2/2 ELEMENTS OF ELECTRICAL ENGINEERING. 



netism due to armature currents is stationary in space (see Arts. 

 63 and 65). 



If polyphase currents are connected to the armature of a poly- 

 phase alternator or synchronous converter at stand-still, the ro- 

 tating magnetism produced in the armature core acts upon the 

 field structure in the same way that the rotating stator magnetism 

 of an induction motor acts upon the rotor. The torque action 

 between the armature and field of the alternator under the above 

 conditions sets the armature into motion, the armature increases 

 in speed until it is nearly in synchronism, and the slight torque 

 due to hysteresis causes the speed to come up to full synchronism. 



This induction motor action is frequently used in the starting 

 of polyphase synchronous motors and converters. The poly- 

 phase supply voltages are reduced to a fraction of the normal 

 value by means of a starting compensator (see Art. 128), and 

 connected to the armature terminals of the polyphase machine. 

 The armature being entirely free to rotate, speeds up and reaches 

 full synchronous speed as above explained, then the supply voltage 

 is increased to its full value, and the field magnet of the polyphase 

 machine is then excited by closing the field switch. 



The rotating armature magnetism produces a pulsating flux 

 through the field coils of a polyphase machine, which is being 

 started in the manner here described, and this pulsating flux 

 induces a very large electromotive force in the many turns of 

 wire which constitute the field winding. To avoid the rupture 

 of the insulation of the field winding by this large electromotive 

 force, a multi-pole switch is arranged to break the field circuit at 

 a number of points so as to avoid the accumulation of the total 

 induced electromotive force at the terminals of the complete 

 winding where it would endanger the insulation. 



133. The single-phase induction motor. The polyphase induc- 

 tion motor, once in operation, continues to run and carry a mod- 

 erate load when all of the stator windings (phases) but one are 

 disconnected from the supply mains. When so operated, the 

 induction motor is called a single-phase induction motor. The 



