The armature and the magnets

Like the coil of the motor you built, an ordinary DC motor has a coil. But because a motor is used to turn a load, its coil must be strong enough to have an outside load connected to it.

Armature

So the coil is wound round a core made of steel or iron, as in figure 13.11. This core with its coil is called the armature. The job of the armature is to be made to turn by the couple set up when an electric current passes through it.

Figure to be added

Figure 13.11 — an armature: many coils of copper wire wound in the slots of a laminated iron core on a shaft, with the commutator at one end.

Magnetic poles

A magnetic field is needed so that a force acts on the coil when a current flows through it. In an ordinary DC motor this field comes from permanent magnets arranged round the armature, as in figure 13.12.

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Figure 13.12 — magnetic poles: two curved permanent magnets lining the inside of the motor's round case.

Why an iron core and curved magnets

Iron is a magnetic material, so the core does more than hold the coil in shape: it gathers the magnet's field lines and carries them through the coil, making the field where the wires sit much stronger. From the earlier page, a stronger field means a bigger force, so the motor is more powerful for the same current. The magnets are curved to follow the round armature, so the gap between them is small and even all the way round, and the wires meet a strong field over as much of each turn as possible. Many turns of wire in the slots add up the length in the field, the third factor in the force.

Armature and poles

The armature is the coil on an iron core that turns; permanent magnets round it give the field.