page_title: Gramme machine
section_title: Drum windings
hierarchical_section_title: Gramme machine / Drum windings
caption_attribution_description: Original text of illustration: Figure 249: Modern form of the Gramme ring armature. The core consists of a number of thin flat rings of well-annealed charcoal iron, the outer diameter of each ring or disc being 11.5 inches and its inner diameter 9.25 inches. Sheets of thin paper insulate each disc from its neighbors to prevent the flow of eddy currents. The armature is mounted on a steel shaft to which is keyed a four-armed metal "spider", the extremes of whose arms fit into notches cut in the inner edges of the soft iron core rings, so that a good mechanical connection is obtained between the core and the shaft. The spider is made of a non-magnetic metal, to reduce the tendency to leakage of lines of force across the interior of the armature. The armature inductors consist of cotton-covered copper wire of No. 9 standard wire gauge, wound around the core in one layer and offering a resistance, from brush to brush, of 0.048 ohm. There are two convolutions in each section, the adjacent ends of neighboring sections being soldered to radial lugs projecting from the commutator bars. Public Domain source: Hawkins Electrical Guide, Volume 1 Chapter 17: The Armature, Page 224. Copyright 1917 by Theo. Audel &amp; Co. Printed in the United States Scanned by Dale Mahalko, Gilman, WI, USA DMahalko (talk) 22:01, 12 January 2008 (UTC)
context_page_description: A Gramme machine, Gramme ring, Gramme magneto, or Gramme dynamo is an electrical generator that produces direct current, named for its Belgian inventor, Zénobe Gramme, and was built as either a dynamo or a magneto. It was the first generator to produce power on a commercial scale for industry. Inspired by a machine invented by Antonio Pacinotti in 1860, Gramme was the developer of a new induced rotor in form of a wire-wrapped ring and demonstrated this apparatus to the Academy of Sciences in Paris in 1871. 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. The winding requires twice the number of turns and twice the number of commutator bars as an equivalent drum-wound armature.
context_section_description: While the Gramme ring permitted a more steady power output, it suffered from a technical design inefficiency due to how magnetic lines of force pass through a ring armature. The field lines tend to concentrate within and follow the surface metal of the ring to the other side, with relatively few lines of force penetrating into the interior of the ring.
  Consequently, the interior windings of each small coil are minimally effective at producing power because they cut very few lines of force compared with the windings on the exterior of the ring. The interior windings are effectively dead wire and only add resistance to the circuit, lowering efficiency.
  Initial attempts to insert a stationary field coil within the center of the ring to help the lines penetrate into the center proved too complex to engineer.  Further, if the lines did penetrate the interior of the ring any e.m.f. produced would have opposed the e.m.f. from the outside of the ring because the wire on the inside was orientated in the opposite direction to that on the outside having turned through 180 degrees as it was wound.
  Eventually it was found to be more efficient to wrap a single loop of wire across the exterior of the ring and simply not have any part of the loop pass through the interior. This also reduces construction complexity since one large winding spanning the width of the ring is able to take the place of two smaller windings on opposite sides of the ring. 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. Neighboring windings overlap, as can be seen in almost any modern motor or generator rotor that has a commutator. In addition, windings are placed into slots with a rounded shape (as seen from the end of the rotor). 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.
  While the hollow ring could now be replaced with a solid cylindrical core or drum, the ring still proves to be a more efficient design, because in a solid core the field lines concentrate in a thin surface region and minimally penetrate the center. For a very large power-generation armature several feet in diameter, using a hollow ring armature requires far less metal and is lighter than a solid core drum armature. The hollow center of the ring also provides a path for ventilation and cooling in high power applications.
  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.
caption: Modern design of the Gramme ring, wrapped only around the exterior of the core.[12]
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