270 SYNCHRONOUS MOTORS. 



R Field regulating resistance. 



S l Main switch.* 



S t Field switch. 



L Synchronising lamp. 



General. Although it is possible in some cases to switch 

 small synchronous motors directly on to the supply after 

 running them approximately up to speed without syn- 

 chronising them, it is a practice which cannot be adopted 

 with all motors, and which is liable to produce considerable 

 fluctuations in the supply voltage. It is very dangerous 

 to attempt to do this with any but very small motors. The 

 form of synchroniser to be employed may be either the 

 simple lamp shown in the diagram (Fig. 128) above, or it 

 may be the form of synchroniser shown in the diagram 

 (Fig. 125). The instructions given below can be readily 

 modified to suit the latter case by referring to those given 

 on page 264 for paralleling alternators. 



Instructions. Connect the armature of the motor to 

 the source of alternating supply through a switch in series 

 with an ammeter. Connect a voltmeter to the machine 

 terminals. Excite the magnets with direct current through 

 a field regulator and ammeter. For synchronising, connect a 

 lamp, or several lamps in series, across the terminals of the 

 main switch. Remember that these lamps must be able to 

 stand a voltage equal to twice, that of the supply, if a single 

 pole switch is employed. 



Before closing the mam switch, run the motor up to full 

 speed by an auxiliary motor or other means. Adjust the 

 excitation until the motor voltage is equal to that of the 

 supply. Vary the speed until the lamp fluctuates slowly, 

 and becomes quite extinguished between each period 

 of brightness. Close the switch when the lamp is out. Then 

 adjust the field until the current taken by the armature is a 

 minimum. The machine may now be loaded without falling 

 out of step. 



Although it is generally advisable to excite the motor so 

 that its armature voltage before switching in is as nearly as 

 possible equal to the voltage of the source of supply, the 

 motor will start with either more or less excitation than this. 



* A double-pole switch with a lamp suitable for the line voltage across each 

 pole may be employed, and is preferable in the case of permanently connected 

 motors. 



