4.5.5 The d.c. motor
- Syllabus
- 0625–2026–2027
- Topic
- 4.5.5
- Level
- —
Learning objectives
4.5.5.1A current-carrying coil in a magnetic• Know: a current-carrying coil in a magnetic field may experience a turning effect and that the turning effect is increased by increasing: (a) the number of turns on the coil (b) the current (c) the strength of the magnetic field4.5.5.2Operation of an electric motor• Describe the operation of an electric motor, including the action of a split-ring commutator and brushes
Why a current-carrying coil turns
In a magnetic field, current flows in opposite directions along the two opposite sides of a coil. The motor-effect forces on those sides act in opposite directions, forming a couple that turns the coil.
| Change, with other factors fixed | Turning effect | Why |
|---|---|---|
| more turns on the coil | increases | more current-carrying sides experience force |
| larger current | increases | the magnetic force on each active side is larger |
| stronger magnetic field | increases | the magnetic force on each active side is larger |
A twofold increase in one factor gives a twofold increase in the turning effect when the other conditions are unchanged. A decrease in a factor decreases the turning effect in the same qualitative way.
The turning effect also changes as the coil rotates. It is greatest when the forces have the largest perpendicular distance from the axis and becomes zero at the position where their lines of action pass through the axis.
A turning effect is produced by separated opposite forces, not by a single resultant force pushing the whole coil sideways. More turns, current or field strength increase the turning effect; they do not by themselves decide its direction.
How a d.c. motor keeps turning
| Part | Function |
|---|---|
| permanent magnet or electromagnet | provides the magnetic field |
| current-carrying coil on an axle | experiences opposite forces that create a turning effect |
| two brushes | maintain sliding electrical contact between the stationary supply and rotating commutator |
| split-ring commutator | swaps the supply connections to the coil every half-turn |
When current enters the coil, Fleming's left-hand rule gives opposite forces on its two active sides. These forces turn the coil about its axle.
After each half-turn, the two halves of the split ring contact the opposite brushes. The current in the coil reverses just as the sides exchange positions. Each side's force therefore also reverses, so the turning effect continues in the same rotational direction.
At the position where the turning effect is momentarily zero, the coil's motion carries it through. Commutation then restores a turning effect in the same rotational direction, allowing continuous rotation.
Reversing the supply polarity reverses the current relative to the fixed field, so the motor rotates in the opposite direction.
A split-ring commutator reverses the coil current every half-turn; the brushes only maintain contact. Slip rings do not perform this reversal and are used for a simple a.c. generator, not this d.c. motor function.