162 TREATISE ON ALTERNATING CURRENTS. 



tional to the resistance per circuit of the rotor winding. Thus, 

 while satisfying the relation (23), both the reactance and resistance 

 of the rotor windings should be small if a large torque is desired. 



At starting, a monophase motor is acting as a di-phase motor, 

 and, by equation (15), the reactance and resistance of the rotor 

 winding should be equal, and both small, to produce a large start- 

 ing torque. If, as is usually the case, the reactance is greater than 

 the resistance, then the maximum starting-torque is obtained by 

 increasing the resistance until it numerically equals the reactance. 

 This can be done by winding the rotor with a tri-phase star wind- 

 ing, terminating in collector rings, and by means of three bushes, 

 inserting resistance temporarily in each circuit. The best resist- 

 ance to insert is that which makes the total resistance per rotor 

 circuit equal to the reactance per circuit. 



The reactance, however, causes the rotor currents to lag behind 

 the E.M.F.s producing them. The lagging components of these 

 currents will tend to demagnetize the stator, and a smaller torque 

 results. 



The insertion of the resistance cuts down the rotor currents, 

 and brings them more and more nearly into phase with the E.M.F.s 

 induced in the rotor. Wattless current, in either the stator or 

 rotor, are useless ; and when occurring in the rotor, are not only 

 useless, but, by their demagnetizing action . on the stator, diminish 

 the torque. The author has devised a means of cutting down the 

 wattless rotor currents without interfering with the energy current, 

 by the insertion of E.M.F.s in the rotor circuits, leading a right- 

 angle before the induced E.M.F.s. There is, however, difficulty 

 in obtaining these counter E.M.F.s in exactly the correct phase. 



The one defect of the method of inserting non-inductive resist- 

 ances in the rotor circuits is that the energy current is cut down 

 simultaneously with the wattless currents. A better result is 

 obtained in polyphase induction motors by connecting the secondary 

 circuits of suitable transformers between the several pairs of brushes 

 of the rotor, the primaries being placed in series with the stator 

 windings. Another method consists in inserting the primaries 

 of a tri-phase transformer in the rotor circuits, while the three 

 secondary circuits of the transformer are short-circuited. In each 

 case the rotor is short-circuited on itself when full speed is attained. 



Another method employed for producing a high-starting torque 

 in induction motors is as follows : Instead of using non-inductive 



