THE INDUCTION MOTOR. 



267 



started by inserting resistance in the rotor windings, as explained 

 in Art. 123 (Fig. 224). The advantages of using a high resist- 

 ance in the rotor windings at starting are (a) that the starting torque 

 is thereby increased and (&) the stator currents at starting are 

 reduced in value. 



(c) The starting compensator 

 consists of two or three auto- 

 transformers (two for two- 

 phase supply and three for 

 three-phase supply) with 

 switches arranged for conve- 

 niently supplying the motor 

 with low voltage polyphase 

 currents at starting, which 

 voltage is increased in steps 

 as the motor speeds up, until 

 finally the full supply-voltage 

 is connected to the motor. 

 The objection to the use of 

 the starting compensator is 

 that the starting torque is very 

 small when the voltage is 

 small ; indeed, the starting 

 compensator can scarcely be 

 used in the case of a motor 

 which is to be started under 

 load. The advantage of the 

 starting compensator is that 

 the motor takes very much 

 smaller currents from the sup- 

 ply mains at starting than it 

 would if connected directly to 

 the supply mains. 



The starting compensator is nearly always used when it is de- 

 sired to start a polyphase alternator or a polyphase synchronous 



Tnree-phase Starting Compensator. 

 Fig. 237. 



