SYNCHRONOUS MOTORS. 283 



alternating current, and the "hunting " of these machines 

 is mainly due to the want of stability of the conditions 

 under which such machines work, as shown in the curves 

 in Figs. 130 to 132. 



Effect of Change of Load at Constant Excitation. Having shown 

 the effect of an alteration in exciting current at various 

 constant loads, it is of interest to ascertain how a change 

 of load will affect the performance of a motor excited with 

 a constant current. This forms the subject of the next 

 experiment. 



EXPERIMENT XLIII. DETERMINATION OF EFFECT OF 

 VARIATION OF LOAD UPON A SYNCHRONOUS MOTOR HAVING 

 CONSTANT EXCITATION. 



DIAGRAM OF CONNECTIONS. 

 Same as Fig. 129 for Experiment XLII. 



Instructions. Make the same connections as described for 

 Experiment XLII., page 271. After exciting the field 

 and synchronising the motor, close the main switch in the 

 armature circuit, and disconnect the starting motor. 

 Vary the excitation until the value is found corresponding 

 to minimum current when unloaded. Keep this value of 

 the excitation constant, and gradually load the motor by 

 a brake, or by making it drive a generator and increasing 

 the output of this machine. For each value of the load 

 take readings of the current and watts supplied to the 

 motor. Bead also the terminal voltage, which should be 

 kept as constant as possible. 



A series of similar readings should then be taken for 

 several different values of the excitation both above and 

 below the most favourable value, the excitation being kept 

 constant through each series. 



The results should be entered in tabular form. The 

 table given for the preceding experiment gives suitable 

 headings. Three sets of curves should be plotted from 

 the results, load being measured horizontally in each case 

 The curves should show armature current, watts input, 

 and values of cos < respectively, plotted vertically. 



The curves in Figs. 133 to 135 show the results of a test, 

 made on the same motor as that from which the curves in 

 Figs. 130 to 132 were obtained. The curves were taken for 



