Transformer Chapter PDF

TRANSFORMERS Introduction Transformer is a static device that transfers electrical energy from one circuit to another by electromagnetic induction. The electrical energy is always transferred - without a change in frequency. -but may involve changes in magnitudes of voltage and current....

TRANSFORMERS Introduction Transformer is a static device that transfers electrical energy from one circuit to another by electromagnetic induction. The electrical energy is always transferred - without a change in frequency. -but may involve changes in magnitudes of voltage and current. A transformer works on the principle of electromagnetic induction. Transformer action is possible using an ac voltage as the input source. 01/23/2025 1  Cont……  The transformer winding which receives energy from the supply mains is called primary winding.  The other winding which delivers electrical energy to the load is called secondary winding.  Transformers can be step up or step down.  If the secondary winding has more number of turns than the primary winding, the transformer is step- up.  When the secondary winding has less turns than the primary windings the transformer is step - down.  01/23/2025 A step-up transformer can be used as a step-down transformer. 2  3.1 Basic Operation principle of a Transformer In its most basic form a transformer consists of: A primary coil or winding. A secondary coil or winding. A core that supports the coils or windings. 01/23/2025 3  Cont……. When the secondary is an open-circuit and an alternating voltage V1 is applied to a primary winding. A small current flows and set up a magnetic flux in the core. This alternating flux links both primary and secondary coils and induces in them e.m.f.’s of E1 and E2. The induced e.m.f. E in a coil of N turns is given by 01/23/2025 4  Cont…… In the ideal transformer, the rate of change of flux is the same for both windings. Assuming no losses When the load is connected across the secondary winding, a current I2 flows. In an ideal transformer losses are neglected and a transformer is considered to be 100% efficient. 01/23/2025 5  Cont… Hence input powers = output power or V1I1 = V2 I2 Example A transformer is to be used to provide a 60V output from a 240V A.C supply. Calculate (a) the turns of ratio (b) the number of primary turns, if the secondary is wound with 500 turns. Solution: a) b) 01/23/2025 6 Example 2. Solution: 01/23/2025 7  Construction of Transformers The magnetic core of a transformer is made up of thin laminations (0.35mm) of silicon steel sheets lightly insulated with varnish. The primary and secondary coils are wound on the core and are electrically insulated from each other and from the core. 01/23/2025 8  There are basically two types of transformer based on their constructional feature, I.core-type II.shell-type. The difference b/n them is the manner in which the windings are placed on the magnetic core. I. Core type transformer The primary and secondary windings are wound on two separate legs. 01/23/2025 9  The vertical portions of the core are usually called limbs or legs.  The top and bottom portions are called yoke.  This means that for single-phase transformers, core- type has two-legged core  Most of the flux is confined to high permeability core. 01/23/2025 10  2. Shell type transformer - has three-legged core - The steel core surrounds a major part of the windings. - In this type of transformer the two windings are placed on the central leg of the core. - In both core and shell-type transformers, the individual laminations are cut in the form of long strips of L's, E’s U’s and I's as shown in the Figure. 01/23/2025 11  core 01/23/2025 12  3.2 Application of transformers Transformers are used for: Impedance matching to achieve maximum power transfer between two devices. Voltage/current step-up or step-down. Common mode rejection in balanced architectures Measurement of high voltage and current Based on the value of transformation ratio (a), transformers can be classified 1. Step up 2. Step down 3. Isolation 01/23/2025 13  IDEAL TWO WINDING TRANSFORMERs For a transformer to be an ideal one, the various assumptions are as follows  The primary and secondary windings have zero resistance (Winding resistances are negligible).  All the flux set up by the primary links the secondary windings i.e. all of the flux is confined to the magnetic core.  The core losses (hysteresis and eddy current losses) are negligible.  The core has constant permeability, i.e. the magnetization curve for the core is linear. It may however be noted that it is impossible to realize such a transformer in practice , yet for convenience, we will start with such a transformer and step by step approach an actual transformer.( it is merely to highlight the most important aspect of transformer action.) 01/23/2025 14 3.3 Ideal Transformer With Out Load Primary is connected to the source and secondary is left open. Suppose the current Im due to e1 is sinusoidal Ie = Imsin( wt ) The mmf =NpIe and therefore , the core flux will be sinusoidal. 01/23/2025 15  CONT... the flux will flow the variation of Ie if Ie=0, flux=0, The induced voltage in the primary , due to 01/23/2025 16 :.The RMS value of the induced emf E1 in the primary winding is Since the primary winding resistance is negligible hence e1, at every instant, must be equal and opposite of V1. That is, 01/23/2025 17  Cont... Similarly , the emf induced in the secondary is, Transformation ratio 01/23/2025 18 Ideal Transformer with load  load is connected at the secondary winding  I2 flows  MMf = I2N2 = demagnetizing  Production of flux Ø2 opposes the original flux,Ø1 due to Io  E1 reduces (back emf) 01/23/2025 19  Cont...  Ø2 weakness primary flux,Ø1  For a movement Vp > E1 and hence causes more current to flow in primary  New primary current be I2’= load component of primary  There is MMf = N2I2’, this sets up Ø2’ at primary which is an opposition to Ø2 but the same direction Ø. And in magnitude Ø2=Ø2’  Then the two Ø2’&Ø2 cancels each other So I2 is neutralized by I2’  N2I2=N1I2’  I2’ =(N2/N1)*I2=kI2  Hence when transformer is on load, the primary winding has two currents. Io &I2’.  So the total primary current is the vector sum of Io and I2’ 01/23/2025 20  3.4 Practical Transformer (No load)  The ideal transformers described before can of course never actually be made which means the assumptions that has been applied in ideal transformer may not be functional here.  What can be produced are real transformers- two or more coils of wire physically wrapped around a ferromagnetic core.  The characteristics of a real transformer approximate the characteristics of an ideal transformer,  To understand the operation of a real transformer, refer to Figure below. The Figure shows a transformer consisting of two coils of wire wrapped around a transformer core.  The primary of the transformer is connected to an ac power source, and the secondary 01/23/2025 winding is open- circuited. 21  Cont...  When an ac power source is connected to a transformer as shown in Figure, current flows in its primary circuit, even when the secondary circuit is open- circuited. This current is the current required to produce flux in a real ferromagnetic core. 01/23/2025 22  It consists of two components: 1) The magnetization 2) The core-loss current current Im , which is the which is the current current required to produce required to make up for the flux in the transformer hysteresis and eddy current core losses ( in phase with flux) ( in phase with Vp) When the primary winding is connected to an alternating supply Vp : I. A current I-e (excitation ) begins to flow through it. II. Primary winding introduces a voltage drop across its internal resistance r p (phase with I-e). III. Primary leakage flux induces a voltage in the primary winding this voltage is known as primary leakage reactance voltage drop. ( leads I-e)by 900. 01/23/2025 23  Cont... Total voltage drop in the primary is, I - e ( r p + jx p )= I e z p 01/23/2025 24 Practical transformer with load  The secondary current ( I2 ) produces leakage flux in the secondary  So there is leakage reactance voltage drop Esl =jXsIs leads by 900 secondary winding resistance voltage drop = Isrs in phase with Is 01/23/2025 25  Cont...  Total voltage drop in the secondary Vs =Is (rs + jXs)=Iszs  Kirchhoff’s voltage low in secondary loop is, Es=Vs + Is( rs+ jXs)=Vs+Iszs Where V1=Vp: is the voltage at primary winding V2=Vs : is the voltage at the secondary winding E1 : is the induced voltage at the primary winding E2=Es : is the induced voltage at the secondary winding 01/23/2025 26