OPERATION OF SYNCHRONOUS MOTORS. HUNTING 107 



FIG. 50. 



like the armature of an induction-motor. In that case, the motor 

 starts normally, and with a current which depends on the resistance. 

 In any case this current is higher than in an induction-motor because 

 the air-gap is greater and requires a stronger magnetizing current. 

 For this reason, it is well to start the motors without load, in order 

 avoid excessive starting currents. 

 The production of the revolving field results from the eddy currents, 



it in the absence of these, the pole- 



;ces, even when laminated, give 

 to hysteresis effects which are 



ften sufficient for starting the motor 



without load. In certain motors 



iving laminated poles the starting 



irrent is about double the normal 



11-load current. 

 This effect is explained by the 



ict that, during the rotation of the 



sld, there is a stronger attraction 

 the direction of motion than in 



ic opposite direction owing to the lag of the magnetization behind the 



langes of field producing it. 



Synchronism. In proportion as the armature-speed increases, 

 the magnetic pulsations produced by it in the pole-pieces become less 

 numerous, as can be seen on connecting a lamp to the field-circuit 

 and noting its variations of brightness. It is well to provide a centrifugal 

 regulator which connects this lamp in circuit only when the speed 

 approaches synchronism, because, at lower speeds, it would be sub- 

 jected to excessive voltage. When synchronism is almost attained 

 (the speed generally remains slightly lower,) the fields are excited 

 when the phases come into opposition, the motor falls into step, and 

 the current immediately diminishes, owing to the disappearance of the 

 reactive current which was absorbed up to that time. The more care- 

 fully the time has been selected for closing the excitation-circuit, the 

 more easily the motor will fall, into step. It is well, as a rule, to include, 

 in the circuit, a variable self-inductance, which has the effect of pre- 

 venting excess of current and of damping objectionable harmonics. 



When once the motor is in synchronism, it can be loaded progres- 

 sively, by shifting the belt from the idler to the driving pulley. This 

 should be done very carefully when the mechanical resistance (load) 



