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Chapter 12

Electromagnetic Induction. Chapter 12. Objectives. Demonstrate how to induce a voltage using a coil and a magnet. Explain four things that affect the amount of inductance in a coil. Draw the symbols for a fixed and adjustable inductor.

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Chapter 12

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  1. Electromagnetic Induction Chapter 12

  2. Objectives • Demonstrate how to induce a voltage using a coil and a magnet. • Explain four things that affect the amount of inductance in a coil. • Draw the symbols for a fixed and adjustable inductor. • Calculate the power and current on the secondary of a transformer.

  3. Induced Voltage • Magnet pushed into coil produces slight current • Changing magnetic field induces voltage in coil • Voltage is called induced emf • Counter-electromotive force (cemf) opposes applied voltage

  4. Induced Voltage (continued) • Current creates magnetic field around the coil • Current in ac circuits is always changing • Magnetic coupling

  5. Inductance • Opposition to change in current flow • Circuit with coil takes longer to reach its maximum current than same circuit without coil • Coil in dc circuit only affects circuit when switch is opened or closed • Coil in ac circuit constantly affects circuit because current is always changing

  6. Inductors • Cores • Units • Applications • Factors Inductor Symbols

  7. Inductor Cores and Units • Inductors are made of coils of copper wire over different types of cores • One henry (H)results from one volt being induced while current is changing at rate of one ampere per second

  8. Inductor Applications • Used in radios for tuning circuits, switching power supplies, and where there is need to smooth out ac ripple • Intersections use inductors to change traffic lights

  9. Inductor Factors • Several things that impact inductance in coils are illustrated here

  10. Inductors in Series • Inductor values are given in henrys (H), millihenrys (mH), and microhenrys (µH) • Formula for total inductance for inductors in series is LT = L1 + L2 + L3 + …

  11. Inductors in Parallel • Inductors have no effect on dc circuits except when switches are closing and opening • Formula for inductors in parallel

  12. Mutual Inductance • Mutual inductance results from ac flowing in coil 1, which will induce voltage across coil 2 • Formula for mutual inductance

  13. Transformers • Primary winding • Secondary winding • Symbols for transformers

  14. Transformers (continued) • Names are based on core design used for attaching windings • Closed form transformer • Shell-form transformer

  15. Transformers (continued) • Laminated cores • Step-up transformers • Step-down transformers • Tapped • Phase relationships • High-voltage • Current clamp use

  16. Laminated Cores • Thin strips of metal are laminated and stacked together to build core • Eddy current flow is reduced by using laminated cores • Eddy currents cause excessive heat

  17. Step-Up Transformers • Secondary voltage is larger than primary voltage • Relationship between voltage and number of turns

  18. Step-Down Transformers • Primary voltage is larger than secondary voltage • Use same turns ratio formula • Majority of transformers in electronics field will be step-down

  19. Tapped Transformers • A tap is connected to secondary winding • Allows turns ratio between primary and secondary to be changed, which yields additional voltages

  20. Phase Relationships • “Like-wound” transformers have two dots placed above winding schematic • “Unlike-wound” transformers have two dots placed at opposite ends of primary and secondary

  21. Voltage-Current Relationships • If transformers are 100% efficient and have no losses • PP = PS • Given voltage of primary and secondary, and primary current, it is possible to calculate secondary current

  22. High-Voltage Transformers • High-voltage transformers have ratings of over 24,000 volt-amperes (VA) • Volt-ampere is found by multiplying volts and amps of primary • Primary of 12,000 volts and two (2) amps would provide 24,000 VA • Unit is called 24 kVA transformer

  23. Math Focus: Square Roots • Related to the base of a number that is squared • A square root of a number is the number that when multiplied by itself gives that value • Do other computations in the equation before finding the square root

  24. Practical Application • Current lamp • Current probe • Placed over the wire to measure circuit current • Uses principles of induction and transformer action

  25. Review • How can voltage be induced with a coil and magnet? • Either by passing a magnet through the coil or by holding the magnet stationary and moving the coil over it.

  26. Review • What are the four things that affect the amount of inductance in a coil? • Number of turns of wire in the coil, type of core, the cross-sectional area of the core, and the length of coil compared to the number of turns of wire.

  27. Review • How is the symbol for a continuously adjustable inductor different from the symbol for a basic inductor? • An arrow runs diagonally through the coil.

  28. Review In a transformer, what is the equation used to express the relationship between voltage and the number of turns?

  29. Activities • List the places you find transformers around your house. • Discuss why industrial plants operate so much of their equipment at 460 V instead of 120 V. • Explain the type of voltage used to operate the large electromagnets found in junkyards.

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