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ELECTRICAL MACHINERY 



to 109 volts. Such a fluctuation in the line voltage is 

 prohibitive if lamps are connected to the line and even if 

 there is no lamp load to suffer, synchronous motors or 

 synchronous converters are likely to be disturbed in their 

 operation and may even pull out of step if the fluctuation is 

 excessive. 



Low-starting Torque. In addition to these bad features 

 of the squirrel-cage rotor, we have the additional one that 

 the starting torque is very small, generally but a small frac- 

 tion of the full-load torque. 



Rotor Circuit with Variable Resistance. A high-resistance 

 rotor would have, when starting, none of the bad features 



123456 

 Resistance of rotor circuit 



FIG. 223. Curves Showing the Effect upon the Starting Character- 

 istics of Varying the Resistance in the Rotor Circuit. 



mentioned above, but the efficiency of a motor with a high- 

 resistance rotor is necessarily low and the speed regulation 

 is poor. The effect of rotor resistance upon the starting 

 characteristics of a motor is shown in Fig. 223. These 

 curves were obtained by a laboratory test of a small motor 

 having a wound rotor. Various resistances were inserted in 

 the external circuit of the rotor and the different quantities 

 measured. The rotor itself had a resistance of .5 ohm, 

 so that the quantities could not be measured for a rotor 

 circuit resistance less than this, but the curves were extra- 

 polated, as shown by the dotted lines. 



Effect of Rotor Resistance upon Starting Characteristics. 

 It is seen from these results that, as external resistance is 



