. Development and electrical distribution of water power . Fig. 41. 42 and 43 show the stationary ring carrying the armaturewindings and the rotating field member (or rotor) respectively,of a standard machine of this type. The complete machine isshown in Fig. 44. The connections to the armature are madewithout collectors or brushes. The field-magnet windings allrotate and it therefore is necessary to transmit current to themthrough collector rings, having brushes bearing on them. The ALTERNATING-CURRENT DYNAMOS 79 field current, however, is always very small and the voltage lowas compared with
. Development and electrical distribution of water power . Fig. 41. 42 and 43 show the stationary ring carrying the armaturewindings and the rotating field member (or rotor) respectively,of a standard machine of this type. The complete machine isshown in Fig. 44. The connections to the armature are madewithout collectors or brushes. The field-magnet windings allrotate and it therefore is necessary to transmit current to themthrough collector rings, having brushes bearing on them. The ALTERNATING-CURRENT DYNAMOS 79 field current, however, is always very small and the voltage lowas compared with the output from the armature, and, therefore,the size of the brushes and collector rings is small, and there is no. Fig. 42. difficulty whatever in their operation. This type of machinemay be constructed at a low cost and they are so thoroughlysatisfactory that they are almost exclusively used in the UnitedStates at the present time. Both the inductor and rotating-field machines have one ad-vantage in common, viz., the stationary armature winding anddirect connection from it to the outgoing transmission line without 8o DEVELOPMENT AND DISTRIBUTION OF WATER POWER the use of collector rings and brushes. This admits of insulatingthe armature winding to the same degree that a transformer wind-ing may be insulated, and, in consequence, dynamos may be woundfor extraordinarily high potentials, it being easy to obtain machinesin many standard sizes which deliver 6,600 volts and a few havebeen made which give 13,000 volts. For comparatively short transmissions—say up to fifteenmiles—these potentials are highenough and the use of step-uptransformers and the expense ofpurchasing them are avoided. Practica
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