Topic 5 of 5
The single-coil d.c. motor
A d.c. motor transfers electrical energy into mechanical motion. Its split-ring commutator reverses current in the physical coil every half-turn so that the magnetic forces keep producing the same sense of rotation.
The two active sides of a coil carry opposite currents and experience opposite forces. As a named side moves to the other side of the axle, its force must reverse to keep the turning sense unchanged.
Connect the supply to a rotating coil
- Two magnetic poles
- They provide the external field across the gap, from N to S.
- Coil and axle
- The current-carrying coil rotates about the axle as its two active sides experience magnetic forces.
- Split-ring commutator
- Two conducting segments, insulated from one another, rotate with the coil. Each is connected to one end of the coil.
- Brushes and d.c. supply
- The brushes remain fixed and make sliding contact with the rotating segments. Their supply polarity stays fixed while each segment changes which brush it touches.
The split ring changes each side's connection every half-turn
A and B name the same physical sides throughout. View from the near, commutator end. The field stays rightwards, the right brush stays positive and the left brush stays negative.
1. Initial orientation
A is on the left: its current comes towards the near end, giving an upward force. B is on the right: its current goes away, giving a downward force. The turning effect is clockwise.
2. A quarter-turn: the switching position
A is above the axle and B below. Their force lines would pass through the axle, giving zero turning effect. The brushes are now at the insulating gaps; in this idealised brief interruption, current is zero. Existing rotation can carry the coil through.
The gaps are centred on this zero-turning orientation. A single stationary coil here is not guaranteed to self-start. The brief loss of contact is an idealised switching interval; it is not a third conducting state. The small bridge in lead B crosses the supply wire without joining it.
3. Half a turn from the start
A has moved to the right and B to the left. Their split-ring segments have exchanged brushes, reversing current in each physical side. The forces still give clockwise turning.
Each labelled split-ring segment stays wired to its physical coil side; the stationary brushes exchange segments as the rotor turns. These are two insulated parts of one split ring, not two complete slip rings. The soft-iron cylinder strengthens the field in the coil and its turning effect in this simple model.
Green: field. Brown: current. Purple: force. Blue: turning or existing rotation. O marks the axle. The lower connection view omits the core and separates the leads to expose the complete electrical path.
Follow one named side through a half-turn
- Initial position: A is left of the axle and carries current out of the page, so its force is up. B is right and carries current into the page, so its force is down. With field right, the pair turns the coil clockwise.
- Intervening zero-turning position: A is above the axle and B below it. The force lines would pass through the axle, giving no moment. In the illustrated commutator, the insulating gaps meet the brushes at this position, so current is briefly interrupted.
- After the half-turn: A is now on the right and B on the left. The segments have exchanged brushes. Current in A is into the page and its force is down; current in B is out of the page and its force is up. The pair still gives clockwise turning.
The external supply polarity and magnetic poles stay fixed. It is the connection of each end of the rotating coil that swaps. Without this swap, the named sides would keep their previous currents after exchanging positions, and the forces would tend to turn the coil back.
The switching positions recur every half-turn. Existing rotation can carry the coil through a zero-turning position and the brief contact gap. A single coil resting exactly at such a position is not guaranteed to self-start.
A split ring has two insulated conducting segments that perform this connection swap. Two separate complete slip rings keep each coil end connected to the same brush and do not provide that half-turn reversal.
Why wind the coil on a soft-iron cylinder?
The soft iron becomes magnetised and strengthens the magnetic field through the coil. With current and other conditions comparable, this increases the magnetic forces and the coil's turning effect.
The core strengthens the magnetic effect; the commutator controls the coil-current reversal. These are different jobs. Neither removes the need for a complete current path through the brushes, segments and coil.