--- title: "Losses in magnetic materials, reduction" subject: "Electrical Machine" chapter: 1 key: "semester_4/electrical_machine:losses-alternating-flux" source: https://www.bctengineer.com/q/semester_4/electrical_machine/losses-alternating-flux site: BCT Engineering --- # Losses in magnetic materials, reduction Electrical Machine · Chapter 1: Magnetic Circuits and Induction - Asked in: 2075 Baisakh, 5+3 marks - Video: [Losses in the Magnetic Materials|| Iron or Core Losses||Hysteresis & Eddy Current Losses|AC Machines](https://www.youtube.com/watch?v=rg2vzcTCbaY) — vvsupura - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/electrical_machine/losses-alternating-flux - Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machine/chapter-1 ## Question Explain the losses which occur when magnetic materials are subjected to an alternating flux. How can we reduce these losses practically? ## Short answer Alternating flux causes hysteresis loss (P_h ∝ B_max^1.6 f V) reduced by silicon steel with narrow loop, and eddy current loss (P_e ∝ B_max² f² t² V) reduced by laminating core and using high-resistivity material. ## Answer Two losses occur in a magnetic core carrying alternating flux: **hysteresis loss** and **eddy current loss**. Together they are called the **core (iron) loss**. #### Hysteresis loss - Each cycle of flux reversal turns the magnetic domains of the core against internal friction. The energy used is lost as heat. - It equals the area of the B–H loop multiplied by the volume and the frequency. - **Pₕ = ηₕ B_max^1.6 f V** (Steinmetz's law), in watts. #### Eddy current loss - The alternating flux induces emf in the core itself, which drives circulating currents. These produce I²R loss. - **Pₑ = kₑ B_max² f² t² V**, in watts, where *t* is the lamination thickness. - Since the loss varies as f² and t², thin laminations are very effective. Both depend on B_max and frequency, so they are present in the core of every transformer, motor and generator, and they do not change much with load. #### Practical reduction | Loss | Method of reducing | |---|---| | Hysteresis | Use silicon steel (3–4 % Si) or other soft magnetic alloys with a narrow B–H loop | | Hysteresis | Anneal and use cold-rolled grain-oriented (CRGO) steel | | Hysteresis | Do not operate at too high a B_max | | Eddy current | Build the core from thin laminations (0.35–0.5 mm) | | Eddy current | Insulate each lamination with varnish or oxide coating | | Eddy current | Use material of high resistivity (silicon raises it) | | Eddy current | Use ferrite or powdered-iron cores at high frequency | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which two losses together make up the core (iron) loss? A. Hysteresis loss and copper loss B. Windage loss and stray loss C. Copper loss and friction loss D. Hysteresis loss and eddy current loss Answer: D. Core loss is the sum of hysteresis and eddy current losses in the iron. (Key point: Core loss components) 2. Why are thin laminations effective at reducing core loss? A. They reduce hysteresis loss as t^2 B. They increase the core resistance of the winding C. Eddy loss varies as t^2, so thinner sheets cut it strongly D. They raise the flux density Answer: C. Pe is proportional to t^2, so reducing lamination thickness sharply reduces eddy current loss. (Key point: Thin laminations) 3. Which measure mainly reduces hysteresis loss? A. Silicon steel or other soft material with a narrow B-H loop B. Higher operating frequency C. Thicker laminations D. Varnish insulation between sheets Answer: A. Hysteresis loss is proportional to loop area, so a material with a narrow loop (silicon steel, CRGO) lowers it. (Key point: Reducing hysteresis loss) 4. How does core loss change with load? A. It rises in proportion to load current B. It does not change much with load C. It falls to zero at no load D. It varies as load squared Answer: B. Core loss depends on Bmax and frequency, which stay nearly constant, so it hardly changes with load. (Key point: Load dependence) ## Related in Electrical Machine - [Hysteresis and eddy current losses, reduction](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Ahysteresis-eddy-losses-reduce) — Electrical Machines, chapter 1 - [Hysteresis loop energy, hard and soft magnetic materials](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ahysteresis-area-energy-hard-soft) — Electrical Machines I, chapter 1 - [Constant flux from no load to full load](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Aflux-constant-no-load-full-load) — Electrical Machines, chapter 2 - [Transformer no-load and load operation, constant core flux](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Aideal-transformer-noload-load-constant-flux) — Electrical Machines I, chapter 2, asked 5× - [Transformer operation; flux constant at any load](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Atransformer-load-noload-flux-constant) — Electrical Machine, chapter 2, asked 3× - [No-load and loaded operation, constant flux](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Aideal-transformer-no-load-loaded-flux) — Electrical Machines, chapter 2, asked 3× ## Related in other subjects - [Domains, domain walls and magnetic losses](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_engineering_material%3Adomain-walls-magnetic-losses) — Electrical Engineering Material, chapter 4 - [Magnetic flux from vector potential](https://www.bctengineer.com/question.md?key=semester_3%2Felectromagnetism%3Amagnetic-flux-from-vector-potential-a) — Electromagnetism, chapter 3 - [True/False: magnetic loading and teeth flux density](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Atf-magnetic-loading-teeth) — Electric Machine Design, chapter 5 - [Soft versus hard magnetic materials](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Asoft-hard-magnetic-materials) — Electric Machine Design, chapter 1, asked 5×