394 THE INDUCTION MOTOR. 



From such a set of curves, each drawn for a particular 

 value of the resistance, it is useful to plot curves showing 

 the starting currents and torques of the motor with 

 various rotor resistances, i.e., to plot curves of starting 

 current and starting torque on a base of rotor resistance. 



By the starting current and torque are here meant the 

 values of these quantities as shown on the curves when 

 the motor comes to rest, i.e., when the slip is 100 per cent. 



These curves may be obtained more directly by loading 

 the motor with a brake and weights or spring balance 

 until it comes to rest, the current and retarding couple 

 being then observed. By repeating this process with 

 different rotor resistances, the desired curves are obtained. 



A modification of the plan just suggested is to attach 

 a lever to the motor pulley and load this with weights, or 

 by a spring balance until the turning effort of the motor 

 is exactly balanced. 



Fig. 190 shows several curves of torque and slip 

 obtained with different resistances in the rotor circuit. It 

 will be seen that the effect of increasing the resistance is 

 to make the curve slope upwards less steeply on the right, 

 so that for small values of the torque the slip is increased 

 All the curves reach ultimately the same maximum value 

 of the torque so that the maximum torque exerted by the 

 motor is not affected by the rotor resistance. The maxi- 

 mum torque is, however, reached with a different value 

 of the slip in each case. 



One of the most important results to be drawn from 

 the curves is the variation in the starting torque, that is, 

 the torque at still stand, due to a change in the resistance 

 of the rotor. The point at which the curves cut the 

 vertical line, corresponding to a slip of 100 per cent., 

 gives the turning effort of a motor when started from rest. 

 Evidently the greatest starting torque will be obtained by 

 so choosing the rotor resistance, that the curve has its 

 maximum value on this vertical line. In the case of the 

 motor for which Fig. 190 is drawn, this resistance of rotor 

 winding and starter per phase is seen to be about '18 

 ohms. A resistance greater than this would give a 

 maximum torque at a slip of over 100 per cent., that is, with 

 the motor rotating in a reversed direction. Resistances 

 less than '18 ohms enable the motor to exert its greatest 

 turning effort, at speeds intermediate between still-stand 



