Introduction to Electrical Transformer | Definition, Construction & Parts of  a Transformer | Types of Transformers

What is a Transformer?

In Very Simple words.

A transformer is a device that:

  1. Transfer Electrical power from one electrical circuit to another Electrical circuit.
  2. It’s working without changing the frequency.
  3. Work through on electric induction.
  4. When both circuits take effect of mutual induction.
  5. Can’t step up or step down the level of DC voltage or DC Current.
  6. Can step up or step down the level of AC voltage or AC Current.
1500 kVA Transformer by Siemens | Electrical pattarai


Without transformers, the electrical energy generated at generating stations won’t probably be sufficient enough to power up a city. Just imagine that there are no transformers. How many power plants do you think have to be set up in order to power up a city? It’s not easy to set up a power plant. It is expensive.
Numerous power plants have to be set up in order to have sufficient power. Transformers help by amplifying the Transformer output (stepping up or down the level of voltage or current).

When the number of turns of the secondary coil is greater than that of the primary coil, such a transformer is known as a step-up transformer.

Likewise, when the number of turns of coil of the primary coil is greater than that of a secondary transformer, such a transformer is known as a step-down transformer.

Construction of a Transformer | Parts of  a Transformer

Parts of a Transformer

1Oil filter valve17Oil drain valve
2Conservator18Jacking boss
3Buchholz relay19Stopper
4Oil filter valve20Foundation bolt
5Pressure-relief vent21Grounding terminal
6High-voltage bushing22Skid base
7Low-voltage bushing23Coil
8Suspension lug24Coil pressure plate
9B C T Terminal25Core
10Tank26Terminal box for protective devices
11De-energized tap changer27Rating plate
12Tap changer handle28Dial thermometer
13Fastener for core and coil29Radiator
14Lifting hook for core and coil30Manhole
15End frame31Lifting hook
16Coil pressure bolt32Dial-type oil level gauge

Types of Transformers

There are two basic Types of Transformers

  1. Single Phase Transformer
  2. Three Phase Transformer

Below are the more types of transformers derived via different functions and operations etc.

Types of Transformers w.r.t Cores

  • Core Type Transformer
  • Shell Type Transformer
  • Berry Type Transformer

Types of Transformer w.r.t uses

  • Large Power Transformer
  • Distribution Transformer
  • Small Power Transformer
  • Sign Lighting Transformer
  • Control & Signalling Transformer
  • Gaseous Discharge Lamp Transformer
  • Bell Ringing Transformer
  • Instrument Transformer
  • Constant Current Transformer
  • Series Transformer for Street Lighting

Types of Transformer w.r.t Cooling

  • Self Air Cooled or Dry Type Transformer
  • Air Blast-Cooled Dry Type
  • Oil Immersed, Self Cooled (OISC) or ONAN (Oil natural, Air natural)
  • Oil Immersed, a Combination of Self Cooled and Air blast (ONAN)
  • Oil Immersed, Water Cooled (OW)
  • Oil Immersed, Forced Oil Cooled
  • Oil Immersed, a Combination of Self Cooled and Water Cooled (ONAN+OW)
  • Oil Forced, Air forced Cooled (OFAC)
  • Forced Oil, Water Cooled (FOWC)
  • Forced Oil, Self Cooled (OFAN)

Types of  Instrument Transformers

  • Current Transformer
  • Potential Transformer
  • Constant Current Transformer
  • Rotating Core Transformer or Induction regulator
  • Auto Transformer

Operating & Working Principles of a Transformer

The transformer is a static device (and doesn’t contain rotating parts, hence no friction losses), which converts electrical power from one circuit to another without changing its frequency. it Steps up (or Steps down) the level of AC Voltage and Current.

Transformer works on the principle of mutual induction of two coils or Faraday Law’s Of Electromagnetic induction. When the current in the primary coil is changed the flux linked to the secondary coil also changes. Consequently, an EMF is induced in the secondary coil due to Faraday law’s of electromagnetic induction.

The transformer is based on two principles: first, an electric current can produce a magnetic field (electromagnetism), and, second 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.

A simple transformer has a soft iron or silicon steel core and windings placed on it(iron core). Both the core and the windings are insulated from each other. The winding connected to the main supply is called the primary and the winding connected to the load circuit is called the secondary.

Winding (coil) connected to higher voltage is known as high voltage winding while the winding connected to low voltage is known as low voltage winding. In the case of a step-up transformer, the primary coil (winding) is the low voltage winding, and the number of turns of the windings of the secondary is more than that of the primary. Vice versa for a step-down transformer

Limitation of the Transformer

To understand the main points, we have to discuss some basic terms related to transformer operation. So let’s back to basics for a while.

A transformer is an AC machine that steps up or steps down an alternating voltage or current. A transformer is an AC machine however cannot step up or down a DC voltage or DC current. It sounds a bit weird though. You might be thinking “so are there no DC transformers?”

(1) Why transformers are rated in kVA and not kW?

Since copper loss depends on the current and core loss depends on the voltage, the total loss in the transformer depends on the Volt-Ampere product and not on the phase angle between voltage and current i.e total loss in the transformer is independent of the load power factor.

Hence, the transformer rating is in Volt-Amperes and not in Watts.

(2) What happens if a transformer is connected to the dc supply?

Consider the below single-phase transformer.

When ac is given to the transformer, an alternating flux is produced and an emf E1 is induced in the primary.

But when a dc supply is given to the transformer, the primary draw a steady current and hence produces constant flux. Consequently, no emf is induced in the primary winding   i.e 

Since primary winding has very low resistance R1, therefore, it draws excessive current given by   I1 = ( V1/R1 ). The high current produces excessive heating in the primary winding. The result is that primary winding burns out.

Therefore, care must be taken not to connect the primary transformer to dc.

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