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Note for Basic Electrical And Electronics Engineering - BEEE by Namachivayam S

  • Basic Electrical And Electronics Engineering - BEEE
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Namachivayam S
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Construction of DC Generator 1)Yoke 2)Poles 3)field winding 4)Armature 5)Brushes 6) Bearing Yoke of DC Generator Yoke of the DC generator serves two purposes, (i) It holds the magnetic pole cores of the generator and acts as cover of the generator. (ii) It carries the magnetic field flux. In small generator, yoke are made of cast iron. Cast iron is cheaper in cost but heavier than steel. But for large construction of DC generator, where weight of the machine is concerned, lighter cast steel or rolled steel is preferable for constructing yoke of DC generator. Normally larger yokes are formed by rounding a rectangular steel slab and the edges are welded together at the bottom.

Text from page-2

Pole Cores and Pole Shoes of DC Generator There are mainly two types of construction available. One: Solid pole care, where it made of a solid single piece of cast iron or cast steel. Two: Laminated pole core, where it made of numbers of thin, limitations of annealed steel which are riveted together. The pole shoes spread out the magnetic flux in the air gap and reduce the reluctance of the magnetic path. Due to their larger cross – section they hold the pole coil at its position. Pole Coils: The field coils or pole coils are wound around the pole core. These are a simple coil of insulated copper wire which placed on the pole which placed between yoke and pole shoes. Armature Core of DC Generator The purpose of armature core is to hold the armature winding and provide low reluctance path for the flux through the armature from N pole to S pole. Although a DC generator provides direct current but induced current in the armature is alternating in nature. That is why, cylindrical or drum shaped armature core is build up of circular laminated sheet. Air ducts are also punched off on each lamination for circulation of air through the core for providing better cooling. Armature Winding of DC Generator These are first wound in the form of flat rectangular coils and are then pulled into their proper shape in a coil puller. Various conductors of the coils are insulated from each other. The conductors are placed in the armature slots, which are lined with tough insulating material. Lap winding Lap winding is the winding in which successive coils overlap each other. It is named “Lap” winding because it doubles or laps back with its succeeding coils.

Text from page-3

In this winding the finishing end of one coil is connected to one commutator segment and the starting end of the next coil situated under the same pole and connected with same commutator segment. Wave winding In this winding the end of one coil is connected to the starting of another coil of the same Polarity as that of the first coil.. In this type of winding the coil side(A-B) progress forward around the armature to another coil side and goes on successively passing through N and S pole till it returns to a conductor (A1-B1) lying under the starting pole. Commutator of DC Generator It collects current from armature and sends it to the load as direct current. It actually takes alternating current from armature and converts it to direct current and then send it to external load. It is cylindrical structured and is build up of wedge – shaped segments of high conductivity,

Text from page-4

hard drawn copper. Each segment is insulated from the shaft by means of insulated commutator segment.Each commutator segment is connected with corresponding armature conductor. Brushes of DC Generator The brushes are made of carbon. These are rectangular block shaped. The only function of these carbon brushes of DC generator is to collect current from commutator segments. Bearing of DC Generator For small machine, ball bearing is used and for heavy duty dc generator, roller bearing is used. The bearing must always be lubricated properly for smooth operation and long life of generator. Working Principle In the above figure a single loop of conductor of rectangular shape is placed between opposite poles of the magnet. Let’s us consider, the rectangular loop of conductor is ABCD which rotates inside the magnetic field about its own axis. When the loop rotates from its position, it cuts the flux lines of the field. As during this movement two sides, i.e. AB and CD of the loop cut the flux lines there will be an induced emf according to the Faradays first law.

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