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ELECTRICAL MACHINERY 



10% of synchronous speed. The difference between the 

 field speed and the rotor speed (i.e., the slip) increases 

 rapidly with overloads and, if the motor is loaded too much, 

 it stops altogether and is said to be " stalled." Under 

 such conditions the circuit breakers (or fuses) in series 

 with the motor open the circuit and the load must be 

 removed, the circuit closed, and the motor again started. 

 This maximum load point is generally 75 or 100% greater 

 than the rated load of the motor; it varies with different 

 motors, but should always be considerably greater than the 



234 

 Output in H.P. 



FIG. 225. Same as Shown in Fig. 224 but Some Resistance has been 

 Added to the Rotor Circuit. 



rated load, otherwise the motor is likely to be " stalled " 

 frequently. 



97. Effect of Rotor Resistance upon Operating Charac- 

 teristics. The motor whose characteristics are shown in 

 Fig. 224 was equipped with a slip-ring rotor. For the 

 purposes of the test the rotor circuit resistance was increased 

 by inserting extra resistance between the brushes on the 

 slip rings and the motor characteristics were again obtained. 

 The results are given in Fig. 225. They show that an 

 increase in rotor resistance produces, for a given load, an 



