104 COMMERCIAL ELECTRICAL TESTING 



and field amperes and volts. If current transform- 

 ers are used with the wattmeters, duplicate trans- 

 formers must be used in the other phases of the machine 

 to prevent unbalancing caused by the resistance and 

 inductance of the transformers. With the machine 

 running in synchronism at rated speed and zero load, 

 and all meters connected, the alternating current volts 

 impressed on the rotary should be held constant and 

 careful readings taken of all instruments. The currents 

 and volts in each phase should be read as a check on 

 the wiring and balancing of all phases. All instruments 

 should also be carefully checked for stray fields and 

 any instruments affected by these must be protected 

 by iron shields, or their location changed. With full 

 load, the test for stray field should be repeated, since any 

 instrument affected will give misleading and erroneous 

 results. With the no load minimum input field current 

 held constant, the alternating current input, as shown 

 by the wattmeters, should be carefully read as a check 

 on the no load losses. 



As efficiency is usually guaranteed at J^, J/2, %, 1 

 and 1}^ full load, careful readings must be taken at 

 these loads. Each time the load is changed, the rotary 

 field excitation must be changed to the minimum input 

 value for that load, which is shown when the sum of 

 the wattmeter readings is exactly equal to the kv-a. 

 input. To obtain this condition for each load, several 

 trials and considerable time is usually required, so that 

 an efficiency test made in this way is more expensive 

 than one made by the separate loss method. The 

 likelihood of error is also greater. This method, there- 

 fore, is not satisfactory for rotary efficiencies at other 

 than full load. 



The method employed to calculate the efficiency 

 of a standard rotary converter is similar to that used 

 for direct current generators, except for the additional 

 C 2 R and friction losses of the alternating current brushes. 

 Because of the neutralizing action of the motor and 

 generator currents it should be noted that only a certain 

 percentage of the current as given by the instruments 

 must be used for calculating the C 2 R loss in the arma- 



