Sambungan Motor 3 Fasa

Sambungan Bekalan 3 Fasa Ke Motor
Motor aruhan 3 fasa sangkar tupai
KENDALIAN

Apabila stator motor diberikan bekalan 3 fasa, dengan sendirinya putaran medan magnet terbentuk pada stator itu. Laju putaran medan magnet berputar bergantung kepada
frekuensi bekalan dan bilangan kutub.

Gelombang 3 fasa dibekalkan kebelitan stator

Penghasilan medan magnet di stator pada ketika ‘A’ dan ‘B’

Penghasilan medan magnet di stator pada ketika ‘C’ dan ‘D”

Arus/voltan akan teraruh dalam pengalir rotor oleh pemotongan fluks magnet berputar

Medan magnet yg terhasil di sekeliling bar sangkar tupai

Motor Tiga Fasa

Motor Aruhan Tiga fasa

Motor 3 fasa dapat memberikan kecekapan yang lebih baik, faktor kuasa yang lebih tinggi, mempunyai sifat permulaan yang lebih baik dan harga yang lebih murah jika dibandingkan dengan motor 1 fasa yang mempunyai keupayaan dan saiz yang sama.

Penggunaan motor 3 fasa memang digalakkan untuk mengelakkan ketidakimbangan beban pada sesalur bekalan. Dalam bab ini akan dibincangkan motor aruhan 3 fasa jenis rotor sangkar tupai (squirrel cage rotor) dan rotor berlilit @ gelang gelincir
( wound rotor @ slip-ring ). Termasuk juga motor segerak @ synchronous motor.

Motor aruhan 3 fasa sangkar tupai (Squirrel cage induction motor 3 phase)
Motor ini mempunyai stator(pemegun) berbelit dengan 3 belitan iaitu satu belitan setiap fasa dan mempunyai rotor(pemutar) sangkar tupai. Pada rotor tidak terdapat sebarang sambungan belitan, melainkan bar-bar pengalir yang diperbuat drpd aluminium @ kuprum yang dipateri dikedua-dua hujungnya agar berlitar-pintas.

Terdapat 3 bahagian utama:-
1. Pemegun (stator)

a. Stator atau utama ialah bahagian yg pegun dalam motor. Stator pada motor aruhan pada prinsipnya sama seperti stator pada pengulang-alik.
b. Ia diperbuat drpd lapisan keluli dan mempunyai lurah-lurah alur(lubang alur) hampir mengelilingi keseluruhan belitan stator.
c. Stator disokong oleh rangka motor yg diperbuat drpd besi tuang atau plat keluli.
d. Terdapat 3 belitan satu fasa di stator. Ia diletakkan pada sudut 60° mekanik atau 120 ° elektrik dan disambungkan samada secara bintang atau delta.

e. Di kotak pengkalan motor terdapat 6 terminal bagi maksud untuk sambungan kebekalan. Setiap 2 terminal mewakili satu belitan.
f. Tugas utama stator ialah sebagai teras untuk menghasilkan medan magnet berputar disamping sebagai pelengkap litar magnet.

Bahagian pegun dikenali sebagai pemegun/stator. Hanya stator disambung kpd bekalan 3 fasa.

Pemutar (rotor)
1. Rotor atau sekunder ialah bahagian berputar. Ia berbentuk selinder dan dibina
daripada lapisan-lapisan keluli yg bertebat.

2. Bagi motor jenis rotor berlilit, terdapat 3 set belitan pada lubang alur rotor yg diakhiri dengan 3 gelang gelincir. Belitan ini disambung secara bintang.
Rotor sangkar tupai /squirrel cage rotor.

Binaan asas rotor sangkar tanpa teras berlamina

Teras rotor sangkar tupai adalah kukuh dan lasak, binaan teras adalah berlamina. Bar-bar sangkar tupai terletak dalam teras

Rotor double squirrel cage / dwi sangkar tupai

3. Motor aruhan sangkar tupai mempunyai batang-batang kuprum atau
aluminium dipasang selari dengan aci berhampiran dengan permukaan rotor.

4. Kedua-dua hujung kesemua batang-batang itu akan dikimpal atau pateri
secara gelang agar kesemua berlitar pintas. Batang-batang pengalir ini tidak
ditebat (bersentuh dengan teras).
5. Kadangkala terdapat juga batang-batang pengalir tidak dipasang selari dengan
aci tetapi condong dengan sudut beberapa darjah.
6. Ini bertujuan untuk mendapat daya kilas yg lebih sekata dan mengurangkan
bunyi dengung semasa motor bekerja.

7. Tugas rotor ialah untuk memutar aci motor. Bagi membolehkan ia berputar ia
perlulah menjadi medan magnet dan seterusnya magnet ini dapat berputar
mengikut putaran medan magnet di stator.

Perisai hujung
1. Perisai ini digunakan untuk memegang alas bebola yg menyokong aci rotor.
2. Perisai juga untuk melindungi bahagian-bahagian dalaman motor daripada
disentuh atau kerosakan mekanik.
3. Terdapat berbagai bentuk perisai dikiri dan kanan kuk motor. Bentuk-bentuk ini
disesuaikan dengan jenis motor; misalnya jenis tutupan rapi, tutupan separuh
terbuka, tutupan kalis api, bahan letupan, tahan panas dsbnya.

Synchronous Motor

Synchronous Motor Speed

There are two ways to define motor speed. First is synchronous speed. The synchronous speed of an AC motor is the speed of the stator's magnetic field rotation. This is the motor's theoretical speed since the rotor will always turn at a slightly slower rate.
The other way motor speed is measured is called actual speed. This is the speed at which the shaft rotates. The nameplate of most AC motors lists the actual motor speed rather than the synchronous speed.
Standard AC induction motors depend on the rotor trying, but never quite succeeding, to catch up with the stator's magnetic field. The difference in the speed of the rotor and the synchronous speed of the stator's rotating magnetic fields is called the slip. Different motor designs will produce different amounts of slip.
AC motors are designed with various numbers of magnetic poles. Standard motors have two, four, six, or eight poles. These poles play an important role in determining the synchronized speed of an AC motor.

A motor's synchronous speed can be computed using this formula: synchronous speed equals 120 times the operating frequency, divided by the number of poles.
-->For example: A six-pole motor's synchronous speed is 120 x 60 = 7200 divided by 6, or 1200 RPM.
A four-pole motor's synchronous speed will be 1800 RPM. Use this formula to determine other speed/pole relationships.

Three-Phase Motors

Three-Phase Motors
Three phase AC power is comprised of three independent voltages. Each phase is displaced 120 degrees from the others.

When phase one (A) is at zero volts, phase two (B) is near its maximum voltage and flowing in the positive direction. The third phase (C) is near its maximum voltage as well, but flows in the negative direction.
These three phases will change from positive to negative as the AC power cycles. A rotating magnetic field is produced if each of the three phases is connected to an electrically independent winding in an AC motor stator.

In this example, using time 1 as our reference point, the current flow in the green phase A winding is positive and pole A1 is north. The opposite pole, A2 is magnetically south. The resultant magnetic field is shown moving from north to south.
The current flow in the blue phase B winding is negative, so pole B2 is north and B1 is south. The resultant magnetic field is shown flowing from B2 to B1.
There is no current flow in red phase C, so these poles are not magnetized. They are neutral. The result is that there is no magnetic field being produced in this winding.
These magnetic fields produce a rotating force in the direction shown by the arrow. This arrow represents the turning of the rotor.

Moving to time 2, the red phase C current is negative going, thus poles C1 and C2 are south and north respectively. Their blue phase B current is positive going and poles B1 and B2 are north and south, respectively. Because the green Phase A is at zero, the A poles are neutral. The arrow represents rotation in the direction of the magnetic field.

Finally, at time 3, we see that the green Phase A is positive going and the red phase C is negative going. Their respective poles are energized with the resultant magnetic fields producing a continuation of the rotating magnetic field. This force is what creates the motion of the rotor.

AC power cycles 60 times per second between positive and negative. In a fraction of a second, the phases have shifted 60 degrees causing the relationship of the north and south poles to change at the same rate. Because the motor has established an induced magnetic field, the opposite fields of the rotor and stator attract each other, causing the rotor to follow the stator's magnetic field change.

As the rotor continues to follow the stators magnetic field, the three phases will shift yet another 60 degrees. It is this continuous change in polarity that causes the rotation of a motor.

How Electric Motors Work

Magnetism

Magnetism is the force that creates rotation for a motor to operate. The poles of a permanent magnet are connected by magnetic lines of force. The principle of magnetism states that unlike poles are attracted to one another while like poles repel. AC motors operate on this principle.
When two bar magnets come into close proximity, the resulting attraction and repulsion create force. The magnet on the left is stationary and cannot move. The one on the right is free-turning and rotates. As the "North" pole of the rotating magnet moves away from the like pole of the stationary magnet, the "South" pole of the rotating magnet is attracted towards the opposite pole of the stationary magnet. Since unlike poles attract, the turning magnet rotates until the "N" and "S" poles come together. When this occurs, both magnets are satisfied and no further action will occur.

How Electric Motors Works

Electric motors function on the principle of magnetism; where like poles repel, and unlike poles attract.

In a simple motor, a free-turning permanent magnet is mounted between the prongs of an electromagnet. Since magnetic forces travel poorly through air, the electromagnet has metal shoes that fit close to the poles of the permanent magnet. This creates a stronger more stable magnetic field. (The electromagnet functions as the stator, and the free-turning magnet is the rotor.) Fluctuating polarity in the electromagnet causes the free-turning magnet to rotate. The poles are changed by switching the direction of current flow in the electromagnet.
The direction of current flow can be changed in one of two ways. In a DC motor, connections must be interchanged at the battery. AC current oscillates on its own.

The stator in an AC motor is a wire coil, called a stator winding. It's built into the motor. When this coil is energized by AC power, a rotating magnetic field is produced.
When a magnetic field comes close to a wire, it produces an electric current in that wire. This is called induction. In induction motors, the induced magnetic field of the stator winding induces a current in the rotor. This induced rotor current produces a second magnetic field necessary for the rotor to turn.

Induction motors are equipped with squirrel rotors, which resemble the exercise wheels often associated with pet rodents like gerbils. Several metal bars are placed within end rings in a cylindrical pattern. Because the bars are connected to one another by these end rings, a complete circuit is formed within the rotor.

Consider this close-up of a 2-pole stator and one of its rotor bars. Alternating current flowing in the stator causes the poles to change rapidly, from north to south and back again. If the rotor is given a spin, the bars cut the stator lines of force. This causes current flow in the rotor bar. This current flow sets magnetic lines of force in circular motion around the rotor bars. The rotor lines of force, moving in the same direction as those of the stator, add to the magnetic field and the rotor keeps turning.

Electromagnetism

How Electromagnets Work

Electromagnets are similar to permanent magnets, but produce much stronger magnetic fields. Electric motors require this extra capacity.
To make an electromagnet, an iron rod is wrapped with insulated wire. The rod is called a "core".
Electric current flows through the wire when it is connected to a battery. This current magnetizes the iron core. Once magnetized, the core has both "N" and "S" poles. The poles of an electromagnet can be reversed by changing the direction of current flow.

When one or both ends of the wire at the battery are disconnected, current flow stops and the core loses its magnetism.
Alternating current changes directions on its own, causing the poles in the electromagnet to switch.

Types of DC Motors

Shunt Wound DC Motors

Shunt wound DC motors provide medium starting torque, 125% to 200% full load, and are capable of delivering 300% of full load torque for short periods. With excellent speed control, shunt wound motors generally drive loads requiring speed control and low starting torque.
Some applications include fans, blowers, centrifugal pumps, conveyors, elevators, printing presses, woodworking machines, and metalworking machines.
There are two basic types of shunt wound DC motors. Self-excited shunt wound motors have a shunt field and armature connected to the same power supply.
In separately excited shunt wound motors, shunt field and armature connect to separate power supplies.

Series Wound DC Motors

A series wound DC motor has its armature and field connected in a series circuit. These type motors normally drive loads that require high torque and do not require precise speed regulation. Series DC motors are ideal for traction work where the load requires a high breakaway torque. Such are locomotives, hoists, cranes, automobile starters, or oil drilling rig applications.

Starting torque developed in series motors normally ranges between 300% and 375% of full load, but attain 500% of full load torque. These motors deliver this high starting torque because their magnetic field operates below saturation.
An increase in load results in an increase in both armature and field current. As a result, the armature flux and field flux increase simultaneously. Since the torque developed in DC motors is dependent upon the interaction of armature and field flux, torque increases by the square of current increase.
Speed regulation in series motors is inherently less precise than in shunt motors. If motor load diminishes, current flowing in both the armature field circuits reduces as well, effecting a reduction in flux density.
This results in a greater increase in speed than realized in shunt motors. Removal of mechanical load from series motors results in indefinite speed increase whereby centrifugal forces generated by the armature eventually destroy the motor.

Compound Wound DC Motors
Whenever an operation requires speed regulation characteristics unavailable in series or shunt motors, compound wound motors perform well. With medial starting torque capability, between 180 and 260% of full load, they deliver constant operating speeds under any percentage of full load.

This characteristic is a result of placing part of the field circuit in series with that of the armature. When under load, increased series winding current raises the level of field flux. Enlarged field flux in compound wound motors yields greater reduction in speed than in a shunt motor.
The compound wound DC motor comprises both series and shunt windings. The shunt winding connects in parallel with armature and series windings. Some associated applications include punch presses, shears, crushers, and reciprocating compressors.
Permanent Magnet DC Motors

Permanent magnet motors are well fit for use where response time is a factor. Their speed characteristics are similar to those of shunt wound motors. Built with a conventional armature, they use permanent magnets rather than windings in the field section. DC power is supplied only to the armature.
Permanent magnet motors are not expensive to operate since they require no field supply. The magnets, however, lose their magnetic properties over time, and this effects less than rated torque production. Some motors have windings built into the field magnets that re-magnetize the cores and prevent this from happening.
Automobiles have installed DC permanent magnet motors that control power seats, windows, and windshield wipers. DC permanent magnet motors produce high torque at low speed, and are self-braking upon disconnection of electrical power. Permanent magnet motors cannot endure continuous operation because they overheat rapidly, destroying the permanent magnets.

Universal DC Motors
Universal motors seldom exceed one horsepower, and do not run at constant speeds. The speed of a universal motor varies with its load. Among the applications using these motors are vacuum cleaners, food mixers, portable drills, portable power saws, and sewing machines.
In most cases, little more than a few hundred rpm is reached with heavy loads. When the motor operates with no load, the speed may attain 15,000 rpm.
The universal series motor differs in design from a true induction motor. The rotor of a universal motor is made of laminated iron wound with wire coils. The ends of the coils, or loops, connect to a commutator. Electric current in the motor flows through a complete circuit formed by the stator winding and rotor winding. Brushes ride on the commutator and conduct the current through the rotor from one stator coil to the other. Directed by these brushes the rotor current interacts with the magnetic field of the stator causing the rotor to turn. When the direction of current flow changes in the stator, it changes in the rotor. Since the magnetic field is reversed, the rotor continues to turn.
Universal motors have series wound rotor circuitry similar to that of DC motors. They have high starting torque and high starting current. The name universal derives from the motor's capability of operating on either AC or DC power sources.
Universal, variable speed motors slow down with increased loads. High horsepower-to-size ratio is characteristic of their design. Due to the brush/commutator setup, universal motors require more maintenance than other motor designs.

Advantages of DC Motors

DC motors provide excellent speed control for acceleration and deceleration with effective and simple torque control. The fact that the power supply of a DC motor connects directly to the field of the motor allows for precise voltage control, which is necessary with speed and torque control applications.
DC motors perform better than AC motors on most traction equipment. They are also used for mobile equipment like golf carts, quarry and mining equipment. DC motors are conveniently portable and well suited to special applications, such as industrial tools and machinery that is not easily run from remote power sources

Types of Motors
There are several kinds of DC motors commonly used in industrial applications. The motors have similar external appearances but are different in their internal construction and output performance. When selecting a DC motor for a given application, two factors must be taken into consideration:
1.The allowed variation in speed for a given change in load.
2 The allowed variation in torque for a given change in load.

Principles of Operation of DC Motor

DC motors comprise four principal components a) field, b) armature, c) commutator, and d) brushes.
The field is the equivalent of a stator in an AC motor, and the armature functions as the rotor.

The brushes act as contacts between an external power source and the commutator. The design of these carbon brushes allows them to move up and down on a brush holder, to compensate for the irregularities of the commutator surface. Thus they are said to ride the commutator.

Each section of the commutator is connected to an armature coil, essentially a conductive loop of wire. A current induced in the armature coil, by way of the brushes and commutator, creates a magnetic field around the armature. Since the current flowing through the armature flows at a right angle to the field's magnetic lines of flux, the two magnetic forces interact. This interaction creates a third magnetic field that tends to rotate counter clockwise.

The commutator regulates current flow in the armature coils, allowing it to flow in one direction only. Each segment of the commutator is directly connected to an armature coil, so the commutator rotates with the armature. As it rotates, each segment of the commutator is constantly breaking contact with one brush, while simultaneously connecting with the other. Every time contact with a new brush occurs, the flow of current reverses in the armature coil.
The interaction of magnetic force from the armature and field poles is renewed each time the armature completes one-half of a rotation. This causes the armature to rotate for as long as current is maintained in the coils.

DC Motors

Brushed DC motors

The classic DC motor design generates an oscillating current in a wound rotor, or armature, with a split ring commutator, and either a wound or permanent magnet stator. A rotor consists of one or more coils of wire wound around a core on a shaft; an electrical power source is connected to the rotor coil through the commutator and its brushes, causing current to flow in it, producing electromagnetism. The commutator causes the current in the coils to be switched as the rotor turns, keeping the magnetic poles of the rotor from ever fully aligning with the magnetic poles of the stator field, so that the rotor never stops (like a compass needle does) but rather keeps rotating indefinitely (as long as power is applied and is sufficient for the motor to overcome the shaft torque load and internal losses due to friction, etc.)
Many of the limitations of the classic commutator DC motor are due to the need for brushes to press against the commutator. This creates friction. At higher speeds, brushes have increasing difficulty in maintaining contact. Brushes may bounce off the irregularities in the commutator surface, creating sparks. (Sparks are also created inevitably by the brushes making and breaking circuits through the rotor coils as the brushes cross the insulating gaps between commutator sections. Depending on the commutator design, this may include the brushes shorting together adjacent sections—and hence coil ends—momentarily while crossing the gaps. Furthermore, the inductance of the rotor coils causes the voltage across each to rise when its circuit is opened, increasing the sparking of the brushes.) This sparking limits the maximum speed of the machine, as too-rapid sparking will overheat, erode, or even melt the commutator. The current density per unit area of the brushes, in combination with their resistivity, limits the output of the motor. The making and breaking of electric contact also causes electrical noise, and the sparks additionally cause RFI. Brushes eventually wear out and require replacement, and the commutator itself is subject to wear and maintenance (on larger motors) or replacement (on small motors). The commutator assembly on a large machine is a costly element, requiring precision assembly of many parts. On small motors, the commutator is usually permanently integrated into the rotor, so replacing it usually requires replacing the whole rotor.
Large brushes are desired for a larger brush contact area to maximize motor output, but small brushes are desired for low mass to maximize the speed at which the motor can run without the brushes excessively bouncing and sparking (comparable to the problem of "valve float" in internal combustion engines). (Small brushes are also desirable for lower cost.) Stiffer brush springs can also be used to make brushes of a given mass work at a higher speed, but at the cost of greater friction losses (lower efficiency) and accelerated brush and commutator wear. Therefore, DC motor brush design entails a trade-off between output power, speed, and efficiency/wear

There are four types of DC motor:
1. DC series motor
2. DC shunt motor
3, DC compound motor -

there are also two types:
(a) cumulative compound
(b) differentially

4. Permanent Magnet DC Motor

Alatubah (penyejukan)

KAEDAH PENYEJUKAN PENGUBAH KUASA TIGA FASA

Kehilangan yang berlaku di transformer menyebabkan wujudnya kepanasan. Jika kepanasan tidak dicegah dengan segera ia boleh menyebabkan penebat belitan terbakar dan seterusnya berlaku litar pintas antara litar belitan.
Haba yang disebabkan oleh kehilangan kuprum dan teras mestilah di kurangkan bagi menjaga hayat dan kecekapan alatubah. Oleh kerana pengubah tidak mempunyai bahagian berputar yang membolehkan pengalih udaraan, ia sukar untuk disejukkan berbanding dengan mesin-mesin elektrik yang lain.

Bagi pengubah yang kecil ( kurang dari 12KW ), ia mungkin dapat di sejukkan oleh udara biasa melalui permukaan luar pengubah. Tetapi bagi pengubah besar, cara ini tidak memadai. Cara tambahan lain perlu dilakukan agar haba itu dapat dipindahkan keluar dari pengubah iaitu seperti dengan merendamkan pengubah itu ke dalam tangki yang berisi minyak khas. Permukaan tangki ini mungkin di buat beralun atau mempunyai tiub-tiub tambahan.

Bagi pengubah melebihi 500KW, keadaan penyejukan tambahan di buat dengan menggunakan air yang dialirkan kedalam tangki khas. Sekirannya ketiadaan air, bagas udara boleh digunakan untuk menyejukkan pengubah disamping minyak. Terdapat juga pengubah yang mengabungkan semua kaedah penyejukan tersebut.

Cara-cara penyejukan Alatubah
1. Dry type
2. Oil immersed, oil cooled
3. Oil immersed, self cooled
4. Air blast
5. Oil immersed, air cooled

Kerumitan Penyejukan
Suhu oil immersed tranformer yang disejukkan dengan bantuan peralatan luar akan ke paras bahaya jika berlaku kerosakan pada peralatan penyejukan itu. Sebagai pemabaukan sementara jika kerosakan ini berlaku, maka beban alatubah ini perlu dikurangkan supaya alatubah ini tidak menjadi terlalu panas.

Dalam alatubah penyejukan air, tiub saluran air boleh tersumbat disebabkan pengumpulan keladak daripada bekalan air. Jika saluran ini tidak dibersihkan jumlah pengaliran air aka berkurangan dan dengan itu suhu alatubah akan naik.

Dalam sistem penyejukan alatubah, kerosakan boleh berlaku jika air bocor ke dalam tangki minyak. Kebocoran ini berlaku disebabkan oleh hakisan dinding saluran yang telah berkarat. Untuk mengelakkan adalah elok jika saluran copper digunakan untuk menggantikan tiub besi.

Jika penyejukan luaran digunakan, risiko kebocoran air ke dalam minyak di atasi dengan meletakkan tekanan minyak di tahap yang tinggi daripada air dengan menggunakan pam. Cara ini kita boleh mempastikan yang setiap kebocoran berlaku adalah daripada minyak ke dalam tangki dan bukan sebaliknya.

Di dalam water cooled Transformer risiko yang lebih besar didapati atau di hadapi dengan kehadiran lembapan. Biasanya air yang berada dalam saluran adalah dalam keadaan normal iaitu diparas suhu yang rendah jika di bandingkan dengan minyak. Ini akan menyebabkan lembapan yang terdapat di atas tiub penyejukkan di punca dimana ia memasuki tangki, lembapan ini akan menitik ke dalam minyak. Risiko ini boleh dikurangkan dengan membalut tiub penyejukkan dengan bahan yang tidak bertindak dengan minyak dan tahan panas serta tidak mengalirkan udara.

Simbol-simbol yang biasa digunakan dalam sistem penyejukan

A= Penyejukan oleh udara sekeliling untuk alat ubah kering
N= Penyejukan semulajadi oleh pengaliran udara.
B= Penyejukan secara tiupan angin (dengan bantuan radiator dan kipas)
0= Direndam di dalam minyak
W= Disejukan oleh air
F= Peredaran udara dengan bantuan pam

Minyak mempunyai beberapa kebaikan berbanding dengan udara sebagai media penyejukan.
1. Minyak mempunyai ‘specific heat’ yang tinggi daripada udara iaitu ia menyerap lebih banyak haba untuk kenaikan suhu yang sama.
2. Minyak mempunyai daya pengaliran haba yang lebih baik daripada udara. Ia dapat mengalirkan haba daripada teras dan lilitan ke permukaan tangki dengan lebih cekap.
3. Daya penebat minyak 6 kali ganda lebih kuat daripada udara iaitu minyak bertindak sebagai bahan penebat.
4. Minyak dapat mengurangkan kebisingan daripada teras.

Basic principles of Transformers

Basic principles

The transformer is based on two principles: firstly, that an electric current can produce a magnetic field (electromagnetism) and secondly that a changing magnetic field within a coil of wire induces a voltage across the ends of the coil (electromagnetic induction). Changing the current in the primary coil changes the magnetic flux that is developed. The changing magnetic flux induces a voltage in the secondary coil.

An ideal transformer is shown in the adjacent figure. Current passing through the primary coil creates a magnetic field. The primary and secondary coils are wrapped around a core of very high magnetic permeability, such as iron, so that most of the magnetic flux passes through both primary and secondary coils.

Induction law
The voltage induced across the secondary coil may be calculated from Faraday's law of induction, which states that:

where VS is the instantaneous voltage, NS is the number of turns in the secondary coil and Φ equals the magnetic flux through one turn of the coil. If the turns of the coil are oriented perpendicular to the magnetic field lines, the flux is the product of the magnetic field strength B and the area A through which it cuts. The area is constant, being equal to the cross-sectional area of the transformer core, whereas the magnetic field varies with time according to the excitation of the primary. Since the same magnetic flux passes through both the primary and secondary coils in an ideal transformer the instantaneous voltage across the primary winding equals

Taking the ratio of the two equations for VS and VP gives the basic equation for stepping up or stepping down the voltage