Dummy or Idle Coils:

These are used with wave-winding and are resorted to when the requirements of the winding are not met by the standard armature punchings available in armature-winding shops. These dummy coils do not influence the electrical characteristics of the winding because they are not connected to the commutator. They are exactly similar to the other coils except that their ends are cut short and taped. They provide mechanical balance for the armature because an armature having some slots without windings would be out of balance mechanically. For example, suppose number of armature slots is 15, each containing 4 sides and the number of poles is 4. For a simplex wave-windings, Dummy coils

which does not come out to be an integer as required by this winding. However, if we make one coil dummy so that we have 58 active conductors, then

This makes the winding possible.

Uses of Lap and Wave Windings: The advantage of the wave winding is that, for a given number of poles and armature conductors, it gives more emf than the lap winding. Conversely, for the same emf, lap winding would require large number of conductors which will result in higher winding cost and less efficient utilization of space in the armature slots. Hence, wave winding is suitable for small generators especially those meant for 500-600 V circuits. Another advantage is that in wave winding, equalizing connections are not necessary whereas in a lap winding they definitely are. It is so because each of the two paths contains conductors lying under all the poles whereas in lap-wound armatures, each of the P parallel paths contains conductors which lie under one pair of poles. Any inequality of pole fluxes affects two paths equally, hence their induced emfs are equal. In lap-wound armatures, unequal voltages are produced which set up a circulating current that produces sparking at brushes. However, when large currents are required, it is necessary to use lap winding, because it gives more parallel paths. Hence, lap winding is suitable for comparatively low-voltage but high-current generators whereas wave- winding is used for high-voltage, low-current machines.