THE SYNCHRONOUS MOTOR. 153 



chronous motor increases and decreases, the torque exerted on 

 the armature by the field magnet increases and decreases, this 

 increase and decrease of torque is accompanied by a shifting of 

 the magnetic flux backwards and forwards over the pole faces, 

 and this shifting flux induces current in the short-circuited bars 

 of the damping frame, which current opposes the shifting of the 

 flux and thereby dampens the hunting oscillations. 



A synchronous motor with a belt load is much less prone to 

 hunting oscillations than a rotary converter or a synchronous 

 motor directly connected to a generator, for the reason that the 

 slipping of the belt dampens the hunting oscillations like a loose 

 fly-wheel. 



When the generator which supplies current to a synchronous 

 motor is driven by an engine the slight pulsations of speed of the 

 engine tend to set up hunting oscillations, especially if the fre- 

 quency of the hunting oscillations is the same as the frequency 

 of pulsations of the engine speed. In such a case troublesome 

 hunting may be avoided by increasing the moment of inertia of 

 the armature of the synchronous motor by attaching a flywheel 

 to it. The effect of moment of inertia upon the hunting frequency 

 is shown in the following discussion. 



Theory of the hunting of the synchronous motor. When a synchronous motor is 

 running steadily it takes in power steadily from the mains and gives out power steadily 

 on its belt (or from its direct-current commutator in case of the rotary converter). 

 The pulsations of power intake due to the alternations of electromotive force and cur- 

 rent are extremely rapid in comparison with hunting oscillations and need not be con- 

 sidered, indeed these pulsations do not exist in case of polyphase machines. 



The mean position, at a given instant, of the armature of a synchronous motor 

 which is hunting is the position it would have at that instant if it were turning at a 

 constant angular velocity. When the motor hunts its armature oscillates forwards 

 And backwards through its mean position. 



When the armature is in its mean position the power intake of the motor and its 

 belt load (including friction losses and the like) are equal, and no unbalanced torque 

 acts on the armature. 



When the armature gets ahead of its mean position its intake is lessened, the belt 

 "load of the machine, which is assumed to be constant, exceeds the intake, and an un- 

 balanced retarding torque acts on the armature. 



When the armature falls behind its mean position its intake exceeds its belt load, 

 and an unbalanced accelerating torque acts on the armature. 



