46 COMMERCIAL ELECTRICAL TESTING 



after the open circuited run, that of the armature 

 before and after the short circuited run, and the temper- 

 atures of the windings cold should also be recorded. 

 During both runs volts and amperes field and speed 

 should be recorded. During the open circuit run, volts 

 armature are recorded, and during the short circuit run 

 amperes armature. 



On some of the large induction motors, only about 

 one-fourth of the normal voltage is impressed. The 

 machine is then loaded until the desired current flow's 

 in the stator, the run being continued as described above. 



Another method of making an equivalent load test, 

 used especially with turbo and other large three-phase 

 alternators, is known as the circulating open delta 

 run. The phases of the machine are connected in delta, 

 one side of which is left open. The fields are excited 

 to give the load desired, this excitation being determined 

 from the saturation and synchronous impedance curves. 

 Due to harmonics which may exist in the legs of the 

 delta, an alternating cross current may flow in the 

 winding. This is measured by an alternating current 

 ammeter (with current transformer, if necessary) 

 inserted in the opening of the delta. The difference 

 between the square of this current and the square of 

 the current with which it is desired to load the machine 

 is found, and a direct current to a value equal to the 

 square root of this difference is circulated through the 

 winding. The run is then continued, a careful record 

 of volts armature, direct and alternating amperes 

 armature, volts and amperes field being made. It will 

 be noted that the alternating cross current in one side 

 of the right angled triangle and the direct current in 

 the other are combined vectorially to obtain the load 

 current desired. This method cannot be used success- 

 fully with all designs. Considerable discretion must 

 be used in selecting machines which may be tested by 

 this method. 



Another method of loading an alternating current 

 generator is to give it normal excitation and run an 

 unloaded synchronous motor from its armature circuit. 

 The field of the motor is varied to give a leading or 



