ACTUAL LOAD TESTS 41 



from the alternator (A) which supplies the losses. The 

 sets are started one at a time from the alternating current 

 end and the direct current ends paralleled by means 

 of a voltmeter across switch P. The direct current 

 motor field is weakened until the ammeter in the direct 

 current line indicates that normal current is flowing. 

 The weakening of the motor field allows the speed of 

 the inverted set to increase just enough to load the 

 induction generator, while it also decreases the counter 

 e.m.f. of the motor a sufficient amount to allow full 

 load current to flow in the direct current circuit. This 

 load must be closely watched, as it is unstable. Load 

 unstability is a rather common occurrence in "feeding 

 back," due to either variations in shop voltage or speed. 



It will be noted in the "feeding back" tests described, 

 that it is necessary to weaken or strengthen one of the 

 fields to obtain the load. To conduct the test with the 

 same field excitation on both machines the armature 

 of a separately excited booster may be connected in 

 series with the armatures of the two machines being 

 tested. The machines, connected so that they run at 

 the same speed, are brought up to normal speed by 

 means of the motor supplying the losses. The connecting 

 switch is then closed and the booster field strengthened 

 until normal current flows in the armature circuit, the 

 field current being adjusted to give the same excitation 

 on both fields. The voltage is held across the motor 

 terminals by varying the speed of the loss supply motor. 

 This method, known as the circulating method, is used 

 particularly in the testing of series or railway motors. 

 In the latter case the machines are geared to the same 

 shaft. 



Another method known as "shifting the phase" is 

 used in testing two similar alternators or frequency 

 changer sets. Two similar alternators may be direct 

 connected by means of a coupling and driven by a 

 motor to supply the losses. For example, let a three- 

 phase machine be considered, the phases of which are 

 shown diagrammatically in Fig. 15. The machines should 

 be run at normal speed, the fields connected in series 

 and separately excited to a value corresponding to the 



