---
title: "Short note: Eddy current loss"
subject: "Electrical Machines"
chapter: 1
key: "semester_4/electrical_machines:note-eddy-current-loss"
source: https://www.bctengineer.com/q/semester_4/electrical_machines/note-eddy-current-loss
site: BCT Engineering
---

# Short note: Eddy current loss

Electrical Machines · Chapter 1: Magnetic Circuits and Induction

- Asked in: 2068 Magh, 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/note-eddy-current-loss
- Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines/chapter-1

## Question

Write a short note on eddy current loss.

## Short answer

Eddy current loss is I^2R heating from circulating currents induced in the core by the changing flux. It depends on thickness squared, peak flux density squared, and frequency squared: P_e = k_e B_max^2 f^2 t^2 V. Use laminated cores with insulated sheets to reduce it.

## Answer

**Eddy current loss** is the power wasted as heat in a magnetic core when the alternating flux induces emf in the iron itself and this emf circulates currents inside the core.

**Cause:** by Faraday's law, a changing flux induces an emf in any conducting path linked with it. A solid iron core has low resistance, so closed current loops (eddy currents) flow in planes at right angles to the flux, and they cause $I^2R$ heating.

```text
  Solid core          Laminated core
  +--------+          +--+--+--+--+
  | (  )   |          |  |  |  |  |
  | (  )   |  flux    |  |  |  |  |
  +--------+          +--+--+--+--+
  large loops         thin insulated layers,
                      small loops
```

**Expression:**

$$
P_e = k_e B_{max}^{2} f^{2} t^{2} V\ \text{W}
$$

where $t$ is the thickness of a lamination, $f$ the frequency, $B_{max}$ the peak flux density and $V$ the volume. The loss is proportional to $t^2$ and $f^2$.

**Effects:** lower efficiency, heating of the core, and a demagnetising effect of the eddy currents on the main flux.

**Reduction:**
- Build the core from thin laminations (0.35 to 0.5 mm) insulated from each other by varnish.
- Add silicon to the steel to raise its resistivity.
- Use ferrite or powdered iron cores at high frequency.

**Uses:** eddy currents are useful in induction furnaces, eddy current brakes and energy meters.

## Check yourself

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

1. Why do eddy currents flow in a solid iron core?
   A. Faraday's law: changing flux induces emf in closed conducting paths of low resistance
   B. Hysteresis of domains
   C. Air gaps in the core
   D. DC current in the winding
   Answer: A. The changing flux induces emf, and the low-resistance iron lets large loops of current flow. (Key point: Cause of eddy currents)

2. How does eddy loss depend on lamination thickness t?
   A. Proportional to t
   B. Proportional to 1/t
   C. Proportional to t^2
   D. Independent of t
   Answer: C. P_e = k_e B_max^2 f^2 t^2 V, so thinner laminations cut the loss strongly. (Key point: Dependence on thickness)

3. Which pair reduces eddy current loss?
   A. Thick solid core and low silicon
   B. Insulated thin laminations and added silicon
   C. Annealing and a wide loop
   D. Higher frequency and thicker plates
   Answer: B. Lamination with insulation limits loop size; silicon raises resistivity. (Key point: Reduction methods)

4. Where are eddy currents deliberately used?
   A. Transformer cores
   B. Armature cores
   C. Induction furnaces and eddy current brakes
   D. Permanent magnets
   Answer: C. Induction heating, braking and energy meters make use of eddy currents. (Key point: Useful application)

## Related in Electrical Machines

- [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
- [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
- [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 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

## Related in other subjects

- [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
- [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
- [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