Electric Transformer Active and auxiliary Parts

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.