INDUCTION MOTOR 



347 



It should be used where frequent starts under load are necessary, 

 or where the motor is large enough to have a bad effect on the 

 regulation of the system due to the large starting current at low 

 power factor, as for cranes, elevators, hoists, etc. 



The squirrel-cage motor with a comparatively high resistance 

 rotor can be used where fairly large starting torque is required 

 and where a wound-rotor motor is not advisable as, in cement mills, 

 etc. 



Fig. 325 shows the characteristic curves of a three-phase, 60- 

 cycle, 220-volt, 20-horse-power induction motor with a squirrel-cage 

 rotor. 



220. Speed Control of Induction Motors. The induction 

 motor is inherently a constant-speed motor. It has been seen 

 above that the speed can be varied by using a wound rotor and 

 connecting resistances in series with the rotor windings, but this 

 is a very wasteful method since the efficiency of a motor is always 

 less than its speed in per cent of synchronous speed. 



To vary the speed efficiently the synchronous speed must be 

 varied. This can be accomplished by means of special windings 

 arranged with different numbers of poles. Such windings are 

 very complicated and, expensive and the number of speeds is limited 

 to three. A squirrel-cage rotor must be used. 



FIG. 326. Cascade control of induction motors. 



A second method of varying the synchronous speed is by con- 

 catenation or cascade control. Two similar motors with wound 

 rotors are rigidly connected to the same shaft. The stator of the 

 first motor is connected to the line; the stator of the second motor 

 is connected to the rotor winding of the first motor and receives 

 power from it; the rotor of the second motor is closed through 

 starting resistances. (Fig. 326.) 



The frequency of the e.m.fs. generated in the rotor of an indue- 



