---
title: "Slip ring motor: starting, no-load, full-load currents"
subject: "Electrical Machines"
chapter: 5
key: "semester_4/electrical_machines:im-4pole-slipring-1490-1300-currents"
source: https://www.bctengineer.com/q/semester_4/electrical_machines/im-4pole-slipring-1490-1300-currents
site: BCT Engineering
---

# Slip ring motor: starting, no-load, full-load currents

Electrical Machines · Chapter 5: Three Phase Induction Machines

- Asked in: 2073 Bhadra (old course), 8 marks
- Video: [✅️Slip-ring Motor Starting & Shorting Current | Induction Motor No Load Current](https://www.youtube.com/watch?v=ygzWA39bHNY) — The Engineering Waley
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/electrical_machines/im-4pole-slipring-1490-1300-currents
- Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines/chapter-5

## Question

A 4-pole, 50 Hz 3-phase slip ring induction motor has star connected stator and rotor windings. The rotor winding has resistance $0.8\ \Omega$ and reactance of $4\ \Omega$ per phase at standstill. The emf induced between slip rings at standstill is 400 V. The stator to rotor turn ratio is 4. The motor runs at 1490 rpm at no-load and 1300 rpm at full-load. Calculate:
i) Starting current
ii) No-load current
iii) Full load current [the rest of the list is cut off in the scan, [?]]

## Short answer

Rotor phase voltage 230.94 V. Slips: 0.00667 (no load), 0.1333 (full load). Rotor currents: start 56.61 A, no load 1.92 A, full load 32.03 A. Equivalent stator currents: 14.15 A, 0.48 A, 8.01 A.

## Answer

**Given:** 4-pole, 50 Hz, star stator and rotor, $R_2 = 0.8\ \Omega$, $X_2 = 4\ \Omega$ per phase at standstill, slip-ring emf $= 400$ V at standstill, turns ratio (stator : rotor) $= 4$, no-load speed 1490 rpm, full-load speed 1300 rpm. The list is cut off after (iii), so the rotor and the equivalent stator-side currents are given for the three conditions, with magnetising current neglected.

$$
N_s = 1500\ \text{rpm}, \qquad E_2 = \frac{400}{\sqrt3} = 230.94\ \text{V per phase}
$$
The rotor current at slip $s$ is $I_2 = \dfrac{sE_2}{\sqrt{R_2^2 + (sX_2)^2}}$, and the corresponding stator-side current is $I_2' = I_2/4$.

| Condition | Slip $s$ | $sE_2$ (V) | $Z_{2s}$ ($\Omega$) | $I_2$ (A) | $I_2' = I_2/4$ (A) |
|---|---|---|---|---|---|
| Starting | 1 | 230.94 | 4.079 | 56.61 | 14.15 |
| No load | 0.00667 | 1.540 | 0.8004 | 1.92 | 0.48 |
| Full load | 0.1333 | 30.79 | 0.9615 | 32.03 | 8.01 |

Slips: $s_{NL} = \frac{1500-1490}{1500} = 0.00667$ and $s_{FL} = \frac{1500-1300}{1500} = 0.1333$.

#### Results
- (i) Starting current: rotor $= 56.61$ A (stator side $= 14.15$ A)
- (ii) No-load current: rotor $= 1.92$ A (stator side $= 0.48$ A)
- (iii) Full-load current: rotor $= 32.03$ A (stator side $= 8.01$ A)

Rotor frequency at full load: $f_r = sf = 0.1333\times 50 = 6.67$ Hz.

**Answer:** rotor currents are 56.61 A, 1.92 A and 32.03 A at start, no load and full load (equivalent stator currents 14.15 A, 0.48 A and 8.01 A).

## Check yourself

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

1. The slip-ring emf at standstill is 400 V with a star rotor. What is the standstill emf per phase E2?
   A. 400 V
   B. 400 x sqrt(3) = 692.8 V
   C. 400/sqrt(3) = 230.94 V
   D. 400/3 = 133.3 V
   Answer: C. For a star winding the line value is sqrt(3) times the phase value, so E2 = 400/sqrt(3) = 230.94 V. (Key point: Rotor emf per phase)

2. The synchronous speed is 1500 rpm. What is the slip at the full-load speed of 1300 rpm?
   A. 0.1333
   B. 0.0667
   C. 0.1500
   D. 0.8667
   Answer: A. s = (1500-1300)/1500 = 0.1333. (Key point: Slip from speed)

3. Which expression gives the rotor current per phase at slip s?
   A. sE2 / sqrt(R2^2 + X2^2)
   B. E2 / sqrt(R2^2 + (sX2)^2)
   C. sE2 / sqrt(R2^2 + (sX2)^2)
   D. sE2 / sqrt((R2/s)^2 + X2^2)
   Answer: C. Both the rotor emf (sE2) and the rotor reactance (sX2) scale with slip, while R2 does not. (Key point: Rotor current at slip s)

## Related in Electrical Machines

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