8-A] OPERATION AND LOAD TEST. 2/1 



including belt loss, for the particular speed and excitation. These 

 losses may be taken as practically proportional to speed, for small 

 variations in speed when the excitation is constant.* (Compare 

 Fig. 2, Exp. 2-B.) 



(4) Increase the load and take readings for a number of dif- 

 ferent loads up to say 25 per cent, overload, or until the motor 

 comes to a standstill.! 



43. Resistance Measurements on Motor. The resistance of 

 the motor primary is found by the direct current fall of potential 

 method, 17, Exp. i-A. This should be taken after the run is 

 made and the motor is heated up. For commercial tests, standard 

 specified conditions in regard to temperature should be followed : 

 see A. I. E. E. Standardization Rules. 



For a single-phase motor the resistance measurement is made 

 as in any single-phase circuit. 



For a 2-phase motor each phase is measured in this same 

 manner. 



For a 3-phase motor the resistance is measured between any 

 two terminals. (For accuracy it is measured between each pair 

 of terminals and averaged.) One-half the resistance thus meas- 

 ured gives the equivalent single-phase resistance, see 29, Exp. 

 6-A, irrespective of whether the connection is star or delta. For 

 a star connection, this is the resistance of one phase between one 

 line terminal and the neutral. For a delta connection, it has no 

 physical existence. 



*(42a). When the test is made by varying the field excitation so that 

 the terminal voltage is constant, these losses may be taken as practically 

 constant for different speeds through the small range of speed used in the 

 test; the decrease in losses due to decrease in speed is roughly compen- 

 sated for by the increase in losses due to the increase in excitation. This 

 makes the test simpler but less accurate. 



t(42b). As motors are usually designed, the excessive current pro- 

 hibits loading to standstill at full voltage, although it may be done at a 

 lower voltage say half voltage. 



