98 COMMERCIAL ELECTRICAL TESTING 



Inverted Rotaries 



The speed of a rotary when running from the alter- 

 nating current side is determined by the line frequency. 

 The same machine running as an inverted rotary and 

 delivering alternating current operates as a direct cur- 

 rent motor. Its speed depends upon the field excitation 

 and load, and it will deliver a variable frequency, 

 particularly if compound wound. When run inverted, 

 a compound wound machine should have its series 

 field almost, if not entirely, short circuited when part 

 of its load is inductive, since a lagging current will 

 weaken the field and increase the speed, sometimes 

 causing a runaway. For this reason care must always 

 be taken when running a rotary inverted to see that 

 sufficient shunt field excitation has been obtained to 

 prevent excessive speed, particularly when another 

 machine is operating as a rotary from the inverted 

 machine. 



Starting Tests from the Alternating Current End 



The rotary should be wired to an alternating current 

 generator of sufficient capacity to start it without 

 overloading. If transformers are needed in order to 

 get the correct voltage, they should be placed between 

 the dynamometer board and the generator. 



A rotary, when starting from the alternating current 

 end, is similar in action to a transformer. The armature 

 corresponds to the primary, and the field, which has a 

 large number of turns, to the secondary. Hence the 

 induced volts on the field may be very high, often 

 3000 or 4000 volts. In all cases, therefore, the field 

 connection must be broken in two or more places to 

 keep this voltage within safe limits. A potential trans- 

 former and voltmeter should be connected across one 

 or two spools in series for reading the induced volts 

 field, and a record made as to the number of poles 

 included in the reading. 



Starting tests should be made from several different 

 positions of the armature with respect to the field. A 

 scale, corresponding in length to the distance between 

 collector ring taps, should be laid off on the armature 



