There are advantages of simplicity in the entire generating, primary, and secondary distribution systems for single phase roads. These advantages are so great that they justify considerable expense, looked at from the railway point of view only, the single phase system throughout may be considered as offering the most advantage.
Ques. What are the objectionable features of single phase alternators?
Ans. This type of alternator has an unbalanced armature reaction which is the cause of considerable flux variation in the field pole tips and in fact throughout the field structure.
In order to minimize eddy currents, such alternators must accordingly be built with thinner laminations and frequently poorer mechanical construction, resulting in increased cost of the machine. The large armature reaction results in a much poorer regulation than that obtained with three phase alternators, and an increased amount of field copper is required, also larger exciting units. These items augment the cost so that the single phase machine is considerably more expensive than the three phase, of the same output and heating.
Fig. 2,149.—Elementary alternator developing one volt at frequencies of 60 and 25, showing the effect of reducing the frequency. Since for the same number of pole, the R.P.M. have to be decreased to decrease the frequency, increased flux is required to develop the same voltage. Hence in construction, low frequency machines require larger magnets, increased number of turns in series on the armature coils, larger exciting units as compared with machines built for higher frequency.
Ques. What factor increases the difficulties of single phase alternator construction?
Ans. The difficulties appear to increase with a decrease in frequency.
The adoption of any lower frequency than 25 cycles may result in serious difficulties in construction for a complete line of machine, especially those of the two or four pole turbine driven type where the field flux is very large per pole.