140 ELECTRICAL MACHINERY 



capacity of a machine is fixed by the sparking at the com- 

 mutator. As was explained in the discussion of commuta- 

 tion, the current in any coil has to reverse during the short 

 interval of time during which it is short circuited by the 

 brush. Now the liability to spark at the commutator 

 was shown to depend upon the rate of change of current 

 necessary during the commutating period. As the time 

 during which commutation has to take place is constant 

 (for a given speed), the rate of change of current depends 

 directly upon the load the machine is carrying and so the 

 liability to spark depends directly upon the load. It has 

 been shown, however, how the commutating pole, produc- 

 ing in the short circuited coil an e.m.f. proportional to the 

 load, overcomes this difficulty and in commutating pole 

 machines the limit of capacity is set by heating and not by 

 commutator sparking. 



Special Insulation Increases Capacity. There is of 

 course one other method by which a machine of given 

 size may have its rating much increased and that is by 

 using only insulating materials which will stand higher 

 temperatures. The limit of 50 C above room temperature 

 is proper for such insulating materials as cotton, shellac, 

 oiled cambric, etc. If a machine could be built with 

 nothing but mica and asbestos for insulation, it might run 

 safely at very high temperatures. Asbestos has been used 

 to some extent but it is difficult to work this material into 

 uniform sheets and coverings and, also, it absorbs moisture 

 readily. If some good heat resisting insulator could be 

 discovered the possible output of electric machinery could 

 be much increased. 



There is one bad feature connected with this high 

 temperature of operation; the resistance of the windings 

 increase quite rapidly with the temperature and so results 

 in an increased PR loss and therefore in a lower efficiency. 



28. Operation of C-C. Generators in Parallel. A 

 generating station has ordinarily several generators, all of 



