Types of D.C. Armature Windings
The different armature coils in a d.c. armature Winding must be connected in series with each other by means of end connections (back connection and front connection) in a manner so that the generated voltages of the respective coils will aid each other in the production of the terminal e.m.f. of the winding. Two basic methods of making these end connections are:
- Simplex lap winding
- Simplex wave winding
For a simplex lap winding, the commutator pitch YC = 1 and coil span YS ~ 2 pole pitch. Thus the ends of any coil are brought out to adjacent commutator segments and the result of this method of connection is that all the coils of the armature .ire in sequence with the last coil connected to the first coil. Consequently, closed circuit winding results. This is illustrated in Fig. (1.21) where a part of the lap winding is shown. Only two coils are shown for simplicity. The name lap comes from the way in which successive coils overlap the preceding one.
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. 1.14]. 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.
Armature Winding Terminology
(i) Back Pitch (YB)
It is the distance measured in terms of armature conductors between the two sides of a coil at the back of the armature. It is denoted by YB For example, if a coil is formed by connecting conductor 1 (upper conductor in a slot) to conductor 12 (bottom conductor in another slot) at the back of the armature, then back pitch is YB = 12 – 1 = 11 conductors
(ii) Front Pitch (YF)
It is the distance measured in terms of armature conductors between the coil sides attached to any one commutator segment .It is denoted by YF For example, if coil side 12 and coil side 3 are connected to the same commutator segment, then front pitch is YF = 12 – 3 = 9 conductors.
(iii) Resultant Pitch (YR)
It is the distance (measured in terms of armature conductors) between the beginning of one coil and the beginning of the next coil to which it is connected. It is denoted by YR. Therefore, the resultant pitch is the algebraic sum of the back and front pitches.
(iv) Commutator Pitch (YC)
It is the number of commutator segments spanned by each coil of the armature winding.
For simplex lap winding, YC = 1
(v) Progressive Winding
A progressive winding is one in which, as one traces through the winding, the connections to the commutator will progress around the machine in the same direction as is being traced along the path of each individual coil. (i) shows progressive lap winding. Note that YB > YF and YC = + 1.
(vi) Retrogressive Winding
A retrogressive winding is one in which, as one traces through the winding, the connections to the commutator will progress around the machine in the opposite direction to that which is being traced along the path of each individual coil. Fig. (1.15) (ii) shows retrogressive lap winding. Note that YF > YB and YC = – 1. A retrogressive winding is seldom used because it requires more copper.
It is the distance measured in terms of number of armature slots (or armature conductors) per pole. Thus if a 4-pole generator has 16 coils, then number of slots = 16.
\ Pole pitch =16/4= 4 slots
General Rules for D.C. Armature Windings
In the design of d.c. armature winding (lap or wave), the following rules may be followed:
- The back pitch (YB) as well as front pitch (YF) should be nearly equal to pole pitch. This will result in increased e.m.f. in the coils.
- Both pitches (YB and YF) should be odd. This will permit all end connections (back as well as front connection) between a conductor at the top of a slot and one at the bottom of a slot.
- The number of commutator segments is equal to the number of slots or coils (or half the number of conductors).
No. of commutator segments = No. of slots = No. of coils
It is because each coil has two ends and two coil connections are joined at each commutator segment
- The winding must close upon itself i.e. it should be a closed circuit winding.
Relations between Pitches for Simplex Lap Winding
In a simplex lap winding, the various pitches should have the following relation:
- The back and front pitches are odd and are of opposite signs. They differ numerically by 2,
YB =YF + 2 for progressive winding
YB =YF – 2 for retrogressive winding
- Both YB and YF should be nearly equal to pole pitch.
- Average pitch =(YB + YF)/2. It equals pole pitch (= Z/P).
- Commutator pitch, YC = ± 1
YC = + 1 for progressive winding
YC = – 1 for retrogressive winding
- The resultant pitch (YB) is even, being the arithmetical difference of two odd numbers viz., YB and YF.
- If Z = number of armature conductors and P = number of poles, then,
Polr – pitch = Z/P
Since YB and YF both must be. about one pole pitch and differ numerically by 2,
For progressive winding
For retrogressive winding
It is clear that Z/P must be an even number to make the winding possible.
Developed diagram is obtained by imagining the cylindrical surface of the armature to be cut by an axial plane and then flattened out. Fig. (1.16) (i) shows the developed diagram of the winding. Note that full lines represent the top coil sides (or conductors) and dotted lines represent the bottom coil sides (or conductors).
The winding goes from commutator segment 1 by conductor 1 across the back to conductor 12 and at the front to commutator segment 2, thus forming a coil. Then from commutator segment 2, through conductors 3 and 14 back to commutator segment 3 and so on till the winding returns to commutator segment 1 after using all the 40 conductors.
Position and number of brushes
We now turn to find the position and the number of brushes required. The brushes, like field poles, remain fixed in space as the commutator and winding revolve. It is very important that brushes are in correct position relative to the field poles. The arrowhead marked “rotation” (i) shows the direction of motion of the conductors. By right-hand rule, the direction of e.m.f. in each conductor will be as shown.
In order to find the position of brushes, the ring diagram shown in Fig. (1.16) (ii) is quite helpful. A positive brush will be placed on that commutator segment where the currents in the coils are meeting to flow out of the segment. A negative brush will be placed on that commutator segment where the currents in the coils are meeting to flow in. Referring to Fig. (1.16) (i), there are four brushes two positive and two negative. Therefore, we arrive at a very important conclusion that in a simplex lap winding, the number of brushes is equal to the number of poles. If the brushes of the same polarity are connected together, then all the armature conductors are connected in four parallel paths; each path containing an equal number of conductors in series. This is illustrated in Fig. (1.17).
Since segments 6 and 16 are connected together through positive brushes and segments 11 and 1 are connected together through negative brushes, there are four parallel paths, each containing 10 conductors in series. Therefore, in a simplex lap winding, the number of parallel paths is equal to the number of poles
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