108 SYNCHRONOUS MOTORS 



In the particular case where it is possible to start the generator and 

 the motor at the same time, the operation of starting becomes easier 

 and there is no necessity of suppressing the excitation of the motor. 

 The motor starts of itself by giving it a slight impulse, it falls into step 

 at the very first period, owing to the low reactance at low speed, and 

 its motion then accelerates synchronously with that of the generator, 

 increasing gradually to full speed. 



Observations on the E.M.F. Induced in the Poles. During the 

 starting period, the fields are the seat of an important alternating 

 flux whose frequency attains, at first, the same frequency as the alternat- 

 ing currents themselves. The result is the production of a very high 

 E.M.F. in the field-winding, which is generally made of fine wire. 

 If, therefore, this winding remains on open circuit, its insulation should 

 be particulary good, and contact should not be made with its terminals. 

 It is preferable to short-circuit this winding. This alternating E.M.F. 

 naturally vanishes when synchronism is attained. It is obvious that 

 the armature-brushes on the D.C. side must also be disconnected, 

 since, otherwise, the pulsating current would produce a braking effect 

 which would prevent the motor from starting. 



Accessory Starting Apparatus. Installation of Synchronous 

 Motors. The starting process involves the use of two kinds of accessory 

 apparatus, i.e., phase-indicators and current-controllers. 



It is also well to provide fuses and automatic circuit -breakers which 

 break the main circuit whenever there is no excitation. 



Phase-Indicators. Phase-indicators play the same role in this 

 case as in coupling alternators in parallel. They are of the same kind 

 and are connected in the same way. Their use is recommended for 

 large-size motors. 



When a synchronous motor is started by a direct current or by an 

 auxiliary motor, a lamp of higher voltage than that of the supply-source 

 may be connected in series with the motor. Owing to its high resistance, 

 this lamp burns feebly at first under the action of the E.M.F. of the 

 source of supply. In proportion as the motor increases in speed, if 

 the field-winding is connected with a D.C. source, the E.M.F. which 

 acts on the lamp, being the resultant of the generator and motor E.M.F.'s, 

 will produce beats or pulsations which will gradually become less fre- 

 quent. The proper time to connect the armature directly to the cir- 

 cuit is when the beats have become extremely slow, and when the lamp 

 goes out, thereby indicating that the E.M.F.'s are in opposition. 



When the motor is started without field-excitation, as an induction- 



