There are a number of considerations which govern the selection of the materials to be used in a particular machine, such as initial cost, weight, efficiency, etc.
Ques. In the construction of field magnets, what governs the choice of materials?
Ans. For cores, wrought iron is most desirable, as requiring the smallest amount of material for a given flux. There is a saving in copper due to using wrought iron for the core since, on account of its small size, the length of each turn of the magnetizing coil is reduced. For heavy yokes, where lightness is not essential, but very often the reverse, cast iron is used, as its cross section can be made larger than that of the cores, this increase in area serving to give strength and rigidity to the machine. Cast steel occupies a place intermediate between cast iron and wrought iron both in cost and magnetic properties.
Ques. Name two forms of yoke in general use.
Ans. The solid, and divided types as shown in figs. 205 and 206.
Ques. What is the object of dividing a yoke?
Ans. To permit access to the armature, where the construction does not admit of removal of the latter from the side.