Cambridge IGCSE Physics 0625 – 4.5.5 The d.c. motor
4.5.5 The d.c. motor
Objective
Describe the operation of an electric motor, including the action of a split‑ring commutator and brushes.
Key Concepts
A d.c. motor converts electrical energy into mechanical (rotational) energy.
The motor is essentially a coil of wire (armature) placed in a magnetic field.
When current flows through the coil, a torque is produced according to the motor effect:
\$\tau = N I A B \sin\theta\$
where \$N\$ = number of turns, \$I\$ = current, \$A\$ = area of the coil, \$B\$ = magnetic flux density, \$\theta\$ = angle between \$B\$ and the normal to the coil.
The split‑ring commutator and brushes ensure that the direction of current in the armature reverses each half‑turn, keeping the torque in the same rotational direction.
Components of a Simple d.c. Motor
Component
Function
Armature (coil)
Conducts the current; experiences a magnetic force that produces torque.
Permanent magnets (or field coils)
Provide a uniform magnetic field \$B\$ across the armature.
Split‑ring commutator
Divides the circuit into two halves; reverses the direction of current in the armature each half‑turn.
Brushes (usually carbon)
Maintain electrical contact with the rotating commutator while allowing it to turn freely.
Axle and bearings
Support the rotating armature and transmit mechanical output.
Step‑by‑Step Operation
When the motor is connected to a d.c. source, current \$I\$ flows from the positive terminal, through one brush, into one half of the split‑ring commutator, and then through the armature coil.
The magnetic field \$B\$ from the permanent magnets interacts with the current‑carrying coil, producing a force on each side of the coil (Fleming’s left‑hand rule). The forces are opposite in direction, creating a torque that starts the coil rotating.
As the coil rotates, the commutator turns with it. When the coil reaches the point where the current would produce a torque in the opposite direction, the split‑ring commutator swaps the connections:
The brush that was connected to the positive terminal now contacts the opposite segment of the commutator.
The direction of current in the coil reverses, but the magnetic forces still act in the same rotational sense.
This reversal happens twice per revolution (once for each half‑turn), resulting in continuous rotation as long as the supply voltage is maintained.
Why a Split‑Ring Commutator Is Needed
Without a commutator, the torque would reverse every half‑turn because the angle \$\theta\$ in the torque equation would change sign. The commutator ensures that the product \$I\,\sin\theta\$ remains positive, keeping the torque direction constant.
Common Misconceptions
“The magnetic field moves the coil.” – The field is static; the forces on the moving charges in the coil cause motion.
“The commutator is a switch.” – It is a continuously rotating contact that reverses current automatically, not a manual switch.
“Brushes wear out quickly.” – Modern carbon brushes have long life; wear is mainly due to friction and sparking, which can be minimized by proper alignment.
Performance Factors
The speed \$n\$ (in revolutions per minute) of a simple d.c. motor is approximately given by:
\$n = \frac{V - I R}{k \Phi}\$
where \$V\$ is the applied voltage, \$I R\$ the internal voltage drop, \$k\$ a constant of the motor construction, and \$\Phi\$ the magnetic flux per pole. Increasing \$V\$ or decreasing the load (reducing \$I\$) raises the speed, while increasing the magnetic field strength \$\Phi\$ lowers the speed but increases torque.
Suggested Diagram
Suggested diagram: Cross‑section of a simple d.c. motor showing the armature coil, split‑ring commutator, brushes, and magnetic field lines.
Summary
A d.c. motor works by exploiting the motor effect: a current‑carrying conductor in a magnetic field experiences a force. The split‑ring commutator and brushes continuously reverse the direction of current in the armature each half‑turn, ensuring that the torque always acts in the same rotational direction, thus producing steady rotary motion.