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Why transformers and alternators rated in KVA, not in KW?

Usually we have seen that transformers and generators are rated in KVA instead of KW. We Know that KVA is a product of voltage and current and KW is a product of voltage, current and power factor. For better understanding of this topic first you have to know there are three types of loads Resistive, Inductive and Capacitive. At the of designing of transformer and alternator designer don’t know the nature of load whether it is resistive, inductive or capacitive. There is a term power factor cosΦ that multiplied with KVA and make it KW. 

Simply power factor cosΦ is defined as the cosine of the angle between voltage and current. Larger the angle between voltage and current, greater current drawn by the machine which results in losses are increased. 

From the above explanation, we have seen with change in current copper losses are changed. Copper losses are directly proportional to the square of current. Second quantity is voltage, voltage causes iron losses in the machine.

We have concluded it Copper losses ( I²R)depends on Current which passing through transformer winding while Iron Losses or Core Losses depends on Voltage. So the Cu Losses depend on the rating current of the load so the load type will determine the power factor P.F ,Thats why the rating of Transformers and alternators in kVA,Not in kW.

Energy transfer device such as transformer and energy generated device such as alternator are rated in KVA. Motors are rated in KW

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Power Transformer and Distribution Transformer

Power Transformer and Distribution Transformer

Transformer plays an important role in the area of electrical engineering. Transformer is used either for raising or lowering the voltage of ac supply. It is used at electrical energy generating station where basically it raise the voltage level of an ac supply and it is also used at substations where it is used to lowering the voltage at suitable level. In power engineering it occupies a very important place.In this article, we will study about power transformer and distribution transformer. The power transformer are located at power generating stations whereas the distribution transformers are installed in the localities of the city.


The power transformers are installed at the sending and receiving end of the high voltage transmission lines whereas the distribution transformers are installed at the near to the load centre to provide utilization at the consumers premises. Distribution transformers are basically pole mounted.


Power transformers are usually operated at full load or nearly full load in simple words we can say that the power transformer gives high efficiency at full load or nearly full load. On the other hand distribution transformers are operates at light load during major part of the day.


The rating of a power transformer is very high in the orders of MVA and power transformer generally rated above 100MVA. The rating of distribution transformer is smaller and distribution transformer are generally less than 100MVA.


The insulation required in case power transformer is more in comparison to the distribution transformer because the power transformer generally operates at very high voltage in order of 400KVA, 220KVA, 132KVA etc.


The power transformer is basically larger in size as comparatively distribution transformer.

Copper Losses and Iron Losses

The power transformers are operated at nearly full load so iron losses are less so the power transformer are designed to have low copper losses. However, the load on the distribution transformer vary time to time so these transformer designed to have low iron losses.


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Transformer on No-Load

Transformer on No-Load

Transformer on No-Load transformer, there is no iron losses and copper losses. The ideal transformer is not practically possible. In case actual transformer these losses are present. When an actual transformer put on load, there is iron loss in the core and copper loss in the windings. When secondary of the transformer is kept open and primary winding is connected to alternating source, the transformer is called transformer on no-load.

When primary is connected to source and transformer is on no load, the transformer draws some current which is not wholly reactive. The primary input current under no load conditions has to supply iron losses in the core and copper losses in the windings (both primary and secondary).


When transformer is connected to source. It draws no load current IO at voltage VI. The current lags the voltage by angle φO which is less than 900. No load current IO has two components. One component is called Active or Working Component and second component is called Reactive or Magnetizing Component.

transformer on no-load

Active or Working component IW is in phase with input voltage V1. Active component supplies the iron loss and small quantity of copper loss. This component is also known as wattfull component or iron loss component.

phasor diagram of transformer on no load

IW = IO Cos φO

Reactive or Magnetizing component is in quadrature with VI. The Function of reactive component is to sustain the alternating flux in the core. This component is also known as wattless component because it does not consume energy.

      Im = IO Sin φO           


  1. The no-load primary current IO is very small in value as compared to the full load primary current. It value may be 2 or 3% of the full load primary current.
  2. The value of input primary current IO is very small. Hence, copper loss is negligibly small. It means input power is equal to the iron loss in the transformer.

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Three Phase Transformers

Three Phase Transformers

For Generation of electrical power three-phase alternators are employed at the power generating station. Usually large power is generated at high voltage. Transmission is generally accomplished at higher voltage of 132KV, 220KV, 400KV, and 765KV. For which purpose 3-phase transformers are necessary to step up the generated voltage to that of the transmission line. At load centre, these voltages are stepped down to 66KV, 33KV, or 11KV, for distribution purpose. For distribution purpose 3-phase transformer also required. So that three phase transformers plays very important role in transmission and distribution of electric power.

Advantages of Three Phase Transformers

  • It is light in weight.
  • It occupies less floor space.
  • Its cost is less than 3 single phase transformers of equal rating.

Disadvantages of Three Phase Transformer

The main disadvantage of this transformer is that one of the phases becomes faulty, and then whole of transformer is to be replaced.

Three Phase Transformers Connections

The windings of three phase transformers may be connected in Y(star) or Δ(delta) in the same manner as for three single phase transformers. There are various type of 3-phase transformers available according to arrangement of connections. The most common connections are given below.

  1. Star – Star (Y – Y) connection.
  2. Delta – Delta (Δ – Δ) connection.
  3. Star – Delta (Y – Δ) connection.
  4. Delta – Star (Δ – Y) connection.
  5. Open – Delta (V – V) connection.
  6. Scott connection or T-T connection.

Star – Star (Y –Y) connection

This connection is most economical for small, high-voltage transformers. The insulation required is minimum for these connections. It given line voltage  times phase voltage. There is no phase shift between primary and secondary voltage.

Delta – Delta (Δ – Δ) connection

This connection is used for large current and low voltage. The conductor is required of smaller x-section as the phase current is 58% of the line current.

Star – Delta (Y – Δ) connection

This connection is used at the substation end of the transmission line where the voltage is to be stepped down. There is a 300 shift between primary and secondary line voltages.

Delta – Star (Δ – Y) connection

This connection is used at generating station because voltage is generated at low level and it need to be stepped up for transmission purpose. So that, it is used at the beginning of the high tension transmission system. This connection is also used in distribution because it give neutral point at secondary side. It is useful for 3-phase 4-wire service. It can be used to serve both the 3-phase machine and single phase lighting circuits.

three phase transformers connections delta star

Open – Delta or V – V connection

If one of the phase is removed and 3 – phase supply is connected to the primary of the transformer. In this case 3 – phase voltage will appear at secondary. The arrangement is called open-delta or V-V connection.

open delta connections

Scott Connections

The conversion of three phase transformer into single phase or two phase such available transformer is called scott connection transformer.

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EMF Equation for Transformer

EMF Equation for Transformer

For drive an emf equation for transformer we will consider the case of an ideal transformer. An ideal transformer one which have no copper loss(I2R losses) and magnetic leakage flux. In other words, we can say that an ideal transformer consists of windings which have zero ohmic resistance and loss-free core.

In previous article, we have studied about basics of transformer such as working principle of transformer and necessity or need of transformer at generating power station, substations etc. In this article we will review the last article, by considering ideal case of transformer.

Consider an ideal transformer, whose secondary is open and primary is connected to alternating supply source. The alternating current flows in the primary winding. Since coils of primary winding is purely inductive and it draws magnetizing current only which is necessary to set up magnetic flux only. This magnetizing current is small in magnitude and lags primary voltage V1 by 900. The magnetic flux Φ sets up in primary circuit links with secondary circuit. According to faraday’s laws of electromagnetic induction induced emf is produced in secondary winding. If the secondary winding is closed current start flowing through the load.

Let the sinusoidal variation of flux Φ be expressed as

Φ = Φmax sinωt

emf equation for transformer

Φmax = maximum value of flux in webers

ω = angular frequency in rad/sec

The emf e1 induced in primary winding by the alternating flux Φ is given by

E1max  = N ω Φmax

Primary induced emf

E1max  = N1 ω Φmax

E1max = 2⊓ fN1 Φmax Volts.

According to above equation

I. Induced emf is directly proportional to no. of primary or secondary turns.

II. Induced emf is directly proportional to rate of change of flux linkage.

III. Induced emf is depends upon supply frequency.

Voltage Transformation Ration (K)

It is the ration of secondary voltage to the primary voltage.


Secondary induced emf to the primary induced emf.


Secondary number of turns to the primary number of turns.


Primary current to the secondary current

K is called transformation ratio of the transformer.

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Transformer on Load

Transformer on Load

When some load is connected across the secondary of the transformer, then it is said to be transformer on load. The current I2 flows through load and secondary winding. The magnitude of current I2 depends upon the terminal voltage V2 and impedance of load. The angle between current I2 and voltage V2 is depends upon the nature of load. Whether it resistive, inductive and capacitive. In this article, we will study about the transformer when it is loaded or certain load is connected to the secondary side. We will explain it step by step.

When certain load is connected to secondary side of the transformer. It draws no load current I0. The no load current I0 produces and mmf N1I0 which sets up magnetic flux in the core, shown in the figure.

transformer on load

The secondary current I2 sets up it own mmf (N2I2) and hence flux Φ2. This set up flux oppose the main primary flux Φ which is setup by no load current I0.

As secondary flux Φ2 oppose the primary flux, therefore the resultant flux also decreases and cause in reduction in self induced emf E1. This results the transformer draws additional current I1′  from supply main and flux in the core restored to its original value. So that V1 becomes equals to E1. The additional current draws by the primary winding is called counter balancing current. The additional current I1 produces and mmf N1I1 which sets up the flux Φ which is same in the direction of primary current and cancels the flux Φ2.

The flux produce secondary current is neutralized by flux produce by current I1. The flux produce secondary current is neutralized by flux produce by current I1. The total primary current I1 is the vector sum of current I0 and I1.

I1 = I0 + I1


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Transformer Tests

Transformer Tests

Transformer tests ( such as Open circuit and short circuit test ) are performed to determine the transformer circuit parameter (RO and XO), efficiency and voltage regulation. Without actually loading the transformer, there tests are very convenient and economical.

Open Circuit Test

Open circuit test is conduct to determine the core loss, Pi and no-load current IO.

Open circuit test is always performed on low voltage side and secondary side or high voltage side is kept open circuited.

To measure no load current, no load input power and applied voltage instruments are connected in primary side are ammeter, voltmeter and wattmeter normal rated voltage V1 is applied to the primary winding.

Small no load current IO start flows through the winding. The value of this primary current is very small, usually 2% to 10% of full load current.

Copper loss is very small in primary and nil in secondary. The wattmeter reading indicates the core loss under no-load conditions.

poen circiut transformer tests

The open circuit test is also called no-load test.

Short Circuit Test

Short circuit test is carried out to determine copper losses.

Copper losses are required for the calculations efficiency of the transformer.

Short circuit test is usually carried out on the high voltage side of the transformer and low voltage side is short circuited by a thick strip. A variable voltage is applied across the low voltage side (say primary winding). The input voltage is gradually increased till at voltage VSC, full load current II flows in the primary.

The test is performed low voltage so that the value of core losses is very small with the result the wattmeter reading show the full load copper loss or I2 R losses for the whole transformer.

Under short-circuit condition, there is no output from the transformer. Therefore all input is dissipated in loss and this loss is almost copper loss because the value of iron loss or core loss is very small.

It may be neglected for performing short circuit test the instruments such as ammeter, voltmeter and wattmeter are connected on high-voltage side.

short circuit transformer test

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The Transformer is a static device that transfers electrical energy from one electrical circuit to another electrical circuit without change in the frequency but voltage level is usually changed. It may be stepup or step down transformer.

stepup transformer

The transformer has two winding one is called primary winding and other is called secondary winding.

  • The source is connected to the primary winding.
  • The load is connected to the secondary winding.
  • The transformer is an electromagnetic energy conversion device, since the energy received by the primary is first converted to magnetic energy and then it is reconverted into electrical energy in the secondary circuit. Thus the primary and secondary winding of a transformer are not connected electrically, but are coupled magnetically.
  • The transformers may be stepped up or stepped down. If the secondary winding has more turns than the primary winding, then the secondary voltage transformer is called a stepup transformer. If the primary winding has more turns than the secondary winding, then the secondary voltage is lower than the primary voltage and the transformer is called a step down transformer.

Necessity of Transformer

The electrical power is generated at 11kv. For economical reasons, ac power is transmitted at very high voltages over long distances. Therefore, a step up transformer is used at generating station to raise the voltage level. Then voltages are stepped down to suitable level by a transformer at various substations. Ultimately for utilization of electrical power, the voltage is stepped down to 400/230 for safely reasons.


Transformer is a static device, owing to the lack of rotating parts. There are no friction or windage losses, so that the efficiency of a transformer is high. Typical transformer efficiencies at full load lie between 95% and 98%.

Transformer on DC

A transformer cannot operate on dc supply and never be connected to a dc source. If the primary winding of a transformer is connected to a dc supply mains the flux produced 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. There will be no induced emf in the primary winding and therefore, a heavy current will be drawn from the supply mains which may result in the burning out of the transformer winding.

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Cooling of Transformers

Cooling of Transformers

When transformer is connected to the supply, some heat is always produced due copper losses (or I2 R losses) and core losses (iron losses). The output of the transformer limited by the rise in temperature. So, there becomes a necessity to cool down the transformer for their normal operation. The cooling of transformers of transformers cooling is necessary.

In rotating electrical machines, cooling fan provided on their shaft. When the machine rotates and thus causes the circulation of our and hence provides cooling. But Transformer is a static device. Therefore, it is difficult to cool down the inner parts of a transformer. The removal of heat from the transformer is called cooling.

The different methods of cooling the transformer are given below

Methods of cooling of transformers

Air Natural Cooling

Small transformer of a rating 10 to 15 KVA is cooled by the circulation of natural air through the core and transformer winding. The heat generated in the transformer dissipated by conduction, convection and radiation.

Oil Immersed Natural Cooling (ON TYPE)

In this case, the assemble of core and winding of a transformer is placed in a tank filled with oil known as transformer oil or insulating oil. The insulating oil or transformer oil perform two function.

  • It provides necessary insulation to the winding.
  • It helps to cool the transformer. It takes the heat from the core and winding and by natural circulation of the oil it cool down the transformer.

cooling of transformers

The hot oil become lighter in weight and goes up from where comes down through the pipes to the bottom of the tank after cooling.

Oil Immersed Forced Oil-Circulation Natural Cooling (OFN)

In this type, transformer core and windings are immersed in transformer oil and oil is circulated through the transformer with the help of pump and heat extracted by the oil from the core and winding and gives it to the tank surface from where the air takes away the heat.

Air Blast Cooled Transformer (AB TYPE)

In this method of cooling, forced air flows through the core and winding. The transformer is placed over an chamber and air is maintained under pressure with the help of compressor.

Oil Immersed Water Cooled Transformer (OW TYPE)

The rating above 500KVA forced water cooling is employed. In this method of cooling, the core and transformer winding is placed in transformer oil and cold water passed through the pipe kept in the transformer oil. The cold water absorbs the heat from the oil and caries away the heat of the oil.

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Autotransformer | Construction Advantage and Application

Working or Operating Principle and Construction of Autotransformer

The operating principle of autotransformer is same as two winding transformer. It  is one winding transformer i.e. the primary and secondary windings are inter-related.The construction of autotransformer is totally differ from conventional two winding transformer. It is a single winding transformer which is common for primary and secondary side.  There is no electrical isolation between primary and secondary windings. In a conventional two winding transformer, the primary and secondary windings are completely insulated from each other but are magnetically coupled by a common core. But in autotransformer primary and secondary windings are connected electrically as well as magnetically. Its working is same as conventional two winding transformer.

Show the connections of step up autotransformer and step down autotransformer. In case of step up transformer, winding PQ having N1 turns is the primary winding and winding QR having N2 turns is the secondary winding. In case of step down transformer winding QR having N1 turns is primary winding and winding PQ having N2 turns is the secondary winding. In case of this type of transformer the power transferred from primary to secondary conductively as well as inductively (transformer action). In an autotransformer primary and secondary voltages related in the same way as in a conventional two winding transformer. Diagram of auto transformer is given below

autotransformer construction

The autotransformer looks like a resistance type potential divider. But its operation it quite different from it. The autotransformer can be stepped up or stepped down the voltage level. But in case of potential divider it is not possible. In case of potential divider power loss occurs whereas it is not so in case of autotransformer. Voltage regulation also poor in case of a potential divider. Less conductor material is required in case of auto transformer as compared to two winding transformer. The cross sectional area of conductor is proportional to the current to be carried and length is proportional to number of turns. Here only one winding is used in autotransformer so that length is required less and hence saving in conductor material. An auto transformer used as a variac.


The reduction in conductor material also reduces the core length. It means less core size is required in case of auto transformer.

It is cheapest than two winding transformer. In case of auto transformer saving in both conductor and core material which reduce the cost.

The losses are low in case of auto transformer as comparatively two winding transformer. Less losses results in high efficiency.


It is used as a starter for induction motor.

It is used in electrical testing laboratory.

It is used as booster to rises the voltage level.

It is used in locomotives for control equipment.

It is used as furnace transformers for getting a suitable supply from normal 230V supply.

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