PRACTICAL OPERATION OF DYNAMOS. l8l 



however is seldom or never the case in actual machines, because the 

 connections from the armature conductors to the commutator seg- 

 ments are seldom or never radial. That position of the brushes 

 which gives a maximum electromotive force between them as 

 indicated by a voltmeter is the proper position. 



2. Excessively low voltage of a shunt generator which is driven 

 at full speed is due to an excess of resistance in the shunt field 

 circuit. 



3. Reversal of a generator. When a machine is standing idle 

 its residual magnetism may be reversed by proximity to another 

 dynamo, or by stray current from some outside source. When 

 the generator is next started it will build up in a direction the 

 reverse of that desired. This reversal of a generator may be cor- 

 rected by stopping the machine, exciting its field for a moment in 

 the proper direction from some outside source, and then starting 

 it up again. 



When a number of generators are to be run in parallel, the 

 shunt field windings of all of them should be connected to the 

 bus * bars, for then it is not possible for one of the machines to 

 become reversed with respect to the others. 



VIII. Symptoms of trouble peculiar to a motor are abnormal 

 variations of speed, and, inasmuch as the speed of a series motor 

 varies greatly under ordinary running conditions, the following 

 simple statements apply primarily to the shunt motor. 



1 . Failure to start. This is most likely to be due to a faulty 

 connection, but it may be due to excessive starting friction, or to 

 a combination of moderate starting friction with an under-excita- 

 tion of the field magnet. 



2. Running at low speed. This might conceivably be due to 

 a greatly over-excited field, but it is more probably due to an 

 over-load, or to the combination of a heavy load with an under- 

 excited field. 



3. Running at excessive speed. This never occurs on a very 



* Omnibus bars, the bars which run along the back of the switchboard panels and 

 which receive all the current from all of the generators. 



