308 SYNCHRONOUS MACHINES. [Exp. 



product of current and flux that is, the instantaneous torque 

 is always positive. Both the current and electromotive force 

 change sign when the conductor is between two poles. 



Again, when the current and the electromotive force are not 

 in phase, their instantaneous product that is, the instantaneous 

 power is negative for a part of each cycle; also, since the cur- 

 rent in this case reverses while a conductor is under a pole, the 

 instantaneous product of current and flux that is, the instan- 

 taneous torque is negative for a part of each cycle. Power 



and torque are both seen to be pulsating. 







The pulsating power and torque in a single-phase motor is 

 taken care of by the fly-wheel effect of the moving parts; in a 

 polyphase motor, the phases so overlap as to give uniform power 

 and uniform torque (2, Exp. 6-A, and I, Exp. 7~A). 



9. Hunting. When the armature of a synchronous motor 

 drops back or advances to assume a running position (4) in 

 which it will develop the power demanded by the load, the inertia 

 of the armature causes it to go past the proper running position 

 and then to approach it by a series of oscillations of a definite 

 period which become damped only by losses which they occasion. 



Any variation in the supply current or in the load will produce 

 such oscillations and when the cause is periodic, with a period 

 approximately equal to the natural period of the motor, the 

 effect becomes cumulative and the oscillations become very great. 

 Such a condition of oscillations is called hunting- when very 

 bad it may make it impossible to keep the motor in step. The 

 oscillation of the armature is accompanied by surging of the 

 current and pulsation of the power. In the laboratory this 

 makes testing unsatisfactory on account of the fluctuation of the 

 instruments. 



A remedy for hunting may sometimes be found in an adjust- 

 ment of the prime-mover or its governor, thus changing the 

 period of the cause. 



