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Transformers Transformer It is a static device. It transfers electrical energy from one electrical circuit to other with desired change in voltage and current, without changing the frequency(f=50Hz) and power. Constant flux device Magnetically coupled and electrically isolated Electro magnetic conversion device. Principle of operation It is based on principle of MUTUAL INDUCTION. According to which an e.m.f. is induced in a coil when current in the neighbouring coil changes. Constructional detail : Shell type Parallel magnetic circuit Windings are wrapped around the central limb of a laminated core. Sandwitch winding to reduce the leakage flux Less amount of copper & more amount of insulation is required Constructional detail : Core type Series magnetic circuit Windings are wrapped around two sides of a laminated square core. More amount of copper & less amount of insulation is required. Economical for high voltage applications Sectional view of transformers Note: High voltage conductors are smaller cross section conductors than the low voltage coils Core type Fig1: Coil and laminations of core type transformer Fig2: Various types of cores Shell type Fig: Sandwich windings • The HV and LV windings are split into no. of sections • Where HV winding lies between two LV windings • In sandwich coils leakage can be controlled Cut view of transformer Transformer with conservator and breather Working of a transformer 1. When current in the primary coil changes being alternating in nature, a changing magnetic field is produced 2. This changing magnetic field gets associated with the secondary through the soft iron core 3. Hence magnetic flux linked with the secondary coil changes. 4. Which induces e.m.f. in the secondary. Ideal Transformers • Zero leakage flux: -Fluxes produced by the primary and secondary currents are confined within the core • The windings have no resistance: - Induced voltages equal applied voltages • The core has infinite permeability - Reluctance of the core is zero - Negligible current is required to establish magnetic flux • Loss-less magnetic core - No hysteresis or eddy currents Ideal transformer V1 – supply voltage ; V2- output voltgae; Im- magnetising current; E1-self induced emf ; I1- noload input current ; I2- output current E2- mutually induced emf Phasor diagram: Transformer on Noload Transformer on load assuming no voltage drop in the winding Fig shows the Phasor diagram of a transformer on load by assuming 1. No voltage drop in the winding 2. Equal no. of primary and secondary turns Transformer on load Fig. a: Ideal transformer on load Fig. b: Main flux and leakage flux in a transformer Equivalent circuit of a transformer No load equivalent circuit: Equivalent circuit parameters referred to primary and secondary sides respectively Transferring secondary parameters to primary side Cu loss after transfer = cu loss before transfer I 12 R2' I 22 R2 I2 R I1 R2 2 k ' 2 2 R2 Where R21 - Equivalent secondary resistance w.r.t primary R01 = R1 + R21 Where R01 – Total primary resistance referred to secondary Equivalent circuit referred to primary side : Transferring primary parameters to secondary side : Cu loss after transfer = cu loss before transfer I 22 R1' I12 R1 I1 ' R1 I 2 2 R1 = k2 R1 Where R11 - Equivalent primary resistance w.r.t secondary R02 = R2 + R11 Where R01 – Total secondary resistance referred to primary Equivalent circuit referred to secondary side : Equivalent circuit w.r.t primary : where Approximate equivalent circuit Since the no load current is 1% of the full load current, the no load circuit can be neglected Transformer Tests •The performance of a transformer can be calculated on the basis of equivalent circuit •The four main parameters of equivalent circuit are: - R01 as referred to primary (or secondary R02) - the equivalent leakage reactance X01 as referred to primary (or secondary X02) - Magnetising susceptance B0 ( or reactance X0) - core loss conductance G0 (or resistance R0) •The above constants can be easily determined by two tests - Oper circuit test (O.C test / No load test) - Short circuit test (S.C test/Impedance test) •These tests are economical and convenient - these tests furnish the result without actually loading the transformer Electrical Machines Open-circuit Test In Open Circuit Test the transformer’s secondary winding is open-circuited, and its primary winding is connected to a full-rated line voltage. Core loss Woc V0 I 0 cos 0 cos 0 Woc V0 I 0 I c or I w I 0 cos 0 R0 V0 Iw V0 I I G0 w V0 X0 • Usually conducted on H.V side I m or I I 0 sin 0 I 02 -I w2 I • To find I B0 I 0 V0 Y0 ; Yo 0 V0 V0 (i) No load loss or core loss W (ii) No load current Io which is Woc V02 G 0 ; Exciting conductanc e G 0 oc2 V0 helpful in finding Go(or Ro ) and Bo (or Xo ) & Exciting susceptanc e B0 Y02 G02 Short-circuit Test In Short Circuit Test the secondary terminals are short circuited, and the primary terminals are connected to a fairly low-voltage source The input voltage is adjusted until the current in the short circuited windings is equal to its rated value. The input voltage, current and power is measured. Full load cu loss Wsc I sc2 R01 • Usually conducted on L.V side • To find (i) Full load copper loss – to pre determine the efficiency (ii) Z01 or Z02; X01 or X02; R01 or R02 - to predetermine the voltage regulation R 01 Wsc I sc2 Z 01 Vsc I sc X 01 Z 012 R012 Voltage regulation recall no - load voltage full - load voltage no - load voltage Vs N s Vp N p Secondary voltage on no-load N2 V2 V1 N1 V2 is a secondary terminal voltage on full load N2 V2 V1 N1 Voltage regulation N2 V1 N1 Substitute we have Formula: voltage regulation In terms of secondary values I 2 R02 cos 2 I 2 X 02 sin 2 0 V2 V2 % regulation 0 V2 0 V2 where ' ' for lagging and '-' for leading In terms of primary va lues V1 V2' I1 R01 cos 1 I1 X 01 sin 1 % regulation V1 V1 where ' ' for lagging and '-' for leading Transformer Efficiency Transformer efficiency is defined as (applies to motors, generators and transformers): Pout 100% Pin Pout 100% Pout Ploss Types of losses incurred in a transformer: Copper I2R losses Hysteresis losses Eddy current losses Therefore, for a transformer, efficiency may be calculated using the following: VS I S cos x100% PCu Pcore VS I S cos Electrical Machines Losses in a transformer Core or Iron loss: Total cu losses = = = Condition for maximum efficiency : The load at which the two losses are equal = All day efficiency : ordinary commercial efficiency all day out put in watts input in watts output in kWh ( for 24 hours) Input in kWh •All day efficiency is always less than the commercial efficiency