THREE-PHASE CURRENT MOTOR. 



84. The ordinates of the full line curve in Fig. 24 measure the output 

 of the motor for an armature of small resistance, provided with ex- 

 ternal starting resistance. 



85. The ordinates of the broken line curve measure the output of the 

 motor having a squirrel-cage armature with considerable resistance 

 six times larger than that of the armature with external resistance 

 in order to start under load. A glance at the curves shows the tre- 

 mendous reduction of the output with which we have to buy the ad- 

 vantage of dispensing with slide rings. This is even more con- 

 spicuous in the curves of Fig. 25. The abscissae here represent the 

 energy in kilowatts consumed by the motor, while the ordinates rep- 

 resent the field current, the output, the efficiency, and the power factor 

 as functions of the energy consumed. As in Fig. 24, the full line 

 curves represent the different quantities for the motor with external 

 starting resistance, while the broken line curves show these quantities 

 for the motor with squirrel-cage armature. 



86. The field current, of course, remains unaltered, and so does the 

 power factor. But the output, and therefore the efficiency, of the 

 motor with squirrel-cage armature are powerfully influenced by the 

 high resistance of the armature. For a starting torque equal to the 

 torque at normal load, we need a current which is almost four times 

 as large as the current at normal load ; indeed, a poor result. 



87. To emphasize this point once more, the abolition of an external 

 starting resistance either revolving with the armature, or placed 

 outside the armature and being connected to it by means of slide 

 rings brings with it inevitably a considerable reduction of the out- 

 put, and a lowering of the efficiency. The reduction of the output, as 

 a glance at Figs. 24 and 25 shows, is equivalent to a reduction of the 

 overloading capacity of the motor, diminishing the margin of power 

 and thus tending to pull the motor out of step at a small temporary 

 overload. For these reasons European designers have abandoned 

 the squirrel-cage armature in all larger motors. 



49 



