Parts of DC Machine 

The simple loop generator has been considered in detail merely to bring out the basic principle underlying construction and working of an actual generator illustrated in (Fig.a) which consists of the following essential parts:

Names of Parts of DC Machine 

1. Yoke or Magnetic Frame
2. Pole-Cares and Pole-Shoes
3. Armature Core
4. Pole Coils or Field Coils
5. Commutator
6. Armature Windings or Conductors
7. Brushes and Bearings 

Of these, the yoke, the pole cores, the armature core and air gaps between the poles and the armature core or the magnetic circuit whereas the rest form the electrical circuit.

1-Yoke

The outer frame or yoke serves double purpose :

It provides mechanical support for the poles acts as a protecting cover for the whole machine and (f) It carries the magnetic flux produced by the poles. In small generators where cheapness rather than weight is the main consideration, yokes are made of cast iron. But for large machines usually cast steel or rolled steel is employed. The modern process of form ing the yoke consists of rolling a steel slab round a cylindrical mandrel and then welding it at the bottom. The feet and the terminal box etc. are welded to the frame afterwards. Such yokes possess sufficient mechanical strength and have high permeability.

2-Pole Cores and Pole Shoes

The field magnets consist of pole cores and pole shoes. The pole shoes serve two purposes.

(i) They spread out the flux in the air gap and also, being of larger cross-section, reduce the reluctance of the magnetic path.

(ii) They support the exciting coils (or field coils) as shown in (Fig.f).

There are two main types of pole construction. 

(a) The pole core itself may be a solid piece made out of either cast iron or cast steel but the pole shoe is laminated and is fastened to the pole face by means of counter sunk screws as shown in (Fig.b).

(b) In modem design, the complete pole cores and pole shoes are built of thin laminations of annealed steel which are rivetted together under hydraulic pressure (Fig.c). The thickness of laminations varies from 1 mm to 0.25 mm. The laminated poles may be secured to the

yoke in any of the following two ways:

(1) Either the pole is secured to the yoke by means of screws bolted through the yoke and into the pole body or

(2) The holding screws are bolted into a steel bar which passes through the pole across the plane of laminations (Fig.d).

3-Pole Coils

The field coils or pole coils, which consist of copper wire or strip, are former-wound for the correct dimension (Fig.e). Then, the former is removed and wound coil is put into place over the core as shown in (Fig.f).

When current is passed through these coils, they electromagnetise the poles which produce the necessary flux that is cut by revolving armature conductors.

4-Armature Core

It houses the armature conductors or coils and causes them to rotate and hence cut the magnetic flux of the field magnets. In addition to this, its most important function is to provide a path of very low reluctance to the flux through the armature from a N-pole to a S-pole. It is cylindrical or drum-shaped and is built up of usually circular sheet steel discs or laminations approximately 0.5 mm thick (Fig.g). It is keyed to the 

shaft. The slots are either die-cut or punched on the outer periphery of the disc and the keyway is located on the inner diameter as shown. In small machines, the armature stampings are keyed directly to the shaft. Usually, these laminations are perforated for air ducts which permits axial flow of air through the armature for cooling purposes. Such ventilating channels are clearly visible in the laminations shown in (Fig.h) and (Fig.i).

Up to armature diameters of about one metre, the circular stampings are cut out in one piece as shown in (Fig.h). But above this size, these circles, especially of such thin sections, are difficult to handle because they tend to distort and become wavy when assembled together. Hence, the circu lar laminations, instead of being cut out in one piece, are cut in a number of suitable sections or segments which form part of a complete ring (Fig.i).

A complete circular lamination is made up of four or six or even eight segmental laminations. Usually, two keyways are notched in each segment and are dove-tailed or wedge-shaped to make the laminations self-locking in position.

The purpose of using laminations is to reduce the loss due to eddy currents. Thinner the laminations, greater is the resistance offered to the induced e.m.f., smaller the current and hence lesser the I²R loss in the core.

5-Armature Windings

The armature windings are mually former wound. These are first wound in the forms 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 the slot and is secured in place by special hard wooden or fibre wedges.

6-Commutator

The function of the commuator is to facilitate collection of current from the armature conduc tors. As shown in Art. 26.2, is rectified Le converts the alternating current induced in the armature conductors 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. These segments are insulated from each other by thin layers of mica. The number of segments is equal to the number of armature coils. Each commutator segment is connected to the armature conductor by means of a copper lug or strip for risers To prevent them from flying out under the action of centrifugal forces, the segments have V-grooves, these grooves being insulated by comical micanite rings. A sectional view of commutator is shown in (Fig.j) whose general

appearance when completed is shown in (Fig.k).

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 a rectangular block. These brushes are housed in brush-holders usually of the box-type variety. As shown in (Fig.L), the brush-bolder 

Because of their reliability, ball-bearings are frequently employed, though for heavy duties, roller bearings are preferable. The hall and rollers are generally packed in hard oil for quieter operation and for reduced bearing wear, sleeve bearings are used which are lubricated by ring oilers fed from oil reservoir in the bearing bracket.