Here is an essay on ‘Transformer’ for class 9, 10, 11 and 12. Find paragraphs, long and short essays on ‘Transformer’ especially written for school and college students.
Essay on Transformer
Essay contents:
- Essay on the Meaning of Transformer
- Essay on the Operating Principle of Transformer
- Essay on Transformer as Two Port Network
- Essay on the Effects of Voltage and Frequency Variations on Transformers
- Essay on Transformer Losses
- Essay on the Testing of Transformers
Essay # 1. Meaning of Transformer:
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Electrical energy is generated at places where it is easier to get water head, oil or coal for hydro-electric, diesel or thermal power stations respectively. Then energy is to be transmitted at considerable distances for use in villages, towns and cities located at distant places. Transmission of electrical energy at high voltages is economical, therefore, some means are required for stepping up the voltage at generating stations and stepping down the same at the places where it is to be used.
Electric machine used for this purpose is “transformer” our country the electrical energy is usually generated at 6.6 or 11 or 33 kV, stepped up to 132, 220, 400, or 765 kV with the help of step-up transformers for transmission and then stepped down to 66 kV or 33 kV at grid substations for feeding various substations, which further step down the voltage to 11 kV for feeding distributing transformers stepping down the voltage further to 415/240 volts for the consumer uses.
Transformers are extensively used like, the instrument transformers for metering purposes. One-to-one ratio transformers (i.e., the transformers having equal primary and secondary voltages) are used to electrically isolate the two parts of an electrical circuit. In high voltage laboratories the transformers are used to provide very high voltages for testing purposes, termed as testing transformers.
In electric communication circuits transformers are employed for a variety of purposes e.g., as an impedance transformation device to allow maximum transfer of power from the input circuit to the output device.
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In radio and television circuits input transformers, inter-stage transformers and output transformers are widely employed. Transformers are also employed in telephone circuits, instrumentation circuits and control circuits. Indeed the transformer is a device which plays a vital and essential role in many facets of electrical engineering.
Transformer is an ac machine that:
(i) Transfers electrical energy from one electric circuit to another
(ii) Does so without a change of frequency
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(iii) Does so by the principle of electro-magnetic induction, and
(iv) Has electric circuits that are linked by a common magnetic circuit.
The energy transfer usually takes place with a change of voltage, although this is not always necessary. When the transformer raises the voltage i.e., when the out-put voltage of a transformer is higher than its input voltage, it is called the step-up transformer and when it lowers the voltage it is called the step down transformer.
Since its basic construction requires no moving parts, so it is often called the ‘static transformer’ and it is very rugged machine requiring the minimum amount of repair and maintenance. Owing to the lack of rotating parts there are no friction or windage losses. Further, the other losses are relatively low, so that the efficiency of a transformer is high.
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Typical transformer efficiencies at full load lie between 96% and 97% and with extremely large capacity transformers the efficiencies are as high as 99 %. The cost per kVA output of transformers is quite low as compared with other electrical machines. As there are no teeth, slots or rotating parts, and the windings can be immersed in oil, it is not difficult to insulate transformers for very high voltages.
Essay # 2. Operating Principle of Transformer:
An elementary transformer consists of a soft iron or silicon steel core and two windings placed on it. The windings are insulated from both the core and each other. The core is built up of thin soft iron or silicon steel laminations to provide a path of low reluctance to the magnetic flux. The winding connected to the supply main is called the primary and the winding connected to the load circuit is called the secondary.
The winding connected to higher voltage circuit is called the high-voltage (hv) winding while that connected to the lower- voltage circuit is called the low-voltage (IV) winding. In case of a step-up transformer, low-voltage winding is the primary and high voltage winding is the secondary while in case of a step-down transformer the high-voltage winding is the primary and low-voltage winding is the secondary.
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The action of a transformer is based on the principle that energy may be efficiently transferred by induction from one set of coils to another by means of a varying magnetic flux, provided that both the sets of coils are on a common magnetic circuit. In a transformer, the coils and magnetic circuit are all stationary with respect to one another. The emfs are induced by the variation in the magnitude of flux with time, as illustrated in Fig. 10.1.
Although in the actual construction the two windings are usually wound one over the other, for the sake of simplicity, the figures for analysing transformer theory show the windings on opposite sides of the core as in Fig. 10.1.
When the primary winding is connected to an ac supply mains, a current flows through it. Since this winding links with an iron core, so current flowing through this winding produces an alternating flux ɸ in the core. Since this flux is alternating and links with the secondary winding also, so induces an emf in the secondary winding. The frequency of induced emf in secondary winding is the same as that of the flux or that of the supply voltage.
The induced emf in the secondary winding enables it to deliver current to an external load connected across it. Thus the energy is transformed from primary winding to the secondary winding by means of electro-magnetic induction without any change in frequency. The flux ɸ of the iron core links not only with the secondary winding but also with the primary winding, so produces self-induced emf in the primary winding.
This induced emf in the primary winding opposes the applied voltage and, therefore, sometimes it is known as back emf of the primary. In fact the induced emf in the primary winding limits the primary current in much the same way that the back emf in a dc motor limits the armature current.
Transformer on DC:
A transformer cannot operate on dc supply and never be connected to a dc source. If a rated dc voltage is applied to the primary of a transformer, the flux produced in the transformer core will not vary but remain constant in magnitude and, therefore, no emf will be induced in the secondary winding except at the moment of switching on. Thus the transformer is not capable of raising or lowering the dc voltage.
Also there will be no self-induced emf in the primary winding, which is only possible with varying flux linkage, to oppose the applied voltage and since the resistance of primary winding is quite low, therefore, a heavy current will flow through the primary winding which may result in the burning out of the primary winding. This is reason that dc is never applied to a transformer.
Ideal Transformer:
For a better understanding and an easier explanation of a practical transformer, certain idealizing assumptions are made which are close approximations for a practical transformer. A transformer having these ideal properties is hypothetical (has no real existence) and is referred to as the ideal transformer. It possesses certain essential features of a real transformer but some details of minor significance are ignored which will be introduced step-by-step while analysing a transformer.
The idealizing assumptions made are as follows:
(i) No winding resistance i.e., the primary and secondary windings have zero resistance. It means that there is no ohmic power loss and no resistive voltage drop in an ideal transformer.
(ii) No magnetic leakage i.e., there is no leakage flux and all the flux set up is confined to the core and links both the windings.
(iii) No iron loss i.e., hysteresis and eddy current losses in transformer core are zero.
(iv) Zero-magnetizing current i.e., the core has infinite permeability and zero reluctance so that zero magnetizing current is required for establishing the requisite amount of flux in the core. From the above discussion an ideal transformer is supposed to consists of two purely inductive coils wound on a loss-free core.
Voltage and Current Transformation Ratios:
Referring to Eq. (10.1), it is clear that the volts per turn is exactly the same for both the primary and secondary windings i.e., in any transformer, the secondary and primary induced emfs are related to each other by the ratio of the number of secondary and primary turns.
Thus,
V2/V1 = E2/E1 = N2/N1 = K …(10.4)
The same relationship can be derived by dividing Eq. (10.3) by Eq, (10.2).
The constant K in Eq. (10.4) is called the voltage transformation ratio.
For step-up transformer, V2 > V1 or voltage transformation ratio, K > 1.
For step down transformer, V2, < V1 or voltage transformation ratio, K < 1.
In an ideal transformer, the losses are negligible, so the volt-ampere input to the primary and volt- ampere output from secondary can be approximately equated i.e.:
Output VA = Input VA
Or V2 I2 = V1 I1
Or I2/I1 = V1/V2 = E1/E2 = N1/N2 = 1/K … (10.5)
i.e., Primary and secondary currents are inversely proportional to their respective turns.
Note:
For a transformer the true ratio of transformation (or turn-ratio) is constant, while the voltage ratio (V2/V1) varies about 1 to 8 percent depending upon the load and its power factor.
Essay # 3. Transformer as Two Port Network:
A box representing two-port network is illustrated in Fig. 10.4. The terminal behaviour of a two port device may be specified by two voltages and two currents (voltage V1, and current I1 at the input port and voltage V2, and current I2, at the output port). The conventional positive polarities of voltage V1 and V0 and currents I1 and I2, are shown in the figure.
Out of four quantities (V1, V2, I, and I2) any two may be selected as independent variables and the remaining two can be expressed in terms of the selected independent variables. In general, we are not free to select the independent variables arbitrarily for example in case of an ideal transformer.
Since the network is linear, the superposition principle applies. Each port voltage is the sum of two components, one arising from I1, the other from I2.
Thus the voltages V1 and V2 can be written as:
The physical significance of z parameters can be realized by substituting one or the other of two ports on open circuit.
If output port is open-circuited, I2 is zero and the Eqs. (10.6) provide:
Thus z11 is defined as the input impedance of the network when the output terminals are open- circuited. Z21 is defined as the transfer impedance with output terminals open-circuited (the open-circuit output voltage resulting from the flow of unit current at the input).
Similarly with input port open-circuited, I1 = 0 Eqs. (10.6) provide:
Thus z22 is defined as the output impedance of the network when the input terminals are open- circuited. z12 is defined as the transfer impedance with input terminals open-circuited (the open-circuit voltage at the input terminals resulting from the flow of unit current at the output. All passive linear bilateral networks obey the principle of reciprocity and, therefore z12 = z21.
Transformer is a two-port network but in a transformer two voltages V2 and V1 (or two currents I1 and I2) cannot be selected as the independent variables because their ratio is a constant and is equal to the turn-ratio of the transformer. Primary terminals of transformer are input terminals while secondary terminals are output terminals.
Transformer on No Load:
When the primary of a transformer is connected to the source of ac supply and the secondary is open, the transformer is said to be at no-load (there is no load on secondary).
Consider an ideal transformer whose secondary winding secondary side is open and the primary winding is connected to a sinusoidal alternating voltage V1. The alternating voltage applied to the primary winding will cause flow of alternating current in the primary winding. Since the primary coil is purely inductive and there is no output (secondary being open) the primary draws the magnetising current Im only. The function of this current is merely to magnetize the core. If the transformer is truly ideal, the magnitude of Im should be zero by virtue of assumption (iv).
Since the reluctance of the magnetic circuit is never zero, Im has definite magnitude. The magnetising current, Im is small in magnitude and lags behind supply voltage V1 by 90˚. This magnetising current Im produces an alternating flux ɸ which is, at all times, proportional to the current (assuming permeability of the magnetic circuit to be constant) and, hence, in phase with it.
Let the instantaneous linking flux be given as:
ɸ = ɸmax sin ωt … (10.11)
The varying flux is linked with both of the windings (primary and secondary) and so induces emfs in the primary and secondary windings.
The instantaneous values of induced emfs in the primary and secondary windings will be:
Since primary winding has no ohmic resistance, (as assumed), therefore, applied voltage to primary winding is to only oppose the induced emf in the primary winding, hence instantaneous applied voltage to primary will be given by:
Comparing Eqs. (10.11), (10.12), (10.13), and (10.14) we conclude that:
(i) Induced emfs in primary and secondary windings, E1 and E2 lag behind the main flux ɸ by π/2, so these emfs (E1 and E2) are in phase with each other, as shown in Fig. 10.6 vectorially.
(ii) Applied voltage to the primary winding leads the main flux by π/2 and is in phase opposition to the induced emf in the primary winding, as shown in Fig. 10.6 vectorially.
(iii) Secondary voltage V2, = E2 as there is no voltage drop in secondary.
The instantaneous value of applied voltage, induced emfs, flux and magnetising current, in case of an ideal transformer, are illustrated by sinusoidal waves in Fig. 10.7.
However, when a varying flux is set up in magnetic material, there will be power loss, called the iron or core loss. So the input current to the primary under no-load condition has also to supply the hysteresis and eddy current losses (iron losses) occurring in the core in addition to small amount of copper loss occurring in primary winding (no copper loss occurs in secondary winding on open circuit or on no-load). Hence, the no-load primary current I0 does not lag behind applied voltage V1 by 90° but lags behind V1 by angle ɸ0 < 90˚.
Input power on no-load, P0 = V1I0 cos ɸ0 where cos ɸ0 is the primary power factor under no-load conditions.
As seen from phasor diagram shown in Fig. 10.8, input current to the primary I0, called the exciting current, has two components- (i) in-phase, active or energy component, Ie used to meet the iron loss in addition to small amount of copper loss occurring in the primary winding and (ii) quadrature component or watt less component, called the magnetizing component, Im used to create the alternating flux in the core.
The equivalent circuit of a transformer on no-load is illustrated in Fig. 10.9, where in two components of no-load current; Ie and Im are represented by currents drawn by a non-inductive resistance R0 and a pure inductive reactance X0 respectively. Both these currents are drawn at induced emf E1 = V1 for resistance-less, no-leakage primary coil; even otherwise E1 = V1.
The worth-noting points are given below:
1. The no-load primary current I0, called the exciting current, is very small in comparison to the full-load primary current. It ranges from 2 to 5 per cent of full-load primary current.
2. The exciting or no-load current I0 is made up of a relatively large quadrature or magnetizing component Im and a comparatively small in-phase or energy component Ie so the power factor of a transformer on no-load is very small (usually varies between 0.1 and 0.2 lag). The phase angle between I0 and V1 is about 78° to 87˚.
3. No-load primary copper loss i.e., I20 R1 is very small and may be neglected. Thus the no-load primary input power is practically equal to the iron loss occurring in the core of the transformer.
4. Phase angle ɸ0 is also known as the hysteresis angle of advance because it is principally the core- loss which is responsible for shift in the current phasor.
5. Since the permeability of the core material varies with the instantaneous value of the exciting current, so the waveform of the exciting or magnetizing current is not truly sinusoidal. As such, it should not be represented by a phasor because only sinusoidal varying quantities are represented by rotating phasors. But, in practice, it makes no appreciable difference.
Transformer on Load:
When the secondary circuit of a transformer is completed through an impedance, or load, the transformer is said to be loaded, and current flows through the secondary and the load. The magnitude and phase of secondary current I2 with respect to secondary terminal voltage V2 will depend upon the characteristic of load i.e., current I2, will be in phase, lag behind and lead the terminal voltage V2 respectively when the load is non-inductive, inductive and capacitive.
When the transformer is on no load, as shown in Fig. 10.5, it draws no-load current I0 from the supply mains. The no-load current I0 sets up an mmf N1 I0 which produces flux ɸ in the core. When an impedance is connected across the secondary terminals, as shown in Fig. 10.10, current I2 flows through the secondary winding. The secondary current I2 sets up its own mmf and hence creates a secondary flux ɸ2.
The secondary flux ɸ2 opposes the main flux ɸ set up by the exciting current I0 according to Lenz’s law. The opposing secondary flux ɸ2 weakens the main flux ɸ momentarily, so primary back emf E1 tends to be reduced. So difference of applied voltage V1 and back emf E1 increases, therefore, more current is drawn from the source of supply flowing through the primary winding until the original value of flux ɸ is obtained.
It again causes increase in back emf E1 and it adjusts itself as such that there is a balance between applied voltage V1 and back emf E1. Let the additional primary current be l’1. The current I’1, is in phase opposition with secondary current I2 and is called the counter-balancing current. The additional current I’1 sets up an mmf N1 I’1 producing flux ɸ1‘ in the same direction as that of main flux ɸ and cancels the flux ɸ2 produced by secondary mmf N2 I2, being equal in magnitude.
So N1 I’1 = N2 I2
Or I’1 = N2/N1 I2
The total primary current I1 is, therefore, phasor sum of primary counter-balancing current I’1 and no-load current I0, which will be approximately equal to I’1 as I0 is usually very small in comparison to I’1.
... I1 = I’1 = N2/N1 I2
Or I1/I2 = N2/N1 = K (transformation ratio)
Hence primary and secondary currents are inversely proportional to their respective turns. Since the secondary flux ɸ2 produced by secondary mmf N2 I2 is neutralized by the flux ɸ’1 produced by mmf N1 I1 set up by counter-balancing primary current I’1, so the flux in the transformer core remains almost constant from no-load to full load.
The phasor diagrams for transformer on non-inductive, inductive and capacitive loads are shown in Figs. 10.11 (a), (b) and (c) respectively.
Since the voltage drops in both of the windings of the transformer are assumed to be negligible, therefore:
V2 = E2 and V1 = – E1
The secondary current I2 is in phase, lags behind and leads the secondary terminal voltage V2 by an angle φ2 for pure resistive, inductive and capacitive loads respectively.
The induced primary current I’1, also known as counterbalancing current, is always in opposition to secondary current I2, and since no-load current I0 is very small, the total primary current I1 is almost opposite in phase to I2 and K times the secondary current I2, where K is transformation ratio.
Note:
In phasor diagrams shown in Figs. 10.11 (a), 10.11 (b) and 10.11 (c) no-load current has been drawn on exaggerated scale for sake of clarity.
Essay # 4. Effects of Voltage and Frequency Variations on Transformers:
Power transformers are not ordinarily subjected to frequency variations and usually are subject to only modest voltage variations, but it is interesting to consider the effects thereof.
Variations in voltage and/or frequency affect the iron losses (hysteresis and eddy current losses) in a transformer.
As long as the flux variations are sinusoidal with respect to line, hysteresis loss (Ph), and eddy current loss (Pe) varies according to the following relations:
Ph α f (ɸmax)x
Where x lies between 1.5 and 2.5 depending on the grade of iron used in transformer core
and Pe α f 2(ɸmax)2
If the transformer is operated with the frequency and voltage changed in the same proportion, the flux density will remain unchanged as obvious from Eq. (10.2) and apparently the no-load current will also remain unaffected.
The transformer can be operated safely at frequency less than rated one with correspondingly reduced voltage. In this case iron losses will be reduced. But if the transformer is operated with increased voltage and frequency in the same proportion, the core losses may increase to an intolerable level.
Increase in frequency with constant supply voltage will cause reduction in hysteresis loss and leave the eddy current losses unaffected. Some increase in voltage could, therefore, be tolerated at higher frequencies, but exactly how much depends on the relative magnitude, of the hysteresis and eddy current losses and the grade of iron used in the transformer core.
Essay # 5. Transformer Losses:
The transformer is a static machine and, therefore, there are no friction or windage losses.
The various power losses occurring in a transformer are enumerated below:
1. Iron or Core Losses:
Iron loss is caused by the alternating flux in the core and consists of hysteresis and eddy current losses.
(a) Hysteresis Loss:
The core of a transformer is subjected to an alternating magnetizing force and for each cycle of emf a hysteresis loop is traced out.
The hysteresis loss per second is given by the equation:
Hysteresis loss, Ph = ƞ’ (Bmax)x f V joules per second or watts … (10.33)
Where f is the supply frequency in Hz, V is the volume of core in cubic metres, ƞ’ is the hysteresis coefficient, Bmax is peak value of flux density in the core and x lies between 1.5 and 2.5 depending upon the material and is often taken as 1.6.
(b) Eddy Current Loss:
If the magnetic circuit is made up of iron and if the flux in the circuit is variable, currents will be induced by induction in the iron circuit itself. All such currents are known as eddy currents.
Eddy currents result in a loss of power, with consequent beating of the material.
The eddy current loss is given by:
Pe = Ke (Bmax)2 f2 r2 V watts or joules per second …(10.34)
From the above expression for eddy current loss in a thin sheet it is obvious that eddy current loss varies:
(i) As the square of maximum flux density
(ii) As the square of the frequency, and
(iii) As the square of thickness of laminations.
The hysteresis and eddy current losses depend upon the maximum flux density in the core and supply frequency. Since it has been determined that the mutual flux varies somewhat with the load (its variation being 1 to 3 % from no-load to full-load), the core losses will vary somewhat with the load and its power factor. It may be emphasized here that core-losses are assumed to remain constant from no-load to full load, the variations in losses from no-load to full-load being very small and negligible.
These losses are determined from the open-circuit.
The input to the transformer with rated voltage applied to the primary and secondary open-circuited is equal to the core loss.
These losses are minimized by using steel of high silicon content for the core and by using very thin laminations (0.3 mm to 0.5 mm) insulated from each other either by insulating varnish or by layer of papers.
2. Copper or Ohmic Losses:
These losses occur due to ohmic resistance of the transformer windings. If I1 and I2 are the primary and secondary currents respectively and R1 and R2 are the respective resistances of primary and secondary windings then copper losses occurring in primary and secondary windings will be I12 R1 and I22 R2, respectively.
So total copper losses will be (I12 R1 + I22 R2). These losses vary as the square of the load current or kVA. For example if the copper losses at full load are Pc then copper losses at one-half or one-third of full load will be (1/2)2 Pc or (1/3)2 Pc i.e., Pc/4 or Pc/9 respectively.
Copper losses are determined on the basis of constant equivalent resistance determined from the short-circuit test and then corrected to 75 °C (since the standard operating temperature of electrical machines is taken 75 °C).
Essay # 6. Testing of Transformers:
The main difficulties encountered in testing of large power transformers by direct loading are- (i) wastage of large amount of energy and (ii) a stupendous (impossible for large transformers) task of arranging a load large enough for direct loading.
The performance characteristics of a transformer can be conveniently computed from the knowledge of its equivalent circuit parameters which, in turn, may be determined by conducting simple tests called the open-circuit or no-load test and short-circuit or impedance test involving very little power consumption (power needed to supply the losses incurred).
Open-Circuit Test (or No-Load Test):
The purpose of this test is to determine the core (or iron or excitation) loss, Pi and no-load current I0 and thereby the shunt branch parameters R0 and X0 of the equivalent circuit.
In this test, one of the windings (usually high voltage winding) is kept open circuited and the rated voltage at rated frequency is applied to the other winding, as shown Fig. 10.22. No doubt, the core loss will be the same whether the measurements are made on lv winding or hv winding so long as the rated voltage of that winding is applied to it but in case the measurements are made on hv winding, the voltage required to be applied would often be inconveniently large while the current I0 would be inconveniently small.
Either an auto-transformer or a voltage divider (VD) is used for varying the voltage applied to the low-voltage winding. Ammeter A and wattmeter W are connected to measure no-load current I0 and input power P0. Voltmeter V is connected to measure the applied voltage.
Since no current flows in the open-circuited secondary, the current in the primary will be merely that necessary to magnetize the core at normal voltage. Moreover, this magnetising current is a very small fraction of the full load current (usually 3 to 10 % of full load current) and may be neglected as far as the copper loss is concerned consequently; the test gives core loss alone practically.
With normal voltage applied to the primary, normal flux will be set up in the core and, therefore, normal iron (or core) loss will occur which are recorded by a wattmeter W.
The open-circuit test gives enough data to compute the equivalent circuit constants R0, X0, no-load power factor cos ɸ0, no-load current I0 and no load power loss (iron loss) of a transformer.
Note:
1. Since no-load current I0 is very small, therefore, pressure coils of wattmeter and the voltmeter should be connected such that the currents drawn by them do not flow through the current coils of the wattmeter and ammeter.
2. Since power factor at no-load is quite low (in the range of 0.1-0.2 lag) a low power factor wattmeter should be used to ensure accurate measurements.
3. The error due to power loss in ammeter can be eliminated by short-circuiting the ammeter while reading wattmeter.
4. Sometimes a high resistance voltmeter is connected across the secondary to indicate the emf induced in the secondary (hv winding). This helps in determination of transformation ratio K.
5. It must, however, be remembered that in making this test, hv side is hot and, therefore, its terminals must be properly insulated.
Separation of Hysteresis and Eddy Current Losses:
Hystersis loss, Ph α (Bmax)1.6 f
And eddy current loss, Pe α (Bmax) 2 f 2
It shows that for constant peak flux density the hysteresis loss varies as the frequency of supply while eddy current loss varies as the square of supply frequency.
For any transformer V/f α Bmax and for a particular value of V/f or of peak flux density core loss per cycle are given as:
Pi/f = A + Bf … (10.40)
Where A is a constant and is equal to Kh (Bmax)1.6 and B is another constant having value of Ke (Bmax).
The values of constants A and B in Eq. (10.40) can be determined by performing open-circuit test on the transformer. The connections are the same as for open- circuit test (Fig. 10.22) except that the primary of the transformer is connected to variable voltage variable frequency source (a dc motor driven alternator).
During this test, the applied voltage V and frequency f are varied together (by adjusting the excitation and speed of the alternator feeding the transformer under test) so as to keep V/f (and, therefore, Bmax) constant. The wattmeter connected in the circuit indicates the iron loss Pr.
For a series of frequencies, with corresponding changes in applied voltage, the measured input power divided by frequency (Pi/f) and plotted against frequency will give a straight line, as shown in Fig. 10.23.
The intercept on the vertical axis will give the value of constant A while the slope of the line will give the value of constant B. Knowing the values of constants A and B the hysteresis and eddy current loss components for any given frequency can be determined.
Short-Circuit Test (or Impedance Test):
The purpose of this test is to determine full-load copper loss and equivalent resistance and equivalent reactance referred to metering side.
In this test, the terminals of secondary winding (usually of low-voltage winding) are short-circuited by a thick wire or strip or through an ammeter (which may serve the additional purpose of indicating secondary rated load current) and variable low voltage is applied to the primary through an auto- transformer or potential divider, as shown in Fig. 10.24. The transformer now becomes equivalent to a coil having an impedance equal to impedance of both the windings.
The applied voltage, Vs to the primary is gradually increased till the ammeter A indicates the full- load (rated) current of the metering side. Since applied voltage is very low (5-8% of the rated voltage) so flux linking with the core is very small and, therefore, iron losses are so small that these can be neglected. Thus the power input (reading of wattmeter W) gives total copper loss at rated load, output being nil. Let the readings of voltmeter, ammeter and wattmeter be Vs, Is and Ws respectively.
The above values are referred to the metering side (high voltage side in above case). If desired, the values could be easily determined referred to the other side.
Determination of Regulation of Transformer from Open-Circuit and Short-Circuit Tests:
Percentage regulation is given as:
Equivalent resistance R02 and reactance X02 referred to secondary can be determined from short-circuit test. I2 and cos ɸ are the load current and power factor (lagging or leading) of the load, so known. No-load secondary terminal voltage is equal to emf induced in the secondary E2.
Note:
Open-circuit test data are not needed for determination of voltage regulation of transformer.