15/09/2017

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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