Chapter 12

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# Chapter 12 - PowerPoint PPT Presentation

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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## PowerPoint Slideshow about 'Chapter 12' - rhonda-caldwell

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Presentation Transcript
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.
• Calculate the power and current on the secondary of a transformer.
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)
• Current creates magnetic field around the coil
• Current in ac circuits is always changing
• Magnetic coupling
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
• Cores
• Units
• Applications
• Factors

Inductor Symbols

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
• 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
• Several things that impact inductance in coils are illustrated here
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 have no effect on dc circuits except when switches are closing and opening
• Formula for inductors in parallel
Mutual Inductance
• Mutual inductance results from ac flowing in coil 1, which will induce voltage across coil 2
• Formula for mutual inductance
Transformers
• Primary winding
• Secondary winding
• Symbols for transformers
Transformers (continued)
• Names are based on core design used for attaching windings
• Closed form transformer
• Shell-form transformer
Transformers (continued)
• Laminated cores
• Step-up transformers
• Step-down transformers
• Tapped
• Phase relationships
• High-voltage
• Current clamp use
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
• Secondary voltage is larger than primary voltage
• Relationship between voltage and number of turns
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
• A tap is connected to secondary winding
• Allows turns ratio between primary and secondary to be changed, which yields additional voltages
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
• 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 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
• 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
• Current lamp
• Current probe
• Placed over the wire to measure circuit current
• Uses principles of induction and transformer action
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.
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.
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.
Review

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

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.