14 March, 2007

Armature and its Windings

Armature :
Gramme -Ring armatureThe old Gramme-Ring armature,now obselete is shown in figure view A. Each coil is connected to two commutator segments as shown. One end of coil 1 goes to segment A, and the other end of coil 1 goes to segment B. One end of coil 2 goes to segment C, and the other end of coil 2 goes to segment B. The rest of the coils are connected in a like manner, in series, around the armature. To complete the series arrangement, coil 8 connects to segment A. Therefore, each coil is in series with every other coil.

View B shows a composite view of a Gramme-ring armature. It illustrates more graphically the physical relationship of the coils and commutator locations.
The windings of a Gramme-ring armature are placed on an iron ring. A disadvantage of this arrangement is that the windings located on the inner side of the iron ring cut few lines of flux. Therefore, they have little, if any, voltage induced in them. For this reason, the Gramme-ring armature is not widely used.
Drum-type armature :
A drum-type armature is shown in figure.The armature windings are placed in slots cut in a drum-shaped iron core. Each winding completely surrounds the core so that the entire length of the conductor cuts the main magnetic field. Therefore, the total voltage induced in the armature is greater than in the Gramme-ring. You can see that the drum-type armature is much more efficient than the Gramme-ring. This accounts for the almost universal use of the drum-type armature in modem dc generators.

Armature Windings :Drum-type armatures are wound with either of two types of windings - the Lap Winding or the Wave Winding. The difference beween the two is merely due to the different arrangement of the end connections at the front or commutator end of armature.Each winding can be arranged progressively or retrogressively and connected in simplex,duplex and triplex.The following rules,however,apply to both types of the windings:

(i)The front and back pitch are each approximately equal to the pole-pitch i.e windings should be full-pitched.This results in increased e.m.f round the coils.For special purposes,fractional-pitched windings are deliberately used.
(ii)Both pitches should be odd, otherwise it would be difficult to place the coils properly on the armature.For example if YB and YF were both even,then all the coil sides and conductors would lie either in the upper half of slots or in the lower half.Hence, it would become impossible for one side of the coil to lie in the upper half of one slot and the other side of the same coil to lie in the lower half of some other slot.
(iii) The number of commutator segments is equa to the number of slots or coils because the front ends of conductors are joined to the segments in pairs.
(iv) The winding must close upon itself i.e if we start from agiven point and move from one coil to another,then all conuctors should be traversed and we should reach the same point again without a break or discontinuty in betwen.

Lap Winding :
View A This type of winding is used in dc generators designed for high-current applications. The windings are connected to provide several parallel paths for current in the armature. For this reason, lap-wound armatures used in dc generators require several pairs of poles and brushes.

In lap winding, the finishing end of one coil is connected to a commutator segment and to the starting end of the adjacent coil situated under the same pole an so on,till all the coils have been connected.This type of winding derives its name from the fact it doubles or laps back with its succeding coils.Following points regarding simplex lap winding should be noted:
  1. The back and front pitches are odd and of opposite sign.But they can't be equal. They differ by 2 or some multiple thereof.
  2. Both YB and YF shpuld be nearly equal to a pole pitch.
  3. The average pitch YA = (YB + YF)/2.It equals pole pitch = Z/P.
  4. Commutator pitch YC = ±1.
  5. Resultant pitch YR is even, being the arithmetical difference of two odd numbers i.e YR = YB - YF.
  6. The number of slots for a 2-layer winding is equal to the number of coils.The number of commutator segments is also the same.
  7. The number of parallel paths in the armature = mP where 'm' is the multiplicity of the winding and 'P' the number of poles.Taking the first condition, we have YB = YF ± 2m where m=1 fo simplex lap and m =2 for duplex winding etc.
  • If YB > YF i.e YB = YF + 2, then we get a progressive or right-handed winding i.e a winding which progresses in the clockwise direction as seen from the comutator end.In this case YC = +1.
  • If YB < size="1">F i.e YB = YF - 2,then we get a retrogressive or left-handed winding i.e one which advances in the anti-clockwise direction when seen from the commutator side.In this case YC = -1.
  • Hence, it is obvious that for
The figures below shows the simplex lap winding in circular form and in development form.

Wave WindingView B, shows a wave winding on a drum-type armature. This type of winding is used in dc generators employed in high-voltage applications. Notice that the two ends of each coil are connected to commutator segments separated by the distance between poles. This configuration allows the series addition of the voltages in all the windings between brushes. This type of winding only requires one pair of brushes. In practice, a practical generator may have several pairs to improve commutation.
When the end connections of the coils are spread apart as shown in Figure a wave or series winding is formed. In a wave winding there are only two paths regardless of the number of poles. Therefore, this type winding requires only two brushes but can use as many brushes as poles. Because the winding progresses in one direction round the armature in a series of 'waves' it is know as wave winding.If, after passing once round the armature,the winding falls in a slot to the left of its starting point then winding is said to be retrogressive.If, however, it falls one slot to the right, then it is progressive.
The figures below shows the simplex wave winding in circular form and in development form.

Points to note in case of Wave winding :
  1. Both pitches YB and YF are odd and of the same sign.
  2. Back and front pitches are nearly equal to the pole pitch and may be equal or differ by 2, in which case, they are respectively one more or one less than the average pitch.
  3. Resultant pitch YR = YF + YB.
  4. Commutator pitch, YC = YA (in lap winding YC = ±1 ). Also YC = (No.of commutator bars ± 1 ) / No.of pair of poles.
  5. The average pitch which must be an integer is given by YA = (Z ± 2)/P = (No.of commutator bars ± 1)/No.of pair of poles.
  6. The number of coils i.e NC can be found from the relation NC = (PYA ± 2)/2.
  7. It is obvious from 5 that for a wave winding, the number of armature conductors with 2 either added or subtracted must be a multiple of the number of poles of the generator.This restriction eliminates many even numbers which are unsuitable for this winding.
  8. The number of armature parallel paths = 2m where 'm' is the multiplicity of the winding.


Anonymous said...

thanks man, keep it up

Anonymous said...