274 SYNCHRONOUS MOTORS. 



The nature of the curves obtained in this experiment 

 will be discussed after a general account of the behaviour 

 of a synchronous motor under variable excitation has been 

 given. 



Behaviour of Synchronous Motors with Variable Excitation. The 

 experiment just described illustrates the most important 

 point in the behaviour of a synchronous motor. The 

 following general reasoning is given as an introduction to 

 the graphical representation of the conditions, which is 

 given later. 



In the direct-current motor the turning effort on the 

 shaft is due to the action of the armature current on the 

 magnetic field, and the magnitude of the turning moment 

 is proportional to the product (armature current x field 

 strength).* Consequently, with a constant load, any change 

 in the strength of the field necessitates an inverse change 

 in the current. This is automatically brought about by a 

 change of speed, which varies the armature back voltage 

 until the current attains the right value. 



Thus in the case of a direct-current motor 



turning effort (field x armature current), 

 when the position of the brushes, and consequently the 

 magnetic axis of the armature, remains unchanged, and 



armature current = p , 



where E = applied electromotive force. 



e = back electromotive force induced by rotation. 

 R = resistance of armature. 



For constant load e varies automatically due to change of 

 speed when the field is changed, so as to maintain the 

 product (field x current) the same as before. 



In the case of an alternating-current synchronous 

 motor the turning effort on the shaft is due to the same 

 causes, and is proportional to the mean force existing 

 between the armature (due to the current in it) and the 

 field. We may, therefore, again in this case write 



turning effort <* (field x armature current), 



but must make the same limitation as before, viz., 

 that the magnetic axis of the armature field produced 



* See Chapter IX. in the author's "Practical Dynamo and Motor Testing." 



