TRANSFORMERSIsolation transformers have turns ratio 1:1 and are used to provide isolation by blocking DC component and allowing AC components to pass. Transformer- Electromagnetic energy conversion device. Electric (primary) – magnetic – electric (secondary). Primary and secondary winding are not connected electrically but are magnetically coupled. Use of transformer: Step up or step down of voltage or current For matching impedance of a source and its load for maximum power transfer in electronic and control circuits For isolating DC while permitting the flow of AC between two circuits Two types of transformers: CORE Type SHELL Type Preferred for high voltage, high power levels low Voltage, low Power Low power transformers- air cooled High power transformers-oil cooled (oil acts as coolant as well as insulator) IDEAL TRANSFORMER V1 = primary voltage V2 = secondary Voltage 𝜑 = core flux E1 = Induced emf in primary E2 = Induced emf in secondary Im= Current in primary windings 𝜑 (Core flux) will follow Im (current) and will be in phase with Im. E1 will be out of phase with Im and V1 𝜑 = 𝜑 max sin 𝜔𝑡 𝐸1 = − 𝐸2 = − 𝑁1 dǾ dt 𝑁2 dǾ dt 𝐸1 = √2 𝜋𝑓𝑁1 𝜑𝑚𝑎𝑥 𝐸2 = √2 𝜋𝑓𝑁2 𝜑𝑚𝑎𝑥 E1/E2 = N1/N2 Practical transformer Practical transformer is different from ideal transformer in many respects. Ideal transformer has no losses but practical transformer have Iron losses(Core loss) Magnetic leakage Winding resistances(Copper loss) 1) IRON LOSS Alternating flux through iron core creates eddy current and hysteresis loss. 2) COPPER LOSS Conductor => resistance (both sides) Hence E1 < V1 & E2 > V2 Transformer phasor diagram at no load The primary voltage V1 has now three components, 1. -E1, the induced e.m.f. which opposes V1 2. I1 R1, the drop across the resistance, in phase with I1 3. I1 X1, the drop across the reactance, leading I1 by 90o The secondary induced e.m.f. has also three components, 1) V2, the terminal voltage across the load 2) I2 R2, the drop across the resistance, in phase with I2 3) I2 X2, the drop across the reactance, leading I2 by 90o The no-load primary current is made up of the following two components: 1). An in-phase current, IE which supplies the core losses (eddy current and hysteresis). 2). A current, IM at 90o to the voltage which sets up the magnetic flux. TRANSFORMER PHASOR DIAGRAM AT UNITY POWER FACTOR LOAD As load power factor is unity, the voltage V2 and I2 are in phase. Steps to draw the phasor diagram are, 1. Consider flux Φ as reference 2. E1 lags Φ by 90o. Reverse E1 to get -E1. 3. E1 and E2 are in phase 4. Assume V2 in a particular direction 5. I2 is in phase with V2. 6. Add I2 R2 and I2 X2 to get E2. 7. Reverse I2 to get I2'. 8. Add Io and I2' to get I1. 9. Add I1 R1 and to -E1 to get V1. Angle between V1 and I1 is Φ1 and cosΦ1 is primary power factor. Remember that I1X1 leads I1 direction by 90o and I2 X2 leads I2 by 90o as current through inductance lags voltage across inductance by 90o. TRANSFORMER PHASOR DIAGRAM AT LAGGING POWER FACTOR LOAD TRANSFORMER PHASOR DIAGRAM AT LEADING POWER FACTOR LOAD