3. Transformers

3.1. Introduction to Transformers

Introduction

A transformer is a static electrical device used to transfer electrical energy between two or more circuits through electromagnetic induction. It is mainly used to increase (step-up) or decrease (step-down) AC voltages. Transformers play a critical role in electrical power distribution, allowing high-voltage transmission for efficiency and low-voltage delivery for safety.

Working Principle

Transformers operate based on Faraday’s Law of Electromagnetic Induction:

When an alternating current (AC) flows through the primary coil, it creates a changing magnetic field, which induces a voltage in the secondary coil.

  • Only works with AC (not DC).

  • No direct electrical connection between primary and secondary windings – energy is transferred via magnetic field.

Basic Structure

  1. Primary Winding – Connected to the input AC supply.

  2. Secondary Winding – Delivers the transformed voltage.

  3. Core – Soft iron material that links magnetic flux between the windings.

Key Formulas

  1. Turns Ratio:

    VpVs=NpNs\frac{V_p}{V_s} = \frac{N_p}{N_s}

    Where:

    • VpV_p

      Vp​,

      VsV_s

      Vs​: Primary and secondary voltages

    • NpN_p

      Np​,

      NsN_s

      Ns​: Primary and secondary turns

  2. Current Ratio:

    IpIs=NsNp\frac{I_p}{I_s} = \frac{N_s}{N_p}

  3. Power Conservation (Ideal Transformer):

    VpIp=VsIsV_p I_p = V_s I_s

Types of Transformers

Type Function Example Use
Step-Up Increases voltage Power stations
Step-Down Decreases voltage Home adapters
Isolation Transformer Voltage remains same, provides safety Medical equipment
Auto-Transformer Single winding acts as both primary & secondary Voltage regulation
Instrument Transformer For measurement & protection CTs and PTs in substations

AC vs DC with Transformer

Feature AC DC
Transformer Use ✅ Yes (essential) ❌ No (doesn't work)
Reason Changing current induces EMF No changing flux in DC

Applications

  • Power transmission & distribution

  • Mobile phone chargers

  • UPS and inverters

  • Audio systems

  • Instrumentation and measurement

Core Types

Core Type Description
Laminated Core Reduces eddy current losses
Toroidal Core Ring-shaped, high efficiency
Shell Type Windings enclosed by core on both sides

Losses in Transformers

  1. Iron Losses – Hysteresis + Eddy current loss (in the core)

  2. Copper Losses –

    I2RI^2R

    I2R losses in windings

  3. Flux Leakage – Not all flux links both windings

  4. Dielectric Loss – In insulation

Efficiency:

Efficiency=Output PowerInput Power×100%\text{Efficiency} = \frac{\text{Output Power}}{\text{Input Power}} \times 100\%

Simple Diagram Explanation

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AC Input | [Primary Coil] |||||| |||| CORE |||| |||||| [Secondary Coil] | AC Output

  • Magnetic flux links the two windings through the iron core.

  • Voltage depends on the turns ratio between coils.