Introduction:

A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core, and thus a varying magnetic field through the secondary winding. This varying magnetic field induces a varying electromotive force (EMF) or "voltage" in the secondary winding. This effect is called mutual induction.

If a load is connected to the secondary, an electric current will flow in the secondary winding and electrical energy will be transferred from the primary circuit through the transformer to the load. In an ideal transformer, the induced voltage in the secondary winding (VS) is in proportion to the primary voltage (VP), and is given by the ratio of the number of turns in the secondary (NS) to the number of turns in the primary (NP) as follows:

By appropriate selection of the ratio of turns, a transformer thus allows an alternating current (AC) voltage to be "stepped up" by making NS greater than NP, or "stepped down" by making NS less than NP.

In the vast majority of transformers, the coils are wound around a ferromagnetic core, air-core transformers being a notable exception.

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Ideal Transformer:

If the secondary coil is attached to a load that allows current to flow, electrical power is transmitted from the primary circuit to the secondary circuit. Ideally, the transformer is perfectly efficient; all the incoming energy is transformed from the primary circuit to the

magnetic field and into the secondary circuit. If this condition is met, the incoming electric power must equal the outgoing power.

Pincoming = IPVP = Poutgoing = ISVS

giving the ideal transformer equation

Transformers are efficient so this formula is a reasonable approximation.

If the voltage is increased, then the current is decreased by the same factor. The impedance in one circuit is transformed by the square of the turns ratio. For example, if an impedance ZS is attached across the terminals of the secondary coil, it appears to the

primary circuit to have an impedance of . This relationship is reciprocal, so

that the impedance ZP of the primary circuit appears to the secondary to be .

Practical Transformer

An ideal transformer is useful in understanding the working of a transformer. But it cannot be used for the computation of the performance of a practical transformer due to the non-ideal nature of the practical transformer. In a working transformer the performance aspects like magnetizing current, losses, voltage regulation, efficiency etc are important. Hence the effects of the non-idealization like finite

permeability, saturation, hysteresis and windingresistances have to be added to an ideal transformer to make it a practical transformer.Conversely, if these effects are removed from a working transformer what is left behind is anideal transformer. Finite permeability of the magnetic circuit necessitates a finite value of the current to be drawn from the mains to produce the mmf required to establish the necessary flux. The current and mmf required is proportional to the flux density B that is required to be established in the core.

Losses

An ideal transformer would have no losses, and would therefore be 100% efficient. In practice energy is dissipated due both to the resistance of the windings (known as copper loss), and to magnetic effects primarily attributable to the core (known as iron loss). Transformers are in general highly efficient, and large power transformers (around 100 MVA and larger) may attain an 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.

The losses arise from:

Winding resistance

Current flowing through the windings causes resistive heating of the conductors. Eddy currents

Induced currents circulate in the core and cause its resistive heating. 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 objects such as the transformer's support structure, and be converted to heat. The familiar hum or buzzing noise heard near transformers is a result of stray fields causing components of the tank to vibrate, and is also from magnetostriction vibration of the core.

Hysteresis losses

Each time the magnetic field is reversed, a small amount of energy is lost to hysteresis in the magnetic core. The level of hysteresis is affected by the core material.

Mechanical losses

The alternating magnetic field causes fluctuating electromagnetic forces between the coils of wire, the core and any nearby metalwork, causing vibrations and noise which consume power.

Magnetostriction

The flux in the core causes it to physically expand and contract slightly with the alternating magnetic field, an effect known as magnetostriction. This in turn causes losses due to frictional heating in susceptible ferromagnetic cores.

Cooling system

Large power transformers may be equipped with cooling fans, oil pumps or water-cooled heat exchangers designed to remove the heat caused by copper and iron losses. The power used to operate the cooling system is typically considered part of the losses of the transformer.

Copper Losses The power lost in the form of heat in the armature winding of a generator is known as COPPER LOSS. Heat is generated any time current flows in a conductor. Copper loss is an I2R loss, which increases as current increases. The amount of heat generated is also proportional to the resistance of the conductor. The resistance of the conductor varies directly with its length and inversely with its cross- sectional area. Copper loss is minimized in armature windings by using large diameter wire.

Different type of transformer:

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. For voltage ratios not exceeding about 3:1, an autotransformer is less costly, lighter, smaller and more efficient than a two-winding transformer of a similar rating.

By exposing part of the winding coils and making the secondary connection through a sliding brush, an autotransformer with a near-continuously variable turns ratio can be obtained, allowing for very small increments of voltage.

Instrument transformers

Current transformers

Current transformers used as part of metering equipment for three-phase 400 ampere electricity supply

A current transformer is designed to provide a current in its secondary which is accurately proportional to the current flowing in its primary.

Current transformers are commonly used in electricity meters to facilitate the measurement of large currents which would be difficult to measure more directly.

Care must be taken that the secondary of a current transformer is not disconnected from its load while current is flowing in the primary as in this circumstance a very high voltage would be produced across the secondary.

Current transformers are often constructed with a single primary turn either as an insulated cable passing through a toroidal core, or else as a bar to which circuit conductors are connected.

Voltage transformers

Voltage transformers (also known as potential transformers) are used in the electricity supply industry to measure accurately the voltage being supplied. They are designed to present negligible load to the voltage being measured.

Different type of transformer test:

There are 12 routine test of transformer in a transformer manufacturer company. Those are:

1. Impulse test;2. Ratio test;3. Insulation test;4. Mega test;5. No load loss test;6. Full load loss test;7. Open circuit test;8. Short circuit test;9. Double frequency test;

Etc..

some of these test are briefly explain below:

Impulse Test

This test is made to prove that the transformer insulation will withstand voltage surges which may be caused by lightning or switching; this includes insulation to ground, insulation between turns and windings, and the flashover value of the associated bushings. A high-voltage wave of standard values, and approximating a lightning surge, is imposed on

the unit to be tested. The surge generator usually consists of a number of capacitors connected so that they be charged in parallel from a relatively low-voltage source and discharged in series to give a high voltage across the test piece. A standard impulse wave is illustrated below;

The standard wave reaches its peak voltage value in 1.5 microseconds and reduces to half the voltage value in 40 microseconds. The value of the voltage applied depends on the rating of the insulation, and may vary from 5 to 30 times the voltage rating of the insulation. Since rather elaborate and costly equipment is needed, impulse tests are usually performed only on large station-type power transformers, and rarely made after they leave the factory.

Short circuit test and open circuit test:

Through the open and short circuit test we can get the value of parameter of a transformer. The parallel parameter values are found with no load connected to the secondary (open circuit) and the series parameter values are found with the secondary terminals shorted (short circuit). It is possible, for convenience in the lab, to make the tests on either the primary or the secondary. Figure 2 shows the equivalents circuits for the two tests. For the open circuit test, the series parameters are neglected for convenience. This is reasonable since the voltage drops are across Req and Xeq are normally small.

Figure . Equivalent circuits for tests. (a) Open circuit. (b) Short circuit.

Expressions for the non-ideal transformer parameters are derived from the equivalent circuits shown in Figure 2. The results are Equations (1), (2), (3), and (4). All parameters are expressed in terms of quantities measured in the open circuit and short circuit tests.

Figure . Connection for open circuit test and short circuit test

Insulation-Resistance Test

This is the often mentioned Megger test. It is meant to give some indication of the condition of the insulation, and is often used in maintenance procedures. The connections are the same as for the high-voltage test. The insulation resistance of a transformer depends largely on the temperature and cleanliness and dryness of the windings. Insulation resistance should be at least 1 megohm (I million ohms) for each 1000 volts of test voltage. If it falls below this figure, presence of dirt or moisture may be indicated.

There are 3 types of current that appear in insulation testing:

A. Capacitance Charging Current      This is the current which is like a condenser, which starts out high and tapers off rapidly to zero.

B. Absorption Current

This is due to the polarization of the insulating materials. It takes longer for absorption current to reach a static point than charging current and, likewise, takes a much longer time to bleed off. On large or long cables it is important to short out the cable after test to eliminate the possibility of shock to the person conducting the test.

C. Leakage Current

This is the current we are really concerned about. It is a steady current leakage through or over the insulation due to moisture, dirt, or other reasons. This test must be continued for one minute or until the reading holds steady for 15 seconds. This assures us that the capacitative and absorption currents have reached a static point. This will vary with the equipment under test. Motors and transformers will take longer than average conductors.

Tour at Energypac: we had a visit at Energypac, a leading company in the sector of power generation and heavy equipment supplying. We watched everything pactically, whatever we learnt in our theory classes. Here is a brief description about the company

Company Profile

Basic InformationCompany Name: Energypac Power Generation Ltd  Business Type: Distributor/Wholesaler  Product/Service

(We Sell):Sales and suppliers of heavy equipments, cng conversion kits sales and suppliers  

Number of Employees: Above 1000 People 

Trade & Market

Main Markets:

North AmericaSouth AmericaWestern EuropeEastern EuropeEastern AsiaSoutheast AsiaMid EastAfricaOceania

Total Annual Sales Volume:

US$1 Million - US$2.5 Million