Explain split-ring commutation in a simple d.c. motor
Trace how a split-ring contact arrangement reverses coil current every half-turn while the supply polarity remains unchanged.
Exam scope and assessment notes
Extended only (Supplement) · Physics 0972, 2026–2028. Use the stated source, operating conditions and meter ranges. Written practice is self-checked. Human subject-teacher review, supervised practical competence and cumulative assessment remain pending. Cambridge states that IGCSE 0972 has the same subject content as IGCSE 0625, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Trace a complete source–coil path.
- Understand a coil turning pair.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which components rotate in the stated simple motor?
What you will learn
- Trace how a split-ring contact arrangement reverses coil current every half-turn while the supply polarity remains unchanged.
- Explain current reversal by split-ring contact and keep source polarity, driving sense and dead-centre limits distinct.
Keep the driving turn in one sense
A simple d.c. motor has a current-carrying coil rotating in a magnetic field, a split-ring commutator connected to the coil ends and stationary brushes connected to the d.c. supply. The separated conducting commutator segments rotate with the coil. Each brush contacts changing segments as the rotor turns.
After a half-turn, the coil sides have exchanged physical positions. The commutator reverses current in the coil at the appropriate crossing, so the force pattern at the physical left and right sides can continue to drive the same turning sense. The external d.c. supply has not reversed polarity; the connection to the rotating coil has changed.
Account for the crossing and input energy
At the ideal dead-centre crossing, the turning moment is zero and real contact geometry may briefly interrupt current. Rotation can carry the rotor through by inertia; the simplified single-coil model is not guaranteed to start from every dead-centre position. Other motor designs use additional coils and arrangements.
The electrical supply provides energy transferred into mechanical work and other transfers such as heating. The permanent magnet supplies the field condition, not an unlimited energy input. Continuous motion also depends on suitable contact, load and source conditions. Do not describe the commutator as two separate continuous slip rings.
Pause and explain
What reverses each half-turn in the coil path?
Worked example
Why does a simple d.c. motor reverse coil current every half-turn while its supply remains d.c.?
Show the worked solution
- The active sides exchange positions after half a turn.
- Without switching, the driving turning sense would change after crossing.
- The split-ring contacts reverse coil current to maintain the intended turning sense.
Answer Coil-current reversal maintains the driving sense while the external supply polarity stays fixed.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Describe the motor’s coil, field, split ring and brushes.
Give me a hint
Assign a role and a moving or stationary status to each part.
Compare my reasoning
- The coil rotates in the imposed magnetic field.
- Its ends connect to separated rotating commutator segments.
- Stationary brushes connect the d.c. source to those segments.
Rotating coil/split ring and stationary supply brushes produce the switching action.
Look for these in your work
- Identify all four parts.
- Name the changing connection.
Explain the half-turn switch without claiming the source becomes a.c.
Give me a hint
Compare the coil-side current labels before and after commutation.
Compare my reasoning
- The coil sides exchange spatial positions.
- Brush contact changes to the opposite segments.
- Coil current reverses while external source polarity stays fixed.
The rotating coil connection switches; the d.c. source does not reverse.
Look for these in your work
- Track labelled coil sides.
- Retain source polarity.
A single ideal coil stops exactly at dead centre. A report says commutation guarantees immediate starting there. Evaluate it.
Give me a hint
Check the turning moment before assuming inertia exists.
Compare my reasoning
- At the ideal position, the turning moment can be zero.
- A stationary rotor has no ongoing rotation to carry it through.
- The simple single-coil model does not guarantee self-starting there.
Dead-centre self-starting is not guaranteed by this simple model.
Look for these in your work
- Separate starting from passing through.
- State the orientation limitation.
Common mistakes
- Coil current and supply polarity differ.
- Segments rotate; brushes stay fixed.
- Account for dead-centre inertia limits.
What can you explain now?
Which part changes the coil-to-supply connections as the motor turns?
Compare with the explanation
The split-ring commutator
Rotating separated segments meet the stationary brushes in the changing contact arrangement.
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Make it stick
Explain current reversal by split-ring contact and keep source polarity, driving sense and dead-centre limits distinct.
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