Magnetic Materials & B-H Curve | AKTU Electrical Machine-1 Notes

🧲 Magnetic Materials & B–H Curve

AKTU B.Tech EE | Electrical Machine-1 | Complete Notes with Tricks

⚡ ELECTRICAL ZINDAGI — Easy Notes

📑 TABLE OF CONTENTS

1. Basic Introduction
2. Important Formula
3. 5 Types of Magnetic Materials
4. B–H Curve Explained
5. Key Terms: Retentivity, Coercivity, Saturation
6. Curie Temperature
7. Quick Comparison Table
8. Real-Life Applications
9. Quick Summary
10. Exam Writing Strategy (AKTU)
11. FAQ

🔹 Basic Introduction

Socho ek transformer ka core hai — usmein baar baar alternating magnetic field apply hoti hai. Is core ka material decide karta hai ki transformer kitna efficient hoga. Agar core material galat choose kiya, toh energy waste, overheating, aur failure — sab ho sakta hai!

Isliye Magnetic Materials samajhna har electrical engineer ke liye must hai. Ye materials external magnetic field ke response mein alag alag tarike se behave karte hain. Kuch strongly attract hote hain, kuch weakly, aur kuch to repel bhi karte hain!

Jab bhi koi magnetic field H (Magnetic Field Intensity) apply ki jaati hai, material mein B (Magnetic Flux Density) develop hota hai. In dono ke beech relationship ko B–H Curve (ya Magnetization Curve) kehte hain — jo is topic ka dil hai.

B = μ × H

B = Magnetic Flux Density (unit: Tesla or Wb/m²)  |  H = Magnetic Field Intensity (unit: A/m)  |  μ = Permeability = μ₀ × μᵣ

μ₀ = 4π × 10⁻⁷ H/m (permeability of free space)  |  μᵣ = Relative permeability of material

🧲 5 Types of Magnetic Materials

Trick to remember: "F P D A F" → Ferro, Para, Dia, Anti, Ferri — Yaad karo "Five People Don't Always Fight" 😄

🔴

1️⃣ Ferromagnetic Materials

Kya hota hai? Ye materials magnetic field lagane par bahut strongly magnetize ho jaate hain. Inka ek unique property hai — inke andar tiny magnetic domains hote hain, jo external field milne par align ho jaate hain.

Properties: Very high permeability (μᵣ = 1000–100,000), strong attraction, hysteresis dikhata hai, aur field hatane ke baad bhi magnetism retain karta hai.

Yaad karo: Ferro = FRIDGE magnet lagta hai ✅

Iron (Fe) Nickel (Ni) Cobalt (Co) Steel

🟢

2️⃣ Paramagnetic Materials

Kya hota hai? Ye materials weakly magnetize hote hain jab field apply hoti hai. Field hatao toh magnetism bhi chali jaati hai. Inke atoms mein unpaired electrons hote hain jo field ki direction mein partially align hote hain.

Properties: Small positive susceptibility, μᵣ slightly greater than 1, no hysteresis, temporary magnetism only.

Yaad karo: Para = PARTly magnetic — field mein hai toh hai, baad mein gone! 🚶

Aluminium Platinum Manganese Oxygen

🔵

3️⃣ Diamagnetic Materials

Kya hota hai? Ye materials magnetic field ke opposite direction mein magnetize hote hain — matlab magnet inhe repel karta hai! Sab atoms ke electrons paired hote hain, isliye net moment zero hota hai.

Properties: μᵣ slightly less than 1 (like 0.999...), negative susceptibility, no permanent magnetism, weak repulsion.

Yaad karo: DIA = opposite mein jaata hai — DIAMOND bhi repel karta hai magnet ko! 💎

Copper (Cu) Bismuth Silver Gold

🟡

4️⃣ Antiferromagnetic Materials

Kya hota hai? In materials mein adjacent atoms ke magnetic moments equal hain but exactly opposite direction mein hain — isliye cancel out ho jaate hain. Net magnetization = ZERO.

Properties: Very small permeability, no strong magnetism, no useful hysteresis, below Neel temperature ye property show karte hain.

Yaad karo: ANTI = Apas mein fight karte hain aur cancel ho jaate hain! ⚔️

MnO NiO FeO CoO

🟣

5️⃣ Ferrimagnetic Materials

Kya hota hai? Antiferro jaisa hi hai — adjacent moments opposite direction mein hain — but unequal hain. Isliye net magnetization zero nahi hota, kuch bacha rehta hai. High frequency applications ke liye best!

Properties: Moderate permeability, high electrical resistivity (eddy current loss kam), lower hysteresis loss than ferro.

Yaad karo: FERRIte = FERRI wheel — opposite sides hain but unequal weight se balance nahi hota! 🎡

Ferrites Fe₃O₄ MnFe₂O₄

📈 B–H Curve of Ferromagnetic Material — Step by Step

Socho ek iron core hai jisme pehle koi magnetism nahi thi (virgin state). Ab hum H badhate jaate hain aur dekhte hain kya hota hai...

Step 1 — Initial Magnetization (O → a)

Initially B = 0 aur H = 0. Jab H badhate hain, B bhi badhta hai — pehle slowly phir fast. Is region mein magnetic domains align hone lagte hain applied field ki direction mein. B ka H ke saath bada aur non-linear relationship hota hai.

Step 2 — Saturation Point (a)

Ek point ata hai jab almost sare domains align ho jaate hain. Ab H badhate raho — B nahi badhta (ya bahut kam badhta hai). Ye hai Saturation Point. Iske baad material "full" ho gaya — aur zyada magnetize nahi hoga.

Step 3 — Retentivity (a → b): H = 0 karo

H ko gradually zero karo. B completely zero nahi hota — kuch flux density bachi rehti hai. Ye bacha hua value hai Retentivity (Br) ya Residual Magnetism. Iron iska "memory" retain karta hai — permanent magnet isi principle se banta hai!

Step 4 — Coercivity (b → c): Reverse H lagao

B zero karne ke liye reverse direction mein H apply karna padta hai. Jitna reverse H chahiye B ko zero karne ke liye, wo hota hai Coercive Force (Hc). Hard magnetic materials ka Hc zyada hota hai (permanent magnets) aur soft materials ka kam (transformer cores).

Yahi process dono directions mein complete karne par jo closed loop banta hai wo hai — Hysteresis Loop. Is loop ka area = Energy loss per cycle (Watt-second/m³).

🔑 Key Terms — Clearly Explained

🧲

Retentivity (Br)

H remove karne ke baad remaining B value. Hard material = High Br (permanent magnet). Soft material = Low Br (transformer core).

⚡

Coercive Force (Hc)

Reverse H required to make B = 0. High Hc = Hard magnet (speaker, motor magnets). Low Hc = Soft magnet (relay, transformer).

🔝

Saturation (Bs)

Maximum possible B in a material. Beyond this, H badhane ka koi faida nahi. Iron ka Bs ≈ 2.1 Tesla hota hai.

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Hysteresis Loss

Energy lost as heat during one complete B–H cycle. Loop area zyada = Loss zyada. AC machines mein ye ek important loss hai. Wh = η × Bmax^1.6 × f × V

🌡️ Curie Temperature — Ek Important Concept!

Ferromagnetic materials ek specific temperature ke baad apni ferromagnetic property kho dete hain aur paramagnetic ban jaate hain — is temperature ko Curie Temperature (Tc) kehte hain.

Iron ka Tc ≈ 770°C  |  Nickel ka Tc ≈ 358°C  |  Cobalt ka Tc ≈ 1115°C

Practical importance: Transformer agar over-heat ho jaaye Curie temperature se upar, core apni magnetic property kho dega — isliye temperature limits important hain!

📊 Quick Comparison Table

Property	Ferro	Para	Dia	Anti-ferro	Ferri
Permeability (μᵣ)	Very High (1000+)	Slightly >1	Slightly <1	Very small	Moderate
Susceptibility	Large positive	Small positive	Small negative	Nearly zero	Medium positive
Net Magnetization	Very High	Low	Opposite direction	Zero	Non-zero (unequal)
Hysteresis	Yes (significant)	No	No	No	Yes (less)
Examples	Fe, Ni, Co	Al, Pt, Mn	Cu, Bi, Ag	MnO, NiO	Ferrites
Used in	Transformer, Motor	Lab purposes	Shielding	Sensors	HF transformers

⚙️ Real-Life Applications of Magnetic Materials

🔌

Transformer Core
(Soft Iron / CRGO Steel)

🔊

Speaker Magnets
(Hard Ferrite)

💾

Hard Disk Drives
(Ferromagnetic thin films)

📱

Inductors & Cores
(Ferrites for HF)

🧲

Permanent Magnets
(AlNiCo, SmCo, NdFeB)

⚡

AC Motor Rotors
(Silicon Steel)

⚡ Quick Summary Points — Last Minute Revision

B = μH → Basic magnetization formula

Ferromagnetic → Strongly magnetized, hysteresis

Paramagnetic → Weakly magnetized, temporary

Diamagnetic → Repelled by magnet, μᵣ < 1

Antiferromagnetic → Equal & opposite moments → Net B = 0

Ferrimagnetic → Unequal opposite moments → Net B ≠ 0

Retentivity (Br) → Remaining flux after H=0

Coercive Force (Hc) → Reverse H to make B=0

Saturation → Max possible B in material

Hysteresis loop area → Energy loss per cycle

Curie Temp (Iron) → 770°C → ferro to para

Ferrites → High resistivity → HF use (less eddy loss)

🎯 AKTU Exam Writing Strategy (10 marks question)

1. Introduction (3–4 lines): Magnetic materials kya hote hain, B = μH formula, permeability kya hoti hai.
2. 5 Short Notes (main body): Har type ke liye — definition, properties (3–4 points), examples, uses. Use bullet points for clarity.
3. B–H Curve Diagram: Clearly labelled axes (B on Y, H on X), mark saturation point (a), retentivity (Br on Y-axis), coercive force (Hc on X-axis), complete hysteresis loop.
4. Retentivity & Coercive Field Definitions: Clearly define both with units.
5. Concluding line: "Soft magnetic materials (low Hc) are used in transformers and motors, while hard magnetic materials (high Hc) are used in permanent magnets."

⏱️ Estimated time: 12–15 minutes | Expected marks: 8–10/10 if diagram is neat

❓ Frequently Asked Questions

Q1. What is the difference between ferromagnetic and ferrimagnetic materials?

Ferromagnetic materials mein sare adjacent magnetic moments parallel (same direction) hote hain → very high magnetization. Ferrimagnetic materials mein opposite direction mein hote hain but unequal hain → moderate net magnetization. Ferrites ferrimagnetic hote hain aur high frequency applications mein use hote hain kyunki inki electrical resistivity zyada hoti hai (less eddy current loss).

Q2. Why is silicon steel used in transformer cores instead of pure iron?

Silicon steel (3–4% Si) mein pure iron se zyada electrical resistivity hoti hai → eddy current loss kam hota hai. Saath hi iska hysteresis loss bhi kam hota hai. Isliye ye transformer cores ke liye economically aur technically better choice hai.

Q3. What is the significance of the area of the B–H hysteresis loop?

Hysteresis loop ka area represent karta hai energy loss per unit volume per cycle (joules/m³). Zyada area = zyada energy loss as heat. Soft magnetic materials (transformers ke liye) mein narrow loop → low loss. Hard magnetic materials (permanent magnets) mein wide loop → high retentivity.

Q4. What happens to a ferromagnetic material above Curie temperature?

Curie temperature se upar jaane par thermal agitation itni strong ho jaati hai ki magnetic domains randomly orient ho jaate hain — net magnetization zero. Material paramagnetic ban jaata hai. Iron ka Curie temperature 770°C hai. Thanda karne par phir ferromagnetic ho jaata hai.

Q5. For AKTU exam — hard magnetic vs soft magnetic material difference?

Soft magnetic: Low coercivity (Hc), low retentivity, narrow hysteresis loop, low energy loss — used in transformer core, relay, electromagnet. Hard magnetic: High coercivity (Hc), high retentivity, wide hysteresis loop — used in permanent magnets, loudspeakers, electric meters.

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