TRANSFORMERS 



163 



grounded, however, unless a current transformer is 

 used. The adjacent ends of the current and potential 

 coils are connected to these binding posts and, unless 

 they are connected to the same side of the line, there is 

 danger of breaking down the insulation between the 

 coils and burning out the wattmeter. Above 2000 

 volts the fuse wire is used to avoid electrostatic effects. 



Core Loss and Exciting Current 



When a transformer is connected to a source of alter- 

 nating current, a loss of energy takes place in the iron, 



2 4 & Q 10 1.2 1.4 is, i.Q 2O 22 24 2.6 2.8 3.0 



Fig. 47. Impedance Curve 



owing to cyclic reversals of the magnetic flux. This 

 loss of energy is known as the core loss ; its value depend- 

 ing on the wave form of the impressed e.m.f., a peaked 

 wave giving a somewhat lower core loss than a flat wave. 

 It is not uncommon to find alternators giving such a 

 peaked wave form that the core loss obtained on trans- 

 formers excited by them is 5 to 10 per cent less than that 

 obtained on the same transformers when excited from 

 generators giving a true sine wave. On the other hand 

 some generators give a very flat wave form, so that the 

 core loss is greater than that obtained when sine wave 

 is used. The core loss test is similar to the impedance 

 test, except that voltage is applied to one winding, the 

 other being left open circuited. Voltage should always 



