13. transformers for electronic and other applications

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Study Unit

Transformers forElectronic and OtherApplicationsBy

Robert L. Cecci

Copyright © 1998 by Education Direct, Inc.

All rights reserved. No part of the material protected by this copyright may bereproduced or utilized in any form or by any means, electronic or mechanical,including photocopying, recording, or by any information storage and retrievalsystem, without permission in writing from the copyright owner.

Requests for permission to make copies of any part of the work should be mailed to Copyright Permissions, Education Direct, 925 Oak Street, Scranton,Pennsylvania 18515.

Printed in the United States of America

05/17/04

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Transformers are very important in industry. They’re used

when the rated voltage of electrical equipment differs from

the voltage available at a voltage source. The increase or

decrease in voltage is made possible by transformers. Some

common uses of transformers are in the transmission of

electric power, in control and signal circuits, and in

electronic and radio equipment.

This study unit will introduce you to the fundamental

concepts of transformers.

When you complete this study unit, you’ll be able to • Explain what the main parts of a transformer are

• Explain how mutual inductance makes it possible to changean AC (alternating current) voltage or current from one valueto another

• Determine the turns ratio when the primary and secondaryvoltages or currents are known

• Calculate primary or secondary voltage or current wheneither one of these and the turns ratio are known

• Explain why transformer cores are laminated (layered)

• Explain the principle of operation of an autotransformer

v

OPERATION OF TRANSFORMERS 1

What Is a Transformer? 1Mutual Inductance 2Step-Down and Step-Up Transformers 3Turns Ratio 4Voltage Ratio and Secondary Voltage 5Conditions in Open and Closed Secondary Circuits 6Power in Primary and Secondary Windings 7Load Current in Primary and Secondary Windings 9Transformer Losses 11Reducing Losses 11Transformer Regulation 12

TYPES OF TRANSFORMERS 14

Transformer Construction 14Core-Form and Shell-Form Transformers 14Power Transformers 16Distribution Transformers 17Instrument Transformers 18Transformers with Two Secondaries 18Autotransformers 19Transformers for Radio and Electronics 21Specialty Transformers 21Inductors 22Saturable Reactors 23Magnetic Amplifiers 23Shielded Transformers 25Constant-Voltage Transformers 25Transformer Insulation 29Transformer Ratings 30Causes of Transformer Problems 30

SELF-CHECK ANSWERS 33

EXAMINATION 35

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1

OPERATION OF TRANSFORMERS

What Is a Transformer?

A transformer changes, or transforms, an alternating voltage

to a higher or lower alternating voltage. A transformer acts

very much like a pump in a water system that changes the

water pressure in the system.

If you take a basic transformer apart, you’ll find two separate

coils wound around an iron core (Figure 1). These are the

main parts of the transformer. The alternating voltage from a

voltage source (an alternator or a distribution power line) is

Transformers for Electronic andOther Applications

FIGURE 1—The primary winding in this transformer has 16 turns and the secondary has eight turns. The second-ary voltage will therefore have one-half the value of the primary voltage. (The lower transformer symbol shown isnow the most commonly used transformer symbol.)

applied to one coil, which is called the primary winding, or

simply, the primary. The other coil is called the secondary

winding, or the secondary. The secondary isn’t connected in

any way to the primary or to any other voltage source. The

secondary is used to connect to the control circuit or elec-

tronic circuit.

Both coils are insulated from the core. The transformer sec-

ondary is normally connected to an electrical load, such as a

lamp or a motor. The primary is in a closed circuit with an

AC (alternating current) voltage source, the secondary is in a

closed circuit that includes the electrical load. The two cir-

cuits are magnetically coupled, but electrically isolated, from

each other. Normally the primary side of the transformer will

contain a fuse in its circuit. The secondary circuit also may

contain a fuse in its circuit.

When a voltage is applied to the primary, a voltage is induced

(or produced) in the secondary winding, and an alternating

current flows through the load. The applied voltage is also

called the primary voltage, and the induced voltage is the

secondary voltage. The induced voltage is due to mutual

inductance, which is an effect of electromagnetic induction.

Mutual Inductance

The relative movement between magnetic lines of force and a

conductor can generate, or produce, a voltage. In a generator,

a group of conductors is moving in a magnetic field, and a

voltage is induced in them. In a transformer, the conductors

don’t move, but the magnetic lines of force change because

the applied voltage changes.

The magnetic lines of force (or flux) created by the applied

voltage are shown by broken lines through the iron core

(Figure 1). The magnetic field indicated by these lines

changes with any variation in the applied voltage. The lines of

force change in number and in direction as they pass over

the turns of the secondary. The change of the magnetic field

has the same effect as movement, and the result is mutual

inductance—or a voltage induced in the secondary winding.

Transformers for Electronic and Other Applications2

Transformers for Electronic and Other Applications 3

The voltage induced in the secondary winding is proportional

to the change in the magnetic lines of force that pass through

the secondary winding.

If a battery (which is a DC voltage source) were connected to

the primary winding in place of the AC voltage source shown

in Figure 1, the magnetic lines of force in the transformer

core would be constant because the magnitude of the voltage

source is constant. The secondary voltage is therefore zero

volts because the magnetic lines of force aren’t changing.

Therefore, an important transformer principle to remember is

that transformers can transform or change only AC voltages;

they can’t transform or change DC voltages.

If the secondary is connected to a load and a closed path is

provided for the current, an alternating current will flow

through the load.

Any change in voltage magnitude and direction in the pri-

mary produces changes in the lines of force, and, therefore,

in the voltage induced in the secondary. If the voltage in the

primary is alternating at a certain frequency, the voltage

in the secondary is also alternating and has the same

frequency.

The value of the voltage induced in the secondary depends on

the ratio of the number of turns in the secondary to the

number of turns on the primary winding. If the secondary

winding has more turns than the primary, the secondary

voltage is greater than the primary voltage. If the secondary

winding has fewer turns than the primary, the secondary

voltage is less than the primary voltage. If the primary and

secondary windings have the same number of turns, the volt-

age on the secondary will be equal to the applied voltage on

the primary windings.

Step-Down and Step-Up Transformers

If a transformer has more turns in the primary winding than

in the secondary winding, the secondary voltage is less than

the primary voltage. This means that the secondary voltage

has been decreased, or stepped down. Such a transformer is

called a step-down transformer. For example, if the voltage

available from the distribution line is 4600 V (volts) and the

motors used in the plant are rated at 240 V, a step-down

transformer is needed. The transformer is placed between the

distribution line and the motor to step down the voltage from

4600 V to 240 V. Since the secondary voltage must be 20

times lower than the primary voltage, the secondary winding

of the transformer must have 20 times fewer turns than the

primary winding.

A transformer can also have the secondary voltage higher

than the primary voltage. Such a transformer is a step-up

transformer. For example, if a voltage generated in an alterna-

tor is 2300 V and the transmission lines carry the electric

power at 230,000 V, a step-up transformer is needed. The

transformer is connected between the alternator and the

transmission line. The secondary of such a step-up trans-

former must have 100 times more turns than the primary.

Figure 2 shows typical step-down and step-up transformers.

Turns Ratio

The change, or transformation, of voltage depends on the

ratio of number of turns in the primary to number of turns in

the secondary. The turns ratio is therefore a very important

factor in transformer structure. If the number of turns in the

primary is indicated by the symbol Np and the number of

turns in the secondary by the symbol Ns, the turns ratio can

be written as follows:

�N

Np

s

� or Np : Ns


FIGURE 2—The ratio of thenumber of turns between the primary and secondarywindings determines if thetransformer is a step-down orstep-up transformer.

Transformers for Electronic and Other Applications

For example, if a step-down transformer has 1000 turns in

the primary and 20 turns in the secondary, the turns ratio

for that transformer is

�N

Np

s

� = �10

2

0

0

0� = �

5

1

0� or 50 : 1 (50 to 1)

That is, the primary voltage in the transformer is 50 times

higher than the secondary voltage.

A second example: If a step-up transformer has 10 turns in

the primary and 400 turns in the secondary, its turns ratio is

�N

Np

s

� = �4

1

0

0

0� = �

4

1

0� or 1 : 40

Here, the secondary voltage in the transformer is 40 times

higher than the primary voltage.

And still another example: If a transformer has 100 turns in

the primary and 100 turns in the secondary, its turns ratio is

�N

Np

s

� = �1

1

0

0

0

0� = 1 or 1 : 1

In this transformer, the primary voltage is equal to the sec-

ondary voltage. Here, the voltage isn’t transformed at all, so

you might wonder why the device is even called a trans-

former. The answer is that such a transformer may be used

for special applications, such as to electrically isolate two cir-

cuits without changing their voltage. These transformers are

called isolation transformers. Isolation transformers are often

used by technicians for safety reasons when they’re working

on ungrounded equipment.

Voltage Ratio and Secondary Voltage

The ratio of the primary voltage and the secondary voltage is

equal to the turns ratio. If the primary voltage is indicated as

Ep and the secondary voltage as Es, the ratio of voltages can

be expressed by the formula

�E

Ep

s

� = �N

Np

s

�

If both the number of turns and the primary voltage are

known, the secondary voltage can be calculated by solving

the voltage formula for Es as follows:

Es = �N

N

p

s� � Ep

5

For example, if a primary voltage (Ep) of 50 V is applied to a

step-up transformer with 100 primary turns (Np) and 1200

secondary turns (Ns), the secondary voltage will be

Es = �N

N

p

s� � Ep Write the formula.

Es = �1

1

2

0

0

0

0� � 50 Substitute the vaules of Ns, Np,

and Ep. Divide 1200 by 100.

Es = 12 � 50 Multiply 12 � 50.

Es = 600 V Answer: Es = 600 V

Without using the formula, you can find the secondary volt-

age by reasoning as follows: The turns ratio is 100 : 1200 =

1 : 12. That means that the secondary has 12 times more

turns than the primary. It must, therefore, have a voltage 12

times higher, or 12 � 50 = 600 V.

Or, if a primary voltage of 24 V is applied to a step-down

transformer with 200 primary turns and 50 secondary turns,

the secondary voltage will be

Es = �N

N

p

s� � Ep = �2

5

0

0

0� � 24 = 6 V

Again, this problem can be solved by reasoning. The turns

ratio is 200 : 50 = 4 : 1, which means that the primary has 4

times more turns and 4 times higher voltage than the sec-

ondary. The secondary voltage is thus one-fourth of the

primary voltage, or 24/4 = 6 V.

Conditions in Open and ClosedSecondary CircuitsFigure 3 shows transformer circuits in a schematic diagram.

Standard symbols are used to indicate the components. In

this example, we’ll use a resistor connected to the trans-

former secondary windings to provide the electrical load.

When the switch in the secondary circuit is open, there’s no

load connected to the transformer and no current flows in

the secondary winding (although a voltage is induced in that

winding). This is the no-load condition of the transformer.



The primary is connected to a an AC voltage source, and a

no-load current flows in the primary circuit. The no-load pri-

mary current is very small and is called the exciting current.

When the switch is closed, a current flows through the

secondary winding and the load. This current is called the

secondary load current. A larger primary current starts to flow

in the primary, automatically adjusting itself to the load

current in the secondary.

When working with transformers, you must be able to distin-

guish between no-load and load conditions. That’s because

you’ll have to know what voltage and current levels should be

present when you make voltage and current measurements

under these two conditions.

Power in Primary and SecondaryWindings

An ideal, (or perfect) transformer is a transformer in which

there are no losses. In an ideal transformer, under load condi-

tions, the power in the primary circuit is equal to the power

delivered to the load by the secondary circuit. You learned

earlier that electric power (P ), in watts, is equal to the voltage

(E ), in volts, times the current (I ), in amperes. Or,

P = E � I

In a transformer, the product of the primary voltage (Ep) and

the primary current (Ip) is equal to the product of the second-

ary voltage (Es) and the secondary current (Is). This is

expressed as a basic formula below.

FIGURE 3—This schematicdiagram uses symbols toindicate the components.The sine wave enclosed bythe circle is the symbol foran AC generator (or alterna-tor). The two scalloped linesrepresent the windings ofthe transformer, and the twostraight lines between thescalloped lines represent theiron core. The switch and theresistor are also shown bytheir standard symbols.

Pp = Ps

or

Ep� Ip = Es � Is

The secondary of a transformer can’t supply any more power

than it receives from the primary, just as a water pump can’t

pump out any more water than it takes in. Step-up and step-

down transformers have different voltages and currents in

their primary and secondary circuits, but the power in each

circuit is the same. Therefore, in a step-up transformer, the

secondary voltage becomes higher than the primary voltage,

and the secondary current must become lower than the pri-

mary current to keep their product the same. Figure 4 shows

an example of this. Here the secondary current (Is) will be

lower than the primary current (Ip).

Similarly, in a step-down transformer, the secondary voltage

is lower than the primary voltage, but the secondary current

is higher than the primary current to keep their product the

same.

The primary current under load can be calculated if the pri-

mary voltage, secondary current, and secondary voltage are

known. (Remember that the power doesn’t change.) Use the

formula

Ip = �E

E

p

s� � Is

For example, if the primary voltage is 14 V, the secondary

voltage 140 V, and the secondary current 20 A (amperes), the

primary current is


FIGURE 4—In this step-uptransformer, the secondaryvoltage will be higher thanthe primary voltage and thesecondary current will belower than the primary current.


Ip = �E

E

p

s� � Is = �1

1

4

4

0� � 20 = 200 A

The secondary voltage is 10 times greater than the primary

voltage, and the primary current must then be 10 times

greater than the secondary current.

Any quantity can be found by using the basic formula if the

other three quantities are known.

Load Current in Primary and SecondaryWindings

If the load in the secondary circuit of a transformer is a

resistance, as shown in Figure 3, the secondary current can

be found by Ohm’s law. The secondary current (Is), in

amperes, is equal to the secondary voltage (Es), in volts,

divided by the load resistance (R), in ohms, or

Is = �E

Rs�

Suppose that in a step-up transformer the turns ratio is

1 : 2. The secondary voltage is twice the primary voltage.

The secondary current must then be one-half the primary

current.

The load currents can also be determined without knowing

the voltage if the turns ratio is known. The magnetic field

produced in the transformer core is the same on the primary

and the secondary side of the transformer. The strength of

the magnetic field, or the magnetomotive force, is measured

in ampere-turns. If the number of primary turns (Np) is

multiplied by the primary current (Ip), the product, Np � Ip, is

the magnetomotive force (in ampere-turns) produced in the

transformer. Since the same magnetic field exists in the

entire transformer, the product of the number of secondary

turns, Ns, and the secondary current Is, must be the same as

the product NpIp, or NpIp = NsIs.

The primary current can then be determined by the formula

Ip = �N

N

p

s� � Is

For example, if a primary has 100 turns and the secondary

200 turns, and if the secondary current is 2 A (as determined

by the load resistance), the primary current is

Ip = �N

N

p

s� � Is

Ip = �2

1

0

0

0

0� � 2 = 4 A

Transformers are often rated in terms of VA (volt-amperes) or

kVA (kilovolt-amperes). This rating system is used because

both the secondary voltage and the current are included in

the transformer’s rating.

Transformers can be purchased in standard kVA sizes from

.05 kVA (small) to thousands of kVA (large).

To determine the capacity of a transformer, you simply

multiply the secondary voltage by the secondary current. For

example, if a transformer has 20 A at 120 V on the second-

ary, the secondary rating would be

VA = Is � Es

VA = 20 A � 120 V = 2400 VA or 2.4 kVA

For this situation, a 3 kVA transformer would be used. A

transformer should always be selected that’s the next higher

standard kVA rating than is calculated. This will prevent heat

buildup and transformer failure. It also allows for future

expansion of the circuit that’s powered by the transformer.

Let’s try a practical example. Suppose a single-phase motor

rated at 12 A at 120 VAC (volts alternating current) will be

attached to a 480 VAC system. The transformer will obviously

be a step-down transformer with a turns ratio of 4 : 1. What

will be the size of the transformer for this application?

VA = Is � Es

VA = 12 A � 120 V

VA = 1440 VA or 1.44 kVA

A 2 kVA transformer should be used.



Transformer Losses

Of course, there’s no such thing as an ideal transformer. If

such a transformer were possible, it would have no losses. A

typical transformer, however, approaches the ideal in effi-

ciency, although it’s not 100 percent perfect.

Here are the main reasons why transformers aren’t perfect:

• The resistance of the windings is never zero. Therefore,

there’s a power loss due to resistance (I2R).

• The lines of force set up by the current in the windings

should all pass through the iron core; however, some of

them follow other paths through the air, producing what’s

known as magnetic leakage.

• Most magnetic materials don’t immediately demagnetize

when the magnetizing force is removed, resulting in

hysteresis losses. Hysteresis losses result due to the

energy required to magnetize or demagnetize a magnetic

material.

• The magnetic core material is also a conductor of

electricity and may form a closed circuit with circulating

currents, known as eddy currents, present in it.

The sum of all losses is subtracted from the input power to

give the output power.

Transformer efficiency may be expressed as:

Efficiency = �o

i

u

n

t

p

p

u

u

t

t

p

p

o

o

w

w

e

e

r

r� � 100

For example, a transformer whose apparent input power is

220 VA when it’s providing 210 VA to a load has an efficiency

of

�2

2

1

2

0

0� � 100 = 95%

Reducing Losses

In a high-quality transformer, the losses are very small. For

example, the coils are wound with a wire large enough to keep

the I2R losses low, and the reluctance of the iron core is low,

keeping magnetic leakage very low. The use of certain alloys

also reduces hysteresis losses. Finally, building the core of

laminations (thin sheets of magnetic iron), instead of using a

solid core helps to reduce eddy current losses. All these fac-

tors are considered in the design of commercial transformers.

Transformer Regulation

The resistance and other losses in a transformer reduce the

secondary voltage, making the value of the secondary voltage

under load different from the no-load value. When no load is

connected to the secondary, the ratio of primary to secondary

voltage is practically equal to the turns ratio. However, when

a load is connected to the secondary and current flows in the

transformer windings, there will usually be a decrease in the

secondary voltage. The change in secondary voltage resulting

from applying a load to a transformer divided by the full-load

secondary voltage is called the regulation of the transformer.

Transformer regulation, which is expressed as a percent is

given by the following formula:

Vs No Load – Vs Full Load% Regulation =

Vs Full Load � 100

In a perfect transformer, which has no losses, the no-load

secondary voltage will be the same as the full-load voltage

and the transformer will have 0% regulation. Special trans-

formers known as constant-voltage transformers will be

discussed later in this study unit. Now, review what you’ve

learned by completing Self-Check 1.



Self-Check 1

At the end of each section of Transformers for Electronic and Other Applications, you’ll beasked to pause and check your understanding of what you’ve just read by completing a “Self-Check” exercise. Answering these questions will help you review what you’ve studied so far.Please complete Self-Check 1 now.

1. The device that can change an AC voltage from one value to another is called a(n)_______.

2. A change of voltage in one coil that leads to a voltage in another nearby coil is a result of (self-, mutual) inductance.

3. If the secondary winding has five times as many turns as the primary winding, the secondary voltage is _______ the primary voltage.

a. one-fifth c. five timesb. the same as d. twenty-five times

4. If the primary winding of a transformer has more turns than the secondary winding, it’scalled a step-_______ transformer. If the secondary has more turns, it’s called a step-_______ transformer.

5. In the transformer shown below, find the secondary voltage if the primary voltage is 12 V.

6. The current that flows in the transformer primary with no load connected to the secondary is called the _______ current.

7. What is the primary current in the transformer of question 5 when the secondary loadcurrent is 4 A?

__________________________________________________________________________

Check your answers with those on page 33.


TYPES OF TRANSFORMERS

Transformer ConstructionA transformer, no matter what its type or form, is relatively

simple. It consists of an iron core, primary and secondary

windings, insulation, mechanical bracing (or other means of

holding the parts together as a unit), cooling means, case,

and bushings.

An iron core is needed in a transformer to provide an intense

magnetic field. An intense magnetic field produces the rated

voltage in the windings with a minimum of exciting current.

The iron core permits more lines of force to concentrate

within its own volume than within the same volume of air or

some other nonmagnetic material.

Cores are always laminated, that is, they’re made of thin steel

sheets called laminations. The laminations make transformers

cost more. They also reduce the eddy currents (currents

induced in the iron parts of the transformer). Laminations

are usually rectangle-shaped. However, on small transform-

ers, L-shaped and E-shaped laminations are sometimes used.

Core-Form and Shell-Form TransformersAccording to the form of core construction, a transformer

may be either core form or shell form. Any transformer can be

built in either form, but, for a given application, one or the

other form is often easier or less costly to use.

In the core form, windings surround the iron core. In the shell

form, there’s a frame (or shell) of iron around the windings.

Figure 5A shows a single-phase transformer of the core form.

It has two core legs, and each leg carries a part of the pri-

mary winding and a part of the secondary winding wound on

top of it. The iron yoke provides a closed path for the flux.

(A single-phase transformer may also be built with all of the

windings on one core leg.)

In the single-phase shell-form transformer (Figure 5B), both

windings are wound on the center leg. The yoke provides a

shell, or return path, for the flux.


Figure 6A shows a three-phase transformer of the core form. It

has three core legs, each carrying one phase of the low-voltage

winding and one phase of the high-voltage winding. Figure 6B

shows a three-phase shell transformer, which has five core

legs. A set of low-voltage and high-voltage windings is wound

on each of the three inner legs. The two outer legs don’t carry

windings.

FIGURE 5—The low-voltagewinding in a transformer isusually next to the iron core,and the high-voltage windingfarther from the core. Thisreduces the possibility of a short circuit, as from high-voltage rupture of theconductor insulation.

FIGURE 6—Each of the three-phase transformers here has three sets of primary windings and three sets of secondary windings.

Power Transformers

Industry uses some transformers for power applications.

These transformers are known as power, distribution, and

instrument transformers.

Power transformers are large transformers usually used at

generating stations to step up the voltage for transmission

systems. Substations use power transformers to step down

the voltage for supplying distribution systems. Figure 7

shows a three-phase power transformer. According to com-

mon usage, any transformer rated over 500 kVA (kilovolt-

amperes) is a power transformer. The high-voltage rating may

be any value up to the highest voltage in use on transmission

lines. The low-voltage rating is usually between 2400 V and

345,000 V. However, the high-voltage and low-voltage ratings

can have any value desired.


FIGURE 7—Transformers ofthis size are seldominstalled inside a building.Much of the equipment onthis transformer is requiredfor cooling the coils.


Distribution Transformers

Distribution transformers are primarily transformers used to

step down the voltage from a distribution voltage to a load

voltage (2400 V, 600 V, or 480 V for industrial plants; and

240 V and 120 V for residential and commercial use).

According to common usage, such transformers are rated

up to 500 kVA. Their high-voltage rating can be up to

67,000 V, and their low-voltage rating can be rated up to

15,000 V. Figure 8 shows a bank of three pole-mounted

distribution transformers.

If you were to open the case of a distribution transformer,

the transformer would appear as shown in Figure 9. Note

that the high voltage is applied to the transformer through

heavily insulated bushings and wires. Also, note that

there’s a surge diverter used to protect the transformer.

This surge diverter protects the transformer and its load

circuit from power surges such as lightning strikes.

FIGURE 8—Distribution trans-former banks like these arecommonly used to serviceindustrial and commercial businesses.

FIGURE 9—An assembleddistribution transformer isshown here in a cutawayview with part of the outsidecover removed.

Instrument Transformers

Instrument transformers are either potential or current

transformers. Potential transformers are small transformers

rated about 200 VA or 500 VA. They’re built to transform the

higher transmission or distribution voltage to a lower distri-

bution voltage. This voltage is usually 120 V or 115 V. They

permit the use of low-voltage measuring instruments such

as meters and relays. Current transformers are small trans-

formers of about 50 to 200 VA with low current (usually 5 A)

on the secondary side. This current is applied to measuring

instruments and relays. Figure 10 shows two current trans-

formers and a potential transformer.

Transformers with Two Secondaries

A transformer may be specially designed for a specific

purpose. The principle of operation is the same for all trans-

formers. However, the forms, connections, and auxiliary

devices differ widely.

One common type of transformer is a single-phase trans-

former with two secondaries (Figure 11). The primary voltage

is 120 V, and the secondary voltages are 240 V and 24 V


FIGURE 10—The potential transformer and two current transformers shown here are examples of instrument transformers. The number on a current transformer is the value of full-load primary current required for a 5A sec-ondary current.


respectively. The number of secondaries is usually not more

than two to four, except for some applications in electronic

equipment.

Autotransformers

The usual transformer has two windings that aren’t wired

directly to each other. In an autotransformer, one of the

windings is connected in series with the other, as shown in

Figure 12A. (This is a step-up autotransformer.)

FIGURE 11—This transformer has two secondaries. Theoretically, there’s no limitto the number of windings a transformercan have. Secondary No. 1 has a step-upturns ratio of 1 : 2, while secondary No. 2has a step-down turns ratio of 5 : 1.

FIGURE 12—Figure 12Ashows an autotransformerwired as a step-up trans-former. Figure 12B showsthis same transformer usedas a step-down transformerby interchanging Ep and Es.

The primary voltage (Ep) is applied to the primary (or

common) winding. The secondary (or series) winding connects

in series with the primary at the junction point, or terminal.

This point may be obtained by a tap. This tap will divide

a single winding into a primary and a secondary of the

autotransformer.

A voltage induced in the secondary winding adds to the volt-

age in the primary winding. The secondary voltage (Es) is

higher than the applied voltage. The transformation ratio

depends on the turns ratio, as in a two-winding transformer.

The autotransformer shown in Figure 12B can be used as a

step-down autotransformer by applying a primary voltage

across both the primary and secondary windings.

Autotransformers are less costly than conventional two-

winding transformers. They also have better voltage regula-

tion and efficiency. They’re used in motor starters, so a

voltage lower than the line voltage may be applied during the

starting period. Autotransformers are also used economically

on high-voltage power lines where the values of the primary

and secondary voltages are about the same. For low-power

applications, variable autotransformers are available in which

the tap is a sliding contact rather than a fixed contact.

In the variable autotransformer shown in Figure 13, the mov-

able tap is a brush, which turns by means of a knob. Holes

are provided for mounting the transformer on a panel.


FIGURE 13—The variableautotransformer requiresperiodic maintenance: thesliding contact, which is acarbon brush, must occasion-ally be replaced. Also, thewindings on which the brushmoves must be kept cleanand smooth.


Adjustable autotransformers in which the load voltage isn’t

continuously variable are also available. Instead, a limited

selection of voltages is available by means of movable taps

operated by a selector switch.

Transformers for Radio and Electronics

Most transformers you’ll work with in plant maintenance

operate at an input power frequency of 60 Hz (hertz).

However, there are small, specially designed transformers

that are used in communications, inductive heating, and

other electronic equipment. These transformers are designed

to operate at audio, intermediate, or radio frequencies. They

include electronic power transformers with one primary

winding and several secondaries. They also include pulse

transformers and many other types of transformers.

Specialty Transformers

Specialty transformers make up a large class of transformers

used for changing line voltage to some particular value best

suited to the load. The primary voltage is generally 600 V, or

less. A sign-lighting transformer is one example of a specialty

transformer in which the 120 V is stepped down to 25 V for

low-voltage tungsten sign lamps. Other examples include

arc-lamp autotransformers, where 240 V is stepped down to

the voltage required for best operation of the arc, and trans-

formers that are used to change 240 V power to 120 V for

operating portable tools, fans, welders, and other devices.

Also included in this specialty class are neon-sign transform-

ers that step the 120 V up to between 2000 and 15,000 V for

the operation of neon signs. Many special step-down trans-

formers are used for small work, such as ringing bells,

running electrical toys, operating battery-charging rectifiers,

and lighting individual low-voltage lamps.

Inductors

An inductor is a coil or winding wrapped around a magnetic

core. Other names for an inductor are choke and reactor.

Inductors are used in AC circuits and oppose any change in

the current value in the circuit.

Figure 14 shows a very common application of an inductor—

a filter circuit. The input to this circuit is an AC voltage. This

AC input is coupled through a transformer to a device called

a rectifier. A rectifier converts an AC voltage into pulsating DC

voltage.

The pulsating DC voltage consists of a DC voltage and several

AC voltages that have frequencies that are multiples of the

frequency of the AC input voltage. This pulsating DC voltage

is supplied to the filter circuit, which consists of an inductor

and a capacitor. The inductor is a low impedance for the DC

component of the pulsating DC voltage, which passes

through the inductor easily. The capacitor is a high imped-

ance to the DC voltage, and doesn’t affect this component of

the pulsating DC voltage. The inductor is a high impedance

to the AC voltages, which are present in the pulsating DC

voltage and therefore resists passage of these AC voltages

through the inductor. The capacitor is a low impedance to

any AC voltages that might get through the inductor and it

will short them to ground. The result is a pure DC voltage at

the junction of the inductor and capacitor.

In electronic equipment, an inductor like the one shown in

Figure 14 is usually referred to as a filter choke.

You may come across other uses for inductors in your work

as well.


FIGURE 14—Pulsating DC isthe voltage output of therectifier. It always has thesame polarity because itvaries between zero and apositive maximum value. Itisn’t constant in value, hav-ing pulses like the tips of anAC wave.


Saturable Reactors

If a second winding is added to a inductor, as shown

in Figure 15, and direct current is supplied to the

winding, the reactance of the AC winding of the induc-

tor will vary as the direct current through the DC

control winding is varied. Such a two-winding inductor

with provision for DC control is referred to as a

saturable reactor. As the DC current, or signal, in the

control winding is increased (by varying the resistor

(R), for example), the reactance of the AC winding

decreases. As the DC signal is decreased, the reac-

tance of the AC winding increases. Thus, a saturable

reactor provides a method of controlling the reactance

in an AC circuit with a DC signal.

Saturable reactors are used in applications similar to

those of ordinary single-winding inductors. However,

unlike ordinary inductors, saturable reactors provide a

method of varying the reactance of the reactor.

Magnetic Amplifiers

A magnetic amplifier uses a saturable reactor to amplify a

small AC or DC control voltage to control a large load, which

may be a bank of lights or a motor. The magnetic amplifier

may consist of just the saturable reactor or the saturable

reactor may be combined with other components. Two or

more magnetic amplifiers may even be cascaded, with the

output from the first amplifier connected to the control input

of a second amplifier.

The saturable reactor shown in Figure 15 is rarely used for

magnetic amplifiers because the magnetic lines of force

developed by the AC winding are directly coupled into the DC

control winding by normal transformer action. Therefore an

AC voltage is introduced into the DC control circuit. This

problem is overcome using the saturable reactor shown in

Figure 16. The saturable reactor in this figure uses an iron

core with three legs. There are two AC windings, one wound

on each of the outer core legs. The DC control winding is

wound on the center leg of the core. The path of the magnetic

FIGURE 15—Saturable reactors arewidely used in industrial control andprocessing equipment. A reactor ofthis type can be magnetically satu-rated just as a sponge can be saturated with water.

lines of force developed by the AC supply voltage is shown as

a solid line in Figure 16. Notice how these lines of force flow

only in the outside portions of the iron core. Since none of

the magnetic lines of force developed by the AC supply volt-

age flow in the center leg of the core, there’s no AC voltage

developed in the DC winding of the saturable reactor. The

magnetic lines of force developed by the DC winding are

shown by the dashed lines of Figure 16. These lines of force

flow in all three legs of the iron core and can saturate the

outer core legs in the same manner as the saturable reactor

shown in Figure 15.

The rectifier and filter shown in Figure 16 are a part of the

magnetic amplifier and convert the AC supply voltage to a DC

voltage. This DC voltage can now be varied by adjusting the

resistor located in the DC control winding circuit. If a small

AC control voltage is now added in series with the DC supply

voltage, the DC output voltage across the load will now vary

in a sinusoidal manner to produce an AC voltage at the load,


FIGURE 16—By the use ofthe magnetic amplifier, asmall AC voltage can controla large AC voltage.


which has the same frequency as the AC control voltage. The

frequency of the AC supply voltage doesn’t have to be the

same as the frequency of the AC control voltage.

The overall effect of the magnetic amplifier is that a small AC

signal on the control winding produces a much larger change

in the AC signal in the AC winding. Any time a small input

signal produces a large change in the output signal, the

device has effectively amplified (or strengthened) the input

signal.

Shielded Transformers

In a typical transformer, the primary and secondary windings

are somewhat linked together by the capacitance between the

windings. This capacitance will allow electrical noise or a

voltage to pass directly from the primary to the secondary

windings without being magnetically coupled through the

transformer core. A special transformer, known as a shielded

transformer, has been designed to eliminate the passage of

this noise.

In a shielded transformer, a metallic shield is placed between

the primary and secondary windings. This shield is then

grounded. The grounding prevents the capacitive coupling

from being present, and all but eliminates the transfer of

electrical noise between the primary and secondary windings

by means of the capacitance between these windings.

Constant-Voltage Transformers

Most modern electronic equipment is very sensitive to voltage

fluctuations. For example, electronic motor controllers con-

tain circuitry that monitors the incoming voltage and will

shut down the controller if the AC voltage becomes too high

or too low. A constant-voltage transformer supplies a constant

AC secondary voltage even if the primary winding voltage

varies over a wide range.

The constant-voltage transformer consists of a transformer

core, a primary winding, a secondary winding, a resonating

winding, and two magnetic shunts. Figure 17A shows a side

view of the transformer core before the windings and

magnetic shunts are installed. The core is made up of many

thin sheets of steel assembled in the same manner as an

ordinary transformer. Figure 17B shows the windings and

magnetic shunts installed on the core. Notice that the pri-

mary winding is separated from the resonating and second-

ary windings by the magnetic shunts. The resonating and

secondary windings are usually wound one on top of the

other. The magnetic shunts are made from thin sheets of

steel assembled in the same manner as the core. Notice that

the magnetic shunts don’t completely touch the inner and

outer core legs. There’s a small air gap between each mag-

netic shunt and the transformer core.


(A)SIDE VIEW OF CONSTANT–VOLTAGE

TRANSFORMER CORE

(B)SIDE VIEW OF

CONSTANT–VOLTAGE TRANSFORMER

FIGURE 17—A side view ofa constant-voltage trans-former. Figure 17A showsthe side view of the trans-former core before the windings and magneticshunts are installed. Figure 17B shows a sideview of the completed transformer. Note the small air gap between themagnetic shunts and thetransformer core.


Figure 18 shows the circuit for a typical constant-voltage

transformer. The capacitor and the resonating winding form a

tuned circuit that’s resonant at the frequency of the voltage

applied to the primary winding. The resonating winding

increases the magnetic lines of force in the lower half of the

transformer core until the core becomes saturated—first in

one direction and then in the opposite direction as the

voltage applied to the primary winding changes direction.

However the magnetic lines of force in the upper part of the

core, which surrounds the primary winding, doesn’t saturate

this area of the core because some of the magnetic lines of

force from the lower part of the core are shunted away from

the upper part of the core by the magnetic shunts. Because

the magnetic lines of force linking the secondary and resonat-

ing windings are greater than the magnetic lines of force that

link the primary winding, the constant-voltage transformer

doesn’t obey the rules regarding the turns ratio between the

primary and secondary windings.

The voltage induced in the resonating winding depends only

on the change in the magnetic lines of force (which is varying

between the positive and negative saturation levels of the

core) and the rate of change in those lines of force (which is

fixed by the frequency of the voltage applied to the primary

winding). As long as the primary winding voltage can supply

enough energy to keep the lower part of the

transformer core in saturation, the resonat-

ing winding voltage will be independent of

the primary voltage. All of the magnetic

lines of force that link the resonating wind-

ing also link the secondary winding (since

they’re wound one on top of the other);

therefore, the turns ratio formula does

apply to the number of turns on the res-

onating and secondary windings. Since the

resonating winding voltage is fixed, the sec-

ondary voltage is also fixed and may be set

to any desired value by adjusting the turns

ratio between the

secondary wind-

ing and the res-

onating winding.FIGURE 18—This is an electrical schematic for a constant-voltage transformer.

There are several advantages in using the constant-voltage

transformer:

• Voltage regulation. The secondary voltage doesn’t change

when the primary voltage varies over a wide range.

• Electrical noise isolation. Because the primary and sec-

ondary windings are physically separated, electrical noise

can’t pass from the primary winding to the secondary

winding. In this respect the constant-voltage transformer

is very similar to a shielded transformer.

• Voltage spike protection. Large voltage spikes which may

appear on the primary winding can’t be passed to the

secondary winding since the value of the secondary

winding voltage depends on the core saturation. A volt-

age spike on the primary winding can’t cause the core

around the secondary winding to become any more

saturated than it already is.

There are also several disadvantages to using the constant

voltage transformer:

• The constant-voltage transformer runs hotter than a

similar-size regular transformer. This is because a large

part of the constant-voltage transformer core is always

in saturation and the transformer core losses increase as

the magnetic lines of force in the core increase. A core in

saturation has the maximum number of lines of force

that are possible for the type of steel used for the core.

In addition, the resonating winding has copper losses

because there’s current flowing in this winding. A regular

transformer doesn’t have this winding.

• The constant-voltage transformer secondary voltage will

vary if the frequency of the primary voltage varies. If the

primary voltage is supplied from a utility, this effect

won’t be seen because the utility frequency is very tightly

controlled by the speed of the utility’s generators.

However if a constant-voltage transformer is connected

to the output of a small portable generator, this effect

will be seen because the generator speed can’t be tightly

controlled as the generator load is changed.



• The constant-voltage transformer secondary voltage won’t

be a pure sinusoidal wave. The output voltage waveform

will tend to have flat tops instead of the rounded ones in

a pure sinusoidal wave. This won’t usually be a problem

for most equipment connected to the constant-voltage

transformer. Several types of compensating constant-

voltage transformers exist in which a small portion of the

primary voltage is added to the secondary voltage to

improve the output voltage waveform and make it more

sinusoidal.

Transformer Insulation

The transformers used in industrial plants

include the dry, the askarel-insulated, and

the oil-insulated types. Figure 19 shows a

dry-type transformer. Dry-type transformers

don’t use any liquid to cool the windings.

They can be installed without fireproof

vaults in all areas except some hazardous

ones. Transformers rated at less than

112 1/2 kVA or 35,000 V can be installed

within a plant without the use of a trans-

former room or vault. However, they must

be kept isolated from combustible materials.

Askarel-insulated transformers contain

special nonflammable liquids within their

cases. These liquids help to insulate the windings and cool

the transformer. Transformers rated in excess of 25 kVA are

provided with pressure-relief valves and require special venti-

lation if they’re installed in poorly ventilated areas. They’re

installed in vaults if the rated voltage exceeds 35,000 V.

Oil-insulated transformers are filled with oil to provide insula-

tion for the windings and to cool the transformer. Generally,

oil-insulated transformers of any voltage are placed in vaults

when used within a building. Oil-insulated transformers are

either self-cooled by radiators exposed to the atmosphere, as

shown in Figure 20, or cooled by fans that circulate the air.

FIGURE 19—Dry-type transformers like this one arefrequently used in industrial plants to step 480 Vdown (to 120 V or 240 V) to supply electric lights.


Transformer Ratings

Small transformers are usually rated in terms of secondary

volts and amperes. Larger transformers are rated in

kilovolt-amperes, or kVA. Transformers that supply lighting

loads shouldn’t have a capacity less than the total connected

load. For incandescent-lamp circuits, the kilovolt-ampere

rating of the transformer should equal the total wattage of

the lamps. For example, a 10 kW (kilowatt) incandescent

lamp load could be supplied by a transformer furnishing

240 V at 41.7 A (240 � 41.7 = 10,008 VA) = 10 kVA.

Power and motor loads should be computed as being equal to

the connected load. In practical applications, 1 kVA of trans-

former rating is to be supplied for each horsepower. For

example, a 5 hp (horsepower) motor will be supplied by a

5 kVA transformer.

Causes of Transformer Problems

The main enemies of transformers (especially larger trans-

formers), are heat, moisture, vibration, a corrosive atmos-

phere, dust, and dirt. Excessive heat and moisture can cause

a breakdown of insulation between turns or windings, or

between windings and the core. Excessive vibration can

loosen the bolts and lock washers that hold laminations

together. A corrosive atmosphere can cause deterioration of

FIGURE 20—These oil-cooled transformersare installed outside the building and are padmounted.


copper conductors and poor contact at terminals. Metallic

particles carried by a ventilating system or dust can result in

poor air circulation, insulation breakdown, or both.

Small, electronic transformers such as those used in small

control circuits need little attention if the equipment they’re

used in is installed in clean, dry, open locations.

The larger transformers (some weighing up to several tons or

more), require much more maintenance. Such maintenance

includes cleaning the air paths and tightening the connec-

tions during downtime periods of the plant.

Now, take a few moments to review what you’ve learned by

completing Self-Check 2.


Self-Check 2

1. The two forms of core construction are _______ form and _______ form.

2. The output of a current transformer is usually

a. 1 A. c. 10 A.b. 5 A. d. 100 A.

3. In the box, draw the symbol for a step-down transformer. Label Ep and Es.

4. Fill in each of the spaces with one of the four words from the list below.

power distribution electronic specialty

a. Small power transformers that have one primary winding and several secondary wind-ings are referred to as _______ transformers.

b. Transformers which are used to change a primary voltage to another value for operat-ing bells, signs, and battery-charging rectifiers are known as _______ transformers.

5. Which current(s) is/are applied to the control winding of a magnetic amplifer?

a. Output current c. AC signal current onlyb. DC control current only d. DC supply and AC control currents

Check your answers with those on page 33.

33

Self-Check 1

1. transformer

2. mutual

3. c

4. down, up

5. 36 V

Es = �N

N

p

s� � Ep

= �6

2

0

0

0

0� � 12

= 3 � 12

= 36 V

6. exciting

7. 12 A

Self-Check 2

1. core, shell

2. b

3.

4. a. electronic

b. specialty

5. d

An

sw

er

sA

ns

we

rs

Self-Check Answers34

NOTES

35

1. What is the efficiency of a transformer that has an input of 600VA and an output to a load of 580 VA?

A. 20 VA C. 83.8%B. 180 VA D. 96.6%

2. A power transformer connected to a 120 VAC line delivers 12VAC at the secondary. What type of transformer is this?

A. A step-up transformer B. A step-over transformer C. A step-down transformer D. A step-out transformer

925 Oak Street

Scranton, Pennsylvania 18515-0001

Transformers for Electronic and Other Applications

When you feel confident that you have mastered the material in this study unit, complete the following examination. Then submitonly your answers to the school for grading, using one of the examination answer options described in your “Test Materials”envelope. Send your answers for this examination as soon as youcomplete it. Do not wait until another examination is ready.

Questions 1–20: Select the one best answer to each question.

EXAMINATION NUMBER:

38700801Whichever method you use in submitting your exam

answers to the school, you must use the number above.

For the quickest test results, go to http://www.takeexamsonline.com

Ex

am

ina

tion

Ex

am

ina

tion

Examination36

3. In a magnetic amplifier, a large AC voltage is controlled by a

A. varistor and resistor combination.B. set of three batteries.C. small AC voltage.D. rectifier.

4. An inductor for which reactance can be varied by supplying current through a separate DCwinding is called a(n)

A. reactance tube. C. autotransformer.B. current transformer. D. saturable reactor.

5. If the turns ratio of a transformer is 4 : 1 and the incoming voltage is 120 VAC, what is thesecondary voltage?

A. 30 VAC C. 240 VACB. 120 VAC D. 480 VAC

6. If the primary in a transformer has more turns than the secondary, the secondary has

A. lower current. C. higher power.B. lower voltage. D. lower ampere-turns.

7. What type of transformer is used to prevent electrical noise from passing from the primarywinding to the secondary winding?

A. Dry-type transformer C. Shielded transformerB. Oil-filled transformer D. Delta transformer

8. You’ll work at times with a transformer that has one winding connected in series with theother to form the equivalent of a single winding. This is called a(n)

A. distribution transformer. C. specialty transformer.B. electronic transformer. D. autotransformer.

9. If the power in the primary circuit of a 4 : 1 transformer is 12 watts, what will be the powerin the secondary of this transformer?

A. 3 watts C. 12 wattsB. 4 watts D. 48 watts

10. What would be the power in the secondary of a transformer that has a voltage of 48 VACand a current of 2.2 A?

A. 52.8 watts C. 121.8 wattsB. 105.6 watts D. 212.6 watts

Examination 37

11. What is the kVA rating of a transformer in which the secondary winding is delivering 10amps at 480 VAC?

A. 4.8 kVA C. 480 kVAB. 48 kVA D. 4800 kVA

12. A basic transformer consists of two separate conductive coils wrapped around a(n) _____core.

A. copper C. ironB. oiled paper D. plastic

13. Inductors are used in AC circuits to oppose changes in current value. What are two com-mon names for inductors?

A. Choke and rectifier C. Choke and reactor B. Rectifier and reactor D. Coil and rectifier

14. The primary current drawn by the transformer shown below will be

A. 2 A. C. 10 A.B. 5 A. D. 50 A.

15. What special installation or maintenance requirements are needed for askarel-insulatedtransformers?

A. Cooling water must be supplied to the transformer core.B. The transformer must be installed in a vault if the voltage exceeds 35,000 VAC.C. The oil must be changed at least once a year.D. The radiator must be installed where air can circulate and the fins must remain clean.

16. The property that allows the change of voltage in one coil to lead to a change of voltage inanother coil is called mutual

A. reactance. C. reduction.B. inductance D. impedance.

17. Why are high-quality transformers wound with large-diameter wire?

A. To lower the hysteresis lossesB. To lower the I2R lossesC. To lower the eddy current lossesD. To lower the magnetic leakage losses

Examination38

18. If 120 VAC is applied to a transformer with a turns ratio of 6 : 1, what type of transformer ispresent and what is the output voltage of the secondary?

A. Step-up, 720 VAC C. Step-down, 20 VACB. Step-up, 600 VAC D. Step-down, 60 VAC

19. When a load is suddenly connected to a transformer, the secondary voltage will bereduced. This drop in voltage is called transformer

A. seduction. C. excitation.B. regulation. D. conductance.

20. Most small electronic devices, such as portable radios, contain an internal 120 VAC trans-former. Usually, what type of transformer is this?

A. Step-down C. Oil-filledB. Step-up D. Air-gap

13. transformers for electronic and other applications

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