Types of Armature Winding | Drum | Lap Winding

Armature winding are always of the non-salient pole type and usually are symmetrically distributed in slots around the complete circumference of the armature. The armature winding, according to the degree of closure produced by the winding to the degree of closure produced by the winding, are of two types of armature winding namely:

(a) Open coil winding (b) Closed coil winding.

Open coil winding is that winding which does not close on itself i.e. a close circuit will not be formed until some external connection is made to a source or load. The open coil winding is never employed in dc machines but is usually employed in ac machine.

Closed coil winding is that winding is that winding which closes on itself. In such a winding if one starts tracing through it, one will come back to the starting point without passing through any external connection. DC machines employ only closed coil winding in order to provide for the commutation of the coils. Though closed type winding are used.

The continuous or closed armature winding are of two types:

(a) Gramme-ring Winding (b) Drum Winding.

The gramme-ring type of armature winding is an early form of winding which has been replaced by the more efficient winding known as drum type winding. Hence our discussing will be restricted to drum winding.

Drum Winding

In this type of winding the conductors are housed in slots over the armature surface and connected to one another by front and back connectors or coil ends. The drum type winding has the following two principal advantages.

  1. All armature copper, except end connections, is active i.e. it cuts flux and, therefore, is active in generating emf.
  2. The coils can be pre-formed and insulated before placing on the armature, hence cost is reduced.

The drum winding may be either single layer or double layer winding. Single layer winding in which one conductor or one coil said is placed in each armature slot, is rarely Used. The layer winding, in which there are two conductors or coil sides per slot arranged in two layers, is usually used for economy reasons. The two layer winding permits the use of coils pre-formed and insulated before being placed on the armature. The coil usually are wound on machine driven forms which give them the proper shape and the turns are then bound together usually with cotton tape. The ends are left bare so that they may be soldered later to the commutator bars. The coils are the dipped into some insulating compounds such as asphaltum and are then dried. For very high temperature operation, other materials, such as mica and paper tape, fiber glass tape, and silicon impregnated insulation are used.

drum winding

Figure 1: Drum Winding

Before placing the coils in the slots, the slots are given U-shaped slot liners of leatheroid or presspahn to ensure mechanical protection of the coils. After the coils have been dropped into the slots, wedges of wood or hard fiber are driven to hold them in place.

In order that the induced in the coil a maximum, other conditions being fixed, the span of the coil should be equal to pole-pitch. However, the span may be reduced to as much as eight-tenths the pole pitch without any serious reduction in the induced emf. When this is done, the winding is called a fractional pitch winding. The advantages of fractional pitch winding are that substantial saving is affected in the copper of end connections and commutation is improved, owing to lesser mutual inductance between the coils.

Usually, one side of every coil lies in the top of one slot and the other side lies in the bottom of some other slot at a distance of approximately one pole pitch along the armature. Thus at least two coil sides occupy each slot are most commonly used for all medium sized machines. In dynamos of larger ratings it is often necessary to place several coil sides or elements in a single slot usually four, six or eight. More than eight  elements in a single slot is rarely used. The coil sides lying at the upper half of the slots are numbered odd 1, 3, 5, 7 etc. while those at the lower half are numbered even. It is to be noted that the coil side 2 lies below coil side 1, 4 below 3 etc. progresses clockwise. Placing of several elements in a single slot givens fewer slots than segments which have got-following advantage.

  1. As the number of slots is reduced the armature core teeth become mechanically stronger, so that, from the standpoint to handing in manufacture, there is less damage to laminations and coils.
  2. As the number of commutator segments in increased, the voltage between those that are adjacent to each other decreases and the number of turns of wire in the coil or coils connected to adjacent segments also decreases. The result is that there is less sparking at decreases. The result is that there is less sparking at the commutator because of the improved commutation.
  3. Assuming that a comparatively large number of segments has been selected so that good commutation will result, the choice of an armature core with one-half, one-third, one-fourth etc. as many slots means that fewer coils will have to be constructed, this reduces the cost of manufacture.

Two layer winding permits the end connections to be easily made as the coil ends can be bent around one another in a systematic manner, passing from the bottom to the top layer by means of the peculiar twist in the ends of the coils.

numbering scheme for two layer winding

Figure 2: Numbering Scheme for Two Layer Winding

In general there are two types of drum armature winding: the lap and wave winding. These are distinguished from each other in several ways, but from the stand point of construction they differ only by the manner in which the coil ends are connected to the adjacent commutator segments while the ends of the wave winding coils are connected to the commutator segments at points separated by approximately twice the distance between adjacent poles.

There is also a third type of drum winding called the frog-leg winding. This winding consist of a lap winding and wave winding placed on the same armature. The wave winding is connected to the commutator segments at equi-potential points so that this winding part can be employed as an equalizer for the lap winding.

Lap Winding

Single turn lap winding is shown in Figs. 1. (c) and (d) of Armature winding. In lap winding finish end of one coil is connected to a commutator segment and to the start end of the adjacent coil situated under the same pole and similarly all coils are connected. This winding is known as lap winding because the sides of successive coils overlap each other. Lap winding may be further classified as being a simplex (single) or multiplex (double or triple) winding.

In the simplex lap winding there are as many parallel paths or circuits through the winding as there are field poles on the machine.

Double and triple winding are used on armature designed for supply of large currents at low voltage. The sole purpose of such a winding is no increase the number of parallel pants enabling the armature to carry a large total current, at the same time reducing the conductor current to improve commutation conditions. A double or duplex winding consists of two similar simplex winding placed in alternate slots on the armature and connected to alternate commutator segments. Each winding carries half the armature current. Likewise, a triple or triplex winding has three similar winding occupying every third slot and connected to every third commutator segment. Hence in duplex lap winding the number of parallel circuits is twice the number of poles. For this reason the lap winding is sometimes called the multiple or parallel winding and is suited for machines that operate at relatively low voltages but with high current outputs.

developed view of 4-pole, single layer, progressive simplex lap winding with 24 coil sides

Figure 3: developed view of 4-pole, single layer, progressive simplex lap winding with 24 coil sides

Important Point Regarding Lap Winding.

1. The coil or back pitch Y_b= must be approximately equal to the pole pitch i.e. Y_b\dfrac{Z}{p}  where Z is the number of conductors on armature and P is the number of poles.

2. The back pitch Y_b should be either lesser or greater than front pitch Y_f  by 2m where m is the multiplicity of the winding.

i.e.  Y_b=Y_f\pm 2m

where m = 1 for simplex winding

m = 2 for duplex winding

m = 3 for triplex winding and so on.

When  Y_b  greater than  Y_f  the winding progresses from left to right and so known as progresses from right to left and, therefore, such a winding is known as retrogressive winding.

3. The back pitch and front pitch must be odd.

4. The average pitch is given by

Y_{av} = \dfrac{Y_b + Y_f}{2}

and should be equal to pole pitch i.e \dfrac{Z}{p}

5. The resultant pitch Y_R  is always even, being the difference of two odd numbers and is equal to 2m where m is the multiplicity of the winding.

i.e. Resultant pitch, Y_R = 2 for simplex lap winding.

Y_R = 4 for duplex lap winding.

and  Y_R= 6   for triplex lap winding.

6. The commutator pitch,  Y_c = m i.e., Y_c is equal to 1, 2, 3, 4 etc., respectively for simplex, duplex, triplex, quadruplex etc. lap winding.

Example 1. Draw the developed winding diagram of progressive lap winding for 4 poles, 24 slots with one coil side per slot, single layer showing therein position of the poles, direction of motion, direction of induced emfs and position of brushes.

Solution: Developed winding diagram is obtained by imagining the armature surface removed and so laid out flat that the slots and conductors can be viewed without the necessity for turning round the armature in order to trace out the armature winding. Such a developed winding diagram is shown Fig. 1.22.

Number of coil sides, Z = 24

Average pitch Y_{av} = \dfrac{Y_b + Y_f}{2} = \dfrac{Z}{P} = \dfrac{24}{4}=6

fig

or   Y_b + Y_f = 12

and for progressive simplex lap winding

 Y_b = Y_f + 2

Solving Eqs. (i) and (ii) we get  Y_b = 7  and   Y_f = 5