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Unit 27 Transformers • Objectives – After completing this chapter, the student should be able to: • • • • Describe how a transformer operates. Explain how transformers are rated. Explain how transformers operate in a circuit. Describe the differences between step-up, stepdown, and isolation transformers. • Describe how the ratio of the voltage, current, and number of turns are related with a transformer. • Describe applications of a transformer. • Identify different types of transformers. • Electromagnetic induction – The action caused when two electrically isolated coils are placed next to each other and an AC voltage is put across one coil, resulting in a changing magnetic field which induces a voltage into the second coil.. – The device used to create this action is called a transformer. • Transformers – The coil containing the AC voltage is the primary winding. – The coil in which the voltage is induced is the secondary winding. – Coefficient of coupling • A number from 0 to 1. – 1 indicating that all the primary flux lines cut the secondary windings. – 0 indicating that none of the primary flux lines cut the windings. • The design of a transformer is determined by: – The frequency at which it will be used. • Low-frequency applications use iron cores. • High-frequency applications use air cores. – The power it must handle. – The voltage it must handle. • Transformers are rated in volt-amperes. • Transformers are wound with tapped secondaries. – Center tapped secondary is equal to two secondary windings. – Used for power supply to convert AC voltages to DC voltages. • Mutual inductance – The primary induces a voltage into the secondary and the secondary induces a voltage back into the primary. • Turns ratio – Determines whether a transformer is used to step up, step down, or pass voltage unchanged. – The number turns in the secondary winding divided by the number of turns in the primary winding. NS – Expressed as: turns ratio = NP – where N = number of turns. • A step-up transformer – A transformer with secondary voltage greater than its primary voltage. – Expressed as: ES NS EP NP – The turns ratio is always greater than one. • A step-up transformer – A transformer that produces a secondary voltage less than its primary voltage. – The turns ratio is always less than one. • When a transformer steps up the voltage, it steps down the current. – This is expressed as: PP = P S (IP)(EP) = (IS)(ES) • The current is inversely proportional to the turns ratio. This is expressed as: IP NS IS N P • Impedance ratio is equal to the turns ratio squared. – This is expressed as: 2 ZP N P 2 ZS N S • Applications for transformers – – – – – – – Stepping up voltage and current. Stepping down voltage and current. Impedance matching. Phase shifting. Isolation. Blocking DC while passing AC. Producing several signals at various voltage levels. • Transformers are used for: – Transmitting electrical power to homes and industry. – Isolating electronic equipment from 120-volts AC, 60-hertz power while it is being tested. • Does not step up or step down the voltage. • Autotransformers – A device used to step up or step down applied voltage. – Both the primary and secondary windings are part of the same core. • In Summary – Transformers consist of: • two coils • a primary winding • a secondary winding – Transformers allow an AC signal to be transferred from one circuit to another. – Transformers allow: • stepping up the signal. • stepping down the signal. • passing the signal unchanged. – Transformers are designed to operate at certain frequencies. – Transformers are rated in volt-amperes. – Turns ratio determines whether a transformer is used to: • step up a voltage. • step down voltage. • pass voltage unchanged. – A step-up transformer: • produces a secondary voltage greater than its primary voltage. • has a turns ratio that is always greater than one. – A step-down transformer: • produces a secondary voltage less than its primary voltage. • has a turns ratio that is always less than one. – The turns ratio determines the amount of voltage that is stepped up or down. – Transformer applications include: • Impedance matching. • Phase shifting. • Isolation. • Blocking DC while passing AC. • Producing several signals at different voltage levels. – Isolation transformers • Pass the signal unchanged. • Used to prevent electric shocks. – Autotransformers • Used to step up or step down voltage. • Do not provide isolation.