INDUCTION MOTORS 127 



Slip is usually measured at full load and running light 

 by means of the slip indicator. During this test, constant 

 speed must be held on the driving alternator, and con- 

 stant voltage on the motor. 



To take slip by the lamp method, an arc lamp is con- 

 nected in the circuit from which the motor is running. 

 On the end of the motor shaft a disk is placed which has 

 as many white and black sectors as there are poles in the 

 motor. As the lamp is running from an alternating cur- 

 rent source, the current wave passes through zero twice 



Slip Disk for 6-Pole Motor 



in each complete cycle. At the zero instant, the light 

 given out by the lamp is a minimum. 



Consider a six-pole 60 cycle motor running at 1200 

 r.p.m., that is to say, at 20 revolutions per second; then 

 20X6 = 120 black sectors passing a stationary point on 

 the circumference of the disk in one second. As the 

 frequency is 60, the number of maximum illuminations 

 will be 120. At each maximum illumination, therefore, 

 the black strips will always occupy the same positions. 

 However, the slip which always occurs in an induction 

 motor will cause the black strip to lag by a small angle 

 behind the position occupied at the previous illumina- 

 tion. These successive differences in position appear as 

 a sector rotating backwards, which can be followed by 

 the eye. The slip, that is, the difference between the 

 actual speed and the synchronous speed of the motor 

 per minute, can thus be counted. 



The resistance of the stator should be measured cold 

 and hot. 



