GRAMME MACHINE

A Gramme machine, Gramme ring, or Gramme dynamo is a type of dynamo, an electrical generator which produces direct current, named for its Belgian inventor, Zénobe Gramme. It was the first generator to produce power on a commercial scale for industry. Gramme demonstrated this apparatus to the Academy of Sciences in Paris in 1871. The Gramme machine used a ring armature, i.e., a series of thirty armature coils, wound around a revolving ring of soft iron. The coils are connected in series, and the junction between each pair is connected to a commutator on which two brushes run. The permanent magnets magnetize the soft iron ring, producing a magnetic field which rotates around through the coils in order as the armature turns. This induces a voltage in two of the coils on opposite sides of the armature, which is picked off by the brushes. Earlier electromagnetic machines passed a magnet near the poles of one or two electromagnets, 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 enough coils, the resulting voltage waveform is practically constant, thus producing a near direct current supply. This type of machine needs only electromagnets producing the magnetic field to become a modern generator.

During a demonstration at an industrial exposition in Vienna in 1873, Gramme accidentally discovered that this device, if supplied with a constant-voltage power supply, will act as an electric motor. A careless workman connected the terminals of a Gramme machine to another dynamo which was producing electricity, and its shaft began to spin. The Gramme machine was the first powerful electric motor useful as more than a toy or laboratory curiosity. Today the design forms the basis of nearly all DC electric motors. Gramme's use of multiple commutator contacts with multiple overlapped coils, and his innovation of using a ring armature, was an improvement on earlier dynamos and helped usher in development of large-scale electrical devices.

Earlier designs of electric motors were notoriously inefficient because they had large, or very large, air gaps throughout much of the rotation of their rotors. Long air gaps create weak forces, resulting in low torque. A device called the St. Louis motor (still available from scientific supply houses), although not intended to, clearly demonstrates this great inefficiency, and seriously misleads students as to how real motors work. 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. The Gramme ring, however, has a comparatively small air gap, which enhances its efficiency. (In the illustration, the large hoop-like piece is probably the permanent magnet; the Gramme ring is rather hard to see.