4. With respect to mode of field excitation, as:
- a. Self-exciting;
- b. Separately excited;
- Exciter direct connected, or gear driven.
- c. Compositely excited.
5. With respect to service requirements, as:
- a. Slow speed;
- b. Fly wheel;
- c. High speed;
- d. Water wheel type;
- e. Turbine driven.
Single Phase Alternators.—As a general rule, when alternators are employed for lighting circuits, the single phase machines are preferable, as they are simpler in construction and do not generate the unbalancing voltages often occurring in polyphase work.
Fig. 1,370.—Elementary four pole single phase alternator. It has four "inductors" whose pitch is the same as the pole pitch. They are connected in series and terminate at the two collector rings as shown. The poles being alternate N and S, it is evident that there will be two cycles of the current per revolution of the armature. For any number of poles then the number of cycles equals the number of poles divided by two. Applying Fleming's rule for induced currents, the direction of the current induced in the inductors is easily found as indicated by the arrows. The field magnets are excited by coils supplied with direct current, usually furnished from an external source; for simplicity this is not shown. The magnets may be considered as of the permanent type.
Ques. What are the essential features of a single phase alternator?
Ans. Fig. 1,370 shows an elementary single phase alternator. It consists of an armature, with single phase winding, field magnets, and two collector rings and brushes through which the current generated in the armature passes to the external circuit.