Transformer A Presentation on

Transformer2 Coil 1 i 1 (t)i 2 (t) Coil 2 M e 1 (t)e 2 (t) S1S1 S2S2 i 1 (t) i 2 (t) The Transformer (Primary has N 1 turns) (Secondary has N 2 turns) V2

Transformer3 The source side is called Primary The load side is called Secondary Ideally 1.The resistance of the coils are zero. 2.The relative permeability of the core in infinite. 3.Zero core or iron loss. 4.Zero leakage flux The Transformer

Transformer4 The Transformer i)Switch ‘S 1 ’ is closed and ‘S 2 ’ is open at t=0 The core does not have a flux at t=0 We will now prove the following on the greenboard: The voltage induced across each coil is proportional to its number of turns.

Transformer5 The Transformer ii) Switch ‘S 2 ’ is now closed A current now starts to flow in resistance R. This current is i 2 (t) (flows out of the dotted terminal). Thus a MMF N 2 i 2 (t) is applied to the magnetic circuit. This will immediately make a current i 1 (t) flow into the dot of the primary side, so that N 1 i 1 (t) opposes N 2 i 2 (t) and the original flux in the core remains constant. Otherwise, N 2 i 2 (t) would make the core flux change drastically and the balance between V 1 and e 1 (t) will be disturbed.

Transformer6 The Transformer We will now prove the following on the greenboard: 1)The current induced in each coil is inversely proportional to its number of turns. 2)Instantaneous input power to the transformer = Instantaneous output power from the transformer.

Transformer7 The Transformer Observation: It was shown that the flux in the core is  m Sin(  t). Since the permeability of the core is infinite ideally zero current can produce this flux! In actuality, a current I m, known as magnetizing current is required to setup the flux in the transformer. This current is within 5% of the full load current in a well designed transformer. L 1 is the primary side self inductance.

Transformer8 Transformer Equivalent circuit I1 I2 INL E1 E2

Transformer9 Transformer Equivalent circuit I1 I2 INL

Transformer10 Transformer Equivalent circuit I1I2 INL

Transformer11 Transformer Equivalent circuit I2' INL I1

Transformer12 Open circuit Test It is used to determine L m1 (X m1 )and R c1 Usually performed on the low voltage side The test is performed at rated voltage and frequency under no load

Transformer13 Short circuit Test It is used to determine Ll p (X eq ) and R p (R eq ) Usually performed on the high voltage side This test is performed at reduced voltage and rated frequency with the output of the low voltage winding short circuited such that rated current flows on the high voltage side.

Transformer14 Transformer Regulation Loading changes the output voltage of a transformer. Transformer regulation is the measure of such a deviation. Definition of % Regulation V no-load =RMS voltage across the load terminals without load V load = RMS voltage across the load terminals with a specified load

Transformer15 Transformer Losses and Efficiency Transformer Losses Core/Iron Loss =V 1 2 / R c1 Copper Loss = I 1 2 R 1 + I 2 2 R 2 Definition of % efficiency = load power factor

Transformer16 Maximum Transformer Efficiency The efficiency varies as with respect to 2 independent quantities namely, current and power factor Thus at any particular power factor, the efficiency is maximum if core loss = copper loss.This can be obtained by differentiating the expression of efficiency with respect to I 2 assuming power factor, and all the voltages constant. At any particular I 2 maximum efficiency happens at unity power factor. This can be obtained by differentiating the expression of efficiency with respect to power factor, and assuming I 2 and all the voltages constant. Maximum efficiency happens when both these conditions are satisfied.

Transformer % full load current pf=1 pf= 0.8 pf= 0.6 At this load current core loss = copper loss Maximum efficiency point 