146 SYNCHRONOUS MOTORS 



action on HI. On the contrary, H 2 and M 2 , both rotating to the 

 right with the same speed -, will produce a regular torque. 



A motor of this type can be started more or less well by the methods 

 of starting already described for ordinary synchronous motors. If 

 the motor is started by means of an induction-motor for two-phase 

 currents, it is well to group the stator-coils so as to reduce the number 

 of poles by one-half, and thus obtain the double speed which is necessary. 



A disadvantage of this system is that it gives rise to upper harmonics 

 in the induced E.M.F. of the motor. Moreover, the field A/i of 

 the armature will induce a harmonic of frequency yut in the field- 

 circuit, while the fields will give, in the armature, an E.M.F. of the 

 form 



^4 

 A sin cot sin 20)t= (cos a>t cos 3<o/), 



i.e., also a harmonic of the same frequency, ^wt. 



Besides, in consequence of the high self-induction, the power- 

 factor is necessarily very low. 



Another type of single-phase synchronous motor with alternating 

 field can also be obtained by employing, in the armature, currents 

 commutated by means of a shell-commutator analogous to that used 

 for the Ganz motors previously described. When once synchronism 

 has been attained, the effect of this commutator is to invert the current 

 in the armature so that the torque resulting from the action of the field 

 on the armature always retains the same sign. This motor is about 

 equivalent to the motor with pulsating field, since the two elements 

 are simply inverted and the armature can be considered as a revolving 

 field. The only difference results from the relative importance of the 

 ampere-turns of the armature in the fields. 



