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185:(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 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.)
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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.
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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
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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
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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
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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
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This illustration shows a simplified one-pole, one-coil Gramme ring and a graph of the current produced as the ring spins one revolution. While no actual device uses this exact design, this diagram is a building block to better understand the next illustrations.
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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
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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
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Electrical Guide Number One, Questions, Answers & Illustrations: a Progressive Course of Study for Engineers, Electricians, Students and those Desiring to Acquire a Working Knowledge of Electricity and its
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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.
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with the first. Because the bottom coil is oriented opposite of the top coil, but both are immersed in the same magnetic field, the current forms a ring across the brush terminals.
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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.
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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.
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A two-pole, four-coil Gramme ring. The coils of A and A' sum together, as do the coils of B and B', producing two pulses of power 90° out of
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A three-pole, six-coil Gramme ring, and a graph of the combined three poles, each 120° out of phase from the other and summing together.
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With more than a few coils on the Gramme ring armature, the resulting voltage waveform is practically constant, thus producing a near
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Diagram of magnetic lines through a Gramme ring, showing the very few magnetic lines of force crossing the center gap.
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requires twice the number of turns and twice the number of commutator bars as an equivalent drum-wound armature.
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Example of a single winding around the exterior of a drum core with no wires penetrating the interior.
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machines passed a magnet near the poles of one or two electromagnets, or rotated coils wound on
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with each other. When coils A and A' are at maximum output, coils B and B' are at zero output.
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A one-pole, two-coil Gramme ring. The second coil on the opposite side of the ring is wired in
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in two of the coils on opposite sides of the armature, which is picked off by the brushes.
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in 1860, Gramme was the developer of a new induced rotor in form of a wire-wrapped ring (
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Early form of the Gramme ring armature with coils penetrating the interior of the ring.
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in 1873, Gramme accidentally discovered that this device, if supplied with a constant-
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which rotates around through the coils in order as the armature turns. This induces a
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Modern design of the Gramme ring, wrapped only around the exterior of the core.
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While the hollow ring could now be replaced with a solid cylindrical core or
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Dynamo-electric machinery: a manual for students of electrotechnics
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and helped usher in development of large-scale electrical devices.
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and only add resistance to the circuit, lowering efficiency.
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lines of force penetrating into the interior of the ring.
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The Gramme machine used a ring armature, with a series of
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During a demonstration at an industrial exposition in
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