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Chapter 12 - PowerPoint PPT Presentation


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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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Presentation Transcript
objectives
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.
induced voltage
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
induced voltage continued
Induced Voltage (continued)
  • Current creates magnetic field around the coil
  • Current in ac circuits is always changing
  • Magnetic coupling
inductance
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
inductors
Inductors
  • Cores
  • Units
  • Applications
  • Factors

Inductor Symbols

inductor cores and units
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
inductor applications
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
inductor factors
Inductor Factors
  • Several things that impact inductance in coils are illustrated here
inductors in series
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 + …
inductors in parallel
Inductors in Parallel
  • Inductors have no effect on dc circuits except when switches are closing and opening
  • Formula for inductors in parallel
mutual inductance
Mutual Inductance
  • Mutual inductance results from ac flowing in coil 1, which will induce voltage across coil 2
  • Formula for mutual inductance
transformers
Transformers
  • Primary winding
  • Secondary winding
  • Symbols for transformers
transformers continued
Transformers (continued)
  • Names are based on core design used for attaching windings
    • Closed form transformer
    • Shell-form transformer
transformers continued1
Transformers (continued)
  • Laminated cores
  • Step-up transformers
  • Step-down transformers
  • Tapped
  • Phase relationships
  • High-voltage
  • Current clamp use
laminated cores
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
step up transformers
Step-Up Transformers
  • Secondary voltage is larger than primary voltage
  • Relationship between voltage and number of turns
step down transformers
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
tapped transformers
Tapped Transformers
  • A tap is connected to secondary winding
  • Allows turns ratio between primary and secondary to be changed, which yields additional voltages
phase relationships
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
voltage current relationships
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
high voltage transformers
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
math focus square roots
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
practical application
Practical Application
  • Current lamp
  • Current probe
  • Placed over the wire to measure circuit current
  • Uses principles of induction and transformer action
review
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.
review1
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.
review2
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.
review3
Review

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

activities
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.