The d.c. motor
Section: Electricity & Magnetism | Syllabus: Cambridge IGCSE Physics (0625)
The Motor Effect in Action
A d.c. motor converts electrical energy to kinetic (rotational) energy using the motor effect. A current-carrying coil in a magnetic field experiences a turning effect (torque), because forces act on both sides of the coil.
Structure of a Simple D.C. Motor
A simple motor consists of a rectangular coil of wire mounted on an axle, which can rotate between the poles of a C-shaped magnet.
A simple d.c. motor: the split-ring commutator and brushes keep current flowing into the coil as it rotates
Key Components
- Coil (armature): current-carrying rectangular coil that rotates
- C-shaped magnet: provides the magnetic field
- Split-ring commutator: two halves of a split copper ring, one connected to each end of the coil, which rotates with the coil
- Brushes: fixed carbon blocks pressed against the commutator by springs, connecting it to the electrical supply
How the D.C. Motor Works
- Current flows into the coil through the brushes and commutator
- One side of the coil carries current in one direction, the opposite side carries it in the other direction
- Applying Fleming's left-hand rule: side ab experiences an upward force, side cd a downward force (no force acts on the other two sides, since they run parallel to the field)
- These two opposite forces produce a turning effect which rotates the coil
- The coil rotates until it reaches the vertical position, where the brushes line up with the gaps between the commutator halves and the current momentarily stops
- Because of its inertia, the coil overshoots the vertical, and the commutator halves swap over which brush they touch, reversing the current through the coil
- The forces on the (now swapped) sides reverse too, so the turning effect continues to act in the same rotational direction
- The coil continues to rotate, repeating this cycle
Why the Split-Ring Commutator?
Without the commutator, the forces on the coil would reverse every half turn without the current reversing, causing the coil to oscillate back and forth instead of rotating continuously. The commutator reverses the current every half turn, ensuring the forces always act to keep the coil turning in the same direction.
Increasing the Turning Effect
The turning effect on the coil increases if:
| Factor | Change | Effect |
|---|---|---|
| Number of turns | Increase | More wires experiencing a force |
| Current | Increase | Greater force on each side of the coil |
| Magnetic field | Stronger magnets | Greater force on each current-carrying side |
The larger the turning effect on the coil, the faster it will turn.
Practical Motors
Practical motors improve on the simple design in several ways:
- A coil of many turns is wound on a soft iron cylinder or core, which rotates with the coil. The coil and core together are called the armature, and this makes the motor more powerful
- Several coils, each in its own slot in the core, each with a pair of commutator segments, giving increased power and smoother running
- An electromagnet (usually) rather than a permanent magnet, to produce the field in which the armature rotates
Most electric motors used in industry are induction motors, which work off a.c. on a different principle from the d.c. motor.
Moving-Coil Galvanometer
A galvanometer detects small currents or small p.d.s, often of the order of milliamperes (mA) or millivolts (mV), using the same turning effect as a motor.
- A coil is pivoted between the poles of a permanent magnet; current enters and leaves the coil by hair springs above and below it
- When there is a current, a turning effect acts on the coil (as in a motor), rotating it until stopped by the springs
- The greater the current, the greater the deflection, shown by a pointer attached to the coil
- A soft iron cylinder fixed at the centre of the coil, together with concave magnet poles, produces a radial field, so the scale is linear (evenly spaced divisions)
The radial field keeps the coil's plane parallel to the field at all deflection angles, giving an evenly spaced (linear) scale
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