Q. Write a short note on types of enclosure of machine.
Sol. The various types of d.c. machine :
(1) Magnetic frame or Yoke
(2) Pole cores and pole shoes
(3) Pole coils and field coils
(4) Armature core
(5) Armature winding or conductors
(6) Commutator
(7) Brushes and bearings
(1) Yoke : The quter frame or yoke serves double purpose :
(i) It provides mechanical support for the poles and acts as a protecting cover fot the whole machine and;
(ii) It carries the magnetic flux produced by the poles. In small generators where cheapness rather than weight is the main consideration yoke are made of cast iron. But for large machines usually cast steel or rolled steel are employed.
(2) Pole cross and pole shoes : The field magnets consists of pole cores and pole shoes. The pole shoes serve two purpose :
(i) They spread out the flux in the air gap and also, being of longer cross - section, reduce the reluctance of the magnetic path.
(ii) They support the exciting coils.
(3) Pole coils : The field coils or pole coils, which consists of copper wire or strip are former wound for the correct dimension. Then the former is removed and wound coil is put into place over the core. When current is passed through these coils, they electro - magnetise the pole which produce the necessary flux that is cut by revolving armature conductors.
(4) Armature core : It houses the armature conductor or coils and causes them to rotate and hence cut the magnetic flux of the field magnets. In adition to this its most important function is to provide a path of very low reluctance to the flux through the armature form an N pole to a S pole.
It is cylindrical or drum - shaped and is built up of usually circular sheet steel discs or lamination approx. 0.5mm thick. It is keyed to the shaft. The slots are either die cut or punched on the outer periphery of the disc and the key way is located on the inner diameter. In small machines the armature stamping are keyed directly to the shaft usually these lamination are performed for air ducts which permits axial flow of air through the armature for cooling purposes.
(5) Armature winding : The armature winding usually former 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 coil 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 the slot and is secured in place by special hard wooden or fibre wedges.
(6) Commutator : The function of commutator is to facilitate collection of current from the armature conductors. It rectifies i.e. converts the alternating current induced in the armature condutors into unidirectional current in the external load circuit. It is of cylindrical structure and is built up of wedge - shaped segments of high conductivity hard - drawn or drop - forged copper.
(7) Brushes and bearings : The brushes whose function is to collect current from commutator are usually made of carbon or graphite and are in the shape of rectangular block. These brushes are housed in brush - holders usually of the box type variety. The brush holder is mounted on a spindle and are made to bear down on the commutator by a spring whose tension can be adjusted by changing the position of level in the notches.