12 COMMERCIAL ELECTRICAL TESTING 



may occur if the windings project beyond the punchings 

 unnecessarily far. In no case should the machines be 

 started until proper clearances are obtained. 



When apparatus is first started it should be brought 

 to speed very slowly and carefully watched to see that 

 everything is correct as the speed increases to normal 

 value. Reliable tachometers, or speed indicating devices 

 should be used in starting to prevent a dangerous 

 increase of speed. Oil rings must be examined at slow 

 speed to see if they are carrying sufficient oil to the 

 bearings. In the majority of cases, oil rings should 

 turn at J4 speed, and properly lubricate the bearings. 



The balance of the rotating parts should be carefully 

 noted until the machine has reached its normal rated 

 speed. If the apparatus does not run without vibration 

 the matter must be remedied before the test proceeds. 

 Vibration due to the running of the machine may 

 indicate lack of balance, whereas it may really be due 

 to improper alignment, or to springing of the shaft. 

 When unbalancing occurs in operating machines run- 

 ing above 1200 r. p.m., correction must be made by dy- 

 namic balancing. This method is described in a subse- 

 quent chapter. 



