Basic Motor Theory (10)

Armature Windings
Gramme Ring Winding
The old Gramme Ring type winding, now obsolete, is shown in Figure 9 and its equivalent circuit in Figure 10. It can be seen that there are an equal number of voltage-generating conductors on each side of the armature and the conductor voltages are additive from bottom to top on each side. There are two paths between the positive and negative brushes and the voltage per path is the generated voltage of the machine. Each path provides half of the current output.


Figure 9. Two Pole Gramme Ring Winding


Figure 10. Equivalent Circuit, Two Pole Gramme Ring Winding

Drum Winding
The Drum type winding is made of coils, one of which is illustrated in Figure 11. The straight portions of the coil are the parts rotating through the magnetic field in which the voltage is induced. Therefore, each single coil has two conductors. This has the advantage over the Gramme Ring winding where only one side of each coil is used as an active conductor. There are two classes of drum windings depending upon how the coils are connected to the commutator.


Figure 11. Drum Type Winding Coil

Lap Winding
When the end connections of the coils are brought to adjacent bars as shown in Figure 12, a lap or parallel winding is formed. In this type winding, there are as many paths through the armature as there are poles on the machine. Therefore, to obtain full use of this type winding, there must be as many brushes as there are poles, alternate brushes being positive and negative. Any winding can be illustrated in one of two forms, the circular form or the development form. A simplex lap winding is shown in Figure 13 (circular form) and Figure 14 (development form.) In this particular circular form, the flux cutting portions of the conductors are shown as straight lines radiating from the center and are numbered for convenience in connecting them to the commutator which is in the center of the diagram. The outermost connecting lines represent the end connections on the back of the armature and the inner connecting lines represent the connections on the front or commutator end of the armature. The development form of winding represents the armature as if it were split open and rolled out flat. It is somewhat simpler to understand but the continuity of the winding is broken. The lap winding is best suited for low voltage, high current ratings because of the number of parallel paths.


Figure 12. Lap Winding connected to commutator bars


Figure 13. Simplex Lap Winding, Circular Form


Figure 14. Simplex Lap Winding, Development Form

Wave Winding
When the end connections of the coils are spread apart as shown in Figure 15 a wave or series winding is formed. In a wave winding there are only two paths regardless of the number of poles. Therefore, this type winding requires only two brushes but can use as many brushes as poles. The simplex wave winding in Figure 16 (circular) and Figure 17 (development) shows that the connections to the armature do not lap back toward the coil but progress forward. The coil voltages are cumulative but it is necessary to travel several times around the armature and to traverse half the total winding in order to trace the path between the positive and negative brush. The wave winding is best suited for high voltage low current ratings since it has only two paths.


Figure 15. Wave Winding connected to commutator bars


Figure 16. Simplex Wave Winding, Circular Form


Figure 17. Simplex Wave Winding, Development Form

Slots and Coils
The number and size of slots depend upon the generator or motor requirements. The slot has to be large enough to hold the correct number of conductors but at the same time, the tooth has to be large enough to pass the necessary magnetic flux. Normally, in a simple winding, there are as many coils as there are slots. This means that each slot contains two coil sides, one side of each coil being at the top of a slot and the other at the bottom of a slot. Each coil may consist of one or more turns depending on the applied or generated voltage of the unit. A typical arrangement of coil sides and slots is shown in Figure 18. Solid lines represent the front end connections to the commutator and dotted lines represent the back end connections.

Slot Pitch
Slot pitch refers to the number of slots spanned by each coil. For example, in Figure 18, the top of coil in slot 1 has its bottom in slot 4, therefore, the slot pitch is 1-4 or 3. Since the top of the coil is directly under the north pole and the bottom is directly under the south pole, the winding is known as a full pitch winding. In many cases, for various reasons, the pitch is reduced to less than full pitch. For example, if the coils in Figure 6 spanned 2 slots instead of three, the winding would become a two-thirds pitch winding.


Figure 18. Coil Sides in Armature Slots

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