THE INDUCTION 



255 



starting. In order to obtain break-down torque at starting, the 

 rotor resistance per phase, /._,, should be approximately equal to 

 the rotor reactance per phase at standstill, Xo. 



An adjustable resistance cannot be readily placed in the 

 squirrel-rage rotor, so that three-phase rotors requiring external 

 ance are usually wound cither two-phase or three-phase. 

 The two-phase windings may be connected either star or mesh 

 and the three-phase windings may he connected either Y or 

 delta. Such rotor windings arc in every way similar to Btatoi 

 windings. The three ends of the three-phase winding are 



Botor Shp-rtui, 



Botor Bheoitat 

 I [<,. L'1.. 1 < lonneotioni for a wound-rotor induction motor. 



brought out to three slip-rings, as shown in Figs. 241, 242, and 

 248. Mru-hes. U-aring (.n each of these three rinn's. Fii:. 'Jl'J. 

 connect to Y-connected external resistances, usually through a 

 controller. The entire resistance <>f each phase is in circuit on 

 starting. Thi> causes the rotor current to be more nearly in 

 phase with tin- air-gap flux, so that a large torque i> ob- 

 taim-d with a moderate value of current. In addition to pmdur- 

 g(M)| starting t<rqur, th<- starting eurn-nt of I he motor 

 does not gi-eatly -xc'-d the rated <Mirrenf. As the nminr conies 

 up to ipeed, the external re>istance is CUt out. The motoi- tlien 



on ourve I 10. 



n without the controller, the wound-rot or type of mo' 



6 than the sipiirn-l-cage motor, due to the greater 



