Section 1 of 5Parts of a DC Motor
An electric motor transfers electrical energy into kinetic energy using the force on a current-carrying wire in a magnetic field.
Coil
- A rectangular coil of wire carries the current and is free to rotate between the poles of a magnet.
Magnets
- Permanent magnets provide a magnetic field across the coil, with the field running from north to south.
Split-ring commutator
- Two halves of a ring are connected to the ends of the coil and reverse the current every half turn.
Brushes
- Carbon contacts press against the commutator, allowing current to flow from the power supply into the rotating coil.
Power supply
- A direct current supply provides the current that creates the forces on the sides of the coil.
Section 2 of 5How the Coil Turns
The motor effect on opposite sides of the coil produces rotation.
1
Current flows through the coil, and the two long sides carry current in opposite directions.
2
Fleming’s left-hand rule shows that the force on one side is upwards and on the other side is downwards.
3
These two forces form a turning effect that rotates the coil.
4
When the coil reaches the vertical position, the commutator swaps contact between the brushes and reverses the current in the coil.
5
The forces keep acting in the same turning direction, so the coil continues to rotate in a continuous way.
Section 3 of 5Changing the Motor
Changing the Motor
Faster rotation
- Increasing the current, using stronger magnets or adding more turns to the coil increases the force and so the speed.
Reverse direction
- Reversing the current, or swapping the magnetic poles, reverses the direction of rotation, but not both together.
Role of the commutator
- Without it the forces would reverse their turning effect after the vertical position,
- so the coil would rock back and forth instead of rotating.
Iron core
- A soft iron core in the coil strengthens the magnetic field and increases the turning effect.
Section 4 of 5Energy and Applications
Energy transfer
- Electrical energy is transferred to kinetic energy, with some wasted as thermal energy in the coil and as sound.
Everyday uses
- Motors turn fans, washing machine drums, electric car wheels and the discs of hard drives.
Efficiency
- Reducing friction at the bearings and using thicker wire reduce wasted energy and so raise efficiency.
Section 5 of 5Exam Checklist & Tips
12.2.6 – Key Points for the Exam
- Motor effect The force on a current-carrying conductor in a magnetic field.
- Commutator A split ring that reverses the current in a motor coil every half turn.
- Brush A contact that connects the power supply to the rotating commutator.
- Fleming’s left-hand rule A rule giving the direction of the force from the directions of field and current.
Common Mistakes
- Saying the commutator reverses the supply current; it reverses the current in the coil only.
- Describing the force as the same direction on both sides of the coil; they are opposite.
- Forgetting that the field of the magnet must be perpendicular to the wire for the largest force.
- Stating that the motor would keep rotating continuously without the split ring.