136 



COMMERCIAL ELECTRICAL TESTING 



as the readings are taken, curves should be plotted with 

 volts as abscissae, and amperes and the algebraic sum of 

 the watts as ordinates. The ampere curve should be a 

 straight line, though sometimes the top portion curves 

 upward very slightly. 



Single-phase check readings should be taken, one 

 above, one below and one at normal amperes, on the two 

 phases containing wattmeters. 



The single-phase impedance current should be 86:5 

 per cent of the three-phase (line) values. The single- 



TABLE XVII 



Impedance on a 100 H.P., 2080 Volt, 6-Pole, 60 Cycle, 

 3-Phase Induction Motor 



phase impedance watts should be approximately half 

 of the three-phase watts. In a quarter-phase motor 

 single-phase impedance is the impedance of one of the 

 two phases. 



On form M motors, when taking impedance, the 

 collector rings should be short circuited either by metal 

 brushes or by metal strips, as the contact resistance 

 varies with carbon brushes. The ratios between the 

 primary and secondary voltage should be taken with 

 the secondary open circuited. 



