332 



ALTERNATING CURRENTS 



136. Synchronous Motors of Very Small Size. Because of 

 their absolutely constant-speed characteristics, synchronous 

 motors are very useful for driving such devices as must be held 

 in absolute synchronism with the supply frequency. Such uses 

 involve the measurement of slip in the induction motor (see 

 page 273), the driving of oscillograph mirrors, stroboscopic 

 devices, mechanical rectifiers, etc. 



As the power required of such motors is extremely small and 

 the matter of low power-factor is of no moment, they are often 

 made to operate without direct-current excitation. In Fig. 306, 

 (a) and (6), are shown motors of this type. In (a), the four-pole 

 armature consists of a cruciform-shaped piece of iron with the 



(a) 4 Poles (I) IB Poles 



FIG. 306. Miniature synchronous motors. 



spaces filled with wood to make the armature cylindrical. The 

 field is made up of U-shaped laminations and is excited from the 

 alternating-current supply 



When the armature is brought up to speed, two diametrically 

 opposite armature poles are attracted to the field poles as the flux 

 is increasing. Because of the inertia of the armature, it continues 

 to rotate when the flux is passing through zero. The next pair 

 of poles are then attracted by the flux as it increases in the 

 opposite direction. Such a motor will therefore run at constant 

 speed, provided the frequency is constant. 



A 16-pole motor operating on the same principle is shown in 

 Fig. 306 (6). 



These motors really operate on the principle of maximum per- 

 meance, although it will be recognized that they are salient-pole 

 synchronous motors of the rotating-field type, having no direct- 

 current field excitation. Their excitation is produced by arma- 

 ture reaction. 



