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Residential Cabling Technologies
Transformers
Copyright © 2005 Heathkit Company, Inc. All Rights Reserved
Objectives
At the end of this presentation,
you will be able to:
2
 Define:
transformer, primary, secondary,
step-up, step-down, signal inversion,
excitation current, mutual inductance,
impedance matching, center-tap,
autotransformer, and phase-splitting.
 Name
the parts of a transformer.
 Calculate
turns ratio, power ratio, and
impedance ratio.
3
 Determine
the relationship between
voltage, current, and impedance ratios.
 State
four uses for the transformer.
 State
the difference between a
transformer and an autotransformer.
 Explain
why the transformer is
considered a safety device.
4
Mutual Inductance
 Mutual
Inductance occurs when the
magnetic field of one inductor induces
a voltage into the coils of a second
inductor.
 Mutual
Inductance is the operating
principle that defines transformer
action.
5
L1
L2
Mutual Inductance
6
 Transformers
are devices designed
specifically to take advantage of
mutual inductance.
 A transformer
typically consists of a
Primary and a Secondary winding.
 The
Transformer Primary is the
winding that connects to the input
voltage source.
7
L1
L2
Primary
Winding
(Input)
Secondary
Winding
(Output)
Cores
Transformer Components
8
 The Transformer
Primary carries the
Primary Current in the circuit.
 The
Transformer Secondary is the
winding which connects to the output.
 The Transformer
Secondary carries the
Secondary Current in the circuit.
 The
Primary and Secondary current are
180 out-of-phase.
9
 By
winding the primary and secondary
in opposite directions we can cause
Signal Inversion.
 Signal
Inversion occurs when the
polarity of the input signal is inverted
in the output.
 The
induced secondary EMF depends
upon the mutual induction between the
coils.
10
 Mutual
Inductance is determined by
the amount of flux linkage between the
coils.
 A more
common term for flux linkage
is “coefficient of coupling.”
 Transformer
input power is always
more than the output power.
11
Transformer Construction

Transformer construction methods vary by
transformer type and application.

The size can vary from several tons to the
size of an eraser.

The iron-core transformer is the type you
are most likely to encounter.
12
 Iron-core
transformers employ a solid
or laminated core.
 The
primary windings are wound
around the iron core.
 The
secondary windings are wound
directly on top of the primary.
 The
iron core increases flux density.
13
Schematic
Symbol
14
Transformer Theory

For a transformer’s output to be useful, it
must be “loaded” or attached to a circuit.

Transformer operation changes from the
loaded to no-load condition.

In a no-load condition, the transformer’s
impedance and applied voltage determines
the primary current.
15
 The
small amount of current flow in
the primary in a “no-load” condition is
called the Excitation Current.
Excitation
Current
Primary
Secondary
“No-Load” Condition
16
 When
a load is attached, the operation
of the transformer changes:
 Primary AC
establishes a fluctuating magnetic
field.
 Field
changes induce CEMF into primary and
EMF into secondary.
 Induced
EMF causes current flow in secondary.
17
We can use this simple circuit as an example.
Induced EMF
Applied EMF
Counter EMF
18
 Secondary
current creates a magnetic field.
 Secondary
flux creates EMF in primary.
 Primary
current increases.
Counter EMF
Induced EMF
Increased Current
19
Transformer Ratios

Several ratios exist which help describe the
operation of the transformer.

The Turns Ratio compares the number of
turns in the primary to the number of turns in
the secondary.

The voltage, power, current, and impedance
ratios can be determined with the turns ratio.
20
 The
turns ratio is related to the output
voltage or current of the transformer.
 A Step-Up
transformer has a higher
output on the secondary.
 A Step-Down
transformer has a lower
output on the secondary.
 The
amount of step-up or step-down is
directly proportional to the turns ratio.
21

Here we have a simple
Step-Up transformer.
Primary
Secondary
2000
Turns
1000
Turns
 To find the Turns Ratio,
divide secondary turns
by primary turns.

The ratio is expressed as
“2 to 1” or 2:1.

The Voltage Ratio
expressed as a formula
is:
NS
2000
or
2
N P 1000
ES
EP

NS
NP
22

We can verify this by
transposing the
previous formula:

In this case, if the
input on the primary
is 115V, the secondary output is 230V.

We can adjust the
turns ratio to obtain a
specific output
voltage.
ES 
NS
NP
x EP
ES  2 x 115 v  230 v
23

We can find the output
of a step-down
transformer in the
same manner.
Primary
Secondary
1000
Turns
2000
Turns

If we have 120V on the
1000
primary, solving gives ES 
x 120  60V
2000
us 60V on the
secondary.

When a transformer
steps up voltage, it also
steps down current.
IP
IS

NS
NP
24



We can transpose the
formula to obtain:
IS 
IP x NP
NS
If we have a step-up
transformer with 100
turns on the primary,
400 on the secondary,
and 1A of primary
current.
1 x 100
IS 
 0.25A
400
If the situation were
reversed for a stepdown transformer:
1 x 400
IS 
 4.0A
100
25


Similarly, we can derive
a formula for the power
ratio.
Assume we have a stepup transformer with 500
turns on the primary,
1000 secondary turns, an
input voltage of 100
VAC, and a secondary
current of 25 mA. What
is the power output?
P
NS
NP
x IS x E P
1000
P
x .025 x 100
500
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 Impedance
matching is one of the most
important transformer applications.
 Maximum
power can only be
transferred when impedance between
the source and load match.
 Impedance
mismatches result in
inefficient operation and power losses.
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 So
far, we have seen that:
 Voltage
is directly proportional to the turns
ratio.
 Current
is inversely proportional to the turns
ratio.
 Power
is unaffected by the turns ratio.
 The
impedance ratio is handled a bit
differently
28

NP 

ZS N 2
S

The Impedance Ratio is
equal to the turns ratio
squared:
ZP

An alternate expression
of the formula is:
NP

If we have an amplifier
with a 100-ohm output
and 4-ohm input
speakers, what turns
ratio is necessary to
match the impedances?
NS
2

ZP
ZS
100 Ω

 25
ZS
4Ω
ZP
29
Transformer Losses
 All
transformers have losses during
normal operation.
 Transformer
losses and efficiency are
inversely related.
 Major
power transformer loses occur
in the iron core.
30
Transformer Applications
 Transformers
are commonly used in
power applications to provide specific
voltage and current needs.
 Other
applications include phaseshifting, phase-splitting, isolation, and
autotransformers.
31
 Transformers
can phase shift the output
180 based upon the winding direction.
Primary
1
3
Secondary
Load
2
4
In-Phase Transformer
32
 By
crossing the output, we can create a
180 phase shift in the output.
Primary
1
3
Secondary
Load
2
4
Out-of-Phase Transformer
33
 A “short
hand” is used in schematic
diagrams to show the phase relation.
 Phase
Dots show the phase relation
between the transformer input and
output.
Phase Dots
In-Phase
Out-of-Phase
34

A Center Tap allows out-of-phase signals
to be taken from the same winding. This is
referred to as Phase Splitting.
1
3
Center Tap
4
2
5
Center-Tapped Secondary
35
 One
of the key functions of a
transformer is electrical isolation.
 Power
transformers are used to isolate
the equipment chassis from the AC
input line.
 This
isolation is a key safety factor
which helps prevent electrical shocks.
36
 An
Autotransformer is a special
transformer which does not isolate the
primary and secondary windings.
 An
autotransformer uses a single coil;
part is the primary, and part is the
secondary.
 Autotransformers
offer advantages
over conventional transformers.
37

Autotransformers may be configured as
step-up or step-down transformers.
1
Primary
Winding
Step-Down
Autotransformer
2
Secondary
Winding
Load
3
38
 In
a Step-up autotransformer, the
configuration is reversed.
1
Step-Up
Autotransformer
Secondary
Winding
2
Load
Primary
Winding
3
39

The autotransformer performs slightly
different than conventional transformers.
1
Primary
Winding
(6000)
2
Secondary
5A Winding
(1000)
1A
Load
3
40
 One
unique type of autotransformer is the
variable autotransformer.
Variable
Autotransformer
Load
41
Residential Cabling Technologies
End
Copyright © 2005 Heathkit Company, Inc. All Rights Reserved