What is Electric Transformer, Its types, Active and Inactive Parts?

What is Electric Transformer, Its types, Active and Inactive Parts?
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What is Electric Transformer?

A transformer is an electrical static machine that steps AC voltage up or down, and transfers AC power from on circuit to another with fixed power and frequency.

You may ask, Why do we consider the transformer a machine while it has no moving parts? The answer is because the rules of electric machines and transformers are the same. The transformer is a static machine.

While I was searching the transformer topics I found a question saying, Does a transformer have moving parts? No, transformers have no moving parts. They are static machines that transfer electricity from one side to another with no moving parts. The only moving thing inside transformers is cooling oil.

Transformer principle of operation

Transformer principle of operation is mutual induction. The voltage in the primary winding generates magnetic flux, as a result an EMF is produced in the secondary winding. It uses the process of electromagnetic induction to transfer energy from one circuit to another.

The iron core plays the role of transmission path of the magnetic flux from the primary to the secondary windings. The iron core is  laminated to reduce eddy current and power losses.

Depending on the turns ratio between the primary and secondary windings, the transformer steps voltage up or down. While the frequency and power still the same.

Transformer consists of three main parts. The primary windings, secondary windings, and magnetic core.

Transformer active and auxiliary parts?

Electric transformer consists of two main groups of parts:

  1. Active parts.
  2. Auxiliary parts.

Transformer Active parts

 Active parts of transformer are the parts that are mainly used to transfer energy. Active parts are primary and  secondary windings and the iron core. These are called active parts because they are responsible of transforming voltage and current which is the transformer main job.

This doesn’t mean the auxiliary parts of the transformer are not important. Auxiliary parts are essential for proper and safe operation of the transformer.


Windings are divided in two. Low and high voltage windings. The side which is connected to power source is called primary and the other one which is connected to the load side is called secondary. A step up transformer has the primary side is the low voltage side and is connected to the source. While the secondary is the high voltage and is connected to the load.

Both windings are copper conductors coils. The number of turns is the main factor of determining the transformation ratio. As in the below formula.

V(In)/V(Out) = n1/n2


  • V(In) is the input voltage on the primary side.
  • V(Out) is the output voltage on the secondary side.
  • n1 is the primary side winding number of turns.
  • n2 is the secondary side winding number of turns.

Iron core

Iron core is made of laminated silicon steel iron with magnetic properties to make a low resistance magnetic path for the flux between primary and secondary windings. The reason to make the iron core laminated is to reduce eddy currents and power losses.

Auxiliary parts (Inactive).

Transformer auxiliary parts are all transformer parts except the active parts. Such as cooling fans, cooling oil, protection relays, pumps, temperature gauge, tank body, silica gel breather, sudden pressure relay and all other parts.

These parts are important also for transformer proper and safe working. These Auxiliary parts are necessary in transformers depending on its power rating and its use. Hereunder some of the transformer auxiliary parts:

  • Cooling oil: Transformers get heated and the main job of the oil is cooling it down. Another job is to isolate winding and core from each others. Also oil is used to protect metallic parts against oxidation. I have answered a important question about transformer oil, you can find it here.
  • Main tank: This tank is to contain the core and windings, protect them and to contain cooling oil.
  • Conservator tank: this is a smaller tank above the main one. Its job is to contain oil expansion and to ensure that the main tank is always full.
  • Silica gel breather: is used to ensure that only dry air enters the tank during breathing in and out process through the silica gel breather.
  • Buchholz relay: Is a protection relay to detect faults and either gives warning or isolate power in case of short circuit faults. For more details about the Buchholz relay I recommend reading my article here.

Different types of transformers are used for different purposes, like utilization, distribution, generation, and transmission of electrical power. The different types of transformer are Step down, step Up, Power , Distribution , and Instrument Transformer, etc. we are going to discuss each in detail.

Transformer Types Based on Core Medium

Transformers different types based on the core medium placed between the primary and secondary windings are classified as Air core and Iron core types.

Air Core Transformer

The flux between primary and secondary windings is linked through the air. Both the primary and secondary are wound on a non-magnetic strip.

Hysteresis and eddy current losses are eliminated in air-core type transformer, but compared to iron core the mutual inductance is less. The reluctance offered to the generated flux is higher in the air transformer type than iron core type.

Iron Core type Transformer

Iron core transformer is a transformer with its windings mounted on a laminated iron core to make a perfect linkage path for the flux between primary and secondary. While windings of primary and secondary are wound on multiple iron plate bunch.

Due to the conductive and magnetic property of the iron, less reluctance to the linkage flux is offered. Iron core type transformers are widely used, in which the efficiency is high compared to the air core one.

Transformers Types Based on Usage

Step Down Transformer

Step down transformer is this type of transformers that converts high voltage level applied on the primary winding to low voltage level on the secondary side. No matter what power rating the transformer is.

This type is used in electronic applications, and also in overhead power lines electrical distribution systems and household devices where low voltages are required for operation.

We can summaries applications of step down transformers as follows:

  • Chargers and adapters to step voltage level down to be suitable for charging cell phones and electronic devices.
  • Control circuits to step down voltage to feed control circuits. This application is widely used in industrial applications. We usually use this step down transformer when we have a control circuit with voltage lower than the main source voltage. For example, we have a 3.3KV soft starter panel, while in the same panel we have low voltage control circuit. In this case its clear to use a step down transformer of 3.3KV/220V to feed the low voltage control circuit.
  • Distribution substations to step down voltages from high to medium or low levels.
  • Before domestic loads and end users of electrical energy to step down voltages from medium to low values.

Step up transformer

The function of Step Up transformer is the opposite of the step-down transformer. Step up transformer increases voltage applied at the primary windings to a higher voltage at the secondary windings.

In this type , the ratio of primary to secondary turns (n1/n2) is less than one. Step Up transformers are used in electrical energy transmission systems.

One of the main uses of step up transformers is electrical transmission through OHTL. The power generation voltage is connected to power transformer, which is a step up transformer, to step up voltages before transmitting it through overhead transmission lines.

Why do we step up voltage? This voltage stepping up is essential because it gives the following advantages:

  • Increasing the transmission voltage decreases the current which in turns decreases the power losses during the long distances power lines.
  • The lower the transmitted current the smaller the wire size and the smaller the size of the wire carrying accessories. i.e lower cost.
  • The lower the wires and its accessories the smaller the towers i.e more money saving.

Also, step up transformers are used in case of voltage stabilizers. In case of voltage drop we use step up transformer to overcome this voltage drop issue.

Read my detailed article about step up and step down transformer and there uses here.

Power Transformer

A power transformer is a step up transformer connected to the power grid from one side and to the OHTL from the other side. Its main job is to step up the generation voltage to be transmitted through OHTL.

The rating of the power transformer is 33 KV, 66KV, 110KV, 220KV,400KV, and so on. Power transformers are installed at power generation stations.

Power transformer loads aren’t variable. This transformer type usually works at its full load capacity.

I have created a YouTube video about what is power transformer. You can find it on YouTube below at the end of the page.

Distribution transformer

This type is used to step down the voltage level. Normally are used at the consumer end. Therefore, the rating of this type of transformer is lower than power transformer, for example 230 V, 440V, 6.6 KV, and 11KV. The main uses are domestic as well as industrial purposes.

Power transformer vs Distribution Transformer

These two types of transformers are the same construction and working principle. The differences are:

  1. The rating of power Transformer is higher than that of distribution Transformer. Although there is no standard that specify the rating of each type. But some references consider that Power Transformers are higher than 500KVA. 
  2. Usage of distribution Transformer is in the distribution system, while power Transformer is mainly used in the power generation plants.
  3. Distribution transformers work on different load values, i.e the load is not fixed and varies during the day intervals. While power transformer loads are usually fixed.
  4. Power transformer works on full load, while distribution transformer can work on no load.

Its also important to know that IEC standard consider that the two types, I mean power and distribution, are one type. Also the American standard consider that all are power type Tr. And the only difference is the rating as follows:

  • Small power Transformer, rating from 500-7500KVA
  • Medium power Transformer, rating 7500KVA-100MVA
  • Large power Transformer, rating 100MVA and up.

Instrument transformer

An instrument transformer is a high accuracy step down transform that isolates or transforms  current or voltage level to use this lower level in measurement or protection devices. This type is used in measuring instruments and protection devices such as voltage meters, current meters and protection relays. Instrument transformers are mainly designed to operate at low voltage and low current.

These transformers also called isolation, because when there is a high voltage at the primary end, It isolates the secondary windings.

The purpose of the isolation is to protect the measuring device which is attached to the secondary side. This are further divided into a current (CT) and a potential transformer(PT). we will discuss both in detail in our next articles.

We can divide instrument transformer into two types. Current transformer (CT), and potential transformer (PT).

Current Transformer (CT)

Current transformer is an instrument transformer that steps down AC current to use it for overcurrent protection and measuring of electric current. It converts the high current into the desired value of the current required in the circuit.

Current transformer is used in swichgear, control panels, electrical generators and other applications where the current readings should be visible instantly. The CT is connected to an AC current gauge and shows the current value.

For more information about current transformer, read my article here.

Potential Transformer (PT)

Potential transformer is a step down transformer that is used to measure voltage when the primary side is connected with medium or high voltages. It is also used to step down voltage into a safe limit.

Potential transformer also is used to energize control and utility circuits in lower voltage than the main bus voltage.

In my work we have a 3.3KV soft starter panel. Inside the panel we have some low voltage contactors and control devices. The solution of feeding these devices is to use a potential transformer. In this case if the low voltage control devices are 110v. Then the potential transformer should be 3.3/110V.

Uses of instrument transformers:

  • In measuring instruments to work as a ratio device between high voltages and current circuits and the low voltage and low current measuring device.
  • To isolate testing devices and the person who is using it from high voltages and currents.

A good example of this transformer type is medium voltage current and voltage measuring devices. We use a medium voltage voltmeter. The over head power line voltage is 11KV. With the help of this medium voltage voltmeter we can make our work safely.

Transformers types according to phases number

Electrical loads and circuits are one of two types, single and three phase. Single phase loads are like domestic loads and small applications single phase motors.

While industrial loads use 3 phase induction motors, heaters and other machines working on 3 phase power. Besides some single phase loads.

  • Single Phase Transformer

This type transforms AC power from one single phase circuit to another. It has two winding. The primary winding connects the AC power supply and the secondary side is connected with the load.

It is mostly used in electronic circuits like televisions set for voltage regulations, chargers and adapters. Other uses are residential and commercial lighting and heating equipment.

  • Three-phase Transformer

Three-phase transformers are used to either step up or step down the voltage level in the power transmission three phase system. It is also used in power distribution grids.

I have written a detailed article about step up and step down transformer, I highly recommend reading it. You can find it here.

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What is electric transformer and it...
What is electric transformer and its active parts?