208 ALTERNATORS 



If the voltage of excitation is 100, the value of each step would 

 100 100 9-2 



Magnetisation Curves with Inductive Load. Th/> curve 

 discussed in the foregoing paragraph is the magnetisation 

 curve obtained with the machine supplying current to a non- 

 inductive circuit. This curve is the most important of 

 the load curves, which can be obtained from an alternator- 

 In most cases, however, alternators have to supply circuits 

 consisting partly of inductive resistances, and it is consequently 

 important to determine in what way the self-induction of the 

 external circuit affects the behaviour of the alternator. In 

 order to do this, a partially-inductive resistance should be 

 substituted for the non-inductive load resistance employed 

 in Experiment XXXV., and the voltage corresponding to 

 the full-load current at various power-factors and with 

 various excitations should be determined. 



Several curves obtained in this way have been drawn in 

 Fig. 97, and show that the terminal voltage of the alternator 

 is diminished by the lag of the current in an inductive circuit. 



Before explaining the reason for this the method of ob- 

 taining the magnetisation curves for inductive loads must be 

 described. 



The difficulty of getting a complete magnetisation curve 

 on an inductive load arises from the fact that it is in practice 

 very difficult to vary the impedance of the external circuit, 

 and to keep the power-factor of the circuit constant, on 

 account of the difficulty of varying the inductive and non- 

 inductive portion of the circuit simultaneously in the required 

 proportion. In the case of large machines, a single reading 

 often is all that can be obtained on full load at a given power- 

 factor. The nature of the curve in the neighbourhood of the 

 point can then be approximately traced by drawing a portion 

 of the curve parallel to the non-inductive load curve. This 

 often suffices for practical purposes, since the actual variation 

 in excitation when running corresponds to a small portion 

 only of the total magnetisation curve. 



If more complete curves are desired corresponding to 

 several values of the power-factor, the following method 

 may be employed. The description may be most simply 

 given by referring to the readings, actually taken in order 

 to obtain the curves shown in Fig. 97. 



