SYNCHRONOUS GENERATORS 295 



corresponding to the field current F E , and it is then only necessary 

 to increase IE in the ratio of the actual field current on the alter- 

 nator at the time of short circuit to F E That is, the sustained 

 short-circuit current at load excitation FI is 



If a voltage regulator is used, the generator field current corre- 

 sponding to the maximum voltage across the collector rings must 

 be taken as FI. 



For water-wheel-driven alternators the sustained short-circuit 

 current based on full-load excitation is generally from two to three 

 times the normal full-load current. 



When a short circuit takes place the current becomes lagging 

 and its effect will be to demagnetize the field poles. Assume, for 

 example, a generator with short-circuit current ratio of ten times 

 the normal full-load current. Then tan = 10 and = 84.5. 

 Thus cos or the power factor under short circuit is equal to 0.09. 

 However, it requires an appreciable time to reduce the magnetic 

 flux to its low short-circuit value, since it is surrounded by the 

 field coils, which act as a short-circuited secondary opposing a 

 rapid change in the field flux, that is, in the moment when the 

 short circuit starts it begins to demagnetize the field, and the 

 magnetic field flux, therefore, begins to decrease. In decreasing, 

 however, it generates an e.m.f. in the field coils, which opposes 

 the change of field flux, that is, increases the field current so as to 

 momentarily maintain the full field flux against the armature 

 reaction. The field flux, however, gradually decreases, and 

 also the field current which increased considerably the first 

 moment. This is clearly illustrated in the oscillograms shown in 

 Fig. 163. 



Armature Connections. Synchronous generators may, as 

 previously mentioned, be connected either single-phase, two-phase 

 or three-phase. Single-phase machines are rarely used, and when 

 two-phase machines are required it is, as a rule, in connection 

 with some existing system. Three-phase machines, on the other 

 hand, are used almost exclusively, due to the many advantages 

 of this system over the other two. 



A three-phase current may be obtained from an ordinary 

 closed coil winding by making connections to point on the winding 



