262 INDUCTION MOTORS. [Exp. 



a 3-phase motor. To save losses, the auto-transformers are 

 cut out when the motor is running. The motor starters are 

 often arranged so as to throw the motor step-by-step on suc- 

 cessively higher voltages, thus avoiding too sudden acceleration 

 and rush of current at any one step. 



13. Any auto-transformer serves to step up the current at 

 the same time that it steps down the voltage and so requires less 

 current from the line; thus, if at % voltage* a motor takes a 

 starting current of 90 amperes, the current drawn from the line 

 is only 30 amperes.f The auto-transformer, therefore, not only 

 reduces the starting current taken by the motor itself on account 

 of the reduced voltage but makes a further proportional reduc- 

 tion in the line current. It serves admirably as a motor starter 

 for all cases except those in which a large starting torque is 

 necessary. 



14. Secondary Starting Resistance. When a large starting 

 torque is necessary, it is best obtained by using a phase-wound 

 secondary with additional resistance for starting; see 9a. As 

 the motor speeds up, this resistance is cut out, either all at once 

 or gradually so as to control the acceleration and current of the 

 motor. In motors with revolving secondaries, this extra resist- 

 ance may be contained within the rotor and controlled by a lever 

 bearing against a sliding collar, or it may be external to the motor 

 with leading-in wires connected through slip rings. 



15. Some motors are provided with a centrifugal device for 

 cutting out this resistance automatically when a certain speed is 

 reached. 



16. Starting Single Phase Motors. When supplied with 

 single-phase current an induction motor has no starting torque, 

 although it will run satisfactorily when once started (56). In 

 some cases small motors may be started by hand, but in gen- 



* (i3a). At full voltage the current would be 270 amperes. 

 t ( I3b). If a series resistance were used to reduce the voltage to 1/3, 

 the current drawn from the line would be 90 amperes. 



