SYNCHRONOUS MACHINERY 271 



The instantaneous short-circuit current is further increased due 

 to the fact that with very heavy armature currents, the path of 

 the true armature-reactance flux becomes very highly saturated 

 and its permeability is therefore decreased. This results in a de- 

 crease of the reactance x\. 



161. Single-phase Short Circuits. In studying armature 

 reaction in single-phase alternators it was shown that a double- 

 frequency pulsation of the field is produced and a third harmonic 

 of e.m.f. generated in the armature. Due to the high self-induc- 

 tance of the field coils these effects are not very marked up to 

 full load. At short circuit, however, the armature m.m.f . is very 

 strong and produces a very large pulsation of the field. As 

 a result large currents and e.m.f.'s are induced in the field 

 winding. 



The same effects are produced in the case of a short circuit of 

 only one phase of a three-phase alternator; the armature reaction 

 consists of a normal three-phase armature reaction with a single- 

 phase reaction superimposed, of intensity corresponding to the 

 increase of the short-circuit current over the normal current. A 

 double-frequency pulsation of the field is produced and a large 

 third harmonic of e.m.f. is generated in the two phases which are 

 not short circuited. 



162. Synchronous Motor. A synchronous motor is exactly 

 the same as an alternator in construction and may be either single 

 phase or polyphase. The single-phase motor is not self-starting 

 and must be brought up to synchronous speed before being con- 

 nected to the supply. It is, therefore, not used except in special 

 cases. The polyphase motor when connected to the supply will 

 accelerate and run up to synchronous speed but only a low voltage 

 should be impressed on it at start or very large lagging currents 

 will be drawn from the supply lines. 



Fig. 249 (a) represents a two-phase, two-pole motor. The 

 armature is stationary and is supplied with two-phase alternating 

 currents, Fig. 249 (6). The armature m.m.f. is constant in value 

 as in the alternator and revolves at synchronous speed in the anti- 

 clockwise direction and produces a revolving field of constant 

 value. Figs. 249 (a), (c) and (d) represent the armature m.m.f. at 

 the instants (1), (2) and (3). 



The speed of the field is directly proportional to the frequency 



