Construction and Working Principle of DC Motor:
Construction and Working Principle of DC Motor is a heteropolar structure with stationary poles and the rotating armature (Fig. 5.8).
DC Machine Basic Structure:
An alternating emf of the same
Construction and Working Principle of DC Motor:
Construction and Working Principle of DC Motor is a heteropolar structure with stationary poles and the rotating armature (Fig. 5.8).
DC Machine Basic Structure:
An alternating emf of the same wave shape as that of B-wave is induced in every coil. As the armature rotates, the emf s induced in the belt of coil-sides under a given pole is unidirectional and the pattern alternates from pole to pole as shown in Fig. 6.11 for a 4-pole DC Machine. For obtaining a constant voltage from armature coils, rectification action is required. This can be achieved by the mechanical rectifier—commutator and brushes.
Construction of DC Machine:
The commutator is a cylindrical assembly of wedge-shaped copper segments (Fig. 6.12(a)) insulated from one another and the shaft by thin mica or micanite sheets. In high-speed Construction and Working Principle of DC Motor, the segments are so shaped that they can be clamped by two cast-iron V-shaped rings as shown in Fig. 6.12(b). Each commutator segment forms the junction between two armature coils (“finish” of one coil and “start” of the other). In large machines flat copper strips known as risers are used forming clip connections to armature bar conductors (Fig. 6.12(b)).
A double-layer winding is universally adopted in Construction and Working of DC Machine. The coils are continuously connected “finish” to “start” to form a closed (re-entrant) winding. Depending upon the type of connection (lap or wave), pairs (one or more) of parallel paths exist. Points of maximum voltage (corresponding to a path) are tapped by means of stationary brushes placed on the commutator. As the armature rotates, the number of series coils tapped remains fixed and their relative disposition with respect to poles does not change. Therefore, constant (dc) voltage appears across the brush pair.
Working Principle of DC Machine:
Coil-sides are numbered continuously—top, bottom, top,... The first top coil-side is numbered 1 so that all top ones are odd and all bottom ones even. Figure 6.13 shows the coil-side numbering scheme for U=4 coil-sides/slot.
In Fig. 6.13 the coil-span Ycs is assumed 4 slots (or the coil spans 4 teeth). Therefore, coil-side 1 will form a coil with coil-side 18 and 3 with 20, and so on. This is depicted in Fig. 6.14. The coil-span in terms of coil-sides is Ycs = 18 – 1 = 17 or 20 – 3 = 17. This indeed is the distance measured in coil-sides between two coil-sides connected at the back end of armature (end away from commutator) to form a coil. It is known as back-pitch denoted as yb. Obviously coil-span
which is odd in this case and must always be so. It indeed equals
Commutation in DC Machine:
The junction of two coils (“finish” – “start”) is connected to one commutator segment.
Number of commutator segments = C (number of armature coils) (6.6)
The number of commutator segments spanned by the two ends of a coil is called commutator-pitch, yc. In Fig. 6.15 (a), yc = 2 – 1 = +1.
The coil-span in terms of slots is always nearly full-pitch. This ensures that coil-side voltages around the coil are additive most of the time (except when coil-sides lie near the magnetic neutral region). Thus
which means that for nonintegral S/P, the coils are short-pitched.
In a lap winding (as in case of ac) the “finish” of one coil (coming from the bottom coil-side) is connected to (lapped on) the “start” of the adjoining coil as illustrated for single-turn coils in Fig. 6.15(a). The coil-side displacement of the front end connection is called the front-pitch, yf. In a lap winding the resultant-pitch
The direction in which the winding progresses depends upon which is more, yb or yf. Thus
yb> yf (Progressive winding, Fig. 6.15(a)) (6.9a)
yb < yf (Retrogressive winding, Fig. 6.15(b)) (6.9b)
There is not much to choose between progressive or retrogressive winding; either could be adopted.
As shown in Figs. 6.15 (a) and (b), the two ends of a coil are connected across adjacent commutator segments. Thus commutator-pitch,
yc = ± 1 (±1 for progressive, –1 for retrogressive) (6.10)
Coils in lap winding are continuously connected as per the above rule and in the end it closes onto itself (as it must). In the process all coils have been connected.
To learn certain further aspects of lap winding—location of brushes, etc., an example is worked out.
EMF Equation of DC Generator:
The three coils (C/P = 12/4) shown in a thick line are located under one pole-pair (N1S1) and are series connected so that their emf’s add up. This constitutes one parallel path. The complete winding can be divided into four such parallel paths lying under four different pole-pairs (N1S1, S1N2, N2S2, S2N1). It is, therefore, concluded that the number of parallel paths (A) in a lap-wound Construction and Working Principle of DC Motor in general equal the number of poles (P), i.e.
A = P (6.11)
The four parallel paths in the winding of Fig. 6.16 electrically form a close ring as shown in Fig. 6.17 in which the parallel path emf’s around the loop alternate.
DC Machine Equivalent Circuit:
The ends of parallel path