134 



SYNCHRONOUS MOTORS 



Figs. 64 and 65 give, for example, these curves obtained by Kolben 

 for an Oerlikon three-phase synchronous motor of 15 H.P. These 



curves show that the minimum current 

 in the armature is obtained with about 

 the same excitation at all loads, and 

 that the efficiency is almost constant 

 within wide limits. For example, the 

 power consumed at full load, to pro- 

 duce 14 k.w., scarcely varies between 

 i and 5 amperes of excitation. These 

 favorable conditions characterize a 

 motor having low armature-reaction, 

 Motors having high armature-reaction 

 give rise to eddy currents in the 



012345 

 Amperes 



FIG. 65. Power-curves for three- 

 phase 30 H.P. synchronous pole-pieces, which involve a consider- 

 motor, run with variable field- , , , 

 excitation. able increase in the power absorbed, 



when the current increases in the 



armature. This peculiarity may be made evident by means of 

 a fourth characteristic curve known as the " characteristic curve of 

 loss by parasite-effect." This 

 curve is obtained by taking as 

 ordinates the power absorbed 

 when running short-circuited, 

 the excitation-currents being 

 taken as abscissae. Figs. 66 and 

 67 show, quite clearly, the differ- 

 ence, in this respect, between a 

 motor having a high armature- 

 reaction and a motor having a 

 low armature-reaction. In the 

 latter the eddy currents, 

 which assume great importance, 

 impart a parabolic form to this 

 characteristic curve. 



f~t.e/a-ExL/-af(.0f7 , Afnperes 



FIG. 66. Curves for 30 H.P. synchronous 



motor having high armature-reaction. 

 A = characteristic curve without load; 

 B = short -circuit current; 

 C= power absorbed when short-circuited. 



Measurement of Efficiency. 

 The most natural method for the 

 measurement of efficiency is 

 the Prony-brake-dynamometer 

 method, used in the same way as for a D.C. motor. The resisting 

 torque produced by the brake is measured, and, at the same time, the 



