8-A] OPERATION AND LOAD TEST. 261 



ning. (High resistance in a squirrel cage armature is some- 

 times obtained by making the short-circuiting rings with high 

 resistance.) 



10. The secondary resistance, besides increasing the torque, 

 acts as a starting resistance (14) and serves to reduce the 

 starting current taken from the line, in the same manner as the 

 starting resistance of a direct current motor. 



The secondary resistance may also be used to vary the speed 

 of an induction motor when running, but this is inefficient, as 

 is the case when the speed of a shunt motor is varied by means 

 of a resistance in series with the armature. See 60. 



11. Starting Polyphase Motors. Polyphase induction motors 

 are self-starting and require no special starting devices in order 

 to obtain a starting torque. There is, however, objection to 

 starting a motor by connecting it directly to the line on account 

 of the excessive current which would thus be taken. Small 

 motors say under 5 horse-power may be started without load 

 in this manner, inasmuch as such a motor forms but a small part 

 of the total load of a system and cannot seriously affect its regu- 

 lation. Furthermore, a small motor comes up to speed so quickly 

 that the momentary excessive current does not overheat the 

 motor. All large motors, however, and small motors when 

 loaded, require some starting device to limit the starting current. 



12. Starting Compensator or Auto-transformer. The most 

 common method of starting is to connect the motor to a low vol- 

 tage for starting and to throw it on to full line-voltage after it 

 has reached, say, half speed. The low starting-voltage is obtained 

 by means of auto-transformers connected across the line and 

 provided with suitable intermediate taps, as in Fig. 2, Exp. 5-A. 

 On a 2-phase circuit, two auto-transformers are used, one on 

 each phase. On a 3-phase circuit, two auto-transformers are 

 also used, these being F-connected, as in Fig. 6, Exp. 7- A. The 

 same starting box is, accordingly, suitable for either a 2-phase or 



