OPEN-CIRCUITS, SHORT-CIRCUITS, ETC. 11 



pole, and the armature revolved through one pole span. 

 The air gap measurement should be taken under every 

 pole tip at the commutator end. The first set of measure- 

 ments are known as the "stationary gap"; the second 

 set as the "revolving gap." 



The maximum air gap on direct current machines, 

 when measured from iron to iron, should not differ 

 from the minimum air gap by more than a few per cent 

 of the average measurement, and but little more when 

 measured from the armature binding wire to the iron 

 of pole piece. 



On commutating pole machines the air gap measure- 

 ment is taken under the center of the commutating pole. 

 It is also necessary to measure the distance between 

 the tip of each commutating pole and the adjacent tip 

 of the main pole. This spacing should be uniform. 



Air gap measurements are taken on alternating 

 current machines in a manner similar to that described 

 above for "stationary" and "revolving gap" on direct 

 current machines; except that the air gap measurement 

 on a-c. machines is taken at the center of the pole piece, 

 both on the front and back ends. In measuring the 

 "revolving gap," it is not necessary to take the air 

 gap measurement at points corresponding to 180 elec- 

 trical degrees on the surface of the armature. The 

 measurement need only be taken at points spaced 45 

 degrees apart. That is, eight (8) sets of measurements 

 are required for the "revolving gap." 



Since the air gaps of induction motors are small, 

 and a uniform gap is important, they are measured by 

 special gauges provided for the purpose. These gauges 

 are passed completely through the motor air gap from 

 end to end of the punchings, measurements being taken 

 at several points about the circumference of the rotor 

 while stationary and while revolved in a manner similar 

 to that described above. 



When air gap measurements are taken, a critical 

 inspection should be made of the clearance between 

 the rotor and windings or other parts, to insure that 

 it is sufficient to allow the machine to operate without 

 any surfaces striking or rubbing together. This difficulty 



