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Difference Between Core Type and Shell Type Transformer

Difference between Core type and Shell type Transformer is as follows:


Core Type

Shell Type


The winding encircles the core.

The core encircles most part of the winding.


It has a single magnetic circuit .

It has a two magnetic circuit.


The core has two limbs.

The core has three limbs.


The cylindrical coils are used.

The multilayer disc or sandwich type coils are used.


The windings uniformly distributed on two limbs hence natural cooling is effective.

The natural cooling does not exist as the windings are surrounded by the core.


The coils can be easily removed from a maintenance point of view.

The coils can not be removed easily.


Core type construction  is preferred for low voltage transformers.

Shell type construction is preferred for high voltage transformers.

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Voltage Regulation of Transformer

When a transformer is loaded, with a constant supply voltage, the terminal voltage changes due to voltage drop in the internal parameters of the transformer i.e. primary and secondary impedances (resistances and inductive reactances). It is observed that the secondary terminal voltage drops from its no load value (E2) to load value (V2) as load and load current increases. The algebraic difference between the no-load and full-load terminal voltage is measured in terms of voltage regulation (known as voltage regulation of transformer).

The regulation is defined as change in magnitude of the secondary terminal voltage, when full load i.e. rated load of specified power factor supplied at rated voltage is reduced to no load, with primary voltage maintained constant expressed as the percentage of the rated terminal voltage.

Let,                                        E2 = secondary terminal voltage on no load

                                                V2 = Secondary terminal voltage at given load

then mathematically voltage regulation at given load can be expressed as,voltage regulation of transformerThe ratio of (E2 – V2/ V2) is called per unit voltage regulation.

The secondary terminal voltage does not depend only on the magnitude of the load current but also an the nature of the power factor of the load. If V2 is determined for full load and specified power factor condition the voltage regulation is called full load regulation.

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Practical Transformer

A practical transformer differs from the ideal transformer in many respects. The practical transformer has (i) iron losses (ii) winding resistances and (iii) magnetic leakage, giving rise to leakage reactances.

Iron losses

 Since the iron core is subjected to alternating flux, there occurs eddy current and hysteresis loss in it. These two losses together are known as iron losses or core losses. The iron losses depend upon the supply frequency, maximum flux density in the core, volume of the core etc. It may be noted that magnitude of iron losses is quite small in a practical transformer.

Winding Resistances

In practical transformer, each winding has some resistance. We can replace a practical transformer with an idealized transformer by including a lumped resistance equal to the winding resistance of series with each winding. R1 and R2, are the winding resistances of the primary and the secondary, respectively. The inclusion of the winding resistances dictates that:Practical transformer

  • The power input must be greater than the power output
  • The terminal voltage is not equal to the induced emf
  • The efficiency (the ratio of power output to power input) of a practical transformer is less than 100%.

Leakage Reactances

In an ideal transformer, alternating flux set up in the core and whole of this flux links with both primary and secondary windings. However, in an practical or actual transformer, both the windings produce some flux that links only with the winding that produces it.

Part of the flux, known as the leakage flux, does complete its path through air. Therefore, when both windings in a transformer carry currents, each creates its own leakage flux. The primary leakage flux set up by the primary does not link the secondary. Likewise, the secondary leakage flux restricts itself to the secondary and does not link the primary. The common flux that circulates in the core and links both windings is termed the mutual flux.Practical transformer 2The primary leakage flux is proportional to the primary current and secondary leakage flux proportional to the secondary current. The primary leakage flux produces self inductance L1  which in turn produces leakage reactance X1 ­. Similarly, secondary leakage flux produces leakage reactance X2

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Transformer on Load

When primary side of transformer is connected to a.c. source and secondary side of transformer is put on load then it is said that transformer is on load. transformer on loadWhen the secondary side of a transformer is loaded, secondary current I2 is flowing through and secondary winding. The magnitude of current I2 depends upon terminal voltage V2 and impedance of the load. The phase angle of the secondary current I2 with respect to V2 depends upon the nature of load to be connected on secondary side.

  • In case of resistive load current I2 is in phase with V2.
  • For inductive load current I2 lags behind the voltage V2.
  • For capacitive load I2 leads the voltage V2

Operation of Transformer On Load

The operation of the transformer on load is explained with help of four steps;


When the primary of transformer is connected to a.c. supply and secondary is open then it will draw no load current from the mains. The no load current I0 produces an mmf  N1Iwhich set up flux in the core.transformer on load 2Step – II

When the transformer is loaded, then secondary current I2 is flowing is secondary winding. I2 produces an mmf N2I2. This mmf further produces Φ2. This flux opposes the flux which is set up by the current I0 according to Lenz’s Law.

transformer on load

Step – III

Since Φ2 opposes the flux therefore, the resultant flux tends to decrease and causes the reduction of self induced emf E1 momentarily. Thus V1 predominates over E1 causing additional primary current I’2 drawn from the supply mains. The amount of this additional current is such that the original conditions i.e. flux in the core must be restored to original value Φ, so that V1 = E1. The current I1 is in phase opposition with I2 and is called primary counter balancing current. This additional current I’1 produces an mmf N1I’1 which sets up flux Φ, in the same direction as that of Φ, and cancels the flux Φ2 set up by N2I2.transformer on loadStep – IV

Thus we have seen that flux is restored to its original value Φ.transformer on loadPhasor Diagram of a Loaded Transformerphasor diagram of transformer on load


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Transformer on No-Load

When the primary side of a transformer is connected to the source of alternating current supply and secondary side is kept open, it is said to be transformer on no-load i.e. there is no load on secondary side. The secondary current I2 is thus zero. In this case, neither the secondary winding has any effect on the magnetic flux in the core nor it has any effect on the primary current.

transformer on no-load

In actual transformer, the losses cannot be neglected. Therefore, if transformer is on no load, a small current I0  called exciting current drawn by the primary. This current has to supply the iron losses (eddy current and hysteresis losses) in the core and a very small amount of copper loss in the primary. As discussed, no current flows in secondary side, so that secondary copper losses are neglected.

Therefore, current I0 lags behind the voltage vector V1 by an angle Φ0 which is less than 900. The angle of lag depends upon the losses in the transformer. The no-load current I0 has two components;

Active or Working Component

This component of current is represented by Iw and in phase with the applied voltage V1. Its function is to overcome the eddy current and hysteresis loss in the core of transformer, secondly a small copper loss I2 in the primary winding. It is also called wattfull component of no load current.

Iw = I0cosΦ0

Reactive or Magnetising Component

This component of I0 is represented by Im and produces alternating flux in the core. This component does not consume any power. Magnetising component of current Im is in phase with flux, so lags the voltage V1 by π/2. It magnetises the core. It is also called wattless component of no-load current. The no-load current I0 is small of the order of 3 to 5 percent of the rated current of the primary. Due to the eddy current and hysteresis loss, the current I0 in primary is not lagging V1 by 900  

Im = I0sinΦ0

From the phasor diagram, when the transformer on no-load

phasor diagram of transformer on no-load

No-load current,

Primary power factor at no-load

Core loss, P0 = V1I0 cosΦ0 = V1Iw watts

Magnetising (reactive) volt amperes = V1I0sinΦ0 = V1Im volt amperes

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Instrument Transformers

For measuring a large current is a d.c. circuit, we use low-range ammeter with suitable shunt. The measurement of high d.c. voltage is made using a low-range voltmeter with a multiplier.

However, shunt and multiplier is not used for the measurement of high alternating currents and voltages respectively for many good reasons.

In order to measure high alternating currents and voltages, we employ specially constructed accurate ratio transformers, called instrument transformers.

There are two types of instrument transformers;

  1. Current Transformers (CT)
  2. Potential Transformers (PT)

Current transformers are employed in the ac circuits where the measuring current exceeds the safe current of measuring instruments.

Potential transformers are employed in the ac circuits where the measurement voltage exceeds 750 volt.

  1. Current Transformers (CT)

Current transformers (CT) are used to measure high alternating current in a power system. The current transformers are basically step-up transformers. The primary winding having one or a few turns of thick wire is connected in series with the line whose current is to be measured. The secondary consists of a large number of turns of fine wire and connected across the ammeter.INSTRUMENT TRANSFORMERS CURRENT TRANSFORMERAs regards voltage, the transformer is of step-up variety but it is obvious that current will be stepped down. Thus, if the current transformer has primary to secondary current ratio of 100:5, then it steps up the voltage 20 times whereas it steps down the current to 1/20th of its actual value.

The working of current transformer is slightly different to that of an ordinary power transformer. In case of current transformer, the ‘Burden’ on the secondary is very small, therefore, it is considered to be short circuited. Hence, current transformer works under short circuit conditions. Moreover, the current in secondary windings is not governed by its load impedance rather it depends upon the current flowing through the primary winding.


From construction point of view, there are two types of current transformers which are commonly used in laboratories and panels. There are two types of current transformers (i) Clamp-on or clip-on type and (ii) Bar type.

  • Clamp-on or clip-on type current transformer: One of the most commonly used current transformers is the one known as clamp-on or clip-on type. It has laminated core which is so arranged that it can be opened out at hinged section by merely pressing a trigger like projection when is core is thus opened, it permits the admission of very heavy current carrying bus bars or feeders whereupon the trigger is released and the core tightly closed by a spring. An ammeter is connected across the secondary winding of the transformer which measures the current flowing through the conductor directly. It is a portable instrument and generally used in the laboratories for testing purposes.


  • Bar type current transformer: It has a circular ring type core over which secondary is wound. An ammeter is connected across the secondary. When a bar conductor or bus bar is inserted through it, the ammeter measures the current flowing through bar conductor directly. These are generally used with the instruments placed on panels or used with protective relays.


  1. Potential Transformers

The potential transformers are basically step down transformers. The connections of a voltmeter when used in conjunction with a potential transformer for measurement of high ac voltages. The voltage to be measured is applied across the primary winding which has a large number of turns.


 In general, they are of the shell-type and do not differ much from the ordinary two-winding transformers, except that their power rating is extremely small. Upto voltage of 5,000V, potential transformers are usually of the dry type, between 5,000 and 11,000 volts, they may be either dry type or oil immersed type, and although for voltages above 11,000V or 11KV they are always oil immersed type.

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An auto-transformer is a transformer with only one winding wound on a laminated core. A part of winding is common to the both primary and secondary circuits.

In a two winding transformer, primary and secondary windings are electrically isolated, but in a auto-transformer the two windings are not electrically isolated.


The winding is wound on laminated silicon steel. Laminations are used to reduce the eddy current losses, whereas, hysteresis loss is reduced by using silicon steel.

According to construction point of view, auto-transformer is divided into two types. In one type of transformers, there is a continuous winding with taps brought out at convenient points determined by the designed secondary voltage. And in other type of auto-transformer, there are two or more different coils which are electrically connected to form a continuous winding. Enamelled copper is used for winding.

A simple arrangement of a step-down auto-transformer is shown in figure, where N1 and N2 are the number of turns between winding AC and winding DC respectively.


At no-load

When we apply a AC voltage to the primary side of a auto-transformer, the exciting current flows from A to C and it establishes a working m.m.f. directed vertically downward in the core.

At load

When switch S is called, the current in winding BC must flow from C to B, in order to create an m.m.f. opposing the exciting or working m.m.f. , according to Lenz’s Law. Since the working m.m.f. in a transformer remains substantially constant at its no-load value, the primary must draw additional current I, from the source, in order to neutralize the current of ICB. In winding AC, I flows from A to C, whereas in winding BC I2 flows from C to B.

I3 = I2 – I1

m.m.f. of winding AB = I1 (N1 – N2)

                                         = (I2 – I1) N2

                                        = I3N2 = m.m.f. of winding CB

Transformed VA = VABIAB = (V1 – V2) I1

Total input VA to transformer = V1I1 = output VA

Conducted VA = total input VA – transformed VA

                            = V1I1 – (V1 – V2)I1 = V2I1

Neglecting internal impedance drops and losses

Saving of Copper Cu

Volume and hence weight of Cu, is proportional to length and area of the cross-section of the conducting materials or conductors. Now, length of the conductors is proportional to the number of turns and cross-section depends on current. Hence weight is proportional to the product of the current and number of turns.

∴  Weight of conductor for winding AB ∝ (N1 – N2) I1

Winding BC carries a current (I2 – I1) and has N2 turns

∴ Weight of conductor for winding BC  ∝ (I2 – I1) N2

­Hence, total weight of conductor in an auto-transformer is

∝ I1(N1 – N­­2) + N2 (I2 – I1)

∝ 2 (I1N1 – I1N2)

∝ 2 (N1 – N2) I1

If two winding transformer were to perform the same duty, then

Weight of copper on its primary ∝ N1I1

Weight of copper on its secondary ∝ N2I2

∴  Total weight of conductor in a two-winding transformer

∝  N1I1 + N2I2

∝ 2 N1I1

Weight of conductor in auto-transformer = (1-k)(weight of conductor in two-winding transformer)

Saving of conductor material if auto-transformer is used = k X Conductor weight in two-winding transformer.

If k = 0.1, saving of conductor material is only 10% and for k = 0.9, saving of conductor material is 90%. Hence the use of auto-transformer is more economical only when the voltage ratio k is more nearer to unity.

Advantages of Auto-transformers

  1. As discussed above, an auto-transformer require less copper than a two winding transformer of the same ratings.
  2. An auto-transformer has smaller size than a two winding transformer.
  3. An auto-transformer has better efficiency than a two winding transformer of the similar rating.
  4. Auto-transformer requires less exciting current than a two winding transformer of the same rating.
  5. An auto-transformer has better voltage regulation than a two winding transformer of the same rating.

Disadvantages of Auto-transformers

  1. It can be operated on light loads.
  2. The short circuit current is much larger than that for the two winding transformer of the same rating.

Applications of auto-transformers

  1. Auto-transformer are used to give a small boost to a distribution cable to the correct voltage drop.
  2. These transformers are used as auto-starter transformers to give upto 50 to 60% of full voltage to an induction motor during starting.
  3. Auto-transformers are used in control equipment for 1-phase and 3-phase electrical locomotives.
  4. These transformers are used as interconnecting transformers in 132KV/400KV system.

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Conditions For Parallel Operation of Transformers

There are certain definite conditions which must be satisfied in order to avoid any local circulating currents and to ensure that the transformers share the common load in proportion to their rated KVA. These conditions are:

  1. The transformer should have same transformation ratio e. the voltage ratings of the primaries and secondaries must be identical.
  2. The transformer should be properly connected with regard to polarities. If this condition is not fulfilled, e. the e.m.f. in the secondary windings of the transformers which are parallel with incorrect polarity will act together in the local secondary circuits and produce the effect equivalent to a dead short circuit.
  3. Primary windings of the transformers should be suitable for the supply system voltage and frequency.
  4. In case of 3-phase transformers, the two transformers must have the same phase-sequence, i.e. the transformers must be properly connected with regard to their phase-sequence.
  5. In case of three phase transformers, the two transformers must have the connections so that there should not be any phase difference between the secondary line voltages i.e., a delta star connected transformer should not be connected with a delta-delta or star-star connected transformer.

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Parallel Operation of Transformers

Parallel Operation of Transformers

For supplying a load in excess of the rating of an existing transformer, a second transformer may be connected in parallel with it.

When the primaries and secondaries of the two or more transformers are connected separately to the same incoming and outgoing lines to share the load, the transformers said to be connected in parallel.

parallel operation of transformers

It is seen that primary windings are connected to the supply bus bars and secondary windings are connected to the load bus-bars. In connecting two or more than two transformers in parallel, it is essential that their terminals of similar polarities are joined to the same bus-bars.

Necessity of Parallel Operation

  1. If the amount of power to be transformed is greater than that which can be handled by one transformer, it becomes necessary to employ two or more units in parallel.
  2. When the load on the transmission lines increases beyond the capacity of the installed transformer. To overcome this problem one way is to replace the existing transformer with the new one having larger capacity and the other way is to place one more transformer is parallel with the existing one to share the load. The cost of replacing the transformer is much more than placing another one in parallel with the existing one.
  3. The cost of standby unit is reduced since spare transformer units are invariably required to ensure continuity of service in case of damage or in the event of fault, it is sometimes found desirable to supply the load through two or more units and thereby to reduce the size of the spare unit.

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Questions and Answers on Single Phase Transformers

  1. What is a transformer?

Ans: – A transformer is a static device which transfers electric power from one circuit to another circuit at same frequency but voltage level is usually changed.

  1. What are step-up and step-down transformers?

Ans: – When the voltage level is raised on the output side, the transformer is called step-up transformer, whereas, the transformer in which the voltages is lowered on the output side, is called a step-down transformer.

  1. Which are the two winding are present in a transformer?

Ans: – Primary and secondary windings

  1. Transformer is called static electrical machine. Why?

Ans: – There is no rotating part in the transformer, therefore, it is called static electrical machine.

  1. What are the functions of transformer oil?

Ans: – In provides additional insulation and protects the insulation from dirt and moisture and it carries away the heat generated in the cores and coils.

  1. Why an iron or steel core is provided in a transformer?

Ans: – To ensure a high permeability of the magnetic circuit. Because of high permeability, the magnitude of exciting current necessary to create the required flux in the core is small. The presence of steel core also causes hundred per cent of magnetic flux.

  1. What is the application of a isolation transformer?

Ans: – An isolation transformer has equal turns in primary and secondary windings. These transformers are employed for isolating the load from supply.

  1. What is the order of magnitude of no-load current?

Ans: – No-load current is transformers ranges from 2 to 5 per cent of full-load primary current.

  1. How is magnetic leakage reduced to a minimum in commercial transformers?

Ans: – By interleaving the primary and secondary windings.

  1. Define voltage transformation ratio.

Ans: –The ratio of secondary voltage to primary voltage is called a transformation ratio. It is denoted by capital letter K.

  1. From the construction point of view, name different types of transformers?

Ans: – According construction point of view, the transformers are divided into two types: (i) Core type and  (ii) Shell type.

  1. In practice, what determines the thickness of the stampings?

Ans: – Frequency.

  1. Why the core of a transformer is laminated?

Ans : – To reduce eddy current loss, the core of the transformer is laminated.

  1. What are the functions of transformer oil?

Ans: – Transformer oil’s primary functions are to insulate and cool a transformer.

  1. What is the cause of noise in a transformer?

Ans: – In a transformer, noise occurs mainly due to loosening of stampings and mechanical forces developed during operation.

  1. Which test gives the copper loss of a transformer ?

Ans: – Short circuit test.

  1. Tap changers are usually employed on H.V. side of a transformer, why?

Ans: – Tap changers are usually employed on H.V. side because, the high voltage side has a large number of turns which allows smoother variation of voltage. It is easily accessible physically and above all, it has to handle low currents.

  1. What would happen, if a power transformer designed for operation on 50Hz, were connected to 5Hz source of the same voltage?

Ans: – The power transformers are designed to operate at 50 Hz. On the other hand, if the primary is connected to a source of 5 Hz frequency, the primary winding will have an insufficient inductive reactance (XL = 2πfL). The result will be that, the primary will have excessive current producing considerable copper losses. There is every possibility that the transformer may start to smoke.

  1. What would happen, if a power transformer designed for operation on 50Hz, were connected to 500 Hz source of the same voltage?

Ans: – The power transformers are designed to operate at a particular frequency, generally at 50 Hz. If the frequency of the supply will be high, it will result greater inductive reactance. This high inductive reactance of the winding will prevent the primary from drawing sufficient power. Moreover, the iron losses i.e. hysteresis and eddy current losses will be excessive.

  1. Silica gel placed in the transformer breather. Why?

Ans: – Silica gel is placed in the transformer breather. The function of the silica gel is to absorb the moisture so that the life of the transformer can be increased.

  1. What is the thickness of laminations?

Ans: – 0.35mm to 0.5mm

  1. Why silicon content in electrical sheet steel is limited to 4.5% to 5%?

Ans: – Silicon content exceeding 5%   makes the sheet steel brittle and so causes in punching.

  1. Why LV winding is placed first on the core and then HV winding in the case of a core type transformer.

Ans: – Placing of LV winding near the core and HV winding around the LV winding in a core type transformer reduces the amount of insulation material required.

  1. Why circular coils are always preferred over rectangular coils for winding in a transformer?

Ans: – Circular coils are preferred for winding a transformer as they can easily wound on machines, conductors can easily be bent and winding does not bulge out due to radial forces developing during operation.

  1. Does the transformer draw any current when its secondary is open?

Ans: – Yes, no load primary current.

  1. Why are iron losses constant at all loads in a transformer?

Ans: – Since the induced primary ampere-turns and secondary ampere-turns always neutralize one another, the flux in the core on load is the same as the flux on no-load. Hence, the iron losses are constant and are independent of load.

  1. Where is shell-type construction suitable for a transformer?

Ans: –In shell type core, both the windings are wound on the central limb. This type of core is used for those transformers which work on poor power factor.

  1. What are the advantages of open and short-circuit tests on a transformer?

Ans: – The efficiency of a transformer is always determined by open-circuit and short-circuit tests due to the following reasons: (i) The power required to carry out these tests is very small. (ii) The tests give the core loss and copper loss separately.

  1. What are the types of bushing used in transformer terminals?

Ans: – (i) Porcelein insulator bushing (ii) Oil filled bushing (iii)  Capacitor bushing

  1. Why is it advantageous to make short circuit test on the high voltage side rather than on low voltage side?

Ans: –  It is due to the fact that the ranges of the meter at the high voltage (HV) side at the short circuit condition of the low voltage (LV) side is much more suitable.s

  1. What is tapped transformer?

Ans: – A tapped transformer is one whose windings are fitted with special taps for changing in voltage or current ratio.

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