PRACTICAL OPERATION OF DYNAMOS. 1 75 



of a set of generators in parallel, as pointed out in a subsequent 

 article. To start the generator shown in Fig. 69 open the main 

 switch, if it is not already open, turn the hand wheel of the field 

 rheostat so as to include all of its resistance in the field circuit, 

 start the engine and generator and bring them slowly up to full 

 speed, cut out resistance from the field rheostat until the generator 

 builds up to nearly full voltage, close the main switch and finally 

 adjust the field rheostat to give the exact voltage desired. Raising 

 the voltage above the proper steady value is likely to damage the 

 incandescent lamps, and is to be carefully avoided. To stop the 

 generator, turn the hand wheel of the field rheostat so as to include 

 all of the resistance of the rheostat in the field circuit, open the 

 main switch and stop the engine. In case of an over-load, due 

 to short circuit out on the mains or to the connecting of too many 

 lamps or motors, the circuit breaker will open the circuit. Before 

 closing the circuit breaker again, open the main switch, then close 

 the circuit breaker, and finally close the main switch ; by pro- 

 ceeding in this way the circuit breaker will operate instantly if 

 the short-circuit on the line still exists. 



Directions for starting and stopping motors are given in articles 

 59 and 61. 



83. Diseases of dynamos. Difficulty in getting a dynamo to 

 operate is most frequently due to a broken connection or a loose 

 contact. Always make sure that the connections are properly 

 made and that the various circuits are free from loose and uncer- 

 tain contacts. Troubles in the dynamo itself may be classified 

 according to their symptoms as follows : 



I. Sparking at the commutator. 

 II. Heating of commutator and brushes 



III. Heating of armature. 



IV. Heating of field magnet. 

 V. Heating of bearings. 



VI. Noise. 



VII. Symptoms peculiar to generators. 

 VIII. Symptoms peculiar to motors. 



