STARTING OF SYNCHRONOUS MOTOR 139 



The auxiliary motor employed in starting up the synchronous 

 motor may be either an induction motor having a smaller number 

 of poles than the synchronous motor, so that it is capable of running 

 up to a speed slightly above the synchronous speed of the main 

 motor, or it may be the exciter used as a continuous-current motor, 

 and supplied with continuous current from any available source. 



The operation of running the motor up to synchronous speed 

 and phase opposition of its e.m.f. relatively to the p.d. is termed 

 synchronization. The operation of synchronizing will be considered in 

 detail in connection with the parallel running of alternators ( 76). 



Polyphase synchronous motors are best started in the same way 

 as single-phase motors by means of a suitable auxiliary motor. 

 There is, however, a method of starting such motors which, although 

 not requiring an auxiliary motor, is not to be recommended, as it is 

 liable to throw severe strains on the motor and to cause serious 

 fluctuations of the generator voltage. The method consists in closing 

 the armature switch, the field circuit being open. In order to limit 

 the current, a transformer with a variable number of turns is used, so 

 that the voltage may be gradually increased as the motor gains speed. 

 The polyphase armature current gives rise to a rotating magnetic 

 field, and the motor starts by virtue of the resistance to relative 

 motion of the rotating field and the field-magnet system caused by 

 hysteresis and eddy currents. It ultimately runs up to synchronous 

 speed. The field current is then switched on. 



With this method of starting, the starting current is generally 

 very large, and seriously disturbs the regulation of the system ; the 

 ammeters and fuses must be short-circuited to protect them against 

 the excessive starting current. Since at the moment of starting the 

 field coils will act as the secondaries of transformers of which the 

 primaries are represented by the armature winding, a very large e.m.f. 

 will be induced in the field winding if the coils are left connected in 

 series. It therefore becomes necessary to open the field winding 

 at a number of points, the connections being re-established when 

 synchronous speed is reached. 



There is a further difficulty connected with this method of 

 starting. When the motor has run up to synchronous speed, the 

 magnetization of its field-poles is due entirely to armature reaction. 

 Now, it may happen that when the field current is switched on, 

 it will reverse the polarity due to armature reaction, thereby also 

 reversing the phase of the motor e.m.f. The motor will be pulled into 

 proper phase relation, but only at the expense of an enormous 

 momentary rush of current and a heavy mechanical strain on the 

 field* 



* This difficulty may be overcome by providing a suitable arrangement for 

 indicating the polarity of the field due to armature reaction. 



