---
title: "Eddy current loss and its minimization"
subject: "Electrical Machines I"
chapter: 1
key: "semester_4/electrical_machines_i:eddy-current-loss-minimize"
source: https://www.bctengineer.com/q/semester_4/electrical_machines_i/eddy-current-loss-minimize
site: BCT Engineering
---

# Eddy current loss and its minimization

Electrical Machines I · Chapter 1: Magnetic Circuits and Induction

- Asked in: 2072 Asoj, 4+4 marks
- Video: [Eddy current & Eddy Current loss | Electrical Engineering](https://www.youtube.com/watch?v=XPk38Tz_UC0) — Gaurav J - TheElectricalGuy
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/electrical_machines_i/eddy-current-loss-minimize
- Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines_i/chapter-1

## Question

What is eddy current loss? How does it take place in ferromagnetic material with ac excitation? How can it be minimized?

## Short answer

Eddy current loss is I²R heat from circulating currents induced in a conducting core by alternating flux, as changing flux induces emf in the core material. It is minimized by laminating the core into thin insulated sheets and by using high-resistivity material such as silicon steel, which reduces induced emf and increases path resistance.

## Answer

**Eddy current loss** is the $I^2R$ heat loss caused by circulating currents (eddy currents) induced in the body of a conducting magnetic core when the flux through it changes.

#### How it takes place with ac excitation

- With an ac supply the core flux alternates: $\phi = \phi_m \sin\omega t$.
- The iron core is itself a conductor. It can be regarded as many closed loops lying in planes perpendicular to the flux.
- By Faraday's law an emf $e = d\phi/dt$ is induced in each loop, so currents circulate in the core in closed paths, like whirlpools ("eddies").
- These currents flow through the resistance of the iron and produce heat $I^2R$. They also set up their own flux opposing the main flux (Lenz's law).

```text
  Solid core              Laminated core
  +-----------+          +-+-+-+-+-+-+
  |  .-----.  |          |o|o|o|o|o|o|
  | (   o   ) |  phi     | | | | | | |  small loops,
  |  '-----'  | (out)    |o|o|o|o|o|o|  high resistance
  +-----------+          +-+-+-+-+-+-+
  large eddy loop        insulated sheets
```

#### Expression

$$
P_e = K_e B_m^2 f^2 t^2 V \quad \text{W}
$$

where $B_m$ = peak flux density, $f$ = frequency, $t$ = lamination thickness, $V$ = core volume, $K_e$ = a constant that falls as the resistivity of the material rises. Since the induced emf ∝ $fB_m t$ and loss ∝ emf², the loss depends on the square of each.

#### Methods of minimising eddy current loss

1. **Laminating the core:** build the core from thin sheets (0.25–0.5 mm) insulated from each other by varnish or oxide, with the sheets parallel to the flux. Each eddy path is confined to a thin sheet, its emf is small and its resistance high. Loss ∝ $t^2$, so halving thickness quarters the loss.
2. **Using high-resistivity material:** adding 3–4% silicon to steel raises its resistivity and lowers the eddy currents (and also reduces hysteresis loss).
3. **Ferrite or powdered-iron cores** for high-frequency use, where resistivity is very high.
4. **Limiting $B_m$ and frequency** in design, since loss ∝ $B_m^2 f^2$.

## Check yourself

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

1. Eddy current loss is:
   A. I^2R heat from circulating currents induced in the core by changing flux
   B. Loss caused by domain rotation
   C. Winding copper loss
   D. Loss in the air gap
   Answer: A. The conducting core has induced emfs (Faraday's law) that drive circulating currents through its resistance. (Key point: What eddy loss is)

2. The eddy loss P_e is proportional to:
   A. B_m f t
   B. B_m^2 f^2 t^2
   C. B_m^1.6 f
   D. t only
   Answer: B. Emf is proportional to f B_m t and loss to emf squared, so each factor is squared. (Key point: Dependence)

3. Halving the lamination thickness changes the eddy loss by:
   A. Halving it
   B. Quartering it
   C. No change
   D. Doubling it
   Answer: B. Loss varies as t^2, so half the thickness gives one quarter of the loss. (Key point: Lamination)

4. Which is a valid way to cut eddy loss?
   A. Use low-resistivity solid iron
   B. Add 3-4% silicon to raise resistivity
   C. Raise the frequency
   D. Laminate with sheets perpendicular to the flux
   Answer: B. Silicon raises resistivity and reduces eddy currents; laminations must lie parallel to the flux. (Key point: Other methods)

## Related in Electrical Machines I

- [Hysteresis with AC excitation](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ahysteresis-ac-excitation) — Electrical Machines I, chapter 1
- [Magnetization characteristic of iron core with AC excitation](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Amagnetization-characteristic-ac) — Electrical Machines I, chapter 1
- [No-load current components, iron loss and core flux](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ano-load-current-10a-0p3pf-500turns) — Electrical Machines I, chapter 2
- [Full-load torque, rotor current and copper loss of 440 V motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Aim-440v-1425rpm-rotor-impedance) — Electrical Machines I, chapter 5
- [Rotor copper loss, frequency, current and efficiency of 11 kW motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Asquirrel-cage-1450rpm-11kw-losses) — Electrical Machines I, chapter 5
- [Hysteresis loop area and energy loss](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ahysteresis-loop-area-energy-loss) — Electrical Machines I, chapter 1, asked 2×

## Related in other subjects

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- [Short note: Eddy current loss](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Anote-eddy-current-loss) — Electrical Machines, chapter 1
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