There are two common armature winding schemes in PMDC motors; lap winding and wave winding. Lap windings are wound as shown in Fig. 4.106. This configuration is for a 12-slot armature with a 2-pole field. In this case, the winding pitch is slots 1 to 6, or a span of 5 teeth. The coil pitch should be greater than the arc of the field coil (or permanent magnet) for good commutation.The first coil goes on slots 1 to 6. The next coil is wound on slots 5 to 12. Then keep shifting 1 slot at a time until all 12 coils are in place. Coil ends are likewise connected to commutator bars in sequence as each coil is wound. As these coils are wound in sequence, the outer coils are necessarily larger than the inner coils because the end turns overlap each other. This results in mechanical imbalance.
FIGURE 4.106 Completed lap winding pattern.
Mechanical imbalance can be overcome to some extent by using a double flier and winding two coils at once, as shown in Fig. 4.107.
Coils 1 to 6 and 12 to 7 are wound simultaneously. Next, coils 2 to 7 and 1 to 8 are wound simultaneously. This process continues until all 12 coils are in place. Coil ends are connected to the commutator bars in sequence as each coil is completed. This is the most common type of winding for PMDC motors.
The lap winding is also used in motors having more than two poles, but one pair of brushes is required for each pair of poles. The number of pairs of brushes can be reduced by using a winding method called wave winding.
In the case of wave winding, the coil ends are not tied to an adjacent commutator bar after each coil is completed. Instead, coils are wound so that a coil under one pole is connected to a coil 180 electrical degrees away which is under a like pole.This allows one pair of brushes to commutate two pairs of poles.
If a two-pole motor is being designed, the winding type is necessarily a lap configuration. A four-pole motor may use lap winding or wave winding. With lap windings, wave windings, or even slot armatures, a pair of brushes is required to commutate each pair of poles. In the case of an odd-slot four- or six-pole motor, the coils can be commutated by a single pair of brushes. The one-pair brush rigging in a
FIGURE 4.107 Double-flier lap winding pattern: (a) first set of coils wound, and (b), second set of coils wound.
wave-wound odd-slot motor will be less complicated than the one brush pair per pair of poles rigging would be. However, the current density in these brushes would be higher because now they would carry all of the current. The area of each brush would have to be increased to get the current densities to reasonable levels.
In lap winding, the finish wire is connected to the next adjacent bar. The wave winding has its start and finish wires connected approximately 360 electrical degrees apart instead of adjacent. Diagrams for the lap and wave winding patterns are shown in Figs. 4.108 and 4.109, respectively.
FIGURE 4.108 Diagram for lap winding pattern.
FIGURE 4.109 Diagram for wave winding pattern.
DC MOTOR WINDINGS (Electric Motors)