372 



ALTERNATING CURRENTS 



Bus 



+BUS 



FIG. 337. Method of obtaining correct 

 polarity by closing equalizer and series-field 

 switches. 



As the speed of the armature increases, the field produced by 

 it must rotate slower and slower in space, although it does not 

 change its speed relative to the armature. The brushes tend to 

 become more and more nearly stationary with respect to (his 

 rotating field, so that their commutating action becomes greater 

 and greater. The frequency of the electromotive force across 

 the brushes becomes less and less, and when the armature finally 

 pulls into synchronism, becomes zero, and a direct-current voltage 

 exists across the brushes. 



If a direct-current voltmeter be connected across the brushes, 



its pointer will tend to os- 

 cillate at line frequency 

 when the alternating cur- 

 rent is first switched on. 

 As the armature speeds up, 

 this frequency becomes less 

 and less, and the pointer is 

 soon able to follow the slow 

 oscillations. When the fre- 

 quency of oscillation be- 

 comes very low and the pointer is just going through zero in 

 the positive direction, the field switch should be closed. This 

 insures the converter's coming in with the correct polarity. A 

 zero-center type of voltmeter is desirable when this method is 

 employed. 



(d) If the converter operates in parallel with others, and equal- 

 izers are used, a weak field of the correct polarity may be pro- 

 duced in the field of the incoming converter by closing a line and 

 an equalizer switch, as indicated in Fig. 337. This tends to make 

 the armature reaction build up fields of the correct polarity and 

 so insures the converter coming in properly. 



153. Starting Synchronous Converter by Means of an Auxili- 

 ary Motor. As was pointed out in Chap. X, one method of 

 starting a synchronous motor is to bring it up to speed with an 

 auxiliary motor and then synchronize. (See page 320, Par. 130.) 

 This same method may be used with the converter. The me- 

 thods of synchronizing are identical with those used with the 

 alternator. (See page 168, Par. 74.) This method of starting 

 is practically obsolete, 



