By the end of this chapter, you will be able to:
Mastering these skills will help you work confidently with electrical devices and systems that rely on magnetism, making you a valuable professional in the electrical and electronics field.
Magnetism and electromagnetism are fundamental principles underlying many electrical and electronic devices used across Kenya’s diverse professional sectors. Understanding magnetic circuits and their components is essential for technicians and engineers working in hospitals, educational institutions, banks, and manufacturing firms, where devices such as transformers, relays, and sensors rely on magnetic principles. This chapter explores the nature of magnetic circuits, key magnetic quantities, calculations involved, analogies with electrical circuits, and the materials used to optimize magnetic device performance.
Magnetic circuits provide a controlled path for magnetic flux, much like electrical circuits guide electric current. They are crucial in devices that convert electrical energy to mechanical energy or vice versa, such as transformers and electric motors. In Kenyan county referral hospitals, for example, MRI machines depend on precisely designed magnetic circuits to generate strong, stable magnetic fields for imaging. Understanding magnetic circuits helps technicians maintain these devices and troubleshoot faults effectively.
A magnetic circuit is a closed loop path made of magnetic material through which magnetic flux flows. Its primary function is to channel magnetic flux with minimum loss, similar to how electrical conductors guide current. Magnetic circuits concentrate flux in specific regions to enhance device performance.
Magnetic circuits typically consist of a core made from ferromagnetic material, an air gap to control flux, and a coil that generates magnetomotive force (MMF). The core’s shape and material influence the circuit’s efficiency and flux density.
Devices such as transformers, inductors, and electromagnets rely on magnetic circuits to operate efficiently. For instance, in a bank’s automated teller machines (ATMs), magnetic relays use magnetic circuits to switch electrical contacts reliably.
While both types of circuits guide a flow (magnetic flux vs. electric current), magnetic circuits deal with magnetic fields and flux, and electrical circuits involve voltage and current. Magnetic circuits have reluctance instead of resistance.
Understanding the key quantities in magnetic circuits is essential for analyzing and designing magnetic devices. These quantities describe how magnetic fields behave and interact within materials and spaces.
Magnetic flux, symbolized by Φ, represents the total magnetic field passing through a given area in the magnetic circuit. It is measured in webers (Wb). In a retail business’s security system, magnetic sensors detect flux changes to trigger alarms.
Magnetic field density, or flux density (B), is the concentration of magnetic flux per unit area, measured in teslas (T). It indicates the intensity of the magnetic field in a specific region. High flux density in transformer cores at a university power lab increases efficiency.
Magnetic field strength (H) quantifies the magnetizing force required to produce a magnetic field, measured in amperes per meter (A/m). It depends on the current flowing through the coil and the number of turns.
Reluctance (R) is the opposition to magnetic flux flow, analogous to electrical resistance, measured in ampere-turns per weber (At/Wb). Air gaps in magnetic circuits, such as those in electromechanical relays at county government offices, increase reluctance.
MMF (F) is the force that drives magnetic flux through a circuit, analogous to electromotive force (voltage) in electrical circuits. It is calculated as the product of current (I) and the number of turns (N) in the coil: F = N × I, measured in ampere-turns (At).
Calculations in magnetic circuits enable the design and analysis of devices to ensure proper functioning under specified conditions. These calculations often use analogies to electrical circuits.
Magnetic flux is calculated using the formula:
Φ = F / R
Where:
Φ = magnetic flux in webers (Wb)
F = magnetomotive force in ampere-turns (At)
R = reluctance in ampere-turns per weber (At/Wb)
Example: If a coil with 200 turns carries a current of 0.5 A and the reluctance of the magnetic circuit is 400 At/Wb, calculate the magnetic flux.
F = N × I = 200 × 0.5 = 100 At
Φ = F / R = 100 / 400 = 0.25 Wb
MMF is found using:
F = N × I
Where:
N = number of turns
I = current in amperes (A)
Reluctance is given by:
R = l / (μ × A)
Where:
l = length of the magnetic path (m)
μ = permeability of the material (H/m)
A = cross-sectional area of the magnetic path (m²)
R = 0.5 / (1.2 × 10⁻³ × 0.01) = 0.5 / 1.2 × 10⁻⁵ = 41,666.67 At/Wb
Flux density is calculated as:
B = Φ / A
Where:
B = flux density (T)
Φ = magnetic flux (Wb)
A = cross-sectional area (m²)
In county government offices, accurate calculation of magnetic circuit parameters ensures electromagnetic locks operate reliably under varying electrical loads.
Magnetic circuits share similarities with electrical circuits, facilitating understanding and problem-solving by drawing parallels.
The analogy maps magnetomotive force (MMF) to voltage, magnetic flux to current, and reluctance to resistance. This analogy aids technicians in visualizing magnetic circuit behavior.
Ohm’s law for electrical circuits (V = I × R) corresponds to magnetic circuits as F = Φ × R. This analogy assists in calculating unknown magnetic quantities.
Magnetic circuits can be arranged in series and parallel configurations similar to electrical circuits. For example, in a hotel’s fire alarm system, magnetic sensors connected in series ensure fault detection.
Electrical circuits store energy in capacitors and inductors, while magnetic circuits store energy in magnetic fields within cores. Understanding this helps in designing inductors used in ICT equipment at universities.
Magnetic materials determine the efficiency, performance, and application of magnetic circuits. They are classified broadly as soft or hard magnetic materials.
Soft magnetic materials, such as silicon steel, have high permeability, low coercivity, and low hysteresis loss. These properties make them ideal for transformer cores and inductors in hospitals where energy efficiency is critical.
Hard magnetic materials, like ferrites and alnico alloys, have high coercivity and retain magnetization, suitable for permanent magnets used in speakers and sensors at retail outlets.
Soft magnetic materials are used in devices requiring rapid magnetization and demagnetization cycles, such as electric motors and transformers in educational institutions.
Hard magnetic materials find application in permanent magnets for magnetic locks and magnetic recording media used in county government offices.
The choice between soft and hard magnetic materials affects device losses, size, weight, and operational stability. For example, soft magnetic cores reduce energy losses in SACCOs’ backup power transformers, improving reliability.
Create a free account to open more of this chapter.
Free: practical guides, quick cards, workplace scenarios and more.
Create a free accountThis chapter explored the fundamentals of magnetic circuits and devices, beginning with the basic concepts of magnetic flux, magnetic field density, magnetic field strength, reluctance, magnetomotive force, and their roles in magnetic circuits. It provided methods for calculations involving these parameters and highlighted the analogies between magnetic and electrical circuits, enhancing understanding of magnetic circuit behavior. The chapter also discussed the characteristics and uses of magnetic materials, distinguishing between soft and hard magnetic materials in electrical applications. The principles of electromagnetic induction were then examined, focusing on Faraday's Law and Lenz's Law, which govern the generation and direction of induced electromotive force. Practical applications such as electric generators and transformers were detailed, including the effects of moving conductors and changing magnetic fields on induced EMF. Further topics included self-induction, mutual induction, and the working principles of transformers, with explanations of step-up and step-down transformers, their power losses, and relevant calculations. The chapter concluded with an overview of energy stored in magnetic fields, completing a comprehensive study of magnetism and electromagnetism in electrical and electronic principles.
Type: Individual
| Tools & Equipment | Materials |
|---|---|
| DC Power supply | Laminated iron U-core |
| Digital multimeter | Laminated iron bar |
| Gauss meter | Copper wire (SWG 20) |
| Steel rule | Dust coat/overall |
| Insulation tape | Safety boots |
| Cleaning brush | |
| Clear safety goggles |
| S/N | Item | Quantity |
|---|---|---|
| 1 | Laminated iron U-core (150mm x 50mm x 25mm) | 1 Pc per Candidate |
| 2 | Laminated iron bar (150mm x 25mm x 25mm) | 1 Pc per Candidate |
| 3 | Copper wire (SWG 20) | 2m per Candidate |
| 4 | DC Power supply (0-12V) | 1 Pc per 2 Candidates |
| 5 | Digital multimeter | 1 Pc per 2 Candidates |
| 6 | Gauss meter | 1 Pc per 3 Candidates |
| 7 | Steel rule | 1 Pc per Candidate |
| 8 | Insulation tape | 1 Pc per Candidate |
| 9 | Clear safety goggles | 1 Pc per Candidate |
| 10 | Dust coat/overall | 1 Pc per Candidate |
| 11 | Safety boots | 1 Pair per Candidate |
| 12 | Cleaning brush | 1 Pc per Candidate |
| Items to be Evaluated | Marks Available | Marks Obtained | Comments |
|---|---|---|---|
| TASK 1: Assembly and Measurement of Magnetic Circuit | |||
| Adhered to prescribed safety procedures: Wore dust coat/overall, safety boots, clear safety goggles. (Award 2 marks for proper use of PPE or zero.) | 2 | ||
| Prepared an operation work plan as per the schematic drawing. (Award 3 marks for a clear, stepwise work plan or zero.) | 3 | ||
| Selected and inspected all required materials and tools. (Award 2 marks for correct selection and inspection or zero.) | 2 | ||
| Wound 100 turns of SWG 20 copper wire neatly on the U-core. (Award 3 marks for correct winding and neatness or zero.) | 3 | ||
| Assembled the magnetic circuit by joining the iron bar to the U-core, securing with insulation tape. (Award 2 marks for correct assembly or zero.) | 2 | ||
| Connected the coil to the DC power supply and set up measuring instruments. (Award 2 marks for correct electrical setup or zero.) | 2 | ||
| Measured coil current using digital multimeter. (Award 2 marks for accurate current measurement or zero.) | 2 | ||
| Measured magnetic field strength (H) and flux density (B) using gauss meter. (Award 2 marks for correct measurement or zero.) | 2 | ||
| Performed housekeeping: Cleaned work area and returned tools. (Award 1 mark for proper housekeeping or zero.) | 1 | ||
| Sub-Total | 19 | ||
| TASK 2: Analysis and Calculation of Magnetic Circuit Parameters | |||
| Recorded all measured values accurately. (Award 2 marks for complete and accurate records or zero.) | 2 | ||
| Calculated magnetomotive force (MMF) using N and I. (Award 3 marks for correct MMF calculation or zero.) | 3 | ||
| Calculated magnetic field strength (H) using core dimensions. (Award 3 marks for correct H calculation or zero.) | 3 | ||
| Calculated magnetic flux (Φ) and flux density (B) using measured values. (Award 3 marks for correct Φ and B calculations or zero.) | 3 | ||
| Calculated reluctance (ℛ) of the magnetic circuit. (Award 2 marks for correct reluctance calculation or zero.) | 2 | ||
| Compared calculated values with measured data and commented on discrepancies. (Award 2 marks for meaningful comparison and comment or zero.) | 2 | ||
| Sub-Total | 15 | ||
| PRODUCT CHECKLIST | |||
| Magnetic circuit assembled as per schematic: U-core (150mm x 50mm x 25mm) and iron bar (150mm x 25mm x 25mm) securely joined. (Award 3 marks for correct dimensions and secure assembly or zero.) | 3 | ||
| Coil wound with exactly 100 turns of SWG 20 copper wire, uniformly distributed. (Award 3 marks for correct number of turns and neat winding or zero.) | 3 | ||
| All measurements (current, B, H) recorded and calculations presented clearly. (Award 3 marks for complete and clear documentation or zero.) | 3 | ||
| Calculated MMF, H, B, Φ, and reluctance match theoretical expectations within 10% tolerance. (Award 4 marks for results within tolerance or zero.) | 4 | ||
| Sub-Total | 13 | ||
| GRAND TOTAL | 47 | ||
Type: Individual
| Tools & Equipment | Materials |
|---|---|
| Bar magnet | Soft iron rod |
| Compass | Steel rod (hard magnetic) |
| DC power supply | Iron filings |
| Insulated copper wire | |
| Switch | |
| Digital multimeter | |
| Retort stand with clamps | |
| Paper sheets | |
| Clear safety goggles | |
| Overall/Dust coat | |
| Safety boots | |
| Cleaning brush | |
| Waste bin |
| S/N | Item | Quantity |
|---|---|---|
| 1 | Soft iron rod (100mm x 10mm x 10mm) | 1 Pc per Candidate |
| 2 | Steel rod (hard magnetic, 100mm x 10mm x 10mm) | 1 Pc per Candidate |
| 3 | Bar magnet | 1 Pc per Candidate |
| 4 | Iron filings | 50g per Candidate |
| 5 | Compass | 1 Pc per Candidate |
| 6 | DC power supply (0-12V) | 1 Pc per 2 Candidates |
| 7 | Insulated copper wire (SWG 22, 2 metres) | 2m per Candidate |
| 8 | Switch | 1 Pc per Candidate |
| 9 | Digital multimeter | 1 Pc per Candidate |
| 10 | Retort stand with clamps | 1 Set per Candidate |
| 11 | Paper sheets (A4) | 2 sheets per Candidate |
| 12 | Clear safety goggles | 1 Pc per Candidate |
| 13 | Overall/Dust coat | 1 Pc per Candidate |
| 14 | Safety boots | 1 Pair per Candidate |
| 15 | Cleaning brush | 1 Pc per Candidate |
| 16 | Waste bin | 1 Pc per 4 Candidates |
| Items to be Evaluated | Marks Available | Marks Obtained | Comments |
|---|---|---|---|
| TASK 1: Preparation and Safety | |||
| Wore all required personal protective equipment: overall/dust coat, clear safety goggles, safety boots (Award 1 mark if all PPE are worn, or zero) | 1 | ||
| Organized tools and materials on the workbench as per procedure (Award 1 mark if all items are arranged and accounted for, or zero) | 1 | ||
| Sub-Total | 2 | ||
| TASK 2: Testing Magnetic Properties | |||
| Wound insulated copper wire uniformly (20 turns) around each rod, leaving leads for connection (Award 2 marks for correct winding and lead preparation, or zero) | 2 | ||
| Connected rods in circuit with DC power supply and switch correctly (Award 2 marks for correct and safe circuit assembly, or zero) | 2 | ||
| Demonstrated magnetization of each rod (soft iron and steel) by passing DC current and using the compass to detect field (Award 2 marks for correct demonstration and compass use, or zero) | 2 | ||
| Used iron filings to visualize magnetic field around each rod (Award 1 mark if iron filings are used correctly on both rods, or zero) | 1 | ||
| Demagnetized rods and tested for residual magnetism (retentivity) (Award 2 marks for correct disconnection and residual field check, or zero) | 2 | ||
| Sub-Total | 9 | ||
| TASK 3: Documentation and Housekeeping | |||
| Recorded observations for each rod: ease of magnetization, field strength, and retention of magnetism (Award 2 marks for clear, complete, and comparative documentation, or zero) | 2 | ||
| Cleaned work area, returned tools, and disposed waste/iron filings appropriately (Award 1 mark if all housekeeping steps are completed, or zero) | 1 | ||
| Sub-Total | 3 | ||
| PRODUCT CHECKLIST | |||
| Tabulated results comparing soft iron rod and steel rod: initial magnetization, field strength, and residual magnetism (Award 3 marks for a complete, clear, and accurate comparison table, or zero) | 3 | ||
| Provided a written conclusion recommending suitable applications for soft and hard magnetic materials in electrical devices, with justification (Award 3 marks for a relevant, accurate, and justified conclusion, or zero) | 3 | ||
| Sub-Total | 6 | ||
| GRAND TOTAL | 20 | ||
At the start of this chapter we promised you would be able to:
Tick each one you can genuinely do.
So, are you there yet?
You're competent when you can confidently do 50% or more of what this chapter promised.
Sign in to record how you're doing.