---
title: "True/false: rotor core loss neglected"
subject: "Electrical Machine"
chapter: 5
key: "semester_4/electrical_machine:tf-rotor-core-loss-neglected"
source: https://www.bctengineer.com/q/semester_4/electrical_machine/tf-rotor-core-loss-neglected
site: BCT Engineering
---

# True/false: rotor core loss neglected

Electrical Machine · Chapter 5: Three Phase Induction Machines

- Asked in: 2068 Bhadra, 1+3 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/electrical_machine/tf-rotor-core-loss-neglected
- Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machine/chapter-5

## Question

State whether the following statement is true or false and justify it: Rotor core loss is often neglected in 3 phase induction motor.

## Short answer

True. Rotor core loss depends on slip frequency f₂ = s·f, which is only 1–2.5 Hz at normal slip (2–5%). Since hysteresis and eddy-current losses are proportional to frequency, rotor core loss is negligible compared to stator core loss and copper losses, so it is omitted from the equivalent circuit.

## Answer

**True.**

Core loss consists of hysteresis loss (∝ f) and eddy-current loss (∝ f²), so it depends strongly on the frequency of the flux alternations in the iron.

- In the stator, the flux alternates at the supply frequency f (50 Hz), so stator core loss is significant. It is included in the equivalent circuit as the core-loss resistance Rc.
- In the rotor, the flux alternates at the slip frequency f₂ = s·f. At normal running slip (2 to 5 %), f₂ = 1 to 2.5 Hz for a 50 Hz supply. At this frequency the hysteresis and eddy-current losses are very small.

For example, at s = 0.04 and f = 50 Hz, f₂ = 2 Hz. The rotor eddy-current loss is about (2/50)² = 0.16 % of that at supply frequency, and the hysteresis loss is about 4 %.

So rotor core loss is negligible compared with stator core loss, copper losses and friction. It is neglected in the equivalent circuit, and the only core loss shown there is the stator core loss. (The statement is true only for the running condition. At standstill, f₂ = f and the rotor core loss is not small, but the motor does not stay in that state.)

## Check yourself

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

1. The statement is?
   A. True
   B. True only at standstill
   C. False for squirrel cage
   D. False
   Answer: A. Rotor core loss is negligible at running slip. (Key point: True or false)

2. What is the rotor frequency at slip s?
   A. f
   B. s·f
   C. (1-s)f
   D. f/s
   Answer: B. f2 = s f. (Key point: Rotor frequency)

3. At s = 0.04 and 50 Hz, what is f2?
   A. 1250 Hz
   B. 0.2 Hz
   C. 50 Hz
   D. 2 Hz
   Answer: D. f2 = 0.04 × 50 = 2 Hz. (Key point: Example)

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