'7 2 TRANSFORMERS. [Exp. 



test is made on a 2,ooo-volt coil, full-load current will be I ampere 

 and the impedance voltage will be 75 volts; for a i,ooo-volt coil, the 

 values become 2 amp. and 37.5 volts, etc. 



In many cases, by a proper connection of coils in series or parallel, 

 one set of instruments may be selected which will be suitable for 

 both open-circuit and short-circuit tests. It is to be understood that 

 this method of selection will give only the approximate range. For 

 frequencies much below normal, it is to be borne in mind that the 

 current will be much greater than at normal frequency and in- 

 struments with say four or five times the current-carrying capacity 

 will be required. 



45. Adjustment of Supply Voltage. The best way to get various 

 voltages for the open-circuit test is by means of a transformer or 

 an auto-transformer with a number of taps. An adjustment of 

 voltage by means of a series resistance distorts the wave form and 

 hence introduces error in the readings of core loss and exciting cur- 

 rent. This error is small if the reduction in voltage caused by the 

 resistance is small, as from no to 104 volts, and in this case the use 

 of series resistance is permissible. It should not be used, however, 

 for large reduction in voltage or in any case when high accuracy is 

 wanted. (In a particular case,* reducing the voltage from 220 to no 

 volts by a series resistance caused a decrease in core loss and in 

 magnetizing current of about 6 per cent.) If a resistance is to be 

 used, less error is introduced when the resistance is bridged across 

 the line and the transformer supply shunted off of part of the resist- 

 ance than is introduced with the resistance in series. It is best, how- 

 ever, to avoid resistance control entirely. 



Another way to vary the voltage is to vary the field excitation of 

 the supply alternator. No other load should be on the alternator, 

 nor should the transformer under test form an appreciable load on 

 the alternator; otherwise change in wave-form may materially change 

 the core loss. (A change of 20 per cent, can be thus produced.) 



46. The two-voltage method (9) obviates the necessity of 

 voltage adjustment. If normal and half voltage are not available, the 

 method may often be used with one voltage only by connecting coils 



*This was for an old transformer. With the new transformer iron 

 and higher densities, the errors due to wave distortion become greater. 



