A DC motor converts electrical energy into kinetic (rotational) energy. It uses the force on a current-carrying conductor in a magnetic field.
The motor effect
When a current flows through a conductor in a magnetic field, a force acts on the conductor. The direction of the force is given by Fleming’s left-hand rule:
- Thumb: direction of force (motion).
- First finger: direction of the magnetic field (N to S).
- Second finger: direction of conventional current.
Structure of a simple DC motor
| Part | Function |
|---|---|
| Permanent magnets | Provide the magnetic field |
| Coil (armature) | Carries the current; experiences a force in the field |
| Split-ring commutator | Reverses the current direction in the coil every half turn, so the coil continues to rotate in the same direction |
| Brushes (carbon contacts) | Maintain electrical contact between the rotating commutator and the external circuit |
How it works
- Current flows through the coil in the magnetic field.
- Each side of the coil experiences a force in opposite directions (one up, one down), creating a turning effect.
- The coil rotates.
- When the coil reaches the vertical position, the split-ring commutator reverses the current direction.
- The forces continue to push the coil in the same rotational direction.
Increasing the speed
- Increase the current.
- Increase the magnetic field strength (stronger magnets).
- Increase the number of turns on the coil.
Common errors and how to correct them
Confusing the split-ring commutator with slip rings. A split-ring commutator has a gap and reverses the current every half turn (DC motor). Slip rings are continuous and maintain the same current direction (AC generator).
Using the right-hand rule instead of the left-hand rule. The motor effect uses Fleming’s LEFT-hand rule. The right-hand rule is for generators (induction).
How to apply this in an exam
Describe the motor effect, state Fleming’s left-hand rule, and explain the role of the split-ring commutator in maintaining continuous rotation. If asked to increase speed, give at least two methods.
Need help with this concept?
A 0625 specialist can work through the student's current question and help identify whether the difficulty is the concept, the calculation or the exam technique.