What is Transformer, Working, Construction, EMF equation

  • The three-phase power transformer is the main device in the power system. Considering the insulation part of the generator, the generation of electrical power is limited 11KV to 22KV, but to transmit power at higher voltage level i.e 132KV or more is economical because of losses in lower voltage transmission is more and larger conductor size is required due to higher current.
    Every ac power transmission line starts with a three-phase electrical transformer and ends with it by stepping down 3 phase high voltage to usable level using a step-down type transformer i.e 415V for three-phase and 215V for single phase user.

    What is the transformer?

    The electrical transformer is a static piece device means it does not have any moving part in it. that's why the efficiency of it is high compared to other electrical machines.
     
    Transformer basically transfers energy from one electrical circuit to other circuits through a magnetic field. Hence also called an electromagnetic energy conversation device. The circuit which transfers energy called primary winding and which received energy refers as the secondary winding of a transformer which further connected to load.
     
    Here mainly to Note that primary and secondary are not connected electrically to each other but connected through magnetically.
     
    Magnetic coupling allows transferring energy from a higher voltage level to a lower level or from low voltage to higher i.e energy transfer in either direction. Transformer converts voltage level from one voltage to other voltage levels without a change in frequency or power.

    Working principle of transformer

    The working principle of transformer is explained below
    transformer circuit diagram working
    transformer circuit diagram working
    Working principle of the transformer is base on the Faraday's law of electromagnetic induction.
    According to Faraday's law, an emf induced in a coil if it links to changing flux. When primary winding is fed with ac supply voltage " V1 " therefore an ac current " IФ " start flowing through winding N1 turns.
     
    Alternating mmf  N1I create alternating flux ϕ. Which start flowing through magnetic core and link to the secondary winding and induced voltage E2 in the secondary winding due to transformer action.

    If a load is connected across secondary of X'mer,  current start flowing in secondary through the load.

    Types of transformer

    There are in general  two types of transformer

    Shell type and core type transformer These two types differ from each other in the manner in which winding wound on the magnetic core.

    Core type

     
    core type transformer diagram
    Core type transformer

    In core type, the magnetic core is built of laminations to form a rectangular frame and the windings are arranged concentrically with each other around the legs or limbs.The top and bottom horizontal portion of the core are called yoke. The yokes connect the two limbs and have a cross-sectional area equal to or greater than that of limbs.

    Each limb carries one-half of primary and secondary windings. The two windings are closely coupled together to reduce the leakage reactance. The low voltage winding is wound near the core and high voltage winding is wound over low voltage winding away from the core in order to reduce the amount of insulating materials required.

    Shell Type 


    SHELL TYPE TRANSFORMER DIAGRAM
                                                                  SHELL TYPE TRANSFORMER
     

    In shell type the windings are wound around the central limb and the flux travels through two side limbs and completes its circuit. The central limb carries total mutual flux while the side limbs forming a part of a parallel magnetic circuit carry half the total flux. The cross sectional area of the central limb is twice that of each side limbs

    Step-up Transformer:

    A transformer which changes voltage from lower voltage to a higher voltage called a step-up transformer. Step-up X'mer has more number of turn in secondary winding than primary hence voltage per turn increase, it increases voltage level, mostly use at generating station to transmit electrical power in transmission line at a higher voltage.

    Step Down Transformer:

    A transformer which lower down voltage to utilization voltage called a step-down transformer, it reduced the voltage from higher to lower voltage level. It has less number of a turn on secondary winding compare to the primary winding. 
     
    It is used in a distribution network, where the transmission line end. mostly delta - star-connected windings, the primary is connected in delta and secondary is in star for deriving neutral point.

    Note that step down x'mer can be used as step up transformer, in which secondary of step down x'mer becomes primary and primary of step down becomes secondary winding.

    Step up or step down working operation of the transformer are decided after installation on site.

    Parts of the transformer :

    Magnetic core:

    The magnetic core is made up of magnetic material like CRGO (cold rolled grain oriented steel). this material has high permeability. The core is a stack of thin silicon - steel lamination's to reduced eddy current losses and separated from each other by using thin layers of varnish.

    Winding :

    Winding is made up of copper conductor and wound on a magnetic core.

    1) In core-type transformer, most of part of the core is surrounded by the winding.
    2) On other hands in shell type transformer core surround major part of the winding. 
    3) The advantage of the core type is it reduced core material but required more copper for winding 
    4) shell type required less conductor material but more core material.
    5) The vertical portion of the X'mer called limb and horizontal portion called yoke. 
    6) 1 phase shell type x'mer has 3 limbs and winding wound on the central limb and core type has two limbs and winding wound on both limbs.

    Conservator :

     
    Transformer Oil Conservator tank
    Oil Conservator tank

    Transformer oil should not be allowed to come in contact with atmospheric air as it may take up moisture resulting reduction in the dielectric strength of oil.also air may cause acidity and sludging of oil. To prevent this transformer provided with a conservator.

    The function of a conservator is take up construction and expansion of transformer oil during working without allowing it to come in contact with outside air. The conservator consists of an airtight metal drum-like structure fitted above the transformer top and connected with it by a pipe.

    The main tank completely filled with the oil when cold. The conservator tank partially filled with oil. So space for expansion of oil is provided and sludge formed remains in the conservator itself and does not go to the main tank.

    Breather :

    When temperature changes, the oil expands or constracts and there is a movement of air take place. When transformer cools down, the oil level goes down and air is drawn inside. this process is known as breathing. The air, drawn in is passed through breather for removing moisture from the air. The breather consist of a small vessel containing a silica gel crystal impregnated with cobalt crystal.

    EMF equation of a transformer :

    Derivation for EMF equation derived below:

    Flux at any instant in the  core is given by,
     
     
    Instantaneous e.m.f induced in a coil of  T1 turn linked by flux is given below, 

    Above equation can be written as


    emf equation transformer  derivation
    transformer emf equation derivation
    above formula called an e.m.f equation of transformer

    Voltage ratio and Turn Ratio :

    The ratio of E/T is called Voltage Per Turn

    Primary voltage per turn equation

    E1/T1=4.44 ϕm f 

    Secondary voltage per turn equation

    E1/T2=4.44ϕf

    from the above equation, we can see that voltage per turn in both windings is the same

     
       

     

    Why transformers rated output is in KVA instead of KW?

    Rated output voltage is express in KVA rather than kilowatt (KW) because the rated output is limited by heating and hence by losses in the transformer. These losses depend on voltage (core loss) and current (I2R) loss and remain unaffected by the load power factor. 
     For this reason, we mention output in KVA instead of KW.

    Advantages of transformer :

    • Used for stepping up or down supply voltage.
    • High efficiency compared to other electrical machines.
    • with the use of a x'mer, long distance ac power transmission is possible.
    • Distribute power at high voltage.
    • Insulate circuits/establish separately derived circuits.
    • Provide 3-wire secondary circuits..
    • Provide electrostatic shielding transient noise protection.

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