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CHAPTER 5

TRANSFORMERS
LEARNING OBJECTIVES
Upon completion of this chapter you will be able to:
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5-1

TRANSFORMERS
The information in this chapter is on the construction, theory, operation, and the various uses of
transformers
...

A TRANSFORMER is a device that transfers electrical energy from one circuit to another by
electromagnetic induction (transformer action)
...
Because a transformer works
on the principle of electromagnetic induction, it must be used with an input source voltage that varies in
amplitude
...

In a preceding chapter you learned that alternating current has certain advantages over direct current
...

As you know, the amount of power used by the load of an electrical circuit is equal to the current in the
load times the voltage across the load, or P = EI
...
However, if a transformer is connected
between the source and the load, the voltage can be decreased (stepped down) to 10 volts and the current
increased (stepped up) to 2 amperes
...
That is, 20
volts times 1 ampere equals the same power as 10 volts times 2 amperes
...
What is meant by "transformer action?"

BASIC OPERATION OF A TRANSFORMER
In its most basic form a transformer consists of:
• A primary coil or winding
...

• A core that supports the coils or windings
...
The magnetic field (flux) builds up (expands) and
collapses (contracts) about the primary winding
...
This
voltage causes alternating current to flow through the load
...


5-2

Figure 5-1
...


Q2
...
In some cases the coils of
wire are wound on a cylindrical or rectangular cardboard form
...
Transformers used at low frequencies, such as 60
hertz and 400 hertz, require a core of low-reluctance magnetic material, usually iron
...
Most power transformers are of the iron-core type
...

• The PRIMARY WINDING, which receives energy from the ac source
...

• The ENCLOSURE, which protects the above components from dirt, moisture, and mechanical
damage
...
Size
limitations and construction costs are also factors to be considered
...
Each of these materials is suitable for particular applications and unsuitable for others
...

Iron-core transformers are usually used when the source frequency is low (below 20 kHz)
...
The iron-core
transformer provides better power transfer than does the air-core transformer
...
The majority of transformers you will encounter in Navy equipment contain laminated-steel cores
...
It takes about 50 such laminations to make a core an inch thick
...
An important point to
5-3

remember is that the most efficient transformer core is one that offers the best path for the most lines of flux
with the least loss in magnetic and electrical energy
...
—Hollow-core construction
...
What are three materials commonly used as the core of a transformer?
Hollow-Core Transformers
There are two main shapes of cores used in laminated-steel-core transformers
...
Figure 5-2 illustrates
this shape of core
...
Figure 5-3 illustrates how
the transformer windings are wrapped around both sides of the core
...
—Windings wrapped around laminations
...
As
shown, each layer of the core consists of E- and I-shaped sections of metal
...
The laminations are insulated from each other and then pressed together
to form the core
...
—Shell-type core construction
...
What are the two main types of cores used in transformers?
TRANSFORMER WINDINGS
As stated above, the transformer consists of two coils called WINDINGS which are wrapped around
a core
...
The winding that is connected to the source is called the PRIMARY
WINDING
...
(Note: In
this chapter the terms "primary winding" and "primary" are used interchangeably; the term: "secondary
winding" and "secondary" are also used interchangeably
...
The primary is wound in layers
directly on a rectangular cardboard form
...
—Exploded view of shell-type transformer construction
...
The wire is coated with varnish so that each turn of the winding is insulated from
every other turn
...


Figure 5-6
...


When the primary winding is completely wound, it is wrapped in insulating paper or cloth
...
After the secondary winding is
complete, it too is covered with insulating paper
...

The leads from the windings are normally brought out through a hole in the enclosure of the
transformer
...
The
figure shows four leads, two from the primary and two from the secondary
...


5-6

Q5
...
A transformer designed for high-voltage applications differs in construction in what way from a
transformer designed for low-voltage applications?

SCHEMATIC SYMBOLS FOR TRANSFORMERS
Figure 5-7 shows typical schematic symbols for transformers
...
Parts (B) and (C) show iron-core transformers
...
Frequently, additional connections are made to the transformer
windings at points other than the ends of the windings
...

When a tap is connected to the center of the winding, it is called a CENTER TAP
...


Figure 5-7
...


Q7
...


5-7

HOW A TRANSFORMER WORKS
Up to this point the chapter has presented the basics of the transformer including transformer action,
the transformer's physical characteristics, and how the transformer is constructed
...

NO-LOAD CONDITION
You have learned that a transformer is capable of supplying voltages which are usually higher or
lower than the source voltage
...

A no-load condition is said to exist when a voltage is applied to the primary, but no load is connected
to the secondary, as illustrated by figure 5-8
...
With the switch open and an ac voltage applied to the primary, there is, however,
a very small amount of current called EXCITING CURRENT flowing in the primary
...
The amount of
exciting current is determined by three factors: (1) the amount of voltage applied (Ea), (2) the resistance
(R) of the primary coil's wire and core losses, and (3) the XL which is dependent on the frequency of the
exciting current
...


Figure 5-8
...


This very small amount of exciting current serves two functions:
1
...

2
...

Exciting current will flow in the primary winding at all times to maintain this magnetic field, but no
transfer of energy will take place as long as the secondary circuit is open
...
What is meant by a "no-load condition" in a transformer circuit?
PRODUCING A COUNTER EMF
When an alternating current flows through a primary winding, a magnetic field is established around
the winding
...
This is the same counter emf that you learned about in the chapter on inductors
...
The counter emf induced in
5-8

the primary has a polarity that opposes the applied voltage, thus opposing the flow of current in the
primary
...

Q9
...
As the exciting current flows through the primary, magnetic lines of force are generated
...
As you remember, a voltage is induced into a coil when magnetic lines cut
across it
...

Q10
...
What causes a voltage to be developed across the secondary winding of a transformer?
PRIMARY AND SECONDARY PHASE RELATIONSHIP
The secondary voltage of a simple transformer may be either in phase or out of phase with the
primary voltage
...
Simply, this means that the two voltages may rise and fall
together or one may rise while the other is falling
...

Dots are used to indicate points on a transformer schematic symbol that have the same instantaneous
polarity (points that are in phase)
...
In part (A) of the figure, both the primary
and secondary windings are wound from top to bottom in a clockwise direction, as viewed from above the
windings
...
This is indicated by the dots on the transformer symbol
...


5-9

Figure 5-9
...


Part (B) of the figure illustrates a transformer in which the primary and secondary are wound in
opposite directions
...
Notice that the top
leads of the primary and secondary have OPPOSITE polarities
...
Thus, the polarity of the voltage at the terminals of the
secondary of a transformer depends on the direction in which the secondary is wound with respect to the
primary
...
What is the phase relationship between the voltage induced in the secondary of an unlike-wound
transformer and the counter emf of the primary winding?
Q13
...


COEFFICIENT OF COUPLING
The COEFFICIENT OF COUPLING of a transformer is dependent on the portion of the total flux
lines that cuts both primary and secondary windings
...

The coefficient of coupling would then be one (unity), and maximum energy would be transferred from
the primary to the secondary
...

Lines of flux generated by one winding which do not link with the other winding are called
LEAKAGE FLUX
...
The voltage induced into the secondary is therefore less than it would
be if the leakage flux did not exist
...
This series LEAKAGE INDUCTANCE is assumed to drop part of the applied voltage, leaving
less voltage across the primary
...
What is "leakage flux?"
Q15
...
Refer to figure 5-10
...
You know
that as lines of flux generated by the primary expand and collapse, they cut BOTH the ten turns of the
primary and the single turn of the secondary
...
This means that if the voltage applied to the primary winding is 10 volts, the counter emf in
the primary is almost 10 volts
...
Since the same flux lines cut the turns in
both the secondary and the primary, each turn will have an emf of one volt induced into it
...


Figure 5-10
...


The transformer represented in part (B) of figure 5-10 has a ten-turn primary and a two-turn
secondary
...

Notice that the volts per turn are the same for both primary and secondary windings
...
This proportion also shows the relationship between the number of turns in each winding
and the voltage across each winding
...
The equation can be written as:

The following formulas are derived from the above equation:

If any three of the quantities in the above formulas are known, the fourth quantity can be calculated
...
A transformer has 200 turns in the primary, 50 turns in the secondary, and 120 volts applied to
the primary (Ep)
...
There are 400 turns of wire in an iron-core coil
...
Sometimes, instead of
specific values, you are given a turns or voltage ratio
...
For example, if a turn ratio is given as
6:1, you can assume a number of turns for the primary and compute the secondary number of turns
(60:10, 36:6, 30:5, etc
...

As a result, there is less voltage across the secondary than across the primary
...
The ratio of a four-to-one step-down transformer is written as 4:1
...
A transformer in which the voltage across the secondary is greater than
the voltage applied to the primary is called a STEP-UP transformer
...
Notice in the two ratios that the value of the primary winding is
always stated first
...
Does 1:5 indicate a step-up or step-down transformer?
Q17
...
If 45 volts are
applied to the primary, what is the voltage developed across the secondary? (Assume no losses)
Q18
...
If 5 volts is developed across the secondary, what is the
voltage applied to the primary? (Note: ES is given, what is EP ?)
Q19
...
If
there are 800 turns on the primary, what is the number of turns in the secondary?
EFFECT OF A LOAD
When a load device is connected across the secondary winding of a transformer, current flows
through the secondary and the load
...
This interaction results from the mutual
inductance between the primary and secondary windings
...

It is also the means by which energy is transferred from the primary winding to the secondary winding
...
The inductance which produces this
flux is also common to both windings and is called mutual inductance
...


Figure 5-11
...


When a load resistance is connected to the secondary winding, the voltage induced into the
secondary winding causes current to flow in the secondary winding
...
Thus, the flux about the secondary cancels some of the flux about the primary
...
The
additional current in the primary generates more lines of flux, nearly reestablishing the original number of
total flux lines
...
The ampere-turn (I × N) is a measure of magnetomotive
force; it is defined as the magnetomotive force developed by one ampere of current flowing in a coil of
one turn
...
Since the flux is the same for both windings, the ampere-turns in both the primary and
secondary windings must be the same
...

This means, a transformer having less turns in the secondary than in the primary would step down the
voltage, but would step up the current
...
Find the current in
the secondary if the current in the primary is 200 milliamperes
...

The above equations can be looked at from another point of view
...
Remember, the turns
ratio indicates the amount by which the transformer increases or decreases the voltage applied to the
primary
...
If the secondary has
one-half as many turns as the primary, the voltage across the secondary will be one-half the voltage across
the primary
...

Thus, a 1:2 step-up transformer will have one-half the current in the secondary as in the primary
...

Example: A transformer with a turns ratio of 1:12 has 3 amperes of current in the secondary
...
A transformer with a turns ratio of 1:3 has what current ratio?
Q21
...
What
is the current flowing in the primary? (Assume no losses)
POWER RELATIONSHIP BETWEEN PRIMARY AND SECONDARY WINDINGS
As just explained, the turns ratio of a transformer affects current as well as voltage
...
Conversely, if voltage is halved in the
secondary, current is doubled in the secondary
...
Refer again to the transformer illustrated in figure 5-11
...
If the input to the primary is 0
...
If the transformer has no losses, 30 watts is delivered to the secondary
...
Thus, the power delivered to the load by the
secondary is P = E × I = 15 volts × 2 amps = 30 watts
...
Hence, more current will flow in the secondary
...
This means the opposition to current is doubled
...
The important thing to remember is
that with the exception of the power consumed within the transformer, all power delivered to the primary
by the source will be delivered to the load
...


TRANSFORMER LOSSES
Practical power transformers, although highly efficient, are not perfect devices
...

5-17

The total power loss in a transformer is a combination of three types of losses
...
This loss is called COPPER loss or I2R loss
...

Copper loss, eddy-current loss, and hysteresis loss result in undesirable conversion of electrical energy
into heat energy
...
What is the mathematical relationship between the power in the primary (PP) and power in the
secondary (PS) of a transformer?
Copper Loss
Whenever current flows in a conductor, power is dissipated in the resistance of the conductor in the
form of heat
...
The greater the value of either resistance or current,
the greater is the power dissipated
...
The resistance of a given winding is a function of the diameter of the
wire and its length
...
Large diameter wire
is required for high-current windings, whereas small diameter wire can be used for low-current windings
...

Whenever the primary of an iron-core transformer is energized by an alternating-current source, a
fluctuating magnetic field is produced
...
The induced voltage causes random currents to flow through the core which dissipates
power in the form of heat
...

To minimize the loss resulting from eddy currents, transformer cores are LAMINATED
...

Hysteresis Loss
When a magnetic field is passed through a core, the core material becomes magnetized
...
If the direction of
the field is reversed, the domains must turn so that their poles are aligned with the new direction of the
external field
...
Each tiny
domain must realign itself twice during each cycle, or a total of 120 times a second when 60 Hz
alternating current is used
...

This loss, called HYSTERESIS LOSS, can be thought of as resulting from molecular friction
...

TRANSFORMER EFFICIENCY
To compute the efficiency of a transformer, the input power to and the output power from the
transformer must be known
...
The output power is equal to the product of the voltage across the
secondary and the current in the secondary
...
You can calculate the percentage of efficiency of a transformer by using the
standard efficiency formula shown below:

Example
...
8 percent, with approximately 40 watts being wasted due
to heat losses
...
Name the three power losses in a transformer
...
The input power to a transformer is 1,000 watts and the output power is 500 watts
...
The
voltage, current, and power-handling capabilities of the primary and secondary windings must also be
considered
...
When a better (and thicker) insulation is used between the windings, a
higher maximum voltage can be applied to the windings
...
If current is excessive in a winding, a higher than ordinary amount of
power will be dissipated by the winding in the form of heat
...
If this happens, the transformer may be permanently
damaged
...
If the
heat can safely be removed, the power-handling capacity of the transformer can be increased
...
The powerhandling capacity of a transformer is measured in either the volt-ampere unit or the watt unit
...
If the frequency applied to a transformer is increased,
the inductive reactance of the windings is increased, causing a greater ac voltage drop across the windings
and a lesser voltage drop across the load
...
But, if the frequency applied to the transformer is decreased, the reactance of the
windings is decreased and the current through the transformer winding is increased
...
For this reason a
transformer may be used at frequencies above its normal operating frequency, but not below that
frequency
...
Why should a transformer designed for 400 hertz operation not be used for 60 hertz operation?

TYPES AND APPLICATIONS OF TRANSFORMERS
The transformer has many useful applications in an electrical circuit
...

POWER TRANSFORMERS
Power transformers are used to supply voltages to the various circuits in electrical equipment
...
The number of windings and
the turns per winding depend upon the voltages that the transformer is to supply
...
95 or more
...
The low-voltage winding usually carries the higher current and
therefore has the larger diameter wire
...

So far you have learned about transformers that have but one secondary winding
...
The schematic symbol for
a typical power-supply transformer is shown in figure 5-12
...
A
winding may be center-tapped like the secondary 350 volt winding shown in the figure
...
Most
power transformers have colored leads so that it is easy to distinguish between the various windings to
which they are connected
...
Usually, red is used to indicate the high-voltage leads, but it is possible for a
manufacturer to use some other color(s)
...
—Schematic diagram of a typical power transformer
...
They range in size from the huge transformers
weighing several tons-used in power substations of commercial power companies-to very small ones
weighing as little as a few ounces-used in electronic equipment
...
Figure 5-13 is a schematic diagram of what is known as an AUTOTRANSFORMER
...
The
voltage across the secondary winding has the same relationship to the voltage across the primary that it
would have if they were two distinct windings
...
That is, when the tap
is at point A, ES is less than EP; when the tap is at point B, ES is greater than EP
...
—Schematic diagram of an autotransformer
...
Audio-frequency
transformers are designed to operate at frequencies in the audio frequency spectrum (generally considered
to be 15 Hz to 20kHz)
...
Because these transformers are subjected to higher frequencies than are power transformers,
special grades of steel such as silicon steel or special alloys of iron that have a very low hysteresis loss
must be used for core material
...
With audio transformers the
impedance of the primary and secondary windings is as important as the ratio of turns, since the
transformer selected should have its impedance match the circuits to which it is connected
...
The windings are wound on a tube of nonmagnetic material, have a special powdered-iron
core, or contain only air as the core material
...
In a short-wave receiver, rf transformers are subjected to
frequencies up to about 20 MHz - in radar, up to and even above 200 MHz
...
One common impedance-matching device is the
transformer
...
The
number of turns on the primary and secondary windings and the impedance of the transformer have the
following mathematical relationship:

Because of this ability to match impedances, the impedance-matching transformer is widely used in
electronic equipment
...
List five different types of transformers according to their applications
...
The leads to the primary and to the high-voltage secondary windings of a power transformer
usually are of what color?

SAFETY
EFFECTS OF CURRENT ON THE BODY
Before learning safety precautions, you should look at some of the possible effects of electrical
current on the human body
...


5-22

AC 60 Hz (mA)

DC (mA)

Effects

0-1

0-4

Perception

1-4

4-15

Surprise

4-21

15-80

Reflex action

21-40

80-160

Muscular inhibition

40-100

160-300

Respiratory failure

Over 100

Over 300

Usually fatal

Note in the above chart that a current as low as 4 mA can be expected to cause a reflex action in the
victim, usually causing the victim to jump away from the wire or other component supplying the current
...

It is important for you to recognize that the resistance of the human body cannot be relied upon to
prevent a fatal shock from a voltage as low as 115 volts or even less
...
Tests have shown that body resistance under unfavorable conditions
may be as low as 300 ohms, and possibly as low as 100 ohms (from temple to temple) if the skin is
broken
...

This is evidenced by the fact that reasonably safe "let-go currents" for 60 hertz, alternating current, are 9
...
0 milliamperes for women, while the corresponding values for direct current
are 62
...
0 milliamperes for women
...
The actual severity of effects will depend on such things as the physical condition of the
work area, the physiological condition and resistance of the body, and the area of the body through which
the current flows
...

ELECTRIC SHOCK
Electric shock is a jarring, shaking sensation you receive from contact with electricity
...
If the voltage and resulting current are sufficiently high, you
may become unconscious
...

RESCUE AND CARE OF SHOCK VICTIMS
The following procedures are recommended for rescue and care of electric shock victims:
1
...
You can do
this by:
• Throwing the switch if it is nearby
• Cutting the cable or wires to the apparatus, using an ax with a wooden handle while taking
care to protect your eyes from the flash when the wires are severed

5-23

• Using a dry stick, rope, belt, coat, blanket, shirt or any other nonconductor of electricity, to
drag or push the victim to safety
2
...
If the victim is not breathing, you must apply
artificial ventilation (respiration) without delay, even though the victim may appear to be
lifeless
...

3
...
The feet should be about 12 inches higher than the head
...
If there is vomiting or if facial injuries have
occurred which cause bleeding into the throat, the victim should be placed on the stomach with
the head turned to one side and 6 to 12 inches lower than the feet
...
Keep the victim warm
...
Keep the victim
covered with one or more blankets, depending on the weather and the person's exposure to the
elements
...

5
...
If necessary, liquids may be administered
...
Alcohol, opiates, and other depressant substances must never be
administered
...
Send for medical personnel (a doctor if available) at once, but do NOT under any circumstances
leave the victim until medical help arrives
...

SAFETY PRECAUTIONS FOR PREVENTING ELECTRIC SHOCK
You must observe the following safety precautions when working on electrical equipment:
1
...
Another person may save your life if you receive an electric shock
...
Work on energized circuits ONLY WHEN ABSOLUTELY NECESSARY
...

3
...

4
...
Remember, a capacitor
is an electrical power storage device
...
When you must work on an energized circuit, wear rubber gloves and cover as much of your
body as practical with an insulating material (such as shirt sleeves)
...

6
...

7
...
Keep the other hand clear of all obstacles that
may provide a path, such as a ground, for current to flow
...
Wear safety goggles
...


5-24

9
...

Carelessness is the cause of most accidents
...

Q28
...
Before working on electrical equipment containing capacitors, what should you do to the
capacitors?
Q30
...

BASIC TRANSFORMER—The basic transformer is an electrical device that transfers alternatingcurrent energy from one circuit to another circuit by magnetic coupling of the primary and secondary
windings of the transformer
...
The coefficient of
coupling (K) of a transformer is dependent upon the size and shape of the coils, their relative positions,
and the characteristic of the core between the two coils
...
The higher the K of the
transformer, the higher is the transfer of the energy
...
This current generates a magnetic field, generating a counter emf (cemf)
which has the opposite phase to that of the applied voltage
...


TRANSFORMER CONSTRUCTION—A TRANSFORMER consists of two coils of insulated
wire wound on a core
...
The secondary winding is then wound on top of the primary
and both windings are wrapped with insulating material
...
Cores come in various shapes and materials
...
The most common types of transformers are the shell-core and the hollow-core types
...


5-25

EXCITING CURRENT—When voltage is applied to the primary of a transformer, exciting current
flows in the primary
...
The moving flux causes a voltage to be induced into the secondary winding,
countering the effects of the counter emf in the primary
...
The primary then draws more current, restoring the
magnetic field to almost its original magnitude
...
If the secondary were wound in the same direction as the primary, the phase would be
the same
...
This is shown on a schematic
drawing by the use of phasing dots
...
The lack of phasing dots on a schematic means a phase reversal
...
When the turns ratio is stated,
the number representing turns on the primary is always stated first
...
In this example, the voltage across the
secondary is two times the voltage applied to the primary
...
This will always be true, regardless of the number of secondary
windings
...

TRANSFORMER LOSSES—Transformer losses have two sources-copper loss and magnetic loss
...
Magnetic losses are caused by eddy currents
and hysteresis in the core
...
Hysteresis loss is constant for a particular voltage and current
...

TRANSFORMER EFFICIENCY—The amplitude of the voltage induced in the secondary is
dependent upon the efficiency of the transformer and the turns ratio
...
Efficiency (in percent) equals
Pout/Pin × 100
...
0 or 100%
...
The transformer is used to supply stepped up and stepped down values of voltage to the various
circuits in electrical equipment
...


AUDIO-FREQUENCY TRANSFORMER—A transformer used in audio-frequency circuits to
transfer af signals from one circuit to another
...

IMPEDANCE-MATCHING TRANSFORMER—A transformer used to match the impedance of
the source and the impedance of the load
...
THROUGH Q30
...
The transfer of energy from one circuit to another circuit by electromagnetic induction
...
Primary winding; secondary winding; core
...
Air; soft iron; steel
...
Hollow-core type; shell-core type
...
Primary to source; secondary to load
...
Additional insulation is provided between the layers of windings in the high-voltage transformer
...

a
...
iron-core transformer
c
...


A voltage is applied to the primary, but no load is connected to the secondary
...


Exciting current is the current that flows in the primary of a transformer with the secondary
open (no load attached)
...


Self-induced or counter emf
...


The magnetic lines generated by the current in the primary cut the secondary windings and
induce a voltage into them
...


In phase
...
The induced voltage of the secondary of an unlike-wound transformer is also 180
degrees out of phase with the primary voltage
...


Note: Remember the dots indicate areas of like polarity, NOT a particular polarity
...


Lines of flux generated by one winding which do not link the other winding
...


It causes K to be less than unity (1)
...


Step up
...


A18
...


A20
...


A22
...


Copper loss, eddy-current loss, and hysteresis loss
...


A25
...
The resulting excessive current would
probably damage the transformer
...

a
...
Autotransformer
c
...
Audio-frequency transformer
e
...


Primary leads-black; secondary leads-red
...


Carelessness
...


Discharge them by shorting them to ground
...


To minimize the possibility of providing a path for current through your body

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