THE ALTERNATOR. 17 



it is called a single-phase commutator motor. This motor is 

 described in Chapter XIV. 



The alternator can be operated as a motor if it is started and 

 brought up to exactly the proper speed by an independent source 

 of power. When so used the alternator is called a synchronous 

 motor. The synchronous motor is described in Chapter VIII. 



The induction motor, which is described in Chapters XII and 

 XIII, must be arranged to operate as a commutator motor at 

 starting, or it must be supplied with two or more distinct alter- 

 nating currents transmitted to the motor over distinct trans- 

 mission lines. The latter arrangement is called the polyphase 

 system of transmission and it is described in Chapter VI. 



Conversion of alternating current into direct current. For many 

 electrochemical processes, direct current only can be used. 

 Thus, storage batteries cannot be charged by alternating current. 

 Furthermore, direct current is quite generally used for operating 

 street railways. When power is transmitted by the alternating- 

 current system and utilized in the form of direct current, the con- 

 version from alternating current to direct current is usually 

 accomplished on a large scale by means of the synchronous con- 

 verter, or rotary converter, as it is often called. The synchronous 

 converter is described in Chapter IX. 



The conversion from alternating current to direct current is 

 sometimes accomplished on a large scale by an alternating-current 

 motor driving a direct-current generator. 



The conversion of alternating current into direct current can 

 be accomplished on a small scale by the mechanical rectifier 

 or by the mercury-arc rectifier. The mechanical rectifier is 

 described in Art. 2 and the mercury-arc rectifier is described in 

 Chapter IX. 



4. Characteristics of alternating current motors. A general 

 idea of the mechanical characteristics of the various kinds of 

 alternating-current motors is necessary for a clear appreciation of 

 the advantages and disadvantages of alternating currents, inas- 



3 



