--- title: "Significance of hysteresis loop" subject: "Electrical Engineering Material" chapter: 4 key: "semester_3/electrical_engineering_material:hysteresis-loop-significance" source: https://www.bctengineer.com/q/semester_3/electrical_engineering_material/hysteresis-loop-significance site: BCT Engineering --- # Significance of hysteresis loop Electrical Engineering Material · Chapter 4: Magnetic Materials - Asked in: 2075 Asoj, 4 marks; 2071 Shrawan, 4 marks; 2070 Chaitra, 4 marks; 2068 Chaitra, 4 marks (asked 4 times) - Video: [The Hysteresis loop explained](https://www.youtube.com/watch?v=ZyVSMfSgOq0) — The Electric Academy - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_3/electrical_engineering_material/hysteresis-loop-significance - Whole chapter: https://www.bctengineer.com/md/semester_3/electrical_engineering_material/chapter-4 ## Question Explain the significance of hysteresis loop while selecting materials for preparing magnetic materials. ## Short answer The hysteresis loop area indicates energy loss per cycle; narrow loops suit soft magnetic cores for low loss, while wide loops with high coercivity and remanence suit permanent magnets. Square loops are used in memory. ## Answer The **hysteresis loop** (B–H loop) shows how flux density $B$ lags behind magnetizing force $H$ when a ferromagnetic material is taken through a full magnetization cycle. Its shape tells the designer which material suits which job. ```text B ^ Bs | _____ Br | / / | / / ---------+-/----/------> H -Hc |/ / Hc /| / _____/ | | ``` #### Quantities read from the loop and their significance | Quantity | Meaning | Significance in selection | |---|---|---| | Area of loop | Energy lost per cycle per m³ | Small area → low hysteresis loss (cores) | | Retentivity $B_r$ | B left when H = 0 | High $B_r$ → strong permanent magnet | | Coercivity $H_c$ | Reverse H needed to make B = 0 | High $H_c$ → magnet hard to demagnetize | | Saturation $B_s$ | Maximum B | High $B_s$ → smaller, lighter cores | | Slope (permeability) | $\mu = B/H$ | High $\mu$ → less magnetizing current | | $(BH)_{max}$ | Max energy product | Figure of merit of a permanent magnet | #### Use in selection - **Soft magnetic materials** (narrow, tall loop): low $H_c$, small loop area, high $\mu$, high $B_s$. Chosen for transformer, motor and generator cores, relays and electromagnets, where the field reverses many times and loss must be low. Examples: silicon steel, permalloy, soft ferrites. - **Hard magnetic materials** (wide, fat loop): high $H_c$, high $B_r$, large $(BH)_{max}$. Chosen for permanent magnets in meters, loudspeakers, PMDC motors. Examples: Alnico, NdFeB, SmCo, hard ferrites. - **Square loop materials**: two stable states $\pm B_r$; used in magnetic memory and switching cores. Hysteresis power loss (Steinmetz): $P_h = \eta B_{max}^{1.6} f V$ watts (loop area × frequency × volume), so for AC machines the material with the smallest loop area is selected. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. The area of the hysteresis loop represents: A. Permeability B. Energy lost per cycle per unit volume C. Retentivity D. Saturation flux Answer: B. Area gives hysteresis loss per cycle per m^3. (Key point: Loop area) 2. For transformer cores choose a material with: A. Narrow tall loop, low Hc, high permeability B. Wide loop, high Hc C. Square loop D. High Br and Hc Answer: A. Small loop area keeps loss low when flux reverses repeatedly. (Key point: Core selection) 3. Which loop properties suit a permanent magnet? A. High Hc, high Br, large (BH)max B. Low Hc, low Br C. Low Hc, high mu D. Zero Br Answer: A. High coercivity resists demagnetization and high Br gives strong field. (Key point: Permanent magnets) 4. Square loop materials are used in: A. Permanent speakers B. Magnetic memory and switching cores C. Power transformers D. Magnetic shielding Answer: B. They have two stable states ±Br. 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