Transformer Construction, Working Principles and Transformation Ratio for SS 3

Class: SS 3
Term: First Term
Week: 1
Age: 16 years
Duration: 45 minutes
Subject: Basic Electricity
Curriculum Theme: Electrical Devices
Previous Lesson:
Topic: TRANSFORMERS
Subject Matter: Identification of a basic constructional feature of a transformer: Working principles of a transformer and Calculation of a transformer ratio

Specific Objectives

By the end of the lesson, pupils/students should be able to:

Cognitive Domain

• Identify the basic constructional features of a transformer.
• Explain the working principles of a transformer.
• State the formulas for transformer ratio.
• Calculate the transformation ratio of a transformer given relevant values.

Affective Domain

• Appreciate the importance of transformers in electrical systems.
• Show interest in learning about electrical devices.

Psychomotor Domain

• Draw a simple diagram of a transformer and label its parts.
• Demonstrate how to connect a transformer to measure input and output values.

Reference Materials

The following resources were used in planning this lesson:

• 2025 Revised 9 Years Basic Education Curriculum
• Relevant State Unified Scheme of Work
• A suitable Basic Electricity textbook for SS 3
• The HeadTeacher Scheme of work

Instructional Materials

The teacher will teach this lesson with the aid of:

• A physical step-down or step-up transformer
• Diagrams of various transformers
• Whiteboard and markers
• Multimeter (AC voltmeter/ammeter)
• Connecting wires
• AC power source

Rationale for the Lesson

This lesson is essential for understanding how electrical energy is efficiently transmitted and distributed. Transformers are fundamental components in nearly all electrical power systems, enabling voltage conversion for domestic and industrial use. Grasping their construction, operation, and ratio calculations provides a strong foundation in practical electricity.

Prerequisite/Previous Knowledge

Students should have a basic understanding of electric current, voltage, magnetic fields, electromagnetic induction, and AC/DC circuits.

Lesson Content/Board Summary

TRANSFORMERS

Basic Constructional Features of a Transformer

A transformer is an electrical device that transfers electrical energy between two or more circuits through electromagnetic induction. It consists of the following key parts:

1. Primary Coil (Primary Winding): This is the coil connected to the input AC power source. It has (N_p) turns.
2. Secondary Coil (Secondary Winding): This is the coil from which the output voltage is taken. It has (N_s) turns.
3. Soft Iron Core: The primary and secondary coils are wound around a laminated soft iron core. This core provides a low-reluctance path for the magnetic flux, ensuring efficient coupling between the coils. The laminations reduce eddy current losses.

Working Principles of a Transformer

Transformers work on the principle of mutual induction. When an alternating current (AC) flows through the primary coil, it produces a continuously changing magnetic flux in the soft iron core. This changing magnetic flux links with the secondary coil, inducing an electromotive force (EMF) across it, according to Faraday’s Law of Electromagnetic Induction.

1. Step-up Transformer: If the number of turns in the secondary coil ((N_s)) is greater than the number of turns in the primary coil ((N_p)), the output voltage ((V_s)) will be higher than the input voltage ((V_p)).
2. Step-down Transformer: If the number of turns in the secondary coil ((N_s)) is less than the number of turns in the primary coil ((N_p)), the output voltage ((V_s)) will be lower than the input voltage ((V_p)).

Transformers only work with alternating current because a steady (DC) current would produce a constant magnetic flux, which cannot induce an EMF in the secondary coil.

Calculation of Transformer Ratio

The transformer ratio, also known as the transformation ratio, relates the number of turns in the primary and secondary coils to their respective voltages and currents. For an ideal transformer (100% efficient), the power in the primary coil equals the power in the secondary coil.

Formulas

The relationship between voltage, current, and turns in a transformer is given by:

( frac{V_s}{V_p} = frac{N_s}{N_p} = frac{I_p}{I_s} )

Where:

1. (V_s) = Secondary voltage (output voltage)
2. (V_p) = Primary voltage (input voltage)
3. (N_s) = Number of turns in the secondary coil
4. (N_p) = Number of turns in the primary coil
5. (I_s) = Secondary current (output current)
6. (I_p) = Primary current (input current)

The ratio ( frac{N_s}{N_p} ) is called the turns ratio or transformation ratio.

Example 1

Question: A step-down transformer has 1000 turns in its primary coil and 200 turns in its secondary coil. If the input voltage is 240 V AC, what is the output voltage?

Solution:

Step 1: Write the formula relating voltage and turns.

( frac{V_s}{V_p} = frac{N_s}{N_p} )

Step 2: Substitute the given values.

( frac{V_s}{240} = frac{200}{1000} )

Step 3: Solve for (V_s).

( V_s = 240 times frac{200}{1000} )

( V_s = 240 times 0.2 )

( V_s = 48 text{ V} )

Answer: ( 48 text{ V} )

Example 2

Question: An ideal transformer has a primary voltage of 120 V and a primary current of 5 A. If the secondary voltage is 600 V, calculate the secondary current.

Solution:

Step 1: Write the formula relating voltage and current.

( frac{V_s}{V_p} = frac{I_p}{I_s} )

Step 2: Substitute the given values.

( frac{600}{120} = frac{5}{I_s} )

Step 3: Solve for (I_s).

( 5 = frac{5}{I_s} )

( I_s = frac{5}{5} )

( I_s = 1 text{ A} )

Answer: ( 1 text{ A} )

Teaching Methods/Instructional Techniques

Discussion, Demonstration, Guided Practice, Question and Answer, Explanation, Observation, Problem Solving, Individual Practice

Instructional Procedures

Step 1: Introduction

Time: 5 minutes

Teaching Skill: Recalling/Engaging

Teacher’s Activity: The teacher greets the students and asks them to recall what they know about electromagnetic induction and its applications. The teacher then introduces the topic of transformers as a practical application of induction.

Pupils’ Activity: Students respond to questions about electromagnetic induction and listen attentively to the introduction.

Learning Point: Electromagnetic induction recall

Step 2: Identification of Transformer Constructional Features

Time: 10 minutes

Teaching Skill: Demonstration/Identification

Teacher’s Activity: The teacher displays a physical transformer (if available) or a clear diagram. The teacher then identifies and explains the basic constructional features: primary coil, secondary coil, and laminated soft iron core. Students are guided to draw a simple diagram.

Pupils’ Activity: Students observe the transformer/diagram, identify its parts, and draw a simple diagram in their notebooks.

Learning Point: Transformer construction features

Step 3: Explanation of Working Principles

Time: 8 minutes

Teaching Skill: Explanation/Illustration

Teacher’s Activity: The teacher explains the working principle of a transformer based on mutual induction, emphasizing the role of alternating current and the changing magnetic flux. The teacher differentiates between step-up and step-down transformers.

Pupils’ Activity: Students listen, ask questions, and contribute to the discussion on how transformers work.

Learning Point: Principle of mutual induction

Step 4: Introduction to Transformer Ratio Formula

Time: 5 minutes

Teaching Skill: Explanation/Formula Derivation

Teacher’s Activity: The teacher introduces the concept of transformer ratio and presents the formulas relating voltage, current, and number of turns. The teacher explains each variable in the formulas.

Pupils’ Activity: Students copy the formulas and the meaning of each variable into their notebooks.

Learning Point: Transformer ratio formulas

Step 5: Guided Practice on Transformer Ratio Calculation (Example 1)

Time: 7 minutes

Teaching Skill: Problem Solving/Guided Practice

Teacher’s Activity: The teacher presents Example 1 on the board and guides students step-by-step through the calculation of output voltage using the turns ratio. The teacher ensures students understand each step.

Pupils’ Activity: Students follow the steps, participate in solving the example, and copy the solution into their notebooks.

Learning Point: Calculating output voltage

Step 6: Independent Practice on Transformer Ratio Calculation (Example 2)

Time: 5 minutes

Teaching Skill: Individual Practice/Application

Teacher’s Activity: The teacher presents Example 2 (calculating secondary current) and allows students to attempt it independently. The teacher moves around to provide support and checks their work.

Pupils’ Activity: Students attempt to solve Example 2 independently and ask for clarification when needed.

Learning Point: Calculating secondary current

Step 7: Evaluation/Review

Time: 5 minutes

Teaching Skill: Questioning/Assessment

Teacher’s Activity: The teacher evaluates the learning by asking the following questions:

1. Mention two basic parts of a transformer.
2. Explain briefly how a transformer works.
3. State the formula for transformer ratio in terms of voltage and turns.
4. A transformer has 500 primary turns and 100 secondary turns. If the input voltage is 220 V, what is the output voltage?

Pupils’ Activity: Pupils answer orally and in writing.

Learning Point: Understanding transformer concepts

Step 8: Note-Taking

Time: 4 minutes

Teaching Skill: Guided Writing

Teacher’s Activity: The teacher guides pupils/students to copy the essential Board Summary notes on transformer construction, working principles, and transformation ratio into their notebooks.

Pupils’ Activity: Pupils/students copy the notes carefully into their notebooks.

Learning Point: Recording lesson information

Step 9: Conclusion

Time: 1 minute

Teaching Skill: Summarising

Teacher’s Activity: The teacher briefly summarises the main points of the lesson, reinforcing the importance of transformers in voltage transformation and power distribution.

Pupils’ Activity: Students listen and prepare for the next lesson.

Learning Point: Transformer lesson summary

Continuous Assessment/Further Study

Type: Homework

Instruction: Answer the following questions in your notebook.

1. Draw a well-labelled diagram of a step-down transformer.
2. Explain why transformers only work with alternating current.
3. A transformer with 1500 turns in the primary coil and 300 turns in the secondary coil is connected to a 240 V AC supply. Calculate the secondary voltage.
4. If the primary current in the transformer from question 3 is 2 A, what is the secondary current (assuming it’s an ideal transformer)?

Lesson Keywords

• Transformer – An electrical device that changes AC voltage levels.
• Primary coil – The input winding of a transformer.
• Secondary coil – The output winding of a transformer.
• Soft iron core – The laminated core that links the magnetic flux between coils.
• Mutual induction – The principle by which a changing current in one coil induces an EMF in an adjacent coil.
• Transformation ratio – The ratio of secondary turns to primary turns, or secondary voltage to primary voltage.
• Step-up transformer – Increases voltage.
• Step-down transformer – Decreases voltage.

Differentiation

For weaker learners: Provide pre-drawn diagrams of transformers for labelling. Offer additional simplified examples for calculating transformer ratios with direct substitution. Pair them with stronger learners for collaborative problem-solving.

For faster learners: Challenge them to research the types of losses in real transformers (e.g., eddy current, hysteresis, copper losses) and how they are minimised. Ask them to explain the concept of an ideal transformer vs. a real transformer.

Suggested Lesson Videos

For further understanding, search on YouTube for: transformer construction working principle SS3 physics

Teacher Guide for Using This Lesson Plan

Before the lesson, ensure you have a physical transformer or very clear diagrams ready. A multimeter is useful for demonstrating voltage readings, even if not fully connected to a live circuit for safety reasons. Begin by linking the lesson to prior knowledge of electromagnetic induction to build a conceptual bridge. When explaining the working principle, use clear analogies to describe the changing magnetic flux. For the calculation part, guide students through the first example thoroughly, breaking down each step. Allow ample time for students to attempt the second example independently, providing support as needed. During note-taking (Step 8), ensure students accurately copy the formulas and examples. Emphasise safety precautions when discussing electrical components, even if not performing live circuit demonstrations. Check understanding frequently through questions and observe students’ participation during the practical activity and problem-solving.