---
title: "Hysteresis and eddy current losses, reduction"
subject: "Electrical Machines"
chapter: 1
key: "semester_4/electrical_machines:hysteresis-eddy-losses-reduce"
source: https://www.bctengineer.com/q/semester_4/electrical_machines/hysteresis-eddy-losses-reduce
site: BCT Engineering
---

# Hysteresis and eddy current losses, reduction

Electrical Machines · Chapter 1: Magnetic Circuits and Induction

- Asked in: 2067 Mangsir (old course), 8 marks
- Video: [65-) Hysteresis Losses & Eddy Current Losses in Core of Transformer and How to Reduce It??](https://www.youtube.com/watch?v=c2WkxGKTQBQ) — ToonNagri
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/electrical_machines/hysteresis-eddy-losses-reduce
- Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines/chapter-1

## Question

What are hysteresis and eddy current losses? What are their significance in the operation of electric machine? Write down different methods to reduce them.

## Short answer

Hysteresis loss (P_h ∝ f B_max^1.6) comes from domain reversal; eddy loss (P_e ∝ f^2 B_max^2 t^2) from induced currents. They cause core heating and efficiency drop. Reduce by using thin laminated silicon steel and narrow-hysteresis materials, and lower B_max.

## Answer

**Hysteresis loss** is the power lost as heat in a magnetic core because its domains are repeatedly turned back and forth by an alternating field. **Eddy current loss** is the $I^2R$ loss caused by circulating currents induced in the core itself by the changing flux.

#### Expressions

$$
P_h = \eta B_{max}^{1.6} f V\ \text{W}, \qquad P_e = k_e B_{max}^{2} f^{2} t^{2} V\ \text{W}
$$

where $t$ is the lamination thickness and $V$ the core volume.

#### Origin

- **Hysteresis:** energy per cycle equals the area of the $B$–$H$ loop, so loss rises with frequency and flux density.
- **Eddy current:** the alternating flux induces emf in the solid core. This drives currents in closed loops inside the iron, which heat it.

#### Significance in machines

1. Together they form the **core (iron) loss**. It is present whenever the core is excited, even at no load, and it is almost constant from no load to full load.
2. It lowers efficiency and creates heat that must be removed, so it limits the rating.
3. It fixes the choice of core material, lamination thickness and the flux density used in the design.
4. In transformers it is the constant loss $P_i$ that sets the condition for maximum efficiency ($P_i = P_{cu}$).

#### Methods of reduction

| Hysteresis loss | Eddy current loss |
|---|---|
| Use materials with a narrow loop (silicon steel, CRGO steel, permalloy) | Build the core from thin laminations (0.35 to 0.5 mm) |
| Add 3 to 4% silicon to the iron | Insulate the laminations with varnish or oxide coating |
| Anneal the steel to remove mechanical stress | Use silicon steel, which has higher resistivity |
| Work at a lower $B_{max}$ | Use powdered iron or ferrite cores at high frequency |
| Reduce core volume | Laminate along the direction of flux |

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is eddy current loss?
   A. Loss from domains turning back and forth
   B. I^2R loss from currents induced in the core by the changing flux
   C. Loss in the winding copper
   D. Loss from flux leakage into air
   Answer: B. Alternating flux induces emf in the iron, driving circulating currents that heat it. (Key point: Eddy current loss definition)

2. How does the core (iron) loss vary from no load to full load?
   A. Rises sharply with load
   B. Falls with load
   C. Is zero at no load
   D. Stays almost constant
   Answer: D. Core loss depends on core excitation, not load current, so it is nearly constant. (Key point: Core loss behaviour)

3. Which material choice reduces hysteresis loss?
   A. Silicon steel with a narrow loop
   B. Hard alnico steel
   C. Solid cast iron
   D. Material with high coercivity
   Answer: A. A narrow loop (silicon steel, CRGO) means small loop area and low loss. (Key point: Reducing hysteresis loss)

## Related in Electrical Machines

- [Hysteresis and eddy losses depend on frequency](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Ahysteresis-eddy-frequency-reason) — Electrical Machines, chapter 1
- [Significance of hysteresis loop, volume dependence](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Ahysteresis-loop-significance) — Electrical Machines, chapter 1
- [Eddy current loss and its minimization](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Aeddy-current-loss-minimize) — Electrical Machines I, chapter 1
- [Short note: Eddy current loss](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Anote-eddy-current-loss) — Electrical Machines, chapter 1

## Related in other subjects

- [Hysteresis and eddy current losses; loop area](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Ahysteresis-eddy-loss-area) — Electrical Machine, chapter 1
- [Hysteresis loss and eddy current loss](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Ahysteresis-eddy-loss) — Electric Machine Design, chapter 1
- [Short note: Eddy current loss](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Ashort-note-eddy-current-loss) — Electrical Machine, chapter 1
- [Losses in magnetic materials, reduction](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Alosses-alternating-flux) — Electrical Machine, chapter 1