364 



ALTERNATING CURRENTS 



148. Experimental Determination of Voltage and Current 

 Relations in a Converter. An instructive laboratory experiment 

 is carried out with a converter connected in the manner shown in 

 Fig. 333. The series reactances may be omitted if the trans- 

 formers themselves have sufficient leakage reactance. Connect 

 instruments to measure the three-phase input, a voltmeter to 

 measure the transformer primary voltage, a voltmeter to meas- 

 ure the slip-ring voltage, ammeters to measure the currents 

 between the transformer secondaries and the converter, direct- 

 current instruments to measure the converter output, and a 

 direct-current ammeter to measure the field current. 



Keep the load on the converter constant at its rated value. 

 Vary its field over the maximum range of operation, reading all 

 instruments. With field current as abscissas, plot as ordinates: 



Transformers 



Shunt Field 



FIG. 333. Connections for testing a synchronous converter. 



1. Voltages Fi, V 2 , V s , and F 4 . 



2. Efficiency of the entire unit. 



3. Power-factor. 



Also, check the currents by the equations of Par. 142, page 350. 

 Note the effect of power-factor on efficiency. 



Other experiments may be performed using these same con- 

 nections, such as keeping the field current constant at its normal 

 no-load value (P.P. = 1.0) and noting the changes in efficiency 

 and power-factor as the load is increased. Plot efficiency and 

 power-factor as ordinates with output as abscissas. 



149. Synchronous -converter Connections. Transformers are 

 usually necessary with synchronous converters. The direct- 

 current voltage is always low and the alternating voltage at the 

 slip-rings must be still less. Moreover, transformers are neces- 

 sary for obtaining a six-phase from a three-phase system. 



