THE INDUCTION MOTOB. 393 



of the rotor resistance. The value of the slip at which it 

 occurs is determined by the rotor resistance. 



The starting current is thus proportional to the rotor 

 resistance, as is also the slip for a given torque when 

 under load. 



This is discussed more fully in connection with the next 

 experiment. 



Effect of Increase of Resistance in Rotor Circuit. The next 

 point to be considered is the effect of increasing the 

 resistance of the rotor. This is best studied by using a 

 motor with wound rotor and slip rings, so that a variable 

 resistance can be put in series with each phase of the 

 windings. It is, of course, necessary to make the resist- 

 ance of all the phase circuits equal. For the purpose of 

 the experiment, the usual three-phase starting resistance 

 may be conveniently employed, provided that its carrying 

 capacity is sufficient to enable it to carry continuously 

 the heavy currents induced in the rotor when the motor 

 is overloaded. 



EXPERIMENT LII. TEST OF AN INDUCTION MOTOR WITH 

 VARIABLE EESISTANCE IN THE ROTOR CIRCUIT. 



DIAGRAM OF CONNECTION. 

 As for Experiment L., Fig. 181, page 370. 



Instructions. The experiment may be carried out exactly 

 as described in the last case, but with a series of different 

 values of the resistance in the rotor circuit. The series 

 of curves are then plotted, each for a different value of 

 the rotor resistance. From these curves a fresh curve 

 may then be obtained, showing the variation of the 

 primary current, torque, efficiency and power factor with 

 the slip. 



For the particular purpose of determining the effect of 

 the rotor resistance on the behaviour of the motor, it is, 

 however, sufficient to take measurements only of the 

 stator and rotor currents, slip and torque, for each of a 

 series of values of the rotor resistance. Complete sets of 

 readings of each of these quantities should be taken and 

 entered up as shown for the previous test, curves being 

 then plotted on a torque base as before. 



A set of torque-slip curves obtained in this way is 

 shown in Fig. 190, 



