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Thursday, 12 December 2019

DC Armature Winding

The complete process of armature winding involves number of operations like insulating the core, inserting the coils into the slots connecting the leads to the commutator, baking and varnishing etc. Without the perfect knowledge of the winding diagram, it would be impossible to wind the armature to that perfection to match the designed requirements. The slots of the armature are either open or semi closed type.

DC Windings are generally classified as under,

Definition of Terms involved in DC Armature Winding:

Certain terms used in connection with the armature winding is defined below:

Pole Pitch:

It is defined as the peripheral slot distance between two adjacent poles. If ‘n’ is the number of slot in the armature and ‘P’ is the number of field poles, then

Pole pitch = No: of slots in armature/ No: of poles in field = n/p
For example if the number of slots in the armature is 24  and the number of poles in the field is 4, then the pitch is 24/4 = 6. If the windings are made for lesser than the pole pitch, the winding is said to be chorded or short pitched. The electrical angle between pole to pole is 180o electrical because it generates half a cycle of emf and the electrical angle between slot to slot is called pitch angle. It is given by

Slot pitch angle = 180oe/No: of slots between poles = 180oe/Pole pitch

Coil Span or Coil Pitch (Ys) :

It Is defined as the distance in terms of armature slots between two sides of a coil. In other words, it is the periphery of the armature spanned by the two sides of the coil. if the coil span is equal to the pole pitch, it is said to be full pitched coil and if the coil span is lesser than the pole pitch or fractional pitched coil, it is then said to be short pitched coil.

Back Pitch and Front Pitch :

Back pitch is defined as the distance in conductors by which a coil advances and is denoted by the letter Yb. Front pitch is defined as the distance in conductors from the end of the first coil to the beginning of the second coil and is denoted by Yf. The resultant pitch is defined as the distance in conductors from the beginning of the first coil to the beginning of the second coil.

Overhang :

The conductor is inserted into the slot and taken back from another slot to form a coil. The ends of the coil are connected to commutator segments. The inductor portion of the coil is the conductor inside the slot responsible for production of emf and the rest of the coil is the overhand as shown in Figure.

Commutator Pitch:

Commutator pitch is defined as the segmental difference between winding connections of any two consecutive oils. It is the distance at commutator between the beginning of first coil and the beginning of second coil.

Single Layer and Double Layer Winding :

In a single layer winding, only one conductor or one coil side is placed in each slot as shown in Fig (a) whereas in the double layer winding, there are two conductors or coil sides arranged in each slot in two layers as shown in Fig (b). In the double layer winding, one side of the coil lies in the upper half of one slot and the other side of the coil lies in the lower half of some other slot at a distance of approximately one pitch away. Double layer windings are most commonly used for all medium sized machines.

Lap and Wave Winding:

There are mainly two types of windings and are known as Lap winding and have winding. The difference between the two is simply due to the dissimilar arrangements of the end connection at the front of commutator end of armature. Each winding can be arranged progressively or retrogressively. However, for both the windings the rules of winding are common. In Fig. shows lap and wave winding for single coil. Note the position of connections at commutator and the distance of the resultant pitch. 

The common rules that apply for both the type of windings are

a. For the full pitched winding, the front pitch and the hack pitch shall be approximately equal to pole pitch in which case increased e.m.f. would result. Fractional pitched windings are also used in special cases.

b. Both pitches should be odd, otherwise placing of coils in the armature would be difficult. if front pitch and hack pitch are even, then both the coil sides would occupy either in the upper half or lower half in which case it would be difficult to wind.

c. The number of commutator segments should be equal to the number of slots or coils as the front end of the conductors are joined in pairs.

d. Upon completing winding. the windings should close by itself after traversing the slots.

e. The number of commutator segments shall be equal to the number of slots.

If two similar windings on the same armature connecting the even numbered commutator bars to one winding and odd numbered commutator bar to second winding, it is termed as duplex winding, and similarly if three windings each connected to one third of the commutator bars are connected, then it is termed as triplex winding.

Armature Winding Diagrams:

In any dc armature winding, a coil is a loop of number of wires in series consisting of two inductor portions and two end turns or overlaps. The inductor portion of the coil is inserted into the slot of the armature. Pitch of the coil means span of the coil i.e the slot distance between two inductor portion of the coil. Back pitch is the distance between the two conductors which forms the loop and front pitch is the distance between the conductor of one loop and the first conductor of the next loop. Both the lap and wave winding pitch distance is explained above.

One side of the armature conductor is fixed to the commutator segments. The end coils have to be connected to these segments. The segmental distance between the two coil ends is called commutator pitch and is denoted b Yc which is given by,

Yc = {Total no. of segment + 1} / P2

In the case of single layer winding the slot contains only one layer of coil whereas in the case of double layer winding the slot contains two layers. The first layer is at the bottom of the slot which is well insulated from the core as well as from the second layer. The second layer is wound on top of the slot over the insulated first layer.

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