Learning outcomes
- Describe the motor effect.
- Use Fleming’s left-hand rule.
- State factors affecting force.
- Explain operation of a simple d.c. motor.
- Explain the split-ring commutator.
19.1 The motor effect
A current-carrying conductor placed in a magnetic field experiences a force if current is not parallel to the field. The force results from interaction between the conductor’s field and the external field. It is greatest when current and field are perpendicular and zero when they are parallel.
Force increases with magnetic field strength, current and length of conductor within the field. Reversing current or field reverses force; reversing both leaves force unchanged.
19.2 Fleming’s left-hand rule
Hold the left thumb, first finger and second finger mutually perpendicular. First finger shows magnetic field from north to south, second finger shows conventional current, and thumb shows force or motion.
Apply the rule only after identifying directions clearly. Electron flow is opposite to the current direction used by the rule.
19.3 Force on a coil
In a rectangular coil between magnetic poles, currents on opposite sides flow in opposite directions. The forces on those sides are opposite and form a couple, producing a turning effect. Forces on the other sides may be parallel to the field and contribute little torque.
Torque varies with coil angle. Without a mechanism to maintain the turning direction, the coil would tend to stop or oscillate near an equilibrium position.
19.4 Simple d.c. motor
A simple motor contains a current-carrying coil in a magnetic field, brushes, a split-ring commutator and a d.c. supply. The motor effect creates opposite forces on the coil sides, causing rotation.
Electrical energy is transferred to kinetic energy, with losses to heating and sound. A load requires torque, so current and input power may increase depending on the motor and supply.
19.5 Split-ring commutator
The split-ring commutator reverses the connection between coil and supply every half-turn. Therefore, current in each side reverses just as the coil passes the position where torque would otherwise reverse. The turning effect continues in the same rotational direction.
Carbon brushes press against the rotating commutator and carry current while allowing motion. Contact friction and sparking cause wear and energy loss.
19.6 Increasing motor turning effect
Torque can be increased by increasing current, magnetic field strength, number of coil turns, coil area or effective conductor length in the field. Adding a soft-iron core strengthens the field through the coil.
Increasing current also raises I²R heating, so practical motors require suitable wire, cooling and protection. Stronger motors are not obtained without trade-offs.
19.7 Motor direction and speed
Reversing either supply polarity or magnetic field reverses rotation. Reversing both leaves rotation direction unchanged. Speed depends on supply, load and motor design rather than a single simple equation at this level.
As a motor spins, it also generates a back e.m.f. opposing the supply. This limits current during normal operation. When stalled, back e.m.f. falls and current can become dangerously large, causing overheating.
Worked examples
Direction change
A motor rotates clockwise. Reversing only the battery reverses current, so forces reverse and the motor turns anticlockwise.
Stronger torque
Doubling current approximately doubles force on each active conductor if field and geometry remain unchanged.
Stalled motor
If the shaft is prevented from turning, back e.m.f. decreases and current rises. The coil may overheat, so motors need protection.
Practical focus
Investigation
Use a motor-effect apparatus with a wire between magnet poles. Observe force direction, reverse current and reverse field separately. Vary current within safe limits and compare displacement. For a model motor, identify brushes and split ring without touching moving parts.
Examination guidance
- Fleming’s left-hand rule uses conventional current.
- The split ring reverses coil current every half-turn.
- Reversing current or field reverses force.
- Include heating when suggesting larger current.
Check your understanding
- When is motor-effect force maximum?
- What is the purpose of the split-ring commutator?
- Give three ways to increase motor torque.
Answers
- When current is perpendicular to the magnetic field.
- It reverses coil current every half-turn so torque remains in the same rotational direction.
- Increase current, field strength, turns, coil area or conductor length in the field.
