TRANSFORMERS 161 



is replaced in alternating current circuits by the equiv- 







alent expression C = 7= where C is the 



\/ J R 2 +(27T/L) 2 , 



current, E the impressed e.m.f., / the frequency, L the 

 coefficient of self-induction, and R the ohmic resistance. 

 The expression V/^+ (27r/"L) 2 is known as the impedance 

 of the circuit and is denned as the apparent resistance 

 of a circuit containing ohmic resistance and self-induction. 

 The term 2?r/L is called the reactance of the circuit. 



The impedance of a transformer is measured by short 

 circuiting one of the windings and impressing an alter- 

 nating e.m.f. on the other winding, taking simultaneous 

 readings of amperes, volts, watts and frequency. The 

 impedance of transformers should be carefully measured 

 for the following reasons: Transformers operating in 

 multiple divide the load inversely as their impedance 

 voltage; i.e., the transformer having the higher imped- 

 ance will take the smaller part of the load and vice versa. 

 When the transformers of different types are operated 

 in multiple, the impedance of one transformer must 

 sometimes be increased by putting a reactive coil in 

 the secondary circuit and adjusting until the desired 

 impedance is obtained. 



Impedance tests show whether a given arrangement 

 of coils is satisfactory or not. In transformers wound 

 with large conductors the impedance watts will differ 

 from the calculated C 2 R of a transformer wound with 

 small wire. 



The following method should be followed in making 

 an impedance test: Place a thermometer on the coils 

 to obtain the temperatures; make a good short circuit 

 on one winding, using as short a cable as possible and 

 one of ample cross section, so that no appreciable losses 

 will occur. Calculate the full load current by dividing 

 the watts capacity by the maximum voltage of the 

 winding in which the meters are placed. Select suitable 

 meters and make connections as shown in Fig. 46, wiring 

 to the alternator through a suitable transformer. The 

 alternator must be operated at as near normal voltage 

 as possible when the normal impedance reading is taken. 



