138 SYNCHRONOUS MOTORS 



angle when running. One of the alternators is driven by a belt run 

 over its driving pulley, and it drives the other, mechanically through 

 the dynamometer, as well as electrically through the circuit. The 

 alternator which is belt-driven acts as a motor, the other acting as a 

 generator; the torsion-dynamometer connected between the two 

 machines measures the excess of power consumed by the generator 

 over and above the energy recovered. The power applied at the 

 pulley represents the total losses of the two machines operating as a 

 power-transmission system. From these data the efficiencv of trans- 

 mission may be deduced. 



Suppose that, for each phase-angle, the excitation-current and the 

 line-current are read, by means of an electrodynamometer E, also 

 that the voltage at the terminals is read, by means of a volt-meter, 

 and that the electric power is read by means of a watt-meter inserted 

 in the circuit, with its shunt-coil connected to the terminals. 



By comparing this "actual" power with the "apparent" power, 

 the phase-angle between the internal E.M.F.'s E\ and 2, is obtained 

 substantially. The magnitudes of these E.M.F.'s are known from the 

 excitation-curves, which are drawn once for all. This method is 

 particularly simple when no transmission-dynamometer is used, when 

 the loads are perfectly steady; but the loads are not then obtained in 

 terms of the mechanical power of the motor. 



When it is impossible to couple the two machines rigidly, or when 

 they do not have the same number of poles, it is not so easy to determine 

 the phase-difference or lag between the two motors, and it is then neces- 

 sary to proceed in one of the two following ways: 



(1) It is possible to mount on the shaft of the machines some small 

 alternators serving as " tell-tales." They are arranged in such man- 

 ner that their E.M.F.'s synchronize in phase with the E.M.F.'s induced 

 when running without load. A triple oscillograph record will then 

 give, directly, for each load, the phase-angles between these E.M.F.'s 

 and the current. 



(2) When the machines have not the same number of poles the 

 phase-angles between the induced E.M.F.'s can be measured mechan- 

 ically by means of the ingenious phase-indicator of Bedell and Moler. 

 The two machines are placed end to end, with their shafts almost 

 touching. The phase-indicator consists of two metallic disks 23 

 centimeters in diameter and 8 millimeters thick, each fastened to one 

 end of the shaft and provided with curved slits symmetrically disposed 

 in the form of an Archimedean spiral. (Fig. 70.) The disks are 



