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

What are Electrical Transformers?

The name itself offers a simple definition. Electrical transformers are used to transform electrical energy. How electrical transformers do so is by altering voltage, generally from high to low. Voltage is simply the measurement of electrons, how many or how strong, in the flow. Electricity can then be transported more easily and efficiently over long distances.
While power line electrical transformers are commonly recognized, there are other various types and sizes as well. They range from huge, multi-ton units like those at power plants, to intermediate, such as the type used on electric poles, and others can be quite small. Those used in equipment or appliances in your home or place of business are smaller electrical transformers and there are also tiny ones used in items like microphones and other electronics.
Probably the most common and perhaps the most necessary use of various electrical transformers is the transportation of electricity from power plants to homes and businesses. Because power often has to travel long distances, it is transformed first into a more manageable state. It is then transformed again and again, or “stepped down,” repeatedly as it gets closer to its destination.
When the power leaves the plant, it is usually of high voltage. When it reaches the substation the voltage is lowered. When it reaches a smaller transformer, the type found on top of electric poles, it is stepped down again. It is a continuous process, which repeats until the power is at a usable level.
You have likely seen the type of electrical transformers that sit on top of electric poles. These, like most electrical transformers, contain coils or “windings” that are wrapped around a core. The power travels through the coils. The more coils, the higher the voltage. On the other hand, fewer coils mean lower voltage.
Electrical transformers have changed industry. Electric power distribution is now more efficient than ever. Transformers have made it possible to transfer power near and far, in a timely, efficient, and more economical manner. Since many people do not wish to live in close proximity to a power plant, there is the added benefit of making it possible for homes and businesses that are quite a distance from power plants to obtain dependable, affordable electricity. Much of the electricity used today will have passed through many electrical transformers before it reaches users.

In electronic equipment, electrical power transformers with capacities in the order of 1 kw are largely used ahead of a rectifier, which in turn supplies direct current to the equipment. Electrical power transformers are usually made of stacks of steel alloy sheets, called laminations, on which copper wire coils are wound. Electrical power transformers in the 1- to 100-W power level are used principally as step-down electrial power transformers to couple electronic circuits to loudspeakers in radios, television sets, and high-fidelity equipment. Known as audio electrical power transformers, these devices use only a small fraction of their power rating to deliver program material in the audible ranges, with minimum distortion. The electrical power transformers are judged on their ability to reproduce sound-wave frequencies (from 20 Hz to 25 kHz) with minimal distortion.
At power levels of 1 milliwatt or less, electrical transformers are primarily used to couple ultrahigh-frequency (UHF), very-high frequency (VHF), radio-frequency (RF), and intermediate-frequency (IF) signals, and to increase their voltage. These high-frequency electrical power transformers usually operate in a tuned or resonant circuit (see Resonance), in which tuning is used to remove unwanted electrical noise at frequencies outside the desired transmission range.Transformer
Toroidal Power Transformers offer significant advantages over conventional laminated transformers:
SPACE SAVING Toroidal power transformers take up 50% less space when supplied with mounting brackets and terminal blocks for drop-in laminate replacement and up to 64% less space when supplied with flying leads instead of terminal blocks. (Many times it is easier to run the lead from the toroidal power transformer to the equipment rather than the reverse) For toroidal power transformers up to 1000VA, savings can be even greater as a centering washer and single centre screw or bolt will usually suffice, eliminating the need for a mounting bracket.
WEIGHT REDUCTION Toroidal power transformer can weigh up to 50% less. The toroidal power transformer core has the ideal shape for producing a transformer with the minimum amount of material. All windings are symmetrically spread over the entire circumference of the toroidal power transformer core, making the wire length very short. This results in lower winding resistance, and higher efficiency.
HIGHER EFFICIENCY Toroidal power transformers are manufactured with the highest quality materials which allow a savings of approximately 50% against conventional laminated transformers, as well as significant space savings.
ENERGY SAVINGS The use of toroidal transformers in place of conventional laminates offers significant energy savings.
FLEXIBLE DIMENSIONS Toroidal transformers offer a high degree of dimensional flexibility compared with conventional laminated transformers. Since toroidal power transformer cores are produced in our own core manufacturing and annealing facilities at each site, it is possible to make a core to virtually any diameter and height.
EASE OF MOUNTING Standard toroidal power transformer mounting for sizes up to 1000VA is with a single metal centering washer and mounting screw or bolt making installation quick and simple. Other popular methods include:
NOISE REDUCTION Because toroidal power transformer cores are manufactured from a continuous strip of high grade steel, there are no air gaps and loose sheets of steel or laminations to cause vibration. This stability is further enhanced by the copper windings of the toroidal power transformer which tightly surround the entire circumference of the core.
LOW STRAYFIELDA toroidal transformer will generally offer a reduction of 8:1 in magnetic interference levels compare with traditional frame style laminate types.
PRICE AND VALUE Highly developed production techniques coupled with material savings resulting from the more efficient design mean that today’s toroidal power transformer is extremely cost-effective when compared with similarly rated conventional units. When taking into account the other hidden benefits of the toroidal power transformer, the advantages become very significant. Generally, with toroidal power transformers the larger the size the lower the cost when compared to traditional types.