Transcript Chapter 12 - Goodheart

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.
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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
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Induced Voltage (continued)
Current creates
magnetic field around
the coil
Current in ac circuits
is always changing
Magnetic coupling
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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
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Inductors
Cores
Units
Applications
Factors
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Inductor Symbols
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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
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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
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Inductor Factors
Several things that
impact inductance in
coils are illustrated
here
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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 + …
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Inductors in Parallel
Inductors have no effect on dc circuits
except when switches are closing and
opening
Formula for inductors in parallel
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Mutual Inductance
Mutual inductance
results from ac
flowing in coil 1,
which will induce
voltage across coil 2
Formula for mutual
inductance
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Transformers
Primary winding
Secondary winding
Symbols for transformers
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Transformers (continued)
Names are based on core design used for
attaching windings
– Closed form transformer
– Shell-form transformer
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Transformers (continued)
Laminated cores
Step-up transformers
Step-down transformers
Tapped
Phase relationships
High-voltage
Current clamp use
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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
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Step-Up Transformers
Secondary voltage is
larger than primary
voltage
Relationship between
voltage and number of
turns
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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
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Tapped Transformers
A tap is connected to
secondary winding
Allows turns ratio
between primary and
secondary to be
changed, which yields
additional voltages
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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
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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
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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
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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
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Practical Application
○ Current lamp
○ Current probe
○ Placed over the wire to measure circuit
current
○ Uses principles of induction and
transformer action
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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.
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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.
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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.
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Review
In a transformer, what is the equation used to
express the relationship between voltage and
the number of turns?
N P EP

N S ES
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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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