52 SYNCHRONOUS MOTORS 



the excitation of the motor (even if it is compounded) has not time to 

 vary. It can, therefore, always be assumed that the motor has a constant 

 induced E.M.F., and the results of experiments made with such 

 motors can always be used as the starting point. 



On this basis, the stability of operation can be calculated in a 

 sufficiently precise manner by simply comparing the effective load of 

 the motor at the maximum power, P m , which it might develop with 

 its E.M.F. 2, and the power, P? max which is given by eq. (22). 



[This value can be determined experimentally by the method set 

 forth in the author's pamphlet on the " Coupling of Alternators ", 

 Paris, 1894, p. 4-] 



Experience shows that it is possible to have stable operation so long 

 as the power P% does not exceed about two-thirds of P m , when the 

 apparatus driven by the motor constitutes a very constant load, or 

 about the half of P m , when the load is .somewhat irregular. [It is 

 assumed, of course, that the apparatus can withstand the corresponding 

 current without overheating.] 



These figures have been verified by various experiments and they are 

 in accordance with the results published by other experimenters. On 

 analyzing the power transmission at Cassel (see La Lumiere Eleclriqtie, 

 Vol. XLV, p. 616) where the motors furnish power for charging bat- 

 teries, we find that the ratio of practical to theoretical maximum 



power is as high as , but this is exceptional. It is safer to assume 

 " 



Variations of Stability with Operating Conditions. It is very 

 evident that, all things being equal, the stability will decrease with 

 the load. To see how it varies with the conditions of the circuit 

 and of the motor-excitation, when the power P^ and the voltage E\ are 

 constant, it is only necessary to study the variations of the power P m , 

 by means of equations (22). With equal resistance it presents a 

 maximum of maximums 



whenever E\, E 2 > Z, and f satisfy the condition 



E l E,Z 

 E 2 =- - = 5-. . . . . . . (24) 



2 COS f 2R 



