Construction of DC Machine PDF

Construction of DC Machine The schematic of a DC Machine: The main parts of a DC Machine are indicated in the diagram in Fig.1. From the diagram we can summarize the parts as follows 1) Frame or Yoke 2) Stator and Pole 3) Field windings 4) Armature 5) Armature Windings 6) Commutator 7) Brushes 8) B...

Construction of DC Machine The schematic of a DC Machine: The main parts of a DC Machine are indicated in the diagram in Fig.1. From the diagram we can summarize the parts as follows 1) Frame or Yoke 2) Stator and Pole 3) Field windings 4) Armature 5) Armature Windings 6) Commutator 7) Brushes 8) Bearings Figure-1 EN7032 (ELECTRICAL MACHINES) 2 Frame or Yoke : Frame of DC Machine serves two purposes, (i) It holds the magnetic pole cores of the machine and acts as cover. (ii) It carries the magnetic field flux. In small machine, the frame is made of cast iron. Cast iron is cheaper but heavier than steel. But for large machines, where weight is a concern, lighter cast steel or rolled steel is preferable. Ref. Fig.2 Figure-2 EN7032 (ELECTRICAL MACHINES) 3 Stator or Pole Core and Pole Shoes: The stator is bolted on the inside of the frame.It consists of Pole core and Pole shoe. Pole core: They are mainly two types of construction. 1. Solid pole core, where it is made of a solid single piece of cast iron or cast steel. 2. Laminated pole core, where it is made up of numbers of thin, laminations of annealed steel which are riveted together. The core carries the wire windings which produce the required magnetic flux. Ref. Fig-3 Laminated cores are used to reduce the eddy current loss (one of the magnetic losses in the core) in them. Figure-3 Pole Shoe: They are attached to pole core and are responsible to make the flux distribution uniform in the air gap (the gap between stator and armature). Field Windings or Pole Coils The pole coils are wound around the pole core. These are a simple coil of insulated copper wire or strip, which wound around the pole and placed between frame and pole shoes. They carry field current and as a result excite the pole cores to produce flux. Ref. Fig.4 Figure-4 EN7032 (ELECTRICAL MACHINES) 4 Armature: The purpose of armature core is to hold the armature windings and provide a low reluctance path for the flux through the armature from North Pole to South Pole. Although a DC machine operates on direct current , the induced current in the armature is alternating in nature with a high ripple. That is why, cylindrical or drum shaped armature cores are manufactured of circular laminated sheets. Ref. Fig.5 Figure-5 In every circular lamination, slots are either cut or punched on the outer periphery and the key way is located on the inner Figure-5 periphery as shown in Fig.5. Air ducts are also punched of cut on each lamination for circulation of air through the core for providing better cooling. Magnetic reluctance, or magnetic resistance, is analogous to resistance in an electrical circuit, but rather than dissipating electric energy it stores magnetic energy. Like the way an electric field causes an electric current to follow the path of least resistance, a magnetic field causes magnetic flux to follow the path of least magnetic reluctance. It is a scalar quantity, akin to electrical resistance. The unit for magnetic reluctance is inverse henry, H−1 EN7032 (ELECTRICAL MACHINES) 5 Armature Winding: Armature windings are generally formed wound. 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. This slot insulation is folded over above the armature conductors placed in it and secured in place by special hard wooden or fiber wedges. Ref. Fig.6 Figure-6 EN7032 (ELECTRICAL MACHINES) 6 Commutator : The commutator plays a vital role in dc generator. It collects current from armature and sends it to the load as direct current or vice versa in a dc motor. It actually takes alternating current from armature and converts it to direct current and then sends it to external load. It is cylindrically structured and is buildup of wedge - shaped segments of high conductivity, hard drawn or drop forged copper. Each segment is insulated from the shaft by means of Figure-7 insulated commutator segment shown. Each commutator segment is connected with corresponding armature conductor through segment riser or lug. Ref. Fig.7 EN7032 (ELECTRICAL MACHINES) 7 Brushes and Bearings: Brushes: The brushes are made of carbon. These are rectangular block shaped. Ref. Fig.1 The only function of these carbon brushes of DC Machine is to collect/give current from/to commutator segments. The brushes are housed in the rectangular box shaped brush holder. The brush face is placed on the commutator segment with attached to the brush holder or box. Ref. Fig.8 Figure-8 Bearing: For small machine, ball bearing is used and for heavy duty dc machine, roller bearing is used. The bearing must always be lubricated properly for smooth operation and long life of machine. Ref. Fig.9 Figure-9 EN7032 (ELECTRICAL MACHINES) 8 Cut-away view of a DC Machine: Figure-10 EN7032 (ELECTRICAL MACHINES) 9