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The principal advantage of the dc machine, particularly the dc. Such an advantage is not claimed by any ac. However dc generators are not as common as they used to be, because direct current, when required, is mainly obtained from an ac. Never the less, an understanding of dc generator is important because it represents a logical introduction to the behaviour of dc motors.
In this chapter, we shall deal with various aspects of dc. An electric generator is based on the principle that whenever flux is cut by a conductor, emf is induced which will cause a current to flow if the conductor circuit is closed.
Therefore, the essential components of a generator are: The function of the field system is to produce uniform magnetic field within which the armature rotates.
It consists of a number of salient poles of course, even number bolted to the inside of circular frame generally called yoke. The yoke is usually made of solid cast steel whereas the pole pieces are composed of stacked laminations.
Field coils are mounted on the poles and carry the dc exciting current. The field coils are connected in such a way that adjacent poles have opposite polarity. Practical dc machines have air gaps ranging from 0. Since armature and field systems are composed of materials that have high permeability, most of the mmf of field coils is required to set up flux in the air gap.
By reducing the length of air gap, we can reduce the size of field coils i. The armature core is keyed to the machine shaft and rotates between the field poles.
It consists of slotted soft-iron laminations about 0. The laminations are individually coated with a thin insulating film so that they do not come in electrical contact with each other. The purpose of laminating the core is to reduce the eddy current loss.
The different armature coils in a dc 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 emf of the winding.
Two basic methods of making these end connections are: Simplex lap winding 2. Simplex wave winding 1. 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.
Consequentlyclosed circuit winding results. This is illustrated in Fig. Only two coils are shown for simplicity. The name lap comes from the way in which successive coils overlap the preceding one. The result is that the coils under consecutive pole pairs will be joined together in series thereby adding together their.
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. Commutator A commutator is a mechanical rectifier which converts the alternating voltage generated in the armature winding into direct voltage across the brushes.
The commutator is made of copper segments insulated from each other by mica sheets and mounted on the shaft of the machine … The armature conductors are soldered to the commutator segments in a suitable manner to give rise to the armature winding.
Depending upon the manner in which the armature conductors are connected to the commutator segments, there are two types of armature winding in a dc. Great care is taken in building the commutator because any eccentricity will cause the brushes to bounce, producing unacceptable sparking.
The sparks may bum the brushes and overheat and carbonise the commutator. Brushes The purpose of brushes is to ensure electrical connections between the rotating commutator and stationary external load circuit.
The brushes are made of carbon and rest on the commutator. The brush pressure is adjusted by means of adjustable springs. If the brush pressure is very large, the friction produces heating of the commutator and the brushes.
On the other hand, if it is too weak, the imperfect contact with the commutator may produce sparking.Subject: Duration of the Examination: Maximum marks of Subject: Minimum marks of the subject for pass: Theory: 3 hrs: Practical Embroidery: 4 hrs.
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