Types of armature windings

Single-layer Winding

It is that winding in which one conductor or one coil side is placed in each armature slot as shown in Fig. 26.27. Such a winding is not much used.

Two-layer Winding

In this type of winding, there are two conductors or coil sides per slot arranged in two layers. Usually, one side of every coil lies in the upper half of one slot and other side lies in the lower half of some other slot at a distance of approximately one pitch away (Fig. 26.28). The transfer of the coil from one slot to another is usually made in a radial plane by means of a peculiar bend or twist at the back end as shown in Fig. 26.29. Such windings in which two coil sides occupy each slot are most commonly used for all medium-sized machines. Sometimes 4 or 6 or 8 coil sides are used in each slot in several layers because it is not practicable to have too many slots (Fig. 26.30). The coil sides lying at the upper half of the slots are numbered odd i.e. 1,3.5,7 etc. while those at the lower half are numbered even i.e. 2. 4, 6, 8 etc.

Degree of Re-entrant of an Armature Winding

A winding is said to be single re-entrant if on tracing through it once, all armature conductors are included on returning to the starting point. It is double re-entrant if only half the conductors are included in tracing through the winding once and so on.

Multiplex Winding

In such windings, there are several sets of completely closed and independent windings. If there is only one set of closed winding, it is called simplex wave winding. If there are two such windings on the same armature, it is called duplex winding and so on. The multiplicity affects a number of parallel paths in the armature. For a given number of armature slots and coils, as the multiplicity increases, the number of parallel paths in the armature increases thereby increasing the current rating but decreasing the voltage rating.

Lap and Wave Windings

Two types of windings mostly employed for drum-type armatures are known as Lap Winding and Wave Winding. The difference between 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 pitch 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 (Art- 26.15).

ii)Both pitches should be odd. otherwise it would be difficult to place the coils (which are former-wound) properly on the armature. For example. if Y_B and Y_F were both even, the all the coil sides and conductors would lie either in the upper half of the slots or in the lower half. Hence, it would become impossible for one side of the coil to lie in The upper 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 equal to the number of slots or coils (or half the number of conductors) because the front ends of conductors arc joined to the segments in pairs.

iv)The winding must close upon itself i.e. if we start from a given point and move from one coil to another, then all conductors should be traversed and w e should reach the same point again without a break or discontinuity in between.

Simplex Lap-winding

It is shown in Fig. 26.25 which employs single-turn coils. 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 and so on, till and the coils have been connected. This type of winding derives us name from the fact it doubles or laps back with its succeeding coils.

Following points regarding simplex lap winding should be carefully noted :

1.The back and front pitches are odd and of opposite sign, But they cannot be equal. They differ by 2 or some multiple thereof

2. Both Y_A and Y_B, should be nearly equal to a polepitch

3. The average pitch Y_A = (Y_{A +} Y_B )/2 .it equals pole pitch = Z/P

4. Commutator pitch Y_c = 1. (In general. Y_c=±m)

5. Resultant pitch Y_R is even, being the arithmetical difference of two odd numbers,i.e..Y_R=Y_B-Y_F

6. The number of slots for a 2-layer winding is equal to the number of coils (i.e. half the number of coil sides). 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 Y_B = Y_F ± 2.

a) ( If Y_B> Y_F i.e. Y_B= Y_F+ 2, then we get a progressive or right-handed winding i.e. a winding which progresses in the clockwise direction as seen from the commutator end. In this case, obviously Y_c=1..

(b) If Y_B< Y_F i.e. Y_B = Y_F-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. Y_c=-1.

c) Hence, it is obvious that

for progressive winding

Y_{F =} (Z/P)-1

Y_{B =} Z/P)+1

for retrogressive winding

Y_{B =} (Z/P)-1

Y_{F =} Z/P)+1

Obviously. Z/P must be even to make the winding possible.

Simplex wave winding
For a simplex wave winding, the commutator pitch YC ~ 2 pole pitches and coil span = pole pitch. The result is that the coils under consecutive pole pairs will be joined together in series thereby adding together their e.m.f.s [See Fig. 1.22]. After passing once around the armature, the winding falls in a slot to the left or right of the starting point and thus connecting up another circuit. Continuing in this way, all the conductors will be connected in a single closed winding. This winding is called wave winding from the appearance (wavy) of the end connections.

Written by John on April 29th, 2009 with 6 comments.
Read more articles on Direct Current Machines and Electrical Machines.

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