346 



ELECTRICAL ENGINEERING 



'If the load to be started is not very great and a large starting 

 current at low power factor is objectionable, resistance R* is used 

 and the motor starts with full-load torque and draws full-load 

 current. 



This second method of starting requires a wound rotor with 

 slip rings and large starting resistances which is much more 

 expensive than a squirrel-cage rotor. 



For the same line current, resistance starting gives about four 

 times the torque given at reduced voltage. 



219. Applications. The constant-speed or squirrel-cage in- 

 duction motor takes the place of the direct-current shunt 

 motor and has very similar characteristics. It is of much more 

 simple and rugged construction than the shunt motor and the wear 

 and danger due to sparking is entirely eliminated. 



10 



100 150 



Torque Ft. Lb, 



FIG. 325. Characteristic curves of a three-phase, 60-cycle, 

 220-volt, 20-horse-power induction motor. 



It should be used where fairly constant power is required for 

 long periods, where good speed regulation is required, where 

 starting is infrequent and only average starting torque is necessary, 

 where the motor is exposed to dust or to inflammable materials 

 or is not easily inspected. It is suitable for driving line shafting, 

 for high- and low-speed centrifugal pumps, blowers, fans, etc. 

 It must be started on reduced voltage except for the smallest 

 sizes. 



The variable-speed induction motor has a wound rotor with its 

 terminals connected to slip rings so that resistance may be intro- 

 duced to vary the speed or to give a large starting torque. 



