CHAPTER IV 

 OPERATION OF SYNCHRONOUS MOTORS. HUNTING 



Synchronous motors are not self -starting, the same as other electric 

 motors, but they have to be started by some artificial means such as 

 described hereinafter. 



Starting by Direct Current. The simplest way, when there is a 

 source of direct current and the synchronous motors tire provided with 

 a commutator for producing direct current for self-excitation, is to 

 start the machine as if it were a D.C. motor, with the armature and 

 fields connected in parallel. This plan can be especially adopted 

 in transformer stations using motor- transformers, so as to start a set 

 of motors operating dynamos as soon as one- has been put in operation. 



Starting with Alternating Current by Polyphase Motors. When 

 there is no D.C. source, the alternating currents themselves must be 

 used for starting the motor, either by mounting on the shaft of the 

 machine a small induction-motor which serves to run it without load, 

 or, more frequently, by using the machine itself as an induction-motor. 

 (When a small motor is used its capacity should be about 10 per cent 

 of that of the machine. It can be even less, if it can withstand over- 

 loading for a few seconds.) 



When using the machine itself as an induction-motor, the first 

 step is to suppress the regular excitation, which would prevent the arma- 

 ture from moving. The second step is to connect the armature with 

 the source of alternating supply, taking care to lower the voltage suf- 

 ficiently to prevent excessive current through the armature. The 

 armature then behaves as if it were the primary winding of a revolving 

 field machine in which the fields, and more especially the pole-pieces, 

 play the role of secondary circuit. To obtain a torque equal to a quarter 

 the normal torque, it usually require's, by this method, a current at 

 least double the normal current. To complete this action, it is well 

 to short-circuit the field-winding. Certain manufacturers have, 

 for this puq>ose, arranged in the pole-pieces, as shown in Fig. 50, a 

 series of sloti containing windings or bars of copper, connected together 



106 



