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Title: Transformers

1
Transformers
• Test
• Friday
• 3/30/12

2
Electromagnetic Induction
Induction is the process of producing (inducing)
a voltage by passing a wire through a magnetic
field.
3
• In induction one of two things must be
happening!!
• The magnetic field is moving.
• The wire is moving.

4
input
output
5
A transformer works with AC voltages, since the
magnetic field must be changing to induce a
voltage in the coils.
6
Transformer
• Designing a Transformer
• Frequency
• Voltage
• Power

7
Transformer Rating
• Transformer are rated in Volt-Amperes (VA)
• Volt Amperes are used to determine the Maximum
Current the transformer can handle.

8
• A transformer consists of two coils of wire wound
around a core of soft iron.

9
• The side connected to the input AC voltage source
is called the primary and has NP turns.

NP NS
10
The other side, called the secondary, is
connected to a load and has NS turns.
NS
11
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
12
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

13
Coefficient of Coupling
14
Mutual Inductance
15
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.
16
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
17
Turns Ratio
NP NS
NS NP
18
Voltage Relationships
The voltages are related by
NP VP NS VS

19
Voltage Relationships
• When NS gt NP, the transformer is referred to as a
step-up transformer.

Voltage Increases
20
Voltage Relationships
• When NS lt NP, the transformer is referred to as a
step-down transformer.

Voltage Decreases
21
Power
The power input into the primary equals, at best,
the power output at the secondary.
Power In Power Out
• IPVP ISVS

22
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.
23
Power Efficiency
• You dont get something for nothing!!!!
• In real transformers, power efficiencies
typically range from 90 to 99 (0.9 to 0.99)

24
VP100v
Vs?
Rs1kO
NP 250
NS 500
25
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26
Power Grid
27
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28
750,000 volts
7,200 volts
240 volts
29
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30
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31
Transformer Applications
• Impedance Matching (O)

More Power is Transfered!!!
32
Phase Shifting
Transformer Applications
33
Isolation
Transformer Applications
• Passes Signal unchanged
• Prevents Electric Shock

34
Blocking DC
Transformer Applications
DC
35
Produce Multiple Voltages Multi-Tap Transformer
Transformer Applications
36
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37
Autotransformer
Transformer Applications
• Step-Up or Step-Down
• No Isolation!!!

38
VP100v
Rs1kO
NP 250
NS 500
39
Ch 12 AC
• Pages 122 128
• 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

40
Ch 18 Transformers
• Pages 168 174