6 



SYNCHRONOUS MOTORS 



to the other practically disappears. Moreover, the " phases are iden- 

 tical," i.e., the poles of like polarity pass at the same time in front of 

 the corresponding portions of the two armatures. 



The induced E.M.F.'s between the corresponding terminals a, 

 b, and A, B, are therefore in unison. If we measure them, on the con- 

 trary, in the direction in which they appear, by following the circuit 

 ab, BA, it will be found that they are exactly opposed to each other. 



Let us now suppose the belt of one of the two machines to be removed. 

 This machine will continue to^urn at the same speed, but it gives indi- 

 cation of a certain very slight delay or falling behind, technically 

 termed "lag," with respect to the other machine. Moreover, the 

 current in the circuit now becomes appreciable. 



If a brake be placed on the pulley and if the load be gradually 

 increased in such a way as to increase the mechanical power produced 



FIG. 5. 



by the motor, the " lag " of the motor will be seen to increase at the 

 same time as the current. 



When this lag approaches a quarter of a period, i.e., half an inter- 

 polar space, the machine slows up all at once and stops as if held fast 

 by the brake. We then say that it is " stalled," or " out of synchron- 

 ism," or " out of step." The current in the circuit rises to a very high 

 value as soon as the machine falls out of synchronism; and it becomes 

 approximately equal to the short-circuit current in the circuit when the 

 machine is stopped. In order to avoid accidents, it is necessary to 

 introduce fuses in the circuit, or to provide some automatic disconnecting 

 device, which will prevent the excessive load. 



It is seen that what characterizes the synchronous motor is the 

 increase of phase-lag with the load and the " stalling " of the motor 

 or its falling out of step beyond a certain maximum load. 



In a good motor, the limiting load should amount to at least 1.5 

 timi-s, or, better, to twice the normal load. This limit is guaranteed 

 by most makers of synchronous motors. 



On the other hand, if the motor is run by a belt in such a way as to 

 give it a " lead in phase," with respect to the machine or the circuit 



