GENERAL PRINCIPLES OF SYNCHRONOUS MOTORS 7 



which supplies it with current, it can be found, by wattmeter measure- 

 ments, that this power changes in sign, i.e., the motor acts as a brake 

 and returns energy to the circuit instead of receiving it therefrom. 



The phenomena become more complicated still on varying the 

 E.M.F. of the motor or of the generator. 



Case of Unequal Electromotive Forces. An interesting and char- 

 acteristic property of synchronous alternating current motors, and 

 which distinguishes them absolutely from direct current motors or from 

 alternating current motors having commutators, is that they can be 

 excited so as to give a voltage greater than that of the supply-circuit. For 

 example, it is possible to feed, from a no- volt circuit, a motor which, 

 driven by belt at the same speed, produces an E.M.F. of 120 to 150 volts 

 at its terminals. But, if the E.M.F.'s are thus unequal, the current pass- 

 ing between the generator and the motor, when the latter is running with- 

 out load, can, instead of being inappreciable, attain a considerable value. 

 Likewise, when the motor is running with load, the current is greater 

 than that which corresponds to the work to be done. The same effects 

 are produced when, instead of giving to the motor an excessive excita- 

 tion, it is given an insufficient induced E.M.F. It is then observed, if 

 the machines are alike, that the potential difference at the terminals 

 assumes a third value, which is the mean of the two E.M.F.'s involved. 



In both cases, the greater the inequality between the two E.M.F.'s 

 the more the current measured will increase, by the change of excita- 

 tion. If we plot a diagram, taking, as abscissa.% the values of the excita- 

 tion of one of the machines, and, as ordinates, the current passing 

 through the circuit, the curve of variation of the latter, as a function 

 of the former, has the form of a V more or less rounded at the bottom 

 (Fig. 6). This form persists, although it may be less marked, when 

 a constant load is placed on the brake. At the same time that the 

 current increases, by reason of an inequality of the E.M.F.'s, it can be 

 noted, by means of an apparatus for indicating phase-difference, that 

 the current undergoes a change of phase, either forward or backward, 

 with respect to the E.M.F. of the motor. This can be expressed in 

 another way by saying that the machine consumes or produces wattless 



current, 1 i.e., current which is "out of phase," being - behind or ahead 



of the E.M.F. This " wattless " current, which has the effect of increas- 

 ing the "apparent" current, is thus named because it produces no 

 work, the load on the brake remaining constant, by hypothesis. 



1 See note at bottom of page 42. 



