THE SYNCHRONOUS MOTOR 331 



involves, such as very accurate alignment, etc., are not pi- 

 when synchronous motors are used. Owing to the salient-pole 

 feature of the synchronous motor, stator coils may be replaced 

 without removing the rotor. Also, the field windings on the 

 salient poles are less subject to injury than the 4 embedded con- 

 ductors in the rotor of an induction motor. The dampers of 

 synchronous motors used for electric propulsion are designed to 

 give moderately high torque on starting, reversing, etc. Both 

 :>eed and the voltage of the generator may be varied, so that 

 the motors have different starting characteristics from those ex- 

 ist intr at constant frequency and constant volt 



ry Changers. It is sometimes necessary to supply 

 electric power from one electric system to another electric system 

 of different frequency. A common method is to use synchronous- 

 motor-alternator sets. The synchronous motor and the alter- 

 nator must have a different number of poles, the number of 

 pol.-< in each being proportional to the frequency of the system 

 to which the particular machine is connected. For example, if 

 the frequency is being changed from 60 to 25 cycles, tin 4 number 

 of poles of the synchronous motor must be to the number of 

 he alternator in the ratio of 60 to 25 or 12 to 5. The 

 hiuhr-t -peed at which this ratio of frequencies can be obtained 

 will require a set having a 21-pole synchronous motor and a 

 10-pole alternator. The set will operate at only oOO r.p.m. 



pt in very large units electrical machines operating at 

 this very low speed would be costly. A 10-pole. 1-pole combina- 

 either a frequency ratio of (>() to 21 cycles or a frequency 

 ratio of r.2..') to -") cycles and opei ;:.() r.p.m. Becm, 



-peed, this combination is often used, even if it does 

 ivc an exact '() to 25 cycle ratio. 



It i< often difficult to syndironi/.e such a set, as it must be 

 synrhroni/ed with both Systems, If the alternator voltai:< 

 out of phase with their respective line voltages, the synchronous 



motor mu-t be made to -lip a pole at a time until the alternator 



in phase with their respective line voltages, The 

 hifted either by advancing the j hase of the system 



supplying the jxiwer. as by opening the turbine governors, or by 



ling in some manner the phase of the voltage in I he system 



