Gramme machine

Although popular in 19th century electrical machines, the Gramme winding principle is no longer used since it makes inefficient use of the conductors.

The portion of the winding on the interior of the ring cuts no flux and does not contribute to energy conversion in the machine.

Earlier electromagnetic machines passed a magnet near the poles of one or two electromagnets, or rotated coils wound on double-T armatures within a static magnetic field, creating brief spikes or pulses of DC resulting in a transient output of low average power, rather than a constant output of high average power.

With more than a few coils on the Gramme ring armature, the resulting voltage waveform is practically constant, thus producing a near direct current supply.

These early inefficient designs apparently were based on observing how magnets attracted ferromagnetic materials (such as iron and steel) from some distance away.

It took a number of decades in the 19th century for electrical engineers to learn the importance of small air gaps.

(In the top illustration, the large hoop-like piece is the laminated permanent magnet; the Gramme ring is rather hard to see at the base of the hoop.)

All modern armatures use this externally wrapped (drum) design, although the windings do not extend fully across the diameter; they are more akin to chords of a circle, in geometrical terms.

At the surface of the rotor, the slots are only as wide as needed to permit the insulated wire to pass through them while winding the coils.

In small armatures a solid drum is often used simply for ease of construction, since the core can be easily formed from a stack of stamped metal disks keyed to lock into a slot on the shaft.