---
title: "Slip ring motor: rotor current at 4% slip"
subject: "Electrical Machines"
chapter: 5
key: "semester_4/electrical_machines:im-slipring-120v-4-slip-current"
source: https://www.bctengineer.com/q/semester_4/electrical_machines/im-slipring-120v-4-slip-current
site: BCT Engineering
---

# Slip ring motor: rotor current at 4% slip

Electrical Machines · Chapter 5: Three Phase Induction Machines

- Asked in: 2069 Bhadra
- Video: [Slip Ring Motor (3 Phase Induction Motor) Electrical Machines (BEE) EEE (1st year Engineering)](https://www.youtube.com/watch?v=oz8sRnnO5Q4) — Gautam Varde
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/electrical_machines/im-slipring-120v-4-slip-current
- Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines/chapter-5

## Question

A 3-phase, slip-ring, induction motor with star-connected rotor has an induced e.m.f. of 120 volts between slip-rings at standstill with normal voltage applied to the stator. The rotor winding has resistance per phase of $0.3\ \Omega$ and standstill leakage reactance per phase of $1.5\ \Omega$. Calculate the current/phase when running short-circuited with 4% slip.

## Answer

**Given:** star-connected rotor, emf between slip rings at standstill $= 120$ V, $R_2 = 0.3\ \Omega$, $X_2 = 1.5\ \Omega$ per phase, slip $s = 0.04$, rotor short-circuited.

Standstill emf per phase:
$$
E_2 = \frac{120}{\sqrt3} = 69.28\ \text{V}
$$
Rotor emf at 4 % slip:
$$
E_{2s} = sE_2 = 0.04\times 69.28 = 2.771\ \text{V}
$$
Rotor impedance per phase at slip $s$:
$$
Z_{2s} = \sqrt{R_2^2 + (sX_2)^2} = \sqrt{0.3^2 + (0.04\times1.5)^2} = \sqrt{0.09+0.0036}=0.3059\ \Omega
$$
Rotor current per phase:
$$
I_2 = \frac{E_{2s}}{Z_{2s}} = \frac{2.771}{0.3059} = 9.06\ \text{A}
$$

**Answer:** rotor current $= 9.06$ A per phase.

## Check yourself

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

1. The slip-ring emf is 120 V with a star rotor. What is the standstill emf per phase?
   A. 120 V
   B. 69.28 V
   C. 207.8 V
   D. 40 V
   Answer: B. E2 = 120/sqrt3 = 69.28 V. (Key point: Phase emf)

2. What is the rotor emf per phase at 4 % slip?
   A. 2.771 V
   B. 69.28 V
   C. 2.77 kV
   D. 0.04 V
   Answer: A. E2s = sE2 = 0.04 x 69.28 = 2.771 V. (Key point: Running emf)

3. Which expression gives the rotor impedance at slip s?
   A. sqrt(R2^2 + X2^2)
   B. sqrt(R2^2 + (sX2)^2)
   C. sqrt((sR2)^2 + X2^2)
   D. R2 + sX2
   Answer: B. Reactance scales with the rotor frequency (sX2) but resistance does not. (Key point: Impedance at slip)

4. What is the rotor current per phase?
   A. 9.06 A
   B. 46.2 A
   C. 2.77 A
   D. 0.30 A
   Answer: A. I2 = 2.771/0.3059 = 9.06 A. (Key point: Current)

## Related in Electrical Machines

- [Starting current and rotor resistance for 150 kW slip ring motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Aim-150kw-3000v-starting-current-30a) — Electrical Machines I, chapter 5
- [Starting, no-load and full-load current of slip ring motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Aslip-ring-0p8-4ohm-400v-ratio4) — Electrical Machines I, chapter 5
- [Slip ring motor: starting, no-load, full-load currents](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Aim-4pole-slipring-1490-1300-currents) — Electrical Machines, chapter 5
- [Delta induction motor: slip, rotor emf, turns ratio](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Aim-delta-440v-1440rpm-rotor-emf) — Electrical Machines, chapter 5, asked 2×
- [Slip ring motor max-torque slip and full-load output](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Aslip-ring-380v-180v-60nm-starting) — Electrical Machines I, chapter 5, asked 4×
- [Maximum torque and torque at 1430 rpm of slip ring motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Aslip-ring-400v-200v-10nm-1430rpm) — Electrical Machines I, chapter 5, asked 3×

## Related in other subjects

- [Slip-ring IM rotor current at 4% slip](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Aim-slipring-rotor-current-4-percent-slip) — Electrical Machine, chapter 5
- [Slip-ring IM starting and running current](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Aim-4pole-slipring-starting-running-current) — Electrical Machine, chapter 5
- [6-pole slip-ring IM no-load current](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Aim-6pole-slipring-standstill-no-load-current) — Electrical Machine, chapter 5
- [Stator design of 120 kW 750 rpm motor](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Astator-120kw-750rpm) — Electric Machine Design, chapter 4, asked 2×