What is an Electric Transformer and its Basic Principle? Emf Equation of Transformer

Transformer


As we know that the devices which are capable of continuous electro-mechanical energy conversion is known Electric Machine.


Transformer is not an electrical machine it is basically a magnetic coupled circuit. Transformer is a static device that transfers electric power at same frequency from one AC voltage level to another AC voltage level. It comprising two coils (Primary and Secondary) which are tightly coupled through a magnetic medium (core) these two coils connected to different voltage levels in an electric system. Electric Transformer can transfer energy in either direction via magnetic field. It is the device that has made the electric system almost universally AC. The electric power is generated at relatively low voltages (maximum up to 33kv) which then stepped up to very high voltage level by means of transformer and then transmitted. High voltages are associated with low currents and reduced transmission losses.


The most important tasks performed by a transformer are



1. Changing voltage and current level at same frequency


2. Matching source and load impedances for maximum power transfer in electronic and control circuitry


3. Electrical isolation between two networks


Electric Transformer
Electric Transformer



Working Principle of Transformer


Transformer is working on the principle of electromagnetic induction. Mutual induction between two set of coils allows for electrical energy to be transferred.


Let us discuss the working principle in details, in the given figure AC voltage is applied to primary winding as this winding is linked with an iron core, its MMF (magneto-motive force) produces an alternating flux ɸ in the core. This alternating flux links with secondary winding. As the flux is alternating, it induces an EMF (electro-motive force) of same frequency as the flux in secondary winding, but its magnitude depends on the number of secondary turns. Due to this induced EMF secondary winding is capable of delivering current. Therefore power is transferred from primary winding to secondary winding.


Since flux plays the main role in transferring the energy so the flux linkage of the two winding should be maximize. In order to ensure the maximum flux linkage of two windings, the core should support the flux and provide a low reluctance path to the flux. So core should made up of high permeability materials like iron or steel alloy.



EMF Equation of Transformer

Electric Transformer on no load
Electric Transformer on no load


Given figure shows the schematic diagram of two winding transformer on no-load i.e. secondary terminal are open while primary is connected to a sinusoidal voltage source of frequency f Hz. Assume the resistance of windings are negligible.

The primary winding draws a small amount of alternating current of instantaneous value of io, called exciting current, from the voltage source. The exciting current establishes flux ɸ in the core. Assume there is no leakage of flux. Consequently the primary winding has flux linkages,


       λ1 = N1ɸ


which induce the emf


    e1  =  dλ/dt  =   N1dɸ/dt


As source is sinusoidal flux must be sinusoidal of frequency f Hz.


    ɸ  =  ɸmaxsin wt    ……… (i)


 ɸmax = maximum value of core flux


  w  =  2πf


e1 = N1dɸ/dt  =  wN1ɸmaxcoswt.   ……….(ii)


From equation (i)  and (ii) it is found that the induced emf leads te flux by 90o.


E1 = √2 πfN1ɸmax    =   4.44fN1ɸmax.


Similarly EMF induced is secondary winding


E2 = 4.44fN2ɸmax.


Dot convention of the transformer


The dots of transformer is assigned such that, when current enters or leave through the dot simultaneously, then the fluxes are additive means if the current enters to the dot in one winding then it should leave through the dot on secondary winding to satisfy the Lenz law. Only the first dot is assigned the remaining dots follow automatically depend upon the sense of the winding.


Transformation Ratio


Transformation ratio

                a  =  Transformation ratio.



Types of Transformer


 Basically there are two types of transformer.

1. Step Up Transformer

2. Step Down Transformer



Step Up Transformer

If number of turns in secondary winding is greater than number of turns in primary winding then EMF induced in secondary winding is greater than in primary winding then the transformer is known as step up transformer. As we know that the electric power is generated at low voltage which then step up to very high voltage by means of step up transformer and then transmitted. In transformer high voltage size is associated with low current so transmission losses are also reduced.

 


Step Down Transformer


If number of turns in secondary winding is less than number of turns in primary winding then induced EMF in secondary winding is less than in primary winding then the transformer is known as step down transformer. As we know that, the transmission voltage is very high which is stepped down by means of step down transformer at the receiving end.



Note:- If number of turns in primary winding is equal to number of turns is secondary winding then primary and secondary voltages are equal, then the transformer is said to have one-to-one ratio. One-to-one transformers are used to electrically isolate two parts of a network it is also known as isolation transformer.



related tags

#_Induction Machine

#_DC Machine




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