THE INDUCTION MOTOR 345 



increased slip, a decreased efficiency, and an increase in 

 current. Also, the maximum load obtainable from the 

 motor is now only 6 hp., whereas with no extra resistance 

 it was nearly 10 hp. 



If still more resistance were added to the rotor circuit, 

 the speed and efficiency for a given output would be still 

 further decreased. If too much resistance is put in the 

 rotor circuit, very little output can be obtained from it; 

 practically all of the power which goes into the stator of the 

 motor is used up as heat in the rotor circuit resistance grids. 



By using a motor with a wound rotor and inserting 

 external resistance, it is possible to obtain good starting 

 characteristics, i.e., low current, high torque and power 

 factor. As the rotor speeds up the external resistance 

 is gradually cut out, and the motor is normally operated 

 with no external resistance in the rotor circuit. Thus the 

 running characteristics are similar to those of the squirrel- 

 cage motor and the starting characteristics are much better 

 than those of the squirrel-cage rotor. 



98. Speed Control. Several methods have been 

 designed for varying the speed of an induction motor for a 

 given output but none of them are very successful; the 

 induction motor is essentially a constant speed motor. 



Variation of Rotor Circuit Resistance. The first method 

 consists of inserting resistance in the rotor circuit. We 

 explained in the last paragraph that this will cut down the 

 rotor speed, for a given load, but that it results also in a low 

 efficiency and a low " pull out " point. Hence the rotor- 

 resistance control scheme can be used economically only 

 during the time the motor is being started; in this way it 

 is similar to the resistance used for the control of series 

 c-c. railway motors. 



Power Supply of Several Frequencies. It was at one 

 time thought feasible to have lines of two or three frequencies 

 and to switch the motor from one line to the other as a 

 change in speed might be desired. We have shown that 



