78 COMMERCIAL ELECTRICAL TESTING 



method should not be used. To perform this test quickly, 

 reversing switches are used in the series and shunt field 

 circuits. Care must be taken, when shifting the brushes, 

 to avoid a dangerous rise of speed. 



When the proper no-load shift has been found for 

 full commutating field, normal load is applied and the 

 commutation and speed noted. If the speed has in- 

 creased under load or the commutation is not sparkless, 

 a german silver shunt is used across the commutating 

 field and adjusted for commutation, the speed for each 

 change in the shunt being noted to ascertain whether 

 the speed is decreasing under load. When the final 

 shunt adjustment is taken, a speed curve reading is 

 taken and the speed and commutation in both directions 

 of rotation, at no-load, full load and whatever overload 

 is required are recorded. At the conclusion of all tests 

 required, while the machine is hot, a hot speed curve 

 covering the same range of load as used in the cold 

 curve is taken. In the case of two-speed machines, 

 this curve should be taken at both speeds. Additional 

 no-load and full-load readings should be taken at full 

 field. If a falling or constant speed is obtained, and 

 commutation is satisfactory, no shunt is necessary; 

 otherwise, a shunt must be placed across the commutat- 

 ing field and adjusted to give these speeds. 



Commutating pole variable speed motors must have 

 the shunt in the commutating pole field adjusted for 

 the highest rated speed. Speed curves and running 

 light tests should be made at both speed limits. 



Shunt wound variable speed motors have the brushes 

 set for commutation at the speed limits. Speed curves 

 and running light tests should be made at both of these 

 speeds. 



Some compound wound variable speed motors are 

 not designed to run light; consequently, before starting, 

 the smallest load the motor is designed to carry should 

 be ascertained. Commutation should be adjusted at 

 the various speeds, series full field readings being taken 

 and the speed carefully recorded. Speed curves should 

 be taken at the different speeds; also running light 

 with the series field disconnected. 



