise versa.

• Why we use transformers? In our country, usually electrical power is generated at 11KV. For economic reasons, AC power is transmitted at a very high voltage 200KV/ 400KV over long distance, there a step up transformer is applied at the generating station then to feed different area voltage is step down to different levels for economic reason by transformer at various substations ultimately for utilization of electrical power the voltage is step down to 400KV/ 220KV/ 132KV/ 66KV/ 33KV/ 11KV and 4KV for safety. Transformer plays an important role in the power system to transmit high levels of power or watts. • An analogy : A transformer can be likened to a mechanical gearbox, which transfers mechanical energy from a high-speed, low torque shaft to a lower-speed, higher-torque shaft, but which is not a source of energy itself. A transformer transfers electrical energy from a high-current, low-voltage circuit to a lower-current, higher-voltage circuit. But power will remain same. • Transformer principle : Coupling by mutual induction The principle of the transformer is illustrated by consideration of a hypothetical ideal transformer. In this case, the core requires negligible Magneto-Motive Force to sustain flux, and all flux linking the primary winding also links the secondary winding. The hypothetical ideal transformer has no resistance in its coils. A simple transformer consists of two electrical conductors called the primary winding and the secondary winding. Energy is coupled between the windings by the time varying magnetic flux that passes through (links) both primary and secondary windings. Whenever the amount of current in a coil changes, a voltage is induced in the neighbouring coil the effect, called mutual inductance, is an example of electromagnetic induction.



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Figure 4

An ideal step-down transformer showing flux in the core if time varying voltage Vp is applied to the primary winding of Np turns, a current will flow in it producing a Magneto-Motive Force (MMF). Just as an electromotive force (emf) drives current around an electric circuit, so mmf tries to drive magnetic flux through a magnetic circuit. The primary MMF produces a varying magnetic flux Φp in the core, and, with an open circuit secondary winding, induces a back electromotive force (emf) in opposition to Vp. In accordance with Faraday's law of induction, the voltage induced across the primary winding is proportional to the rate of change of flux. Vp = Np ( dΦp/ dt ) and Vs = Ns (dΦs/ dt )

Where, Vp and Vs are the voltages across the primary winding and secondary winding,

NP and NS are the numbers of turns in the primary winding and secondary winding, (dΦp/dt ) and (dΦs/dt ) are the derivatives of the flux with respect to time of the primary and secondary windings.

In the hypothetical ideal transformer, the primary and secondary windings are perfectly coupled, or equivalently Φp = Φs. Substituting and solving for the voltages shows that :

Vp/ Vs = Np/ Ns Where,

Vp and Vs are voltages across primary and secondary, Np and Ns are the number of turns in the primary and secondary, respectively.



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Figure 5

Hence in an ideal transformer, the ratio of the primary and secondary voltages is equal to the ratio of the number of turns in their windings, or alternatively, the voltage per turn is the same for both windings. The ratio of the currents in the primary and secondary circuits is inversely proportional to the Turn’s ratio. The EMF in the secondary winding will cause current to flow in a secondary circuit. The MMF produced by current in the secondary winding opposes the MMF of the primary winding and so tends to cancel the flux in the core. Since the reduced flux reduces the emf induced in the primary winding, increased current flows in the primary circuit. The resulting increase in MMF due to the primary current offsets the effect of the opposing secondary MMF. In this way, the electrical energy feed into the primary winding is delivered to the secondary winding. In addition, the flux density will always stay the same as long as the primary voltage is steady. P = EI (Power = electromotive force × current)

Since a direct current by definition does not change, it produces a steady MMF and so steady flux in the core; this quantity does not change and so cannot induce a voltage in the secondary winding. In a practical transformer, direct current applied to the winding will create heat and can damage the insulation of core.

• Transformer Equation : The universal electromotive force (emf) equation : If the flux in the core is sinusoidal, the relationship for either winding between its number of turns, voltage, magnetic flux density and core cross-sectional area is given by the universal emf equation (from Faraday's law)

E = (2 π f N a B)/√2 = 4.44f N a B

Where, E is the sinusoidal rms or root mean square voltage of the winding,

f is the frequency in hertz,



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N is the number of turns of wire on the winding, a is the cross-sectional area of the core in square meter

B is the peak magnetic flux density in Tesla. Other consistent systems of units can be used with the appropriate conversion in the equation.

• Classifications : Transformers are adapted to numerous engineering applications and may be classified in many ways. a) By power level (from fraction of a volt-ampere (VA) to over a thousand MVA).

b) By application (power supply, impedance matching, circuit isolation). c) By frequency range (power, audio, radio frequency (RF)).

d) By voltage class (a few volts to about 750 Kilovolts). e) By cooling type (air cooled, oil immersed, fan cooled, water cooled, etc.).

f) By purpose (distribution, rectifier, arc furnace, amplifier output, etc.).

g) By ratio of the number of turns in the coils.

1. Step-up : The secondary has more turns than the primary.

Figure 6

2. Step-down: The secondary has fewer turns than the primary.

Figure 7

3. Isolating : The secondary has got equal turns than the primary.

Figure 8



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Intended to transform from one voltage to the same voltage. The two coils have approximately equal numbers of turns, although often there is a slight difference in the number of turns, in order to compensate for losses.

• Variable : The primary and secondary have an adjustable number of turns, which can be selected without reconnecting the transformer.

• Circuit symbols : Standard symbols of various transformers given below:

• Transformer Energy losses : An ideal transformer would have no losses, and would therefore be 100% efficient. In practice, real transformers are less than 100% efficient. Energy is dissipated due to both the resistance of the windings known as copper loss or I2 R loss, and due to magnetic effects primarily attributable to the core (known as iron loss). Transformers are in general highly efficient (large power transformers over 50 MVA) may attain efficiency as high as 99.75%. Small transformers, such as a plug-in "power brick" used to power small consumer electronics, may be less than 85% efficient. Various types of transformer energy losses are given below:

1. Winding resistance losses : There are resistive losses in the coils (losing power I2 R). For a given material, the resistance of the coils can be reduced by making their cross section large. The resistivity can also be made low by using high purity copper. Current flowing through the windings causes resistive heating of the conductor (I2 R loss). At higher frequencies, skin effect and proximity effect create additional winding resistance and losses.

Transformer with two windings and iron core.

Transformer with three windings. The dots show the relative winding configuration of the windings.

`

Step-down or Step-up transformer.

The symbol shows which winding have more turns, but does not usually show the exact ratio.

Transformer with three windings. The dots show the relative winding configuration of the windings.



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2. Eddy current losses : Induced eddy current circulate within the core, causing resistive heating. Silicon is added to the steel to help in controlling eddy current. Adding silicon also has the advantage of stopping aging of the electrical steel that was a problem years ago. These can be reduced by laminating the core. Laminations reduce the area of circuits in the core, and so reduce the Faraday’s emf, and so the current flowing in the core, and so the energy thus lost. 3. Hysterisis losses : Each time the magnetic field is reversed, a small amount of energy is lost to hysteresis within the magnetic core. The amount of hysteresis is a function of the particular core material. The Magnetization and Demagnetization curves for magnetic materials are often a little different and this means that the energy required to magnetize the core (while the current is increasing) is not entirely recovered during Demagnetization. The difference in energy is lost as heat in the core. 4. Magnetostriction losses : Magnetic flux in the core causes it to physically expand and contract slightly with the alternating magnetic field (producing a buzzing sound), an effect known as Magnetostriction. This in turn causes losses due to frictional heating in susceptible ferromagnetic cores. In addition to Magnetostriction, the alternating magnetic field causes fluctuating electromagnetic forces between the primary and secondary windings. These incite vibrations within nearby metalwork, creating a familiar humming or buzzing noise, and consuming a small amount of power. 5. Stray losses : Not all the magnetic field produced by the primary is intercepted by the secondary. A portion of the leakage flux may induce eddy currents within nearby conductive objectives, such as the transformer's support structure, and be converted to heat.

• Construction of transformer :

Figure 9



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Core : Laminated core transformer showing edge of laminations at top of unit.

1. Laminated steel cores : Transformers for use at power or audio frequencies typically have cores made of high permeability of silicon steel. Permeability many times that of free space and the core thus serves to greatly reduce the magnetizing current, and confine the flux to a path which closely couples the windings. Early transformer developers soon realized that cores constructed from solid iron resulted in prohibitive eddy-current losses. Later designs constructed the core by stacking layers of thin steel laminations. Each lamination is insulated from its neighbours by a thin non-conducting layer of insulation. The universal transformer equation indicates a minimum cross-sectional area for the core to avoid saturation. The effect of laminations is to confine eddy currents to highly elliptical paths that enclose little flux, and to reduce their magnitude. Thinner laminations reduce losses, but are more laborious and expensive to construct. Thin laminations are generally used on high frequency transformers with some types of very thin steel laminations able to operate up to 10 KHz. Laminating the core greatly reduces eddy-current losses. One common design of laminated core is made from interleaved stacks of E-shaped steel sheets capped with I-shaped pieces, leading to its name of "E-I transformer". Such a design tends to exhibit more losses, but is very economical to manufacture.

2. Solid cores : Powdered iron cores are used in circuits (such as Switch-Mode Power Supplies), that operate above main frequencies and up to a few tens of Kilohertz. These materials combine high magnetic permeability with high bulk electrical resistivity. For frequencies extending beyond the UHF band, cores made from non-conductive magnetic ceramic materials called ferrites are common. Some radio-frequency transformers also have moveable cores (sometimes called 'slugs') which allow adjustment of the coupling coefficient (and bandwidth) of tuned radio-frequency circuits.

3. Toroidal cores :

Small transformer with toroidal core Figure 10



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Toroidal transformers are built around a ring-shaped core which, depends on operating frequency, is made from a long strip of silicon steel or perm alloy wound into a coil, powdered iron, or ferrite. A strip construction ensures that the grain boundaries are optimally aligned, improving the transformer's efficiency by reducing the core's reluctance. The closed ring shape eliminates air gaps inherent in the construction of an E-I core. The cross-section of the ring is usually square or rectangular, but more expensive cores with circular cross-sections are also available. The primary and secondary coils are often wound concentrically to cover the entire surface of the core. This minimizes the length of wire needed, and also provides screening to minimize the core's magnetic field from generating electromagnetic interference. Toroidal transformers are more efficient than the cheaper laminated E-I types for a similar power level. Other advantages compared to E-I types, include smaller size (about half), lower weight (about half), less mechanical hum (making them superior in audio amplifiers), lower exterior magnetic field (about one tenth), low off-load losses (making them more efficient in standby circuits), single-bolt mounting, and greater choice of shapes. The main disadvantages are higher cost and limited rating. Ferrite toroidal cores are used at higher frequencies, typically between a few tens of Kilohertz to a Megahertz,(MHz) to reduce losses, physical size, and weight of switch-mode power supplies. A drawback of toroidal transformer construction is the higher cost of windings.

4. Air core : A physical core is not an absolute requisite and a functioning transformer can be produced simply by placing the windings in close proximity to each other, an arrangement termed an "air-core" transformer. The air which comprises the magnetic circuit is essentially lossless, and so an air-core transformer eliminates loss due to hysteresis in the core material. The leakage inductance is inevitably high, resulting in very poor regulation, and so such designs are unsuitable for use in power distribution. They have however very high bandwidth, and are frequently employed in radio-frequency applications, for which a satisfactory coupling coefficient is maintained by carefully overlapping the primary and secondary windings.

• Winding :

Figure 11

Windings are usually arranged concentrically to minimize flux leakage



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Cut view through transformer windings. White: Insulator. Green spiral: Grain oriented silicon steel. Black: Primary winding made of oxygen-free copper. Red: Secondary winding. Top left: Toroidal transformer. Right: C-core, but E-core would be similar. The black windings are made of film. Top: Equally low capacitance between all ends of both windings. Since most cores are (bad) conductors they also need insulation. Bottom: Lowest capacitance for one end of the secondary winding needed for low-power high-voltage transformers. Bottom left: Reduction of leakage inductance would lead to increase of capacitance. The conducting material used for the windings depends upon the application, but in all cases the individual turns must be electrically insulated from each other to ensure that the current travels throughout every turn. For small power and signal transformers, in which currents are low and the potential difference between adjacent turns is small, the coils are often wound from enameled magnet wire. Larger power transformers operating at high voltages may be wound with copper rectangular strip conductors insulated by oil-impregnated paper and blocks of pressboard. High-frequency transformers operating in the tens to hundreds of Kilohertz often have windings made of braided litz wire to minimize the skin-effect and proximity effect losses. Large power transformers use multiple-stranded conductors as well, since even at low power frequencies non-uniform distribution of current would otherwise exist in high-current windings. Each strand is individually insulated, and the strands are arranged so that at certain points in the winding, or throughout the whole winding, each portion occupies different relative positions in the complete conductor. The transposition equalizes the current flowing in each strand of the conductor, and reduces eddy current losses in the winding itself. The stranded conductor is also more flexible than a solid conductor of similar size, aiding manufacture.

For signal transformers, the windings may be arranged in a way to minimize leakage inductance and stray capacitance to improve high-frequency response. This can be done by splitting up each coil into sections, and those sections placed in layers between the sections of the other winding. This is known as a stacked type or Interleaved Winding. Both the primary and secondary windings on power transformers may have external connections, called taps, to intermediate points on the winding to allow selection of the voltage ratio. The taps may be connected to an automatic on-load tap charger for voltage regulation of distribution circuits. Audio-frequency transformers, used for the distribution of audio to public address loudspeakers, have taps to allow adjustment of impedance to each speaker. A center-tapped transformer is often used in the output stage of an audio power amplifier in a push-pull circuit. Modulation transformers in AM transmitters are very similar. Certain transformers have the windings protected by epoxy resin. By impregnating the transformer with epoxy under a vaccum, one can replace air spaces within the windings with epoxy, thus sealing the windings and helping to prevent the possible formation of corona and absorption of dirt or water. This produces transformers more suited to damp or dirty environments, but at increased manufacturing cost.



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• Coolant :

Figure 12

Three-phase oil-cooled transformer with cover cut away. The oil reservoir is visible at the top to dissipation heat. Extended operation at high temperatures is particularly damaging to transformer insulation.Small signal transformers do not generate significant heat and need little consideration given to their thermal management. Power transformers rated up to a few KVA can be adequately cooled by natural convective air-cooling, sometimes assisted by fans. Specific provision must be made for cooling high-power transformers, the larger physical size requiring careful design to transport heat from the interior. Some power transformers are immersed in specialized transformer oil that acts both as a cooling medium, thereby extending the lifetime of the insulation, and helps to reduce corona discharge. The oil is a highly refined mineral oil that remains stable at high temperatures so that internal arcing will not cause breakdown or fire; transformers to be used indoors must use a non-flammable liquid.

The oil-filled tank often has radiators through which the oil circulates by natural convection; large transformers employ forced circulation of the oil by electric pumps, aided by external fans or water-cooled heat exchanger.Oil-filled transformers undergo prolonged drying processes to ensure that the transformer is completely free of water vapour before the cooling oil is introduced. This helps prevent electrical breakdown under load. Oil-filled transformers may be equipped with Buchholz relays, which detect gas evolved during internal arcing and rapidly de-energize the transformer to avert catastrophic failure.

Polychlorinated biphenyls have properties that once favored their use as a coolant, though concerns over their toxicity and environmental persistence led to a widespread ban on their use. Today, non-toxic, stable silicon-based oils, or fluorinated hydrocarbons may be used where the expense of a fire-resistant liquid offsets additional building cost for a transformer.



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Some "dry" transformers are enclosed in pressurized tanks and cooled by nitrogen or sulphur hexafluoride gas. To ensure that the gas does not leak and its insulating capability deteriorates, the transformer casing is completely sealed. Experimental power transformers in the 2 MVA range have been built with superconducting windings which eliminates the copper losses, but not the core steel loss. These are cooled by liquid nitrogen or helium. • Insulation of windings : The turns of the windings must be insulated from each other to ensure that the current travels through the entire winding. The potential difference between adjacent turns is usually small, so that enamel insulation is usually sufficient for small power transformers. Supplemental sheet or tape insulation is usually employed between winding layers in larger transformers. The transformer may also be immersed in transformer oil that provides further insulation. Although the oil is primarily used to cool the transformer, it also helps to reduce the formation of corona discharge within high voltage transformers. By cooling the windings, the insulation will not break down as easily due to heat. To ensure that the insulating capability of the transformer oil does not deteriorate, the transformer casing is completely sealed against moisture ingress. Thus the oil serves as both a cooling medium to remove heat from the core and coil, and as part of the insulation system. Certain power transformers have the windings protected by epoxy resin. By impregnating the transformer with epoxy under a vacuum, air spaces within the windings are replaced with epoxy, thereby sealing the windings and helping to prevent the possible formation of corona and absorption of dirt or water. This produces transformers suitable for damp or dirty environments, but at increased manufacturing cost. • Shielding : Where transformers are intended for minimum electrostatic coupling between primary and secondary circuits, an electrostatic shield can be placed between windings to reduce the capacitance between primary and secondary windings. The shield may be a single layer of metal foil, insulated where it overlaps to prevent it acting as a shorted turn, or a single layer winding between primary and secondary. The shield is connected to earth ground. Transformers may also be enclosed by magnetic shields, electrostatic shields, or both to prevent outside interference from affecting the operation of the transformer, or to prevent the transformer from affecting the operation of nearby devices that may be sensitive to stray fields such as CRTs.



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• Coolant :

Figure 13

Three phase dry-type transformer with cover removed; rated about 200 KVA, 480 V. Small signal transformers do not generate significant amounts of heat. Power transformers rated up to a few kilowatts rely on natural convective air-cooling. Specific provision must be made for cooling of high-power transformers. Transformers handling higher power, or having a high duty cycle can be fan-cooled. Some dry transformers are enclosed in pressurized tanks and are cooled by nitrogen or sulphur hexafluoride gas. The windings of high-power or high-voltage transformers are immersed in transformer oil a highly refined mineral oil that is stable at high temperatures. Large transformers to be used indoors must use a non-flammable liquid. Formerly, polychlorinated biphenyl (PCB) was used as it was not a fire hazard in indoor power transformers and it is highly stable. Due to the stability and toxic effects of PCB by-products, and its accumulation in the environment, it is no longer permitted in new equipment. Old transformers that still contain PCB should be examined on a weekly basis for leakage. If found to be leaking, it should be changed out, and professionally decontaminated or scrapped in an environmentally safe manner. Today, non-toxic, stable silicon-based oils, or fluorinated hydrocarbons may be used where the expense of a fire-resistant liquid offsets additional building cost for a transformer vault. Other less-flammable fluids such as canola oil may be used but all fire resistant fluids have some drawbacks in performance, cost, or toxicity compared with mineral oil. The oil cools the transformer, and provides part of the electrical insulation between internal live parts. It has to be stable at high temperatures so that a small short or arc will not cause a breakdown or fire. The oil-filled tank may have radiators through which the oil circulates by natural convection. Very large or high-power transformer may have cooling fans, oil pumps and even oil to water heat exchangers. Oil-filled transformers undergo prolonged drying processes, using vapour-phase heat transfer, electrical self-heating, and the application of a vacuum, or combination of these, to ensure that the transformer is completely free of water vapour before the cooling oil is introduced. This helps prevent electrical breakdown under load. Oil-filled power transformers may be equipped with Buchholz relays which are safety devices that sense gas build-up inside the transformer and thus switches off the transformer.



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• Terminals : Very small transformers will have wire leads connected directly to the ends of the coils, and brought out to the base of the unit for circuit connections. Larger transformers may have heavy bolted terminals, bus bars or high-voltage insulated bushings made of polymers or porcelain. A large bushing can be a complex structure since it must provide electrical insulation without letting the transformer leak oil. • Transformer type and uses :

1. Autotransformers An Autotransformer has only a single winding, which is tapped at some point along the winding. AC or pulsed voltage is applied across a portion of the winding, and a higher (or lower) voltage is produced across another portion of the same winding. While theoretically separate parts of the winding can be used for input and output, in practice the higher voltage will be connected to the ends of the winding, and the lower voltage from one end to a tap. For example, a transformer with a tap at the center of the winding can be used with 230 volts across the entire winding, and 115 volts between one end and the tap. It can be connected to a 230 volts supply to drive 115 volts equipment, or reversed to drive 230 volts equipment from 115 volts. As the same winding is used for input and output, the flux in the core is partially cancelled, and a smaller core can be used. For voltage ratios not exceeding about 3-1, an Autotransformer is cheaper, lighter, smaller and more efficient than a true (two-winding) transformer of the same rating.

2. Constant voltage transformer (Ferro-resonance) : By arranging particular magnetic properties of a transformer core, and installing a resonant tank circuit (a capacitor and an additional winding), a transformer can be arranged to automatically keep the secondary winding voltage constant regardless (within some limits) of any variance in the primary supply without additional circuitry or manual adjustment. CVA transformers run hotter than standard power transformers, for the regulating action is dependent on core saturation, which reduces efficiency somewhat.

3. Poly phase transformers : For three-phase power, three separate single-phase transformers can be used, or all three phases can be connected to a single poly phase transformer. The three primary windings are connected together and the three secondary windings are connected together. The most common connections are Y-Δ, Δ-Y, Δ-Δ and Y-Y. A vector group indicates the configuration of the windings and the phase angle difference between them. If a winding is connected to earth (grounded), the earth connection point is usually the center point of a Y winding. If the secondary is a Δ winding, the ground may be connected to a center tap on one winding (high leg delta) or one phase may be grounded (corner grounded delta). A special purpose poly phase transformer is the zig-zag transformer. There are many possible configurations that may involve more or fewer than six windings and various tap connections.



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4. Resonant transformers : A resonant transformer operates at the resonant frequency of one or more of its coils and (usually) an external capacitor. The resonant coil, usually the secondary, acts as an inductor, and are connected in series with a capacitor. When the primary coil is driven by a periodic source of alternating current, such as a square or saw tooth wave at the resonant frequency, each pulse of current helps to build up an oscillation in the secondary coil. Due to resonance, a very high voltage can develop across the secondary, until it is limited by some process such as electrical breakdown. These devices are used to generate high alternating voltages, and the current available can be much larger than that from electrostatic machines such as the Van de Graff generator. • Uses of transformers :

a. For supplying power from an alternating current power grid to equipment which uses a different voltage.

b. For regulating the secondary output of a constant voltage (or Ferro-resonant), in which a combination of core saturation and the resonance of a tank circuit prevents changes in the primary voltage from appearing on the secondary.

c. Electric power transmission over long distances. d. Large, specially constructed power transformers are used for electric arc

furnaces used in steel making. e. Rotating transformers are designed so that one winding turns while the

other remains stationary. A common use was the video head system as used in VHS and beta video tape players. These can pass power or radio signals from a stationary mounting to a rotating mechanism, or radar antenna.

f. Other rotary transformers are precisely constructed in order to measure distances or angles. Usually they have a single primary and two or more secondary and electronic circuits measure the different amplitudes of the currents in the secondary.

g. Sliding transformers can pass power or signals from a stationary mounting to a moving part such as a machine tool head.

h. A transformer-like device is used for position measurement. Example linear variable differential transformer.

i. Some rotary transformers are used to couple signals between two parts which rotate in relation to each other.

j. Small transformers are often used internally to couple different stages of radio receivers and audio amplifiers.

k. Transformers may be used as external accessories for impedance matching; for example to match a microphone to an amplifier.



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• Limitations : Transformers alone cannot do the following :

• Convert DC to AC or vice versa.

• Change the voltage or current of DC.

• Change the AC supply frequency.

Details of measuring instruments used in the product 1. Ammeter : An ammeter is a measuring instrument used to measure the flow of electric current in a circuit. Electric currents are measured in amperes, hence the name it uses magnetic deflection, where current passing through a coil causes the coil to move in a magnetic field. The voltage drop across the coil is kept to a minimum to minimize resistance across the ammeter in any circuit into which it is inserted. Moving iron ammeters use a piece or pieces of iron which move when acted upon by the electromagnetic force of a fixed coil of (usually heavy gauge) wire. This type of meter responds to both direct and alternating currents. To measure larger currents, a resistor called a shunt is placed in parallel with the meter. A large amount of the current flows through shunt.

2. Voltmeter : A voltmeter is an instrument used for measuring the electrical potential difference between two points in an electric circuit. Analog voltmeters move a pointer across a scale in proportion to the voltage of the circuit; digital voltmeters give a numerical display of voltage by use of an analog to digital converter. Voltmeters are made in a wide range of styles. Instruments permanently mounted in a panel are used to monitor generators or other fixed apparatus. Small portable instruments, usually equipped with facilities to also measure current and resistance in the form of a multimeter, are standard test instruments used in electrical and electronics work. Any measurement that can be converted to a voltage can be displayed on a meter that is suitably calibrated; for example, pressure, temperature, flow or level in a chemical process plant.

3. Wattmeter : The wattmeter is an instrument for measuring the electric power or the supply rate of electrical energy (watt) of any given circuit .The traditional analog wattmeter is an electrodynamics instrument. The device consists of a pair of fixed coils, known as current coils, and a movable coil known as the potential coil. The current coils connected in series with the circuit, while the potential coil is connected in parallel. Also on analog wattmeter’s, the potential coil carries a needle that moves over a scale to indicate the measurement. A current flowing through the current coil generates an electromagnetic field around the coil. The strength of this field is proportional to the line current and in phase with it. The potential coil has, as a general rule, a high-value resistor connected in series with it to reduce the current that flows through it. The result of this arrangement is that on a DC circuit, the deflection



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of the needle is proportional to both the current and the voltage, thus conforming to the equation W=VA or P=EI. On an AC circuit the deflection is proportional to the average instantaneous product of voltage and current, thus measuring true power, and possibly (depending on load characteristics) showing a different reading to that obtained by simply multiplying the readings showing on a stand-alone voltmeter and a stand-alone ammeter in the same circuit.

4. Autotransformer : An Autotransformer has only a single winding with two end terminals, plus a third at an intermediate tap point. The primary voltage is applied across two of the terminals, and the secondary voltage taken from one of these and the third terminal. The primary and secondary circuits therefore have a number of windings turns in common. Since the volts-per-turn is the same in both windings, each develops a voltage in proportion to its number of turns. By exposing part of the winding coils and making the secondary connection through a sliding brush, an autotransformer with a near-continuously variable turn’s ratio is obtained, allowing for very fine control of voltage.

• The Importance of Polarity : An understanding of polarity is essential to correctly construct three-phase transformer banks and to properly parallel single or three-phase transformers with existing electrical systems. Polarity test in situations, where the secondary bushing identification is not available or when a transformer has been rewound, it may be necessary to determine the transformer polarity. Knowledge of polarity is also required to connect potential and current transformers to power metering devices and protective relays. The basic theory of additive and subtractive polarity is the underlying principle used in step voltage regulators where the series winding of an autotransformer is connected to either buck or boost the applied line voltage. Transformer polarity refers to the relative direction of the induced voltages between the high voltage terminals and the low voltage terminals, during the AC half-cycle when the applied voltage (or current in the case of a current transformer).

For example, if the transformer is actually rated 480 - 120 volts, the transformer ratio is 4:1 (480 / 120 = 4). Applying a test voltage of 120 volts to the primary will result in a secondary voltage of 30 volts (120 / 4 = 30). If transformer is subtractive polarity, the voltmeter will read 90 volts (120 - 30 = 90). If the transformer is additive polarity the voltmeter reads 150 volts (120 + 30 = 150).

The arrows indicate the relative magnitude and direction of the primary and secondary voltages. Polarity marks to insure correct wiring, control schematics, and three-line power diagrams. The polarity mark is usually shown as a round dot. All instrument transformers are subtractive polarity. While the transformation ratio test is to be done to calculated the voltage in the primary and secondary winding and correspondingly calculated the current and number of turn in both the windings.



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Experiment 1 Objective : Study of Polarity Test on a Single-Phase Transformer Circuit diagrams : 1. Additive Polarity:

Figure 14



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2. Subtractive Polarity:

Figure 15



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Procedure: First of all make sure that the earthing of your laboratory is proper and connected to the terminal provided on back side of the panel. Additive : 1. First make sure that the autotransformer knob is at zero position and Mains

supply is off. 2. First connect terminals 1 to 8 and 2 to 9.

3. Now connect 8 to 10 and 9 to 13. 4. Connect terminals 11 to 12, 15 to 16.

5. Connect 10 to 19, and 14 to 18. 6. Now insert meters in the circuit. For this connect terminals 8 and 9 to Vp1 and

Vp2 respectively. 7. Similarly connect 18 and 19 to Vs3 and Vs4.

8. Switch on A.C supply and adjust the autotransformer to get the desired voltage.

9. Record the voltmeters reading. Note reading of first voltmeter as V1 and that of second as V2

10. Switch off the mains supply.

Subtractive : 1. Remove all connections made before. 2. Make sure that the autotransformer knob is at zero position and Mains supply is

off. 3. First connect terminals 1 to 8 and 2 to 9

4. Now connect 8 to 10 and 9 to 13. 5. Connect terminals 11 to 12, 15 to 16 and 13 to 14.

6. Connect 10 to 18, and 17 to 19. 7. Now insert meters in the circuit. For this connect terminals 8 and 9 to Vp1 and

Vp2 respectively. 8. Similarly connect 18 and 19 to Vs3 and Vs4.

9. Switch on A.C supply and adjust the autotransformer to get the desired voltage. 10. Record the voltmeters reading. Note reading of first voltmeter as V1 and that of

second as V2 11. Switch off the mains supply.



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Observation Table :

Results : You will observe that reading of one of the voltmeter is twice the reading of another voltmeter in case of Additive polarity, and 0 in case of Subtractive polarity.

S. No

V1 V2 Additive/ Subtractive

1

2



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Experiment 2 Objective : Study of Transformation Ratio and Turns Ratio of a Single-Phase Transformer Circuit diagrams :

• Isolation Transformer

Figure 16



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• Step Down Transformer

Figure 17



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• Step Up Transformer

Figure 18

Procedure: First of all make sure that the earthing of your laboratory is proper and connected to the terminal provided on back side of the panel.

• Isolation Transformer 1. First of all make sure that the AC Mains is off and knob of variac is at zero

position. 2. Now connect terminal 1 to 8 and 2 to 9.

3. Connect 8 to 10 and 9 to 13. 4. Short terminals 11 and 12 similarly on secondary side short 15 and 16.

5. Connect terminals 14 to 18 and 17 to 19.



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6. Now insert voltmeters, for this connect terminals 8 and 9 to Vp1 and Vp2. 7. Similarly on the secondary side connect terminals 18 and 19 to Vs3 and

Vs4. 8. Switch on the Mains Supply.

9. Record reading of primary Voltmeter as V1 and that of secondary as V2 into the observation table.


• Step Down Transformer 1. First of all make sure that the AC Mains is off and knob of variac is at zero

position. 2. Now connect terminal 1 to 8 and 2 to 9.

3. Connect 8 to 10 and 9 to 13. 4. Short terminals 11 and 12 of primary.

5. Connect terminals 14 to 18 and 15 to 19. 6. Now insert voltmeters, for this connect terminals 8 and 9 to Vp1 and Vp2.

7. Similarly on the secondary side connect terminals 18 and 19 to Vs3 and Vs4.

8. Switch on the Mains Supply. 9. Record reading of primary Voltmeter as V1 and that of secondary as V2

into the observation table. 10. Switch off the mains supply.

• Step Up Transformer 1. First of all make sure that the AC Mains is off and knob of variac is at 0

position.

2. Now connect terminal 1 to 8 and 2 to 9. 3. Connect 8 to 10 and 9 to 11.

4. Short terminals on secondary side 15 and 16. 5. Connect terminals 14 to 18 and 17 to 19.

6. Now insert voltmeters, for this connect terminals 8 and 9 to Vp1 and Vp2. 7. Similarly on the secondary side connect terminals 18 and 19 to Vs3 and

Vs4. 8. Switch on the Mains Supply.

9. Record reading of primary Voltmeter as V1 and that of secondary as V2 into the observation table.



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Observation Table :

Transformer V1 V2

Isolation

Step Up

Step Down

Calculations : Transformation Ratio of transformer is given by- K= V2/V1

Similarly the turns ratio of transformer is given by- T=V1/V2 or N1/N2

Results : Transformation and Turns Ratios of Isolation Transformer are__________

Transformation and Turns Ratios of Step Up Transformer are___________ Transformation and Turns Ratios of Step Down Transformer are________

• Open circuit and short circuit tests : In a "real- life" transformer, there is always some flux produced by one winding that does not link the other winding. In the equivalent circuit of the transformer, the leakage inductance of each winding accounts for the leakage flux associated with that winding. In addition, each winding has some resistance. Since the magnetic core of the transformer has finite permeability, it has infinite reluctance that accounts for some of the MMF drop in the magnetic circuit. In the equivalent circuit of the transformer, the magnetizing inductance accounts for the MMF drop in the magnetic core of the transformer. Although a transformer is constructed from thin laminations of highly permeable soft magnetic material, there are always magnetic (eddy current and hysteresis) losses in the core of the transformer. Thus, even when the transformer is operating at no load (output current is zero), the input source has to supply power to account for the magnetic (core) loss. The magnetic loss is represented by an equivalent core-loss resistance in the equivalent circuit of the transformer. The purpose of this experiment is to determine the resistance and leakage inductance of each winding, the core-loss resistance and the magnetizing reactance. The following tests are performed to obtain these quantities.



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• Open circuit : A transformer is said to be on no load when secondary of the transformer is open circuited and the secondary current is zero but in actual practice the losses can not be neglected there if transformer is on no load a small current usually 2 to 10% of rated value called exciting current is drawn by the primary this current has to supply an iron losses in the core and a very small amount of copper losses in the primary but comparatively negligible than core or iron losses (hysteresis and eddy current loss). This test is performed by applying the rated voltage at the rated frequency to one winding while the other is left open. Since the open-circuit tester requires the application of the rated voltage, it is usually performed on the low voltage side. With rated voltage applied to the low-voltage side, full rated voltage will appear across the terminals of the high-voltage side. For these reasons, the high-voltage side should be completely isolated to protect the person or persons performing the test. As the secondary is open no load current flow in the circuit which is hardly 2 to 4% of full load current as secondary current is zero secondary copper losses are zero and primary current is very low total copper losses are negligibly small so the total input power is used to supply the iron losses. This power is measured by the wattmeter W0. Hence wattmeter in open circuit test gives the iron losses, which remain constant for all the loads.

• Short circuit : This circuit is performed by applying a rated current at the rated frequency to one winding while a short circuit is placed across the other winding. Since the rated current is smaller on the high-voltage side, the short circuit test is performed on the high-voltage side taking safety into consideration. With the Rated current in the high-voltage winding, the short-circuit current will be the Rated current of the low-voltage winding. This test is performed to measure the effective resistance and leakage reactance of the two windings as referred to the high voltage side. The current flowing through the winding is rated current hence the total copper losses is full load copper loss. Now the applied voltage is very low which is a small fraction of the rated voltage the iron losses are the function of applied voltage so the iron loss is very small. Hence the wattmeter read the power which is equal to the full load copper losses as iron loss are very low.



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Experiment 3 Objective : Study of the Open Circuit Test on Single-Phase Transformer Circuit diagram :

Figure 19



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Procedure : First of all make sure that the earthing of your laboratory is proper and connected to the terminal provided on back side of the panel. • Open circuit test

1. First make sure that the mains supply is off and the knob of variac is at zero position.

2. Connect terminal 1 to 3 and 2 to 6.

3. Now connect 4 to 5 and connect 7 to 8. 4. Connect 6 to 9.

5. Connect 8 to 10 and 9 to 11.(Since Open Circuit Test is performed on low voltage side so low voltage winding is selected).

6. Connect 15 to 16, and secondary terminals 14 and 17 to 25 and 26 respectively(Opened).

7. Now insert meters at their corresponding positions, to insert ammeter connect terminals 3 and 4 to Ap3 and Ap4 (Since No load current is very low hence low range meter is used).

8. Now insert wattmeter, for this connect Wp1, Wp2 and Wp3 to 5, 7 and 6 respectively.

9. Now connect voltmeter by connecting terminals 8 and 9 to terminals Vp1 and Vp2.

10. Now switch on the mains supply.

11. Note the readings of Voltmeter, Ammeter and Wattmeter into the observation table as V0, I0 and W0 respectively.

12. Switch off the supply.

Observation Table :

S. No Voltmeter readings V0 in volt

Ammeter readings I0 in ampere

Wattmeter readings W0 in watt

1 2 3 4



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Calculation : Hence, Power input can be written as

W0 = V0 I0 CosФ0

CosФ0 = W0 / V0 I0

Once CosФ0 is known we can obtain, IC = I0CosФ0

IM = I0 SinФ0 Once IC, IM are known we can determine the exciting circuit parameter

R0 = V0 / IC Ω (No Load Resistance) X0 = V0 / IM Ω (No Load Reactance)

Here, V0 = Rated voltage of the transformer.

I0 = Input current or no load current of the transformer. W0 = Pi = Input power losses(Iron Loss) of the transformer.

CosФ0 = No load power factor of the transformer.



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Experiment 4 Objective : Study of the Short Circuit Test on Single-Phase Transformer Circuit diagram :

Figure 20



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Procedure : First of all make sure that the earthing of your laboratory is proper and connected to the terminal provided on back side of the panel. <System should not remain ON for more than 5 min. in this test(in short condition), otherwise the transformer may burn.>

1. First of all make sure that the Mains is off and the knob of Variac is at zero position.


3. Now connect 4 to 5.


5. Connect 8 to 10 and 9 to 13, short terminals 11 and 12 of primary of transformer. (Since Short Circuit Test is performed on high voltage side, hence high voltage winding is selected).

6. On the secondary side, connect terminal 14 to 25, 15 to 26 and then short terminals 25 and 26.

7. Now insert meters in the circuit. To insert ammeter connect terminals 3 to As1 and 4 to As2.

8. To insert wattmeter, connect terminals 5 to Ws2, 7 to Ws1 and 6 to Ws3. 9. Now connect voltmeter in the circuit, for this connect terminal 8 to Vs1

and 9 to Vs2. 10. Switch on AC supply.

11. Now gradually increase the input voltage so that the reading of ammeter reaches its maximum value (5A).

12. Record the Ammeter, Voltmeter, Wattmeter readings as Isc, Vsc and Wsc in the observation table.

13. Switch off the supply.



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Observation Table :

S. No Voltmeter reading VSC in volt

Ammeter reading ISC in ampere

Wattmeter reading WSC in watts

1. 2. 3. 4.

Calculations : Hence, WSC = VSC ISC CosФSC.

CosФSC = WSC/VSC ISC. Once these factors known we can determine the circuit parameter

R1E = WSC / I 2SC

Z1E = VSC / ISC = 2 2R + X1E 1E

X1E = 2 2Z - R1E 1E

Knowing the transformation ratio K, equivalent circuit parameters referred to secondary also can be determined.

Here, VSC = Reading of Voltmeter.

ISC = Short circuit current of the transformer. WSC = (PCU) F.L. = full load copper loss of the transformer.(reading of Wattmeter) R1E = Equivalent resistance of the transformer at primary side.

X1E = Equivalent reactance of the transformer at primary side. Z1E = Equivalent impedance of the transformer at primary side.

CosФSC = Short circuit power factor.



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• Efficiency of transformers : The efficiency of the transformer can be defined as the ratio of output power to input power. Efficiency is a function of a transformer's power losses, and two factors account for nearly all of these losses. One is winding copper loss. Since you have two sets of windings, you have two components to copper loss: primary and secondary winding copper loss. The second factor accounting for transformer power losses is core loss. You get core losses due to hysteresis fortunately the core losses for any given transformer stay constant.

You obtain maximum efficiency when winding copper loss equals core loss i.e., Copper loss = Core loss.

• Voltage Regulation : When a transformer is loaded with a constant supply voltage, the terminal voltage change depending upon the load and its power factor. The algebraic difference between the no load and full load terminal voltage is measured in terms of voltage regulation at a constant supply voltage the change in secondary terminal voltage from no load to full load with respect to no load voltage, is called voltage regulation of the transformer. In any step down transformer, the secondary current produces voltage drop across the resistive and reactive components of the transformer's secondary side. On the other side, the primary current produces voltage drops across the resistive and reactive components of the transformer's primary side. From this, it is easy to see the primary voltage will be less than the supply voltage, and the secondary (output) will be less than either of those. Let's assume you have no load connected to your transformer. In such a case, no secondary current flows. With no current, you have no voltage drop across those resistive and reactive components of the transformer's secondary side. But, another thing happens. Without a secondary current, the primary current drops to the no-load current—which is nearly zero. This means the voltage drop across the resistive and reactive components of the transformer's primary side becomes very small. What's the net effect? In a no-load situation, the voltage on the primary is almost equal to the supply voltage, and the secondary voltage nearly equals the supply voltage times the ratio of primary windings to secondary windings.

You might assume the transformer's output voltage is highest at no load. It would then make sense that (under loaded conditions) the transformer's resistive and reactive components cause the output voltage to drop below its no-load level. This is a logical assumption, but one that's not necessarily so. Depending on the power factor of the load, the output full-load voltage may actually be larger than the no-load voltage. The voltage regulation of the transformer is the percentage change in the output voltage from no-load to full-load.



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Experiment 5 Objective : To determine the Efficiency and Voltage Regulation of a Single-Phase Transformer by load test. Circuit diagram :


Ap3 Ap4 Vp1 Vp2 Vs2 Vs3 Vs4Vs1

1 3 4 5 7 8 10 14 18 20 22 23 24 25

VariacInput 0-270V50Hz

Load

2 6 9

11

12

13

15

16

17 19 21 26

0

0

0

0

125V 125V

125V 125V

Load

Mains

CC

PC

Wp Ws

CC

PC

As1 As2Ap1 Ap2

Wp1 Wp2

Wp3

Ws1 Ws2

Ws3

Figure 21



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Procedure : First of all make sure that the earthing of your laboratory is proper and connected to the terminal provided on back side of the panel. 1. First of all make sure that the Mains is off and the knob of variac is at zero

position. 2. Connect terminals 1 to 3 and 2 to 6.

3. Connect terminal 4 to terminal 5. 4. Now connect terminals 7 to 8 and 6 to 9.

5. Connect 8 to 10 and 9 to 13. 6. Short transformer terminals 11 to 12 and 15 to 16.

7. Connect 14 to 18, 17 to 19 8. Connect terminal 18 to 23 and 19 to 26.

9. Connect terminal 24 to terminal 25. 10. Now connect terminals 25 and 26 to the load terminals given at the top right

cornor. 11. Now insert meters in the circuit, for this connect terminals 3 and 4 to terminals

Ap1 and Ap2.

12. To insert wattmeter connect terminal 5 to Ws2, 7 to Ws1 and 6 to Ws3. 13. Connect terminal 8 to Vp1 and 9 to Vp2.

14. Connect terminals 18 to Vs3 and 19 to Vs4. 15. Connect terminals 23 to As1 and 24 to As2.

16. Now connect the suitable load to the socket provided on the top right cornor of the panel.

17. Switch ON the load step by step in steps of 100W until you get wattmeter reading at least 300W.

18. Find load fraction using formula- 19. n=Load/Transformer rating

(in this case n will be 300/1000=0.3)



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Observation Table :

Primary Side of the transformer Secondary Side of the transformer

S. No

voltage V0 (V)

current I0 (A)

Power W0 (W)

Voltage V1 (V)

Current I1 (A)

Efficiency η%

1 2 3 4

Calculation : Here, CosФ0 = Power factor of the transformer.=W0/V0I0

R1E = Equivalent resistance of the transformer.(as calculated before) X1E = Equivalent reactance of the transformer.(as calculated before) PI = Iron Loss (As recorded in OC test) PCU = Full load Copper Loss (as recorded in SC test) VA rating of transformer =1 KVA (1000VA) n =Load Wattage/VA rating of transformer Hence, Efficiency and voltage regulation at n load : Efficiency at n load

0 I CU

0n x (VA rating) x CosΦ%η = x 1002n x (VA rating) x CosΦ + P + n x (P ) F.L

Regulation at n load

1E 1E

1

-0 0n x (I) F.L x R x CosΦ n x (I) F.L x X x CosΦ% R = x 100

V

Efficiency and voltage regulation at full load : Efficiency at full load

0 I CU

0n x (VA rating) x CosΦ% η = x 100

(VA rating) x CosΦ + P + (P ) F.L

Regulation at full load

1E 1E

1

-0 0(I) F.L x R x CosΦ (I) F.L x X x CosΦ% R = x 100

V



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Warranty 1) We guarantee the product against all manufacturing defects for 24 months from

the date of sale by us or through our dealers. Consumables like dry cell etc. are not covered under warranty.

2) The guarantee will become void, if a) The product is not operated as per the instruction given in the operating

manual. b) The agreed payment terms and other conditions of sale are not followed.

c) The customer resells the instrument to another party. d) Any attempt is made to service and modify the instrument.

3) The non-working of the product is to be communicated to us immediately giving full details of the complaints and defects noticed specifically mentioning the type, serial number of the product and date of purchase etc.

4) The repair work will be carried out, provided the product is dispatched securely packed and insured. The transportation charges shall be borne by the customer.

List of Accessories 1. 4mm Patch Cord 8” (Red)....................................................................15 Nos.

2. 4mm Patch Cord 8” (Black) .................................................................15 Nos. 3. 4mm Patch Cord 24” (Red)..................................................................5 Nos.

4. 4mm Patch Cord 24” (Black) ...............................................................5 Nos. 5. e-Manual .............................................................................................1 No.

6. Single Phase Mains Chord ...................................................................1 No.



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