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Transformers
Test
Friday
3/30/12
Electromagnetic Induction
Induction is the process of
producing (inducing) a voltage by
passing a wire through a
magnetic field.
Generator
In induction one of two things
must be happening!!
•The magnetic field is moving.
•The wire is moving.
input
output
A transformer works
with AC voltages,
since the magnetic field
must be changing to
induce a voltage in the
coils.
Transformer
Designing a Transformer
Frequency
Voltage
Power
Transformer Rating
Transformer are rated in
Volt-Amperes (VA)
Volt Amperes are used to
determine the Maximum
Current the transformer can
handle.
A transformer consists of two
coils of wire wound around
a core of soft iron.
The side connected to the input
AC voltage source is called
the primary and has NP turns.
NP NS
The other side, called the secondary,
is connected to a load and has
NS turns.
NS
Core
The core is used to increase the
magnetic flux and to provide a
medium for the flux to pass from one
coil to the other
Coefficient of Coupling
The measure of how good the
transformer is.
Scale of 0 to 1
1 – All the magnetic Flux lines cut the
Secondary Winding
0 – None of the magnetic Flux lines
cut the Secondary Winding
Coefficient of Coupling
Mutual Inductance
Mutual Inductance
The fact that a change in the
current of one coil affects the
current and voltage in the second
coil is quantified in the property
called mutual inductance.
Turns Ratio
The turns ratio of a transformer is
the ratio of number of windings of
primary side to the secondary
side of the transformer.
NP
NS
NP
NS
NS
Turns Ratio =
NP
Voltage Relationships
The voltages are related by:
NP V P
=
NS V S
Voltage Relationships
When NS > NP, the
transformer is referred to as
a step-up transformer.
Voltage Increases
Voltage Relationships
When NS < NP, the
transformer is referred to as
a step-down transformer.
Voltage Decreases
Power
The power input into the primary
equals, at best, the power output at
the secondary.
Power In = Power Out
IPVP = ISVS
Power
IPVP = ISVS
(This assumes an ideal transformer.)
If VS increases, as in a step up
transformer, IS must decrease.
If VS decreases, as in a step
down transformer, IS must
increase.
Power Efficiency
•You don’t get something for
nothing!!!!
•In real transformers, power
efficiencies typically range from
90% to 99% (0.9 to 0.99)
Vs=?
VP=100v
Rs=1kΩ
NP= 250
NS= 500
V P NS
VS =
NP
VS
100v X 500
=
250
VS = 200V
Power Grid
750,000 volts
7,200 volts
240 volts
Transformer Applications
Impedance Matching (Ω)
More Power is Transfered!!!
Transformer Applications
Phase Shifting
Transformer Applications
Isolation
Passes Signal unchanged
Prevents Electric Shock
Transformer Applications
Blocking DC
DC
Transformer Applications
Produce Multiple Voltages
Multi-Tap Transformer
Transformer Applications
Autotransformer
Step-Up or Step-Down
No Isolation!!!
VP=100v
Rs=1kΩ
NP= 250
NS= 500
Ch 12: AC
Pages 122 – 128
Answer Question through Chapter
Write down Questions and Answers
 AC Worksheets: 2-1
 Lab 1: Book 2 “DC and AC”
 Problem Worksheets
 4 Simulations – Computer Lab
Due 3/26th Test 3/26th
Ch 18: Transformers
Pages 168 – 174
Answer Question through Chapter
Write down Questions and Answers
 Transformer Worksheets: Lab 2-14
 Lab: 29 Old Book
 Problem Worksheets
 Video Worksheet – “Generating Electricity”
Due 3/26th Test 3/26th