SYNCHRONOUS MOTORS. 



273 



will be necessary to adjust both the speed and excitation of 

 this generator as the current taken by the motor varies. 



After taking a complete set of readings as described with 

 the motor unloaded, several similar sets of observations 

 should be taken with the motor loaded, the load being kept 

 constant for each complete series of readings. The load 

 may be applied either by a band or other form of brake, 

 on the motor pulley, or the motor may be made to drive a 

 dynamo, the current of the dynamo being kept constant during 

 each set of readings, and varied when a change of load is 

 desired. If the efficiency of the dynamo is known, the actual 

 load on the motor can at once be determined by measuring 

 the output of the dynamo. If the efficiency is not known, 

 it is sufficient for the purpose of the present experiment to 

 assume an approximate value of the efficiency in order to show 

 the difference between the curves obtained. The direct- 

 current motor employed to start the synchronous motor in 

 the first place may often be conveniently employed as the 

 dynamo for this purpose. 



If the excitation of the direct-current machine is main- 

 tained constant its armature current will be directly pro- 

 portional to the torque exerted by the motor. 



The results should be entered in tabular form as indicated 

 below. 



EXCITATION CHARACTERISTIC OF SYNCHRONOUS MOTOR. 



Synchronous Motor No Type 



Output . . . .h.p. at ... .volts and . . . .revs, per minute. 

 Periodicity. . . .cycles per second. 



The results of each set of readings should be plotted on 

 squared paper, excitation being plotted horizontally and 

 volts and watts vertically, giving one curve of current and 

 a curve of watts for each load applied to the motor (see 

 Fig. 130, page 278). 



