ELECTRICAL TRANSFORMERS
A TOP GRADED STUDY FOR
STUDENTS AND PROFESSIONALS
RYAN GODSELL
Copyright © 2013 by Ryan Godsell
All rights reserved. This book or any portion thereof may not be reproduced
or used in any manner whatsoever without the express written permission of
the publisher except for the use of brief quotations in a book review.
Table of Contens
Introduction
Principles of Operation
Construction
General
Core Type
Shell Type
Ring Type
Specialised Transformers
Auto Transformers
Voltage Transformers
Current Transformers
Conclusion
Introduction
Transformers, widely used throughout the electrical industry, are static
electromagnetic devices which employ the principles of mutual induction to
increase or decrease voltage levels according to the requirements of their
given application.
First commercialised almost 130 years ago by William Stanley in
Massachusetts, USA, transformers can today be found powering homes and
businesses the world over as part of a sophisticated array of distribution
systems. They also find use in almost every electronic device you can think
of, including your mobile phone charger and the computer used to write this
book!
Evidently, transformers play a crucial role in todays society and the way we
go about our every day lives. But how do they actually work? How are they
made? And what different types are out there? These are all questions which I
aim to answer in this book.
Principles of Operation
When an electrical current flows through a conductor, a magnetic field is
formed around the conductor. This is the basic principle on which
transformers are based.
Magnetic fields, in some regard, can be considered to be lazy. By this, I mean
that they will always choose to travel around the path of least resistance. In
taking a current carrying conductor, wrapping it several times around itself to
form a coil and inserting through the centre a piece of ferrous metal to create
a magnetic circuit, the magnetic field produced by the current can be greatly
intensified. This idea is better shown by the following illustration:
The magnetic flux chooses to flow around the ferrous core due to its high
permeability and low reluctance with respect to air. In other words, the flux
experiences less resistance.
If a D.C. current is applied to the coil, the magnetic field in the core will not
reach its final value instantaneously. Why? Well, as the field expands, its flux
lines will cut the coil perpendicularly, inducing an e.m.f. which acts in the
opposite direction to that of the circuit’s voltage source, limiting current and
therefore magnetic field intensity. This is knows as self - induction.
Mathematically, Faraday’s Law states that the magnitude of this induced
e.m.f. will be equal to the number of turns which make up the coil, multiplied
by the instantaneous rate of change of magnetic flux:
Once the flux reaches its final steady value ( i.e. rate of change = 0 ) an e.m.f.
will no longer be induced and current can flow unimpeded through the coil.
Referring back to the diagram on page three - the magnetic flux travelling
around the ferrous core is constant. That is, the amount of flux in one part of
the core will be the same as in any other part, much like current in a series
circuit. If a second coil of wire were to be wound on the core, it would
become magnetically linked to the first coil. This means that a change in
magnetic flux in the core, produced by a change in current in the first coil,
will induce an e.m.f. in the second coil of a magnitude determined by
Faraday’s Law, forming the principles of mutual induction on which, as
mentioned earlier, all transformers are based.
A problem arises though if the first coil - known as the primary winding - is
powered by a steady direct current as used in my previous example.
With a D.C. source, the magnetic field produced in the transformer core will
only be changing for a very short period of time, which means that an e.m.f.
in the second coil, or secondary winding will be induced for a similarly short
period of time.
The solution to this problem is a simple one - alternating current is supplied
to the primary winding in place of direct current, translating to a constantly
changing magnetic field in the core. Now, an e.m.f. is always being induced
in the secondary winding, the magnitude of which is used to determine
whether the transformer is classified as step up ( secondary voltage greater
than primary ) or step down ( primary voltage greater than secondary ).
As the rate of change of flux experienced by the windings in a transformer
will be the same at any moment in time, it can be deduced from Faraday’s
Law that step up transformers must have a secondary winding whose number
of turns is greater than the primary, and step down transformers must have a
secondary winding whose number of turns is less than the primary. This gives
the equation:
So the ratio of primary voltage to secondary voltage is equal to the ratio of
primary turns to secondary turns. Does this mean that primary current and
secondary current also follow the same rule. Actually, no - they don’t.
The laws of the conservation of energy state that energy cannot be created or
destroyed, it can only change form. Power in an electrical system can be
determined by multiplying together voltage and current. If a step up
transformer were to produce, for example, 500V across its secondary winding
from a 250V source, the transformation ratio would be 1:2. Now, if the
primary current was 10A and the secondary 20A ( 1:2 ratio ), then secondary
power would be four times the primary. And then, in the blink of an eye, the
world’s energy problems would be solved! But they’re not, so this obviously
isn’t the case…
In actual fact, secondary current decreases in a step up transformer and
increases in a step down transformer, maintaining power between the
windings assuming no losses. So, the last equation can be re - written to
include current as follows:
An interesting and important quirk of transformers is that once a load is
connected across the secondary winding, the secondary current will produce
another magnetic field in the core which will act to oppose the main core flux
- that is, it travels in the opposite direction. By observing the circuit
schematic below, the reason behind this becomes more clear:
Current in the secondary winding travels in the opposite direction to the
primary, denoted by Ip and Is respectively, so it stands to reason that the
corresponding magnetic fields will also oppose one - another. Crucially
though, in order to maintain the original quantity of flux in the transformer
core, more current is drawn from the electrical supply. It effectively does not
allow the flux to decrease.
Transformers are of course not 100% efficient. Energy is lost to heat any time
an electrical current flows, and further losses occur through induced eddy
currents, leakage flux and hysteresis effects on the ferrous core. These will be
covered more thoroughly in the construction chapter of this report.
The principles and concepts outlined here give an insight into how single -
phase power transformers work, however many more types exist for various
specialised applications. These too will be covered in another chapter, but for
now it should be easy to appreciate just how useful transformers are in our
use of electricity.
Construction
General
Transformers are not 100% efficient. Just as a car experiences drivetrain
losses which prevent full power transfer from engine to wheels, transformers
experience losses which prevent full power transfer from primary to
secondary windings.
Specifically, losses in transformers come from four main areas: winding
resistance, leakage flux, induced eddy currents and hysteresis effects on the
ferrous core. These can all be minimised though through the use of suitable
materials and refined construction techniques.
Starting from the top - any electrical conductor with a relatively high
resistance will dissipate significant amounts of power as heat energy. Great
for limiting current in various parts of an electronic circuit, but not so great
when designing an efficient transformer. For this reason, enamelled copper or
aluminium ( in heavy - duty applications ) is normally used in transformer
windings due to their high conductivity.
Leakage flux is the term given to magnetic flux produced by the transformer
windings which does not travel around the core, but instead expands and
contracts into space doing no useful work. If this leakage can be reduced,
efficiency can be increased.
By configuring a transformers windings in different formations around the
core, more flux can be forced to link between primary and secondary,
increasing mutual inductance and further reducing losses. With regards to
single phase power transformers, three setups are commonly used namely:
core type, shell type and ring type.
Core Type
A core type transformer has windings formed around opposite core limbs. It
is perhaps the most widely used configuration despite greater amounts of
leakage flux compared with its alternatives.
Here, half of the low voltage windings and half of the high voltage windings
are placed on each limb, separated from each other by an insulting medium. It
can, in a sense, be likened to an electrical series circuit where magnetic flux
in the core can travel around only a single path.
Shell Type
Shell type transformers are preferred for use in extra high - voltage and
higher MVA applications despite being subject to a more labour intensive
manufacturing process. Generally lighter in weight compared with its core
type counterpart, both primary and secondary windings are formed on one
central limb, where flux leakage is less prevalent.
If a core type transformer can be compared with an electrical series circuit,
then the shell type can be compared with a parallel circuit, where magnetic
flux can travel around two separate paths.
Ring Type
Ring type, or toroidal transformer cores are made up of a single strip of
ferrous metal wound tightly into one continuous donut shaped coil. Often
packaged inside an insulating plastic container, they generally find use in low
power applications due to high manufacturing costs. Indeed, it would be
unlikely to come across a toroidal transformer rated at more than a few kVA.
Leakage flux in traditional core and shell type transformers comes mainly
from the high quantity of air gaps around the core and relative flux density
inside the core. Here though, leakage is at a minimum as virtually no air gaps
exist.
Eddy currents, another source of power loss in transformers, are small
circulating currents which are induced in a transformers core by the rapidly
changing magnetic field. In fact, these current can produce quite dramatic
heating effects if left to their own devices.
Electrical resistance is inversely proportional to cross - sectional area. If a
transformers core were to be made of one single chunk of metal, its resistance
would be very low. Eddy current therefore would be of a high magnitude,
experiencing little opposition to flow.
Reducing the magnitude of these currents is key to minimising losses in this
area. For this reason, transformer cores are made up of individual sheets -
known as laminations - which are electrically insulated from one another,
effectively reducing cross - sectional area and increasing resistance.
Now, a voltage and current will be induced in each lamination which will be
of a much smaller value than before. Overall, there will be a considerable
reduction in power loss.
A materials’s magnetic hysteresis loop dictates the quantity of energy which
will dissipate as heat as it magnetises and demagnetises under the application
of a changing magnetic field.
A transformer’s core will always be made of a material which has a tall,
narrow hysteresis loop similar to that shown above on the right hand side.
Characterised by this is a high saturation point and, crucially, the need for
only a small coercive force to become fully demagnetised.
On the left is a loop associated more with a permanent magnet. It represents a
material which retains a high flux density on the removal of an external
magnetising force. A large amount of energy is required in order for it to be
magnetised and demagnetised - not a desirable property when this process
may repeat several times a second.
Today, through the use of specialised materials and refined manufacturing
techniques, transformers can reach up to 98% efficiency. An excellent
example of getting better with age!
Specialised Transformers
Auto Transformers
Auto transformers are a type of transformer in which primary and secondary
share one common winding. They benefit from reduced production costs over
the conventional core and shell types thanks to the need for a lesser amount
of raw materials.
An A.C. supply connected across a given number of turns in the single
winding will yield a specific voltage per turn ratio. This will be constant
through the entire winding so that a load can simply be connected across a
number of turns which will output the desired operating voltage.
In the diagram above, an auto transformer with multiple outputs is shown.
Evident is the fact that it can act in a step - up or step - down manner, perhaps
offering a secondary voltage in the range of 40 - 120% of that provided by
the supply. Although not all are designed designed this way, it is obvious that
the ease by which alternations can be made make auto transformers a very
convenient proposition.
Limitations include the inability to provide electrical isolation between
primary and secondary circuits, rendering the auto transformer useless for
applications involving a particularly sensitive load. Furthermore, a short
circuit within the sole winding can lead to full supply voltage appearing
across the load.
Evidently then, the selection and installation of an auto transformer must be
preceded by careful consideration for how all of its attributes - good and bad
- will fair in the long term.
Voltage Transformers
Transformers now find widespread use in electrical measurement and testing
equipment where large voltages and currents are present.
Standard voltmeters and ammeters are simply not designed to handle what in
some cases may be several thousand volts and amps, which presents a
problem - how are high voltage transmission lines or high - load premises to
be monitored safely and accurately without resorting to the manufacture of
jumbo sized test equipment?
The answer, as touched on, is through the use of specialised transformers
designed with this very purpose in mind.
Voltage transformers - sometimes called potential transformers - are used
where high voltages are to be measured and / or recorded. They are not
dissimilar to standard step down transformers in that the primary winding is
connected in parallel across the electrical supply to be measured, and the
secondary connected across a voltmeter where a high internal resistance
limits excess current flow.
Typically, voltage is stepped down to a maximum of around 120V. The
number of primary and secondary winding turn is calculated precisely to
ensure accurate readings.
Current Transformers
Current transformers ( or C.T.s ) stray noticeably from typical transformer
designs. A few variants exist, from series connected types to clamp types
commonly found in handheld test equipment, although the basic principle
remains the same.
Essentially, a current transformer features only a small number of turns on its
primary winding - typically between one and three - which in the type
mentioned formerly in the paragraph above, is connected in series with the
current carrying conductor from which a reading is to be obtained. Acting as
a step up transformer, current is reduced from its primary value, normally to a
standardised magnitude of up to 1A or 5A.
Although effectively a step up transformer, a C.T’s secondary voltage is
normally very small thanks to the low resistance presented by an ammeter.
Caution must be taken though when removing a series connected current
transformer - if the ammeter is disconnected first, a massive voltage may
suddenly appear across the secondary winding thanks to an increase in
resistance. As shown in the above diagram, a short circuit can be put in place
to prevent this.
Perhaps a more common sight are clamp type current transformers, where a
conductor is placed inside a set of moveable jaws containing what is the
secondary winding. A primary winding in a sense does not exist - the
magnetic field formed around the aforementioned conductor instead being
used to induce an e.m.f. in the secondary winding. There are no actual turns
in most case
In terms of the magnitude of induced voltage, the same principles apply as
before. Here though, the transformation ratio can be quite large due to the
sizeable difference between number of primary and secondary turns.
Usually, a C.T. is rated to give a maximum reading at a given primary current
- this being sufficient to induce maximum secondary current which, as
mentioned, can typically be 1A or 5A. For example, a primary current of
300A may induce a
secondary of 5A.
By forming loops with the primary winding - what was before just a straight
conductor - a smaller primary current can still equate to maximum secondary
current. Certainly a handy trick when working in the field.
Conclusion
To summarise, transformers in all their various shapes and sizes have become
an integral part of how the world today operates.
Without electricity, modern society would grind to a halt. Without
transformers, our use of electricity would be very limited and things we take
for granted would simply not exist. It is therefore easy to appreciate just how
important they are.
I hope that in this book, I have been able to explain clearly the working
principles of transformers, how they are constructed in ways by which to
maximise efficiency and how they have been adapted to find widespread
application in a number of crucial areas.
Thank you very much for reading,
