Chapter 12 Electromagnetic Induction

Permission granted to reproduce for educational use only. © Goodheart-Willcox Co., Inc. 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.

Permission granted to reproduce for educational use only. © Goodheart-Willcox Co., Inc. Induced Voltage  Magnet pushed into coil produces slight current  Changing magnetic field induces voltage in coil  Voltage is called induced emfinduced emf  Counter-electromotive force (cemf) opposes applied voltage Counter-electromotive force

Permission granted to reproduce for educational use only. © Goodheart-Willcox Co., Inc. Induced Voltage (continued)  Current creates magnetic field around the coil  Current in ac circuits is always changing  Magnetic coupling Magnetic coupling

Permission granted to reproduce for educational use only. © Goodheart-Willcox Co., Inc. 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

Permission granted to reproduce for educational use only. © Goodheart-Willcox Co., Inc. Inductors  Cores  Units  Applications  Factors Inductor Symbols

Permission granted to reproduce for educational use only. © Goodheart-Willcox Co., Inc. 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 secondhenry (H)

Permission granted to reproduce for educational use only. © Goodheart-Willcox Co., Inc. 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

Permission granted to reproduce for educational use only. © Goodheart-Willcox Co., Inc. Inductor Factors  Several things that impact inductance in coils are illustrated here

Permission granted to reproduce for educational use only. © Goodheart-Willcox Co., Inc. Inductors in Series  Inductor values are given in henrys (H), millihenrys (mH), and microhenrys (µH)  Formula for total inductance for inductors in series is L T = L 1 + L 2 + L 3 + …total inductance

Permission granted to reproduce for educational use only. © Goodheart-Willcox Co., Inc. Inductors in Parallel  Inductors have no effect on dc circuits except when switches are closing and opening  Formula for inductors in parallel

Permission granted to reproduce for educational use only. © Goodheart-Willcox Co., Inc. Mutual Inductance  Mutual inductance results from ac flowing in coil 1, which will induce voltage across coil 2  Formula for mutual inductance

Permission granted to reproduce for educational use only. © Goodheart-Willcox Co., Inc. Transformers  Primary winding Primary  Secondary winding Secondary  Symbols for transformers

Permission granted to reproduce for educational use only. © Goodheart-Willcox Co., Inc. Transformers (continued)  Names are based on core design used for attaching windings –Closed form transformerClosed form transformer –Shell-form transformerShell-form transformer

Permission granted to reproduce for educational use only. © Goodheart-Willcox Co., Inc. Transformers (continued)  Laminated cores  Step-up transformers Step-up transformers  Step-down transformers Step-down transformers  Tapped  Phase relationships  High-voltage  Current clamp use

Permission granted to reproduce for educational use only. © Goodheart-Willcox Co., Inc. 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

Permission granted to reproduce for educational use only. © Goodheart-Willcox Co., Inc. Step-Up Transformers  Secondary voltage is larger than primary voltage  Relationship between voltage and number of turns

Permission granted to reproduce for educational use only. © Goodheart-Willcox Co., Inc. 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

Permission granted to reproduce for educational use only. © Goodheart-Willcox Co., Inc. Tapped Transformers  A tap is connected to secondary windingtap  Allows turns ratio between primary and secondary to be changed, which yields additional voltages

Permission granted to reproduce for educational use only. © Goodheart-Willcox Co., Inc. 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

Permission granted to reproduce for educational use only. © Goodheart-Willcox Co., Inc. Voltage-Current Relationships  If transformers are 100% efficient and have no losses –P P = P S  Given voltage of primary and secondary, and primary current, it is possible to calculate secondary current

Permission granted to reproduce for educational use only. © Goodheart-Willcox Co., Inc. High-Voltage Transformers  High-voltage transformers have ratings of over 24,000 volt-amperes (VA)volt-amperes  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

Permission granted to reproduce for educational use only. © Goodheart-Willcox Co., Inc. 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

Permission granted to reproduce for educational use only. © Goodheart-Willcox Co., Inc. Practical Application ○Current lamp ○Current probe ○Placed over the wire to measure circuit current ○Uses principles of induction and transformer action

Permission granted to reproduce for educational use only. 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. © Goodheart-Willcox Co., Inc.

Permission granted to reproduce for educational use only. 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. © Goodheart-Willcox Co., Inc.

Permission granted to reproduce for educational use only. 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. © Goodheart-Willcox Co., Inc.

Permission granted to reproduce for educational use only. © Goodheart-Willcox Co., Inc. Review In a transformer, what is the equation used to express the relationship between voltage and the number of turns?

Permission granted to reproduce for educational use only. © Goodheart-Willcox Co., Inc. 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.

Permission granted to reproduce for educational use only. © Goodheart-Willcox Co., Inc. Glossary  Closed-form transformer –A transformer in which the core is a closed ring with the primary wrapped around one side of it and the secondary wrapped around the other side.

Permission granted to reproduce for educational use only. © Goodheart-Willcox Co., Inc. Glossary  Counter-electromotive force (cemf) –The voltage that forms across the coils of an inductor due to the rising and falling of the applied alternating current. Counter- electromotive force opposes the applied voltage.

Permission granted to reproduce for educational use only. © Goodheart-Willcox Co., Inc. Glossary  Henry (H) –The unit of measure for inductance. A coil has one henry of inductance if an emf of one volt is induced when the current through the inductor is changing at the rate of one ampere per second.  Induced emf –The voltage that is induced in a coil due to a changing magnetic field.

Permission granted to reproduce for educational use only. © Goodheart-Willcox Co., Inc. Glossary  Inductance –That property of a circuit or device that opposes any change in current through it. Its symbol is L.  Inductor –An electrical component that has the property of inductance. The most common form of an inductor is a coil of wire.

Permission granted to reproduce for educational use only. © Goodheart-Willcox Co., Inc. Glossary  Magnetic coupling –When the changing magnetic field in one turn of the coil produces an emf in the other turns of the coil through which the field passes.  Mutual inductance –The condition that exists when the magnetic field of one conductor is linked to the magnetic field of a second conductor. In this way, each field has some effect on the emf of the other conductor.

Permission granted to reproduce for educational use only. © Goodheart-Willcox Co., Inc. Glossary  Primary –The winding of a transformer that is connected to the energy source.  Secondary –The winding of a transformer with the induced energy.

Permission granted to reproduce for educational use only. © Goodheart-Willcox Co., Inc. Glossary  Shell-form transformer –A transformer constructed by wrapping the primary around a cardboard core and then wrapping the secondary over the primary. Insulating paper separates each layer of windings from the other, producing a transformer with a high mutual inductance.

Permission granted to reproduce for educational use only. © Goodheart-Willcox Co., Inc. Glossary  Step-down transformer –A transformer used to reduce voltage.  Step-up transformer –A transformer used to increase voltage.

Permission granted to reproduce for educational use only. © Goodheart-Willcox Co., Inc. Glossary  Tap –A fixed electrical connection on a transformer that is used to provide another voltage level.  Total inductance –The combined value of all the capacitor in a circuit.

Permission granted to reproduce for educational use only. © Goodheart-Willcox Co., Inc. Glossary  Transformer –A device used to step up or step down voltage by induction. Also, an electrical device used to transport energy from one circuit to another circuit at the same frequency.  Volt-ampere –The result of multplying volts by amps.