Number of tapping's to equalizer ring is equal to number of

A. Poles

B. Pole pairs

C. Parallel paths

D. Commutator segments

Answer: B. Pole pairs
Explanation: 
  • The brushes in Dc machines are used for collecting the current from the commutator segment. 
  • The brushes are made from carbon. 
  • In Dc machines Number of brushes are equal to the number of parallel paths or the number of poles. 
  • For wave winding, a number of parallel paths are equal to 2 and the number of brushes is required also 2.
  • Inter-poles are similar to the main field poles and are located on the yoke between the main field poles.
  • The inter-pole winding is connected in series with the armature winding because the inter-pole must produce flux that is directly proportional to the armature current.
  • The armature flux in the commutating zone which tends to shift the magnetic neutral axis is neutralized by an appropriate component of inter-pole flux.
  • For lap winding, the number of parallel paths(A) is equal to the number of poles(P) and a number of brushes required in lap winding are equal to the number of poles. 
  • In lap winding, there are equalizer rings are required for good commutation. The equalizer rings are used in lap winding because there are more parallel paths in lap winding than in wave winding. 
  • The equalizer rings are used in lap winding to reduce the unequal current distribution at brushes and reduce the sparking between the carbon brushes and commutator. 
Dummy Coils In Wave Winding:
  • In wave winding, there are dummy coils are used. 
  • The dummy coil provides the mechanical balance to the rotor only and dummy coils are physically connected but electrically not connected. 
  • The Wave winding is called incomplete winding means some armature slots remain empty during the winding. These empty slots are fields with some extra coils and these extra coils are called dummy coils. 
  • The dummy coils are exists only on the wave winding not available in lap winding.
Equalizer Ring In Lap winding
  • The equalizer rings is used in lap winding.
  • In lap windings, Under ideal conditions flux from all poles are the same, hence emf induced by each pole are the same and the current carried out by each parallel path is equal. 
  • But under the practical operating condition, there are variations in air gap length or differences in magnetic properties of poles material which causes a slight unbalance of induced emf in the various parallel paths.
  • As a result, conductors under stronger poles generate greater emf and hence carry a larger current than remaining conductors.
  • It means that some brushes are overloaded and carry more current than normal current while the remaining brushes carry less current due to being underloaded.
  • The current does not reverse completely in a given commutation time period due to overloaded brushes and that will cause sparking between brushes and the commutator surface.
  • The equalizer rings are used to overcome such problems.
  • Hence, the function of equalizer rings is to avoid unequal current distribution at the brushes and to reduce sparking between brushes and commutators.
  • The equalizer rings are a low resistance copper conductor which connects those points in the armature winding which have a potential difference between them. 
  • If there are potential differences between such points, the circulating current flows through equalizer rings rather than brushes.
  • Thus, the equalizer rings relieve the brushes to the circulating current by causing the latter to be passed by equalizer rings.
  • The current does not reverse completely in a given commutation time period due to overloaded brushes and that will cause sparking between brushes and the commutator surface.
Why are equalizer rings used only in lap winding?
  • Equalizer rings are more essential in lap winding than wave winding because of more no of parallel paths in lap winding. 
  • As in lap winding no of the parallel path is more, there would be severe sparking at brushes due to difference in currents in different parallel paths. 
  • But in wave winding no of parallel paths is two and the sparking at commutation is less. 
  • So, equalizer rings are used in lap winding to avoid any unequal distribution.
  • The lap winding is used in low voltage and high current applications. 
  • The wave winding is used in high voltage and low current applications. 
  • The efficiency of the lap winding is less and the efficiency of the wave winding is more. 
  • In simplex lap winding number of parallel paths is equal to the number of poles. 
  • In duplex lap winding, the number of parallel paths is equal to twice the number of poles. 
  • In triplex lap winding, the number of parallel paths is equal to the thrice the number of poles.
  • The wave winding is called incomplete winding and some slots remain empty. these empty slots are filled with dummy coils. 
  • There is more emf induced in the wave winding and less emf induce in lap winding. 
  • The other name for lap winding is parallel winding or multiple winding. 
  • The other name for wave winding is two circuits or series winding.
  • For simplex lap winding range of the winding pitch of (2,-2). For progressive winding pitch = Forward pitch - Backward pitch. 
  • For simplex lap winding, commutator pitch is equal to 1 and -1. 
  • The resultant pitch in lap winding is equal to the difference between the back and front pitches. 
  • The resultant pitch in wave winding equals the sum of the back pitch and front pitch. 
Important Points about Lap and Wave Windings
Wave winding:
For simplex wave winding, the number of parallel paths between the brushes is always 2.
For multiplex wave, the winding number of the parallel path is ‘2m’

Where m is the multiplicity of the winding
m = 1 for simplex winding
m = 2 for duplex winding
m = 3 for triplex winding

Lap winding:
For Simplex Lap Windings
Number of parallel paths (A) = P(Number of poles)

For Duplex Lap Windings
Number of parallel paths (A) = 2P(Number of poles)

For Triplex lap windings
Number of parallel paths (A) = 3P(Number of poles)


Lap & Wave Winding MCQs

In lap winding, the number of brushes is always

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