Blog 9: Transformer Basics: Its Type and Construction

Introduction:

Transformers are devices that transfer electrical energy from one circuit to another using electromagnetic induction. A transformer is used to increase or decrease the voltage level between the circuits. Transformers in the simplest way is described as a device that is used to step up or step down voltage. In step-up transformers, the output voltage increases; in step-down transformers, the output voltage decreases. The step-up transformers will reduce the output current and the step-down transformers will increase the output current to keep the input and the output power of the system equal.

The transformers are voltage control device that is used widely in the distribution and transmission of alternating current power. The idea was first discussed by Michael Faraday in the year 1831 and was carried forward by many other prominent scientific scholars. However, the general purpose of using transformers is to maintain a balance between the electricity that is generated at very high voltages and consumption that is done at very low voltages.

It is a device used in the power transmission of electric energy. The transmission current is AC. It is commonly used to increase or decrease the supply voltage without a change in the frequency of AC between circuits. It works on basic principles of electromagnetic induction and mutual induction.

Different types of Transformer

1)   Transformer on basis of power application

a.    Electrical transformer:

The electrical transformer also known as a high power transformer is designed for a high-power application. They are used at grid stations, power distribution in cities, controlling of high voltage lines, and voltage changes in high voltage lines for home applications. Since electrical voltage in high tension wires works at a voltage of 155000 to 765000 volts while the consumers cannot deal with a high voltage application hence the transformers are used. These transformers are big in size and shape.

b.    Electronic transformer

Electronic transformers are designed especially for low-power applications. They are used in computers, radio frequency (RF) devices, and lighting. Electronic transformers are used to insulate circuits and to provide high to alternating-but-low impedance. Because electronic equipment uses electron tubes, electronic transformers must provide appropriate bias voltage values for proper operation. Since weight and space are important considerations in designing electronic equipment such as handheld devices, electronic transformers are smaller and lighter than older power transformers.  

2)   Transformer on basis of change in voltage level

a.    Step up transformers

Step up transformers are transformer that delivers a higher output voltage and lower output current. This type is determined by the number of turns of wire. So, if the second set has a greater number of turns than the primary side, it means that the voltage will correspond to that, which forms the base of a step-up transformer. They are used between the power generator and the power grid.

b.    Step down transformers

Step-down transformers are transformers that deliver a lower output voltage and a lower output current. This type is typically used for stepping down the level of voltage in the power transmission and distribution network and so its mechanism is the exact opposite of a step-up transformer. These transformers are used to convert high voltage primary supply to low voltage secondary output

3)   Transformer on basis of design

a.    Core type transformers

This transformer features two horizontal sections with two vertical limbs and a rectangular core with a magnetic circuit. Cylindrical coils (HV and LV) are placed on the central limb of the core type transformer

b. Shell-type transformers

The shell-type transformer has a double magnetic circuit and a central limb with two outer limbs.

4)   Transformer on basis of supply

a.    Single phase transformers

A single-phase transformer has only one set of windings. Separate single-phase units can offer the same results as three-phase transfers when they are externally interconnected.

b. Three-phase transformers

A triple-phase (or three-phase) transformer has three sets of primary and secondary windings to form a bank of three single-phase transformers. The three-phase transformer is mainly used for the generation, transmission, and distribution of power in industries

5)   Transformers on basis of use

a.    Power transformers

Typically used to transmit electricity and has a high rating. It is used at power generation stations as they are suitable for high voltage application

b.    Distribution transformers

This electrical transformer has a comparatively lower rating and is used to distribute electricity. Mostly used at distribution lanes for domestic purposes. They are designed for carrying low voltages. It is very easy to install and characterized by low magnetic losses.

c.    Protection transformers

They are used for component protection purposes. In circuits, some components must be protected from voltage fluctuation, etc. Protection transformers ensure component protection.

6)   Transformer on basis of cooling

a.    Oil cooled transformers

This type is generally employed in small transformers of up to 3 MVA and is it designed to cool itself by the surrounding airflow.

b. Water-cooled transformers

This type of electrical transformer employs a heat exchanger to facilitate the transfer of the heat from the oil to the cooling water.

c.    Air cooled transformers

In this type of transformer, the heat that is generated is cooled with the help of blowers and fans that force the circulation of air on the windings and the core.

7)   Transformer on basis of core

a.    Air core transformers

The flux linkage between primary and secondary winding is through the air. The coil or windings wound on the non-magnetic strip.

b.    Iron core transformers

Windings are wound on multiple iron plates stacked together, which provides a perfect linkage path to generate flux

Construction of a single-phase transformer

A transformer consists of two coils as seen in the figure. One is a primary coil or input coil and the other one is a secondary coil of the output coil. From the figure above, it can be seen that in a transformer there is no direct electrical connection between the two coil windings. Since there is no medium or connection between the coils hence it was given the name of an Isolation Transformer.

Generally, the primary winding of a transformer is connected to the input voltage supply which converts or transforms the electrical power into a magnetic field. While the secondary winding uses that magnetic field and converts this alternating magnetic field into electrical power producing the required output voltage as shown.

Transformer Construction (single-phase)

Where:

  VP  –  is the Primary Voltage

  VS  –  is the Secondary Voltage

  NP  –  is the Number of Primary Windings

  NS  –  is the Number of Secondary Windings

  Φ (phi)  –  is the Flux Linkage

A single-phase transformer can operate to either increase or decrease the voltage applied to the primary winding. When a transformer is used to “increase” the voltage on its secondary winding with respect to the primary, it is called a Step-up transformer. When it is used to “decrease” the voltage on the secondary winding with respect to the primary it is called a Step-down transformer.

However, a third condition exists in which a transformer produces the same voltage on its secondary as is applied to its primary winding. In other words, its output is identical with respect to voltage, current and power transferred. This type of transformer is called an “Impedance Transformer” and is mainly used for impedance matching or the isolation of adjoining electrical circuits.

The difference in voltage between the primary and the secondary windings is achieved by changing the number of coils turns in the primary winding ( NP ) compared to the number of coil turns on the secondary winding ( NS ).

As the transformer is basically a linear device, a ratio now exists between the number of turns of the primary coil divided by the number of turns of the secondary coil. This ratio is called the ratio of transformation, more commonly known as a transformer “turns ratio”, (TR). This turns ratio value dictates the operation of the transformer and the corresponding voltage available on the secondary winding.

It is necessary to know the ratio of the number of turns of wire on the primary winding compared to the secondary winding. The turns ratio, which has no units, compares the two windings in order and is written with a colon, such as 4:2 (4-to-2).

This means in this example, that if there are 4 volts on the primary winding there will be 2 volts on the secondary winding, 4 volts-to-2 volt. Then we can see that if the ratio between the number of turns changes the resulting voltages must also change by the same ratio, and this is true.

Transformers are all about “ratios”. The ratio of the primary to the secondary, the ratio of the input to the output, and the turn ratio of any given transformer will be the same as its voltage ratio. In other words for a transformer: “turns ratio = voltage ratio”. The actual number of turns of wire on any winding is generally not important, just the turns ratio, and this relationship are given as:

A Transformers Turns Ratio= Np/Ns= Vp/Vs= n= Turn Ratio

Assuming an ideal transformer and the phase angles:  ΦP ≡ ΦS.

Advantages of transformers:

  • A transformer will increase or decrease basically AC voltage, current or independence.
  • It is efficient for the high-frequency range.
  • The available power cannot change but will slightly decrease depending upon the efficiency of the transformer.
  • It has the advantage of preventing condensed flux leakage as well as iron loss.
  • It offers good mechanical strength.
  • The transformer is widely used in power transmission.

Disadvantages of transformers:

  • It will not work with DC voltage under any condition.
  • The transformer size becomes un widely.
  • The physical size of transformers is directly related to the amount of power to be desired.
  • It is not good to use outdoors.
  • It can be noisy.

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