ROTARY CONVERTERS 109 



up switch." If, after closing the shunt field switch, 

 brushes begin to spark, the residual magnetism left 

 in the poles by the induced voltage at starting is of 

 the wrong polarity. 



Two methods can be used to correct this; first, reverse 

 the field with respect to the armature ; or second, reverse 

 the residual polarity by opening the alternator field 

 circuit, and then closing this circuit and bringing the 

 rotary back to synchronism, repeating the operation 

 if necessary until the field builds up in the right direc- 

 tion. This second method is the more satisfactory 

 since no change of wiring is required. 



Before proceeding further, read the current in each 

 phase to make sure that there is no unbalancing. These 

 currents should not vary over 1 per cent from the 

 average ; any greater variation due to wiring must 

 be remedied at once. After balance is established, 

 both no load and full load phase characteristics are 

 taken. 



These operations complete the preliminary tests 

 and the full load heat run may now be made, care 

 being taken to set the brushes for the best commutation. 

 For the load run, hold full load direct current amperes 

 and volts constant with minimum input field current. 

 The load should be kept on at least one hour after all 

 temperatures are constant. At the end of the run, 

 temperatures must be taken on all parts of the machine 

 and the resistance measured on the armature (alternating 

 current end) and field. If the rotary is a six-phase 

 machine, the armature resistance is measured between 

 rings 1-4, 2-5, 3-6, counting outwards from the arma- 

 ture. 



If an overload run is required, take a few points on 

 the overload phase characteristic to determine the 

 field current required for minimum input; then hold 

 this current and the direct current volts and amperes 

 constant, as on the normal load run. 



After the heat runs, the test should be finished by 

 taking a phase rotation, hot drop on spools, direct 

 current running light at normal voltage, and direct 

 current starting tests. 



