15 September 2017

Dummy coils

  • The wave winding is possible only with particular number of conductor or coils.
  • But sometimes the armature slots available in the armature winding do not meet the requirement of the winding. ( It means that the available number of slots is more than the required number of conductors ). 
  • Some slots are kept without armature winding in that case and dummy coil or coils are employed in that slots.  
  • The dummy coils are similar to other coils except their ends are cut, short and taped. They do not connect with the commutator bars.  
  • The dummy coils are simply to provide mechanical balance for the armature.
  •  As they do not connect with commutator bars they do not affect the electrical characteristics of the winding.
Example
  • Let us consider that the wave winding of 26 slots and 4 – poles.
        Z = 26 × 2 = 52
        Simplex wave winding
        YA = ( Z ± 2 ) / P
              = ( 52 ± 2 ) / 4
              = 25/2 OR 27/2
  • But the YA must be an integer therefore the winding is not possible with 52 conductors. However if the number of coils are 25

        YA = ( Z ± 2 ) / P
              = ( 50 ± 2 ) / 4
              = 12 OR 13
  •  It means that one coil ( 26 – 25 = 1) is inactive, it is called as dummy coil.
Why dummy coils are not necessary in Lap winding?
  • Because the lap winding is designed for any number of conductors.
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