244 ALTERNATORS. 



The resistance of the armature, ammeter, and connecting 

 leads should be determined by measurement with direct 

 current or other means. To the resistance thus measured 

 should be added a percentage depending on the type and 

 speed of the machine, in order to allow for the power spent 

 in producing eddy currents in the pole faces and armature 

 core. It will be shown later that errors in computing the 

 eddy current losses have a very small effect upon the calculated 

 value of the armature drop. 



Since it is not possible to measure directly the amount of 

 the losses due to eddy currents, they can only be estimated 

 from the results of experience in machines of similar type, 

 unless a special experiment can be made to determine them.* 



It is thus possible to construct a right-angle triangle of 

 electromotive force for the alternator on short circuit. The 

 hypotenuse of the triangle will be the total electromotive 

 force generated, i.e., the open circuit voltage. The two other 

 sides will represent respectively (1) the idle voltage, due to 

 self-induction of the armature and loss of voltage by arma- 

 ture reaction, and (2) the energy voltage spent in overcoming 

 armature resistance and eddy current losses. The voltage 

 (1) will be period in advance of the current in phase, while 

 the energy voltage (2) will be coincident in phase with the 

 current. The angle between the energy voltage and total 

 voltage will give the angle of lag for the circuit. 



An example taken from the measurements plotted in Fig. 

 115 will illustrate what has just been stated. 



The no-load magnetisation curve given in Fig. 92 was 

 taken from the same machine while running at the same 

 speed ; consequently the voltages on Fig. 92 give the total 

 voltages producing the currents plotted on Fig. 115 for the 

 same excitation. Thus with -4 amp. excitation, we see from 

 Fig. 92 that the total voltage generated is 52-3. The arma- 

 ture resistance (warm, and including the short-circuiting leads) 

 was -17 ohm. Hence the energy voltage overcoming resis- 

 tance was 25-8 x -17 = 5-79 volts, the short-circuit current 

 corresponding to this excitation being seen from Fig. 115 

 to be 25-8 amps. In the present case the eddy currents are 



*In small machines with low linear velocity the eddy currents are not likely 

 to affect the behaviour of the machine seriously. They may therefore be 

 neglected in comparison with the. loss in armature resistance, or taken into 

 account by adding a percentage to the losses in the resistance. An approximate 

 rule sometimes used for larger machines is to assume the eddy current losses. 

 to be equal to the armature resistance loss. 



