146 THE TRANSFORMER. 



if the losses have been measured on the high-tension side. 

 Frequently the no-load losses are measured on the low- 

 tension side for convenience. In.this case, if a measurement 

 with 100 volts shows an insulation resistance of 100,000 ohms, 



e 2 100 2 



the loss amounting to C*R = -5- = . , .. - , = '1 watt would remain 



K 100,000 



unnoticed when measured in the manner described in the last 

 Experiment. If, however, with the same insulation resistance 

 a measurement is made on the 2,000 volts side, the loss due 



2 OOO 2 



to defective insulation becomes - ^=40 watts, which 



100,000 



would no longer be negligible. 



On this account no transformer intended for a pressure 

 of 200 volts should leave the testing house with a lower 

 insulation resistance than 2 megohms from primary to 

 secondary and iron. These values will be usually con- 

 siderably exceeded. If a newly-constructed transformer 

 shows an unsatisfactory insulation resistance, it should be 

 heated continuously for some time by having its primary 

 connected to a low voltage while its secondary is short- 

 circuited, so that its windings become heated, and the 

 moisture in the insulation of the windings is expelled. 



The following modification of the experiment just given 

 is interesting as a further illustration of the relation between 

 the no-load current and periodicity of a transformer. 



EXPERIMENT XXVI. DETERMINATION OF NO-LOAD 

 CURRENT AND WATTS WITH VARYING FREQUENCY. 



DIAGRAM OF CONNECTIONS. 

 As for Experiment XXV., page 141. 



Instructions. Make connections as for Experiment XXV., 

 connecting the circuit to an alternator, which can be made 

 to supply current of varying frequencies, while maintaining 

 the terminal voltage of the transformer constant, either by 

 adjustment of a resistance in series with it or by regulation 

 of the alternator fields. 



Measure the current and power supplied to the trans- 

 former at various frequencies on open circuit at constant 

 voltage. Enter the results in columns with suitable headings, 

 and plot on a frequency base curves of current and power 

 supplied. 



