---
title: "Testing of DC machines: retardation vs Swinburne's test"
subject: "Electrical Machines I"
chapter: 4
key: "semester_4/electrical_machines_i:dc-testing-retardation-vs-swinburne"
source: https://www.bctengineer.com/q/semester_4/electrical_machines_i/dc-testing-retardation-vs-swinburne
site: BCT Engineering
---

# Testing of DC machines: retardation vs Swinburne's test

Electrical Machines I · Chapter 4: DC Motor

- Asked in: 2081 Chaitra (new course), 3+3 marks
- Video: [Retardation Test - Testing of DC Motor - Electrical Machines 1](https://www.youtube.com/watch?v=kfSoA_UhhYs) — Ekeeda
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/electrical_machines_i/dc-testing-retardation-vs-swinburne
- Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines_i/chapter-4

## Question

Explain the necessity of testing in DC machines. Explain how is the retardation test different from the Swinburne's test?

## Short answer

Testing determines losses, efficiency, heating, commutation and characteristics without full-load testing. Swinburne's test runs the machine as a shunt motor on no load and measures constant losses from electrical input. Retardation test cuts armature supply and measures deceleration to find rotational losses and separate iron and mechanical losses using moment of inertia.

## Answer

#### Necessity of testing DC machines
Testing is done to find the performance of a DC machine without (or before) putting it in service:
- To find **losses** (constant and variable) and hence **efficiency** at any load.
- To check temperature rise and **heating** under load, which decides the rating.
- To check **commutation** and sparking at the brushes.
- To find **characteristics** (OCC, load and speed-torque curves) and confirm the design and name-plate values.
- For acceptance and quality control by manufacturer and buyer.

Direct loading of large machines wastes much energy and needs a large load, so **indirect tests** such as Swinburne's (no-load) test and the retardation test are used.

#### Swinburne's test
The machine runs as a **shunt motor on no load at rated voltage and speed**. The input $V I_0$ minus the small no-load armature copper loss gives the **constant losses** (iron + friction and windage + shunt field copper loss) directly. The efficiency at any load is then calculated.

#### Retardation (running-down) test
The machine is run **above rated speed** and the supply to the armature is **cut off** (field kept excited for iron loss). The armature slows down as its stored kinetic energy supplies the rotational losses. The time taken to fall through a known speed range (say 1.1 $N$ to 0.9 $N$) is measured:

$$
P_{rot} = \omega J \frac{d\omega}{dt}
$$

$J$ is found by repeating the test with a known extra load (or an added flywheel). Running down with the field off gives the mechanical loss alone, so the iron loss can be separated.

#### Difference
| Point | Swinburne's test | Retardation test |
|---|---|---|
| Machine state | Running as motor on no load | Armature disconnected, coasting down |
| Measurement | Electrical: $V$, $I_0$, $I_{sh}$ | Speed vs time (slope $dN/dt$) |
| Losses obtained | Total constant (stray) losses | Rotational losses; mechanical and iron separately |
| Need of $J$ | Not needed | Moment of inertia must be known or found |
| Machines | Shunt and compound only | Shunt machines; also large machines with good inertia |
| Accuracy | Ignores stray load loss and temperature change | Also ignores stray load loss; depends on speed reading accuracy |
| Power needed | Small no-load power | Almost none (uses stored energy) |

## Check yourself

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

1. Why are indirect tests like Swinburne's used instead of direct loading for large machines?
   A. Direct loading wastes much energy and needs a large load
   B. Direct loading cannot measure efficiency
   C. Indirect tests give higher accuracy at full load
   D. Indirect tests do not need any measurements
   Answer: A. Indirect tests find losses at small power and calculate efficiency. (Key point: Why testing)

2. What does Swinburne's test give directly?
   A. Armature copper loss at full load
   B. Constant losses (iron + mechanical) from no-load input minus armature copper loss
   C. Stray load loss
   D. Moment of inertia
   Answer: B. Running as a no-load shunt motor, the input is almost all constant loss. (Key point: Swinburne's test)

3. What is measured in the retardation test?
   A. No-load current at rated voltage
   B. Field current versus voltage
   C. Time for the speed to fall through a known range after the supply is cut off
   D. Temperature rise at full load
   Answer: C. Rotational loss = ωJ dω/dt from the slowing rate. (Key point: Retardation test)

4. Which statement correctly differentiates the tests?
   A. Swinburne's test needs J; retardation test does not
   B. Both need J
   C. Neither test needs a machine running
   D. Retardation test needs the moment of inertia; Swinburne's test does not
   Answer: D. Swinburne's is an electrical measurement; retardation depends on J. (Key point: Difference)

## Related in Electrical Machines I

- [Armature reaction in DC machines and its remedies](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Aarmature-reaction-effects-remedies) — Electrical Machines I, chapter 3
- [Self-excited vs separately excited DC generators](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Aself-vs-separately-excited-classification) — Electrical Machines I, chapter 3
- [Need of DC motor starter and three-point starter](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Adc-starter-necessity-three-point) — Electrical Machines I, chapter 4, asked 4×
- [Back emf and its role in DC motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Aback-emf-dc-motor-role) — Electrical Machines I, chapter 4, asked 6×
- [DC current for iron ring with 4 mm air gap](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Airon-ring-1p5m-4mm-500turns-current) — Electrical Machines I, chapter 1
- [DC current for iron rod ring with 4 mm air gap](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Airon-rod-200sqmm-4mm-500turns-250mwb) — Electrical Machines I, chapter 1

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