162 



COMMERCIAL ELECTRICAL TESTING 



Take ten readings, starting at 50 per cent and raising 

 to 125 per cent full load. Hold the speed constant and 

 take simultaneous readings of amperes, volts and watts. 

 It is essential that the speed be exactly right, since the 

 reactance varies directly with the frequency. This curve 

 should be plotted after the readings are taken (not as 

 they are taken) to see if the curve is smooth ; if the curve 

 is riot smooth, check it at once Plot volts as ordinates 

 and watts and amperes as abscissae. The volt-ampere 

 curve should be a straight line ; the volt-watt curve should 

 be a parabola. (Fig. 47.) In taking the readings, results 

 will be more satisfactory if meters are selected so that 

 no change in them is necessary throughout the curve. 

 On the record sheet, note the temperature of transformer 



Wattmeter Ammeter 



Primary Secondary 

 Fig. 46. Connections for Impedance Test 



coils and constants of all meters used. If a potential 

 transformer is used, record its number and ratio. 



The connections shown in Fig. 46 are used in prefer- 

 ence to those in which the losses of the voltmeter and of 

 the potential coil of the wattmeter are included in the 

 reading of the wattmeter. In Fig. 46 the only extra 

 loss is that in the current coils of the ammeter and 

 wattmeter, and this is negligible. 



A potential transformer or multiplier should be used 

 with a wattmeter when the voltage exceeds 150 volts. 

 It will be noted that the lower binding posts on Thom- 

 son wattmeters must be connected together when 

 neither a potential nor a current transformer is used; 

 and when the voltage of the circuit is above 2000 volts, 

 they should be connected by a small fuse wire. The 

 secondary of the potential transformer should not be 



