Fig, 2 shows the sectional view of four pole D,C. machine,
Fig. 2. Sectional view of a four pole D.C. machine.
- The frame is the stationary part of a machine to which are fixed the main and commutating poles and by means of which the machine is bolted to its bed plate.
- The ring-shaped portion which serves as the path for the main and commutating pole fluxes is called the ‘yoke’,
Cast iron used to be the material for the frame/yoke in early machines but now it has been replaced by cast steel, This is because cast iron is saturated by a flux density of about 0.8 Wb/m2 while saturation with cast steel is at about 1.5 Wb/m2 Thus the cross-section of a cast iron frame is about twice that of a cast steel frame for the same value of magnetic flux. Hence, if it is necessary to reduce the weight of machine, cast steel is used, Another disadvantage with the use of cast iron is that its mechanical and magnetic properties are uncertain due to the presence of blow holes in the casting, Lately, rolled steel yokes have been developed with the improvements in the welding techniques. The advantages of fabricated yokes are that there are no pattern charges and the magnetic and mechanical properties of the frame are absolutely consistent.
It may be advantageous to use cast iron for frames but for medium and large sizes
usually rolled steel is used.
- If the armature diameter does not exceed 35 to 45 cm, then, in addition to the poles, end shields or frame-heads which carry the bearings are also attached to the frame. When the armature diameter exceeds 1 m, it is common practice to use pedestal-type bearings, mounted separately, on the machine bed plate outside the frame.
- The end shield bearings, and sometimes the pedestal bearings, are of ball or roller type. However, more frequently plain pedestal bearings are used.
- In machines with large diameter armatures a brush-holder yoke is frequently fixed to the frame.