THE I\Dl'( "T/o.v MOTt)R 265 



connecting the motors in concatenation u filing is equal to that of 

 a 2-1 -pole motor, or a motor whose poles are equal in number 

 to the xu m of the poles of the two individual motors. When the 

 two motors are connected in opposition, the resulting synchron- 

 ised is equal to that of a 16-pole motor, or a motor whose 

 jxiles are equal in number to the difference of the poles of the two 

 individual motors. 



It will be recognized that the concatenation method of speed- 

 control is very similar to the series-parallel method of speed 

 control for direct -current motors (see Vol. I, page 345, Par. 223). 

 In concatenation, at starting and for intermediate speeds, resist- 

 ance is introduced in the rotor circuit of the second motor. 

 When the motors are connected in parallel across the line, resist- 

 ance is introduced in each rotor circuit and is gradually cut out. 

 Motors operating in concatenation are used abroad to some 

 extent, particularly in railway work. Because of its rather 

 complicated connections, this system of speed-control is not used 

 to any extent in this country. 



110. The Induction Generator. If an induction motor be 

 driven above synchronous speed, the slip becomes negative. 

 The rotor conductors then cut the flux of the rotating field in a 

 direction opposite to that which occurs when the machine operates 

 as a motor. The rotor currents are then reversed with respect 

 to the direction which they had when the machine operated as a 

 motor. By transformer action these rotor currents induce cur- 

 rent- in the stator which are substantially 180 out of phase with 

 the energy component of the stator current which existed when 

 the machine operated as a motor. 



The induction motor, therefore, can be used as a generator, 

 but it has certain limitations which the synchronous alternator 

 docs not possess. 



The machine does not have a definite speed for a given fre- 

 quency as the synchronous alternator has, but the speed with 

 iiit t're(|uency varies with the load. The load is practically 

 proportional tothe>]ip. Because its speed is not in synchronism 

 with line frequency, the machine is often called an asynchronous 

 .itor. The frequency and voltage of the induction 



erator are tlmt of the line to which it is connected, irrespective of 



