Chapter 12

Chapter 12Electromagnetic

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.


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 flowCircuit 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 windingSecondary windingSymbols for transformers


Transformers (continued)

Names are based on core design used for attaching windings– Closed form transformer– Shell-form transformer


Transformers (continued)

Laminated coresStep-up transformersStep-down transformersTappedPhase relationshipsHigh-voltageCurrent 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

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.


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?

S

P

S

P

E

E

N

N


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.

Chapter 12

Documents

Transcript of Chapter 12