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Chapter 18

Chapter 18. 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.

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Chapter 18

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  1. Chapter 18 Transformers

  2. 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, step-down, and isolation transformers.

  3. 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.

  4. 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.

  5. Transformers • The coil containing the AC voltage is the primary winding. • The coil in which the voltage is induced is the secondary winding.

  6. 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.

  7. 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.

  8. Transformers are rated in volt-amperes.

  9. 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.

  10. Mutual inductance • The primary induces a voltage into the secondary and the secondary induces a voltage back into the primary.

  11. 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. • Expressed as: turns ratio = • where N = number of turns.

  12. A step-up transformer • A transformer with secondary voltage greater than its primary voltage. • Expressed as: • The turns ratio is always greater than one.

  13. A step-up transformer • A transformer that produces a secondary voltage less than its primary voltage. • The turns ratio is always less than one.

  14. When a transformer steps up the voltage, it steps down the current. • This is expressed as: PP = PS (IP)(EP) = (IS)(ES)

  15. The current is inversely proportional to the turns ratio. This is expressed as:

  16. Impedance ratio is equal to the turns ratio squared. • This is expressed as:

  17. 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.

  18. 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.

  19. Autotransformers • A device used to step up or step down applied voltage. • Both the primary and secondary windings are part of the same core.

  20. 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.

  21. 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.

  22. 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.

  23. 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.

  24. 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.

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