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Short circuit test of transformer

Short circuit test of transformer

Transformer Turns ratio test

Transformer Turns ratio test

Transformer Testing

Transformer Testing

Phasor diagram of transformer

Phasor diagram of transformer

Transformer equivalent circuit

Transformer equivalent circuit

What is a transformer ?

Every electrical network contains a lot of transformers but do you ask your self about what is transformer ? and why we use transformer ?

In simple words transformer is static device (Don’t have any moving parts) which electric power in one circuit is transformed into electric power of the same frequency in another circuit with the physical basic of  Mutual induction between two circuits ..

Also Electrical transformer is a device which is used mainly for transforming electrical energy. It is able to change the voltage from high to low one (and otherwise). This enables the transportation of the electricity more easily and efficiently.

What is transformer construction?

The transformer is generally composed of two circuits:

1. The electrical circuit

2- the magnetic circuit

  • where the electrical circuit consists of two winding, The primary winding that connects to the source and the secondary winding  that connects the loads … these winding may be copper (common) or aluminum (rarely)
  • The magnetic circuit
    It consists of metal sheets made of high quality silicon steel, where both winding are wrapped

the core is built of metal  sheet steel lamination assembled to provide continuous  magnetic path with a minimum of air-gap
included. The eddy current loss is minimized to reduce heat by laminating the core ..

the core has two types:

1.Core-type :

single phase transformer                                core type transformer

  • The magnetic flux path is of one loop and this loop connects the primary and secondary winding together magnetically .
  • it uses at small transformer because of its flux losses through core …
  • practically Primary winding and secondary winding are wrapped around each other

core type transformer

2. Shell type:

shell type transformer                          shell type transformer

  • The magnetic flux path has two loops
  • it uses at large transformer because of less of its flux losses through core


This fig. illustrate two types of transformer cores

types of cores of transformers

In the shown figure we notice this theory of transformer as it consists of two inductive coils which are separated electrically but linked through magnetic flux .

transformer construction

Function of transformer ?

As I mention before that transformer contains two coils primary and secondary .. if the number of primary coils less than the number of secondary coils that’s mean this transformer is step up transformer .. this transformer can step up voltage according to turns ratio between primary and secondary coils .. and so current is decreased at secondary side as the relation between voltage and current is “The voltage is inversely proportional to the current”  … from this brief We conclude the main role of transformer

The main function of transformer is to decrease currents through Transmission process of electricity from production location to distribution locations


Its economical to decrease current through transmission process because:

  • as current decrease .. cross section of transmission lines decrease
  • losses in transmitted power decrease .. P = I² * R .. as power losses depends on current
  • Voltage drop in transmission lines decreased as voltage drop depends on current ..
  • decrease Size of power transmission towers because transmission lines have small cross section area

.. and after transmit this power  to distribution location we use step down transformer  to decrease voltage and distribute to customers


 Types of transformers ?

transformers can classified into many categories, These classifications based on:

classifications According to function :

classifications According to Type :

  • Power Transformer
  • Distribution Transformer
  • Auto transformer
  • Instrument transformer

classifications According to phases:

  • Single phase transformer
  • Three phase transformer

classifications According to insulation :

  • Oil immersed transformer
  • Dry type transformer (depend on vans to move air)
  • SF6 (Stable gas) Transformer

Equation of  ideal transformer:

what does we mean by ideal transformer ? we mean transformer at no losses condition and unfortunately it does not occur practically.

1. turns ratio of transformer

Assume that :

N1 : number of  primary winding turns

transformer construction
transformer construction

N2 : number of secondary winding turns

V1: Voltage at primary side (source side)

V2: voltage at secondary side (load side)

In this case (single phase ideal transformer)

V1/V2 = N1/N2 = k

Such as “k” called turns ratio .. from above equation we notice that:

If N2 > N1 … the transformer is step up transformer that voltage at primary less than secondary side

If N1 > N2 … the transformer is step down transformer that voltage at primary greater than secondary side

Also for ideal transformer (that means no losses) ..

Input Power = Output power

P1(input power) = V1 * I1  , P2(output power) = V2*I2

So V1 *I1 = V2*I2

From above equations we notice that

V1/V2 = N1/N2 = I2/I1 = K

So if the number of primary turns = 1000 turn and secondary turns = 50 turns .. primary voltage = 100 volt .. calculate secondary voltage .. mention the type of transformer (step up or step down) …


V1/V2 = N1/N2 = k

100/V2 = 1000/50   then V2 = 5 volt ..

this transformer is step down transformer because voltage at primary side greater than secondary side

2. EMF of transformer equation


equation of transformerequation of transformer

as shown in the previous fig. flux increases from zero to its maximum value at the first quarter of the cycle (because it follows the the AC current motion)

equation of transformer

equation of transformer

So , rate of change of flux per turns

equation of transformer

Its called EMF (Electromotive force)

As flux varies sinusoidal , then r.m.s value of induced EMF obtained from

equation of transformer

equation of transformer

Now, r.m.s. value of the induced e.m.f. in the whole of primary winding = (induced e.m.f/turn) × No. of primary turns

equation of transformer

Also EMF at secondary side will be

equation of transformer


From two equation above (i) and (ii) we conclude that at ideal transformer V1/V2 = N1/N2 = E1/E2 this is ideal no load conditions

what is transformer

this article just preface to electrical transformer explanation lectures .. next articles will explain every thing in details about types of transformers and full equation driven and proofs .. if you like this article .. you can share it

What is transformer