Transformer



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TRANSFORMER

Transformer is a static device which transfer electrical energy from one electrical circuit to another through the medium of magnetic field. The electrical circuit to which supply is given is called primary winding and the other electrical circuit which gives electrical energy to the load is called secondary winding.

Transformer is a type of electromagnetic energy conversion device. As, it converts the energy received by primaryis first converted to magnetic energy and then it is converted to useful electrical energy in the other circuit. If primary winding is connected to an alternating voltage source, an alternating flux is produced. Magnitude of this alternating flux depends on the amplitude of the primary voltage, frequency and number of turns of primary winding. Voltage is induced in secondary winding due to the mutual flux linkage whose magnitude will depend on the number of turns in secondary winding and magnitude of mutual flux and frequency.

The essence of transformer action requires only existence of time-varying mutual flux linkage across the windings. Such action can be possible for two windings coupled through air, but coupling between the two windings can be made effective using a core of iron and other ferromagnetic material. Due to this, most of the flux is then confined to a definite, high permeability path linking between the windings. This type of transformer is commonly called iron core transformer. The winding also produce additional flux known as linkage flux which links one winding without linking the other. Although, its value is very small but it plays an important role in determining the behaviour of the transformer.

Construction:

There are two main types of transformer i.e. core type and shell type. They both differ from each other by the way windings are wound around the magnetic core.

  1. Core type- The magnetic core is a stack of thin silicon-steel laminations. To reduce the eddy current losses, these lamniations are insulated to each other by thin layers of varnish. To reduce the core loss, almost all transformers have their magnetic core made from cold rolled grain oriented sheet steel (C.R.G.O.). This material on magnetizing in the rolling direction has low core loss and high permeability. In this type, winding suuround a considerable part of steel core. It also require less iron and more conductor material. It has two legged core for single phase transformer. It is thus achieved by placing half of the low voltage winding over one leg and other half over the second leg or limb. Low voltage winding is placed adjacent to steel core while high voltage winding is placed outside, in order to minimize the amount of insulation required.

  2. Shell type- For single phase transformer, shell type has three legged core. It requires more iron as compared to core type. In shell type, steel core surrounds a major part of the windings. The low voltage and high voltage windings are wound over the limb and are interleaved or sandwiched

 

Note: The shell type transformer is preferred for low voltage and low power applications while the core type is preferred for high voltage and high power applications.

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Types:

Transformer has two types which are:

  1. Step-up Transformer- In step-up transformer, secondary winding has more number of turns than primary winding. Due to this secondary voltage is greater than primary voltage, so it step-up the voltage. (N2> N1 or V2>V1).

  2. Step-down Transformer- In step-down transformer, primary winding has more number of turns than secondary winding. Due to this primary voltage is greater than secondary voltage, so it step-down the voltage. (N2< N1 or V2<V1).

Principle:

Transformer works on the principle of Faraday's Law of Electromagnetic Induction. According to this, when current in primary coil is varied then flux linked to secondary coil also varies. Hence an EMF is induced in secondary coil according to this law which is proportional to the change in flux linkage. Transformer working is based on two principle first an electric current produces magnetic field and second that changing magnetic field in the coil induces voltage.

Open circuit test (or no-load test):

In transformers, these tests are used to determine the voltage regulation, parameters of equivalent circuit and efficiency. In open circuit test, a voltmeter, a wattmeter and an ammeter are connected to low voltage side of the transformer. The high voltage side is left open circuited. Ammeter measures the no-load current or exciting current whose value is quite small. Wattmeter measures the input power which consists of core loss and ohmic loss. In open circuit test, ohmic losses are very negligible. So, it measures only core losses. Hence, wattmeter reading is equal to the transformer core loss.

 


 

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V1 = Applied voltage

Ie = Exciting current

Pc = core loss = V1Ie cost

No load power factor = cost = Pc/V1Ie

Ic = Ie cost

and Im = Ie sint

so, Ic = Pc/V1

Core loss resistance Rc = V1/Ic = V1/Ie cost = V12/ Pc.

Short circuit test:

In short circuit test, the low voltage side of the transformer is short-circuited while the instruments are placed on the high voltage side. The wattmeter measures the ohmic losses or copper losses. In short circuit test, core losses are very negligible and hence can be ignored.

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Vsc = voltmeter reading

Isc = ammeter reading

Psc = wattmeter reading

Equivalent leakage impedance referred to high voltage side is given by-

Zeh = Vsc/Isc

Equivalent resistance referred to high voltage side is

reh = Psc/I2sc

and equivalent leakage reactance referred to high voltage side

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xeh = (Z2eh + r2eh)1/2

Back-to-back test (Sumpner's test):

This is another test carried in transformer other than short circuit or open circuit test.It is also used to calculate the voltage regulation, parameters of equivalent circuit and efficiency. This type of test gives more accurate results than OC or SC test. A lead test on a transformer is important if its maximum temperature rise is to be determined. Two identical transformers are required to carry out this test. Primary side of both transformer is connected in parallel and are energized with rated voltage and rated frequency. Secondary side of both transformer is connected in series with their polarities in phase opposition, which can be checked by a voltmeter. Range of this voltmeter should be double of rated voltage of either transformer reading. As secondaries are connected in voltage opposition so both EMFs cancel each other. In this way no load current is carried out as no current flows through the secondary in this case. When a small voltage is iinjected into secondary, rated current flows through the secondary. Thus, in this way short circuit test is carried out.

 


 

If Pc and Psc are the core losses and ohmic losses then the reading of the wattmeter W1 = 2Pc and W2 = 2Psc .Then efficiency = output power/ input power.

In this test, even though the transformers are not supplying any load current, still full iron loss occurs in their cores and full load ohmic loss occurs in their winding. If the temperature rise of two transformers are kept under rated loss conditions for several hours till maximum stable temperature is reached.

Equivalent circuit and Phasor diagram:

 

 

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Voltage Regulation:

It is defined as the change in magnitude of secondary voltage per unit of rated secondary rated voltage, when load at given power is reduced to zero with primary applied voltage held constant.

Voltage Regulation = E2 - V2 /E2 *100

where V2 = secondary terminal voltage at any load.

E2 = secondary terminal voltage at no load.

The change is secondary terminal voltage with load current is due to the primary and secondary leakage impedance. The magnitude of this change depends on the load power factor load current, total resistance and total leakage reactance of the transformer. Thus, when load power factor is leading voltage regulation is zero. Maximum voltage regulation occurs when load power factor is at a lagging load. Magnitude of maximum voltage regulation is equal to the p.u. value of equivalent leakage impedance of the transformer.

Welding Transformer:

Welding transformer have thin primary winding with large number of turns.While secondary winding has more area of cross-section and less number of turns. Thus, there is less voltage and very high current in the secondary. One end of seondary winding is connected to the welding electrode and other end is connected to the pieces to be welded. If any high current flows then heat is produced due to the contact resistance between the electrode and pieces to be welded. The generated heat melts a tip to the electrode and the gap between the two pieces is filled. Its winding is highly reactive. Otherwise, a reactor may be added in series with the secondary winding.

 

Following are the types of reactor used in welding transformer:

  • Tapped reactor

  • Moving coil reactor

  • moving shunt reactor

  • Saturable reactor

Ex 1: A 20kVA, 2500/250V, 50Hz, single phase transformer gave the following test results:

Open circuit test (on l.v. side): 250V, 1.4A, 105W

Short circuit test (on h.vv side): 104V, 8A, 320W.

Compute the parameters of equivalent circuit referred to high voltage and low voltage sides.

Soln: No load power factor = cost = 105/(250*1.4) = 0.3

sint = 0.954

Ic = Ie cost = 1.4*0.3*0.42

Im = Ie sint = 1.4*0.954*1.336

Rc = V1/Ic = 250/0.42 = 595 ohm.

Xm = V1/Im = 250/1.336 = 187 ohm.

From short circuit test,

Zeh = Vsc/Isc = 104/8 = 13 ohm.

reh = Psc/I2sc = 320/8*8 = 5 ohm.

xeh = (Z2eh + r2eh)1/2 = 12 ohm.

Ex 2: Two similar 250kVA single phase transformers gave the following result when tested by sumpner test. Mains wattmeter reading 5kW, secondary series wattmeter reading is 7.5kW at full load current. Calculate the efficiency of individual transformer at 75% full load and 0.8 p.f. leading.

Soln: Total losses for both transformer = 5+7.5 = 12.5kW

Full load loss for each transformer = 12.5/2 = 6.25kW

Copper loss at 75% load = 3/4 * 3/4 * 7.5/2 = 2.11kW

output of each transformer at 75% full load and 0.8 p.f.

= 250*0.75* 0.8 = 150kW

Efficiency = 150 / (150+2.5 +2.11) = 97%


 


 


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