Field Excitation of Alternators.—The fields of alternators require a separate source of direct current for their excitation, and this current should be preferably automatically controlled. In the case of alternators that are not self exciting, the dynamo which generates the field current is called the exciter.
The excitation of an alternator at its rated overload and .8 power factor would not, in some cases, if controlled by hand, exceed 125 volts, although, in order to make its armature voltage respond quickly to changes in the load and speed, the excitation of its fields may at times be momentarily varied by an automatic regulator between the limits of 70 and 140 volts.
Fig. 1,430.—Western Electric armature for self-excited alternator. The main winding is placed at the bottom of the slots, each coil being surrounded by an armour of horn fibre. The exciter winding occupies a very small portion of the slot, being placed on top of the main winding, and connected to the commutator immediately in front of the core and between core and collector rings as shown.
The exciter should, in turn, respond at once to this demand upon its armature, and experience has shown that to do this its shunt fields must have sufficient margin at full load to deliver momentarily a range from 25 to 160 volts at its armature terminals.
It is obvious from the above that an exciter suitable for use with an automatic regulator must commutate successfully over a wide range in voltage, and, if properly designed, have liberal margins in its shunt fields and magnetic circuits.
Alternator fields designed for and operated at unity power factor have often proved unsatisfactory when the machines were called upon to deliver their rated kva. at .8 power factor or lower. This is due to the increased field current required at the latter condition and results, first, in the overheating of the fields and, second, in the necessity of raising the direct current exciting voltage above 125 volts, which often requires the purchase of new exciters.
Ques. What is a self-excited alternator?