. Electric traction and transmission engineering . sults F = ^5=^^(), (4) pi which shows that as Si approaches zero F approaches the limiting speed, 60/ pi + pi Hence the synchronous speed of the two motors connectedin direct concatenation is the same as that of a singlemotor having pi + pi pairs of poles. Two similar induction motors connected in cascadeshare the load with approximate equality; thus the secondmotor utilizes a considerable portion of the energy thatwould otherwise be consumed in the starting resistanceswhen operating at speeds below the synchronous speed ofthe combination.
. Electric traction and transmission engineering . sults F = ^5=^^(), (4) pi which shows that as Si approaches zero F approaches the limiting speed, 60/ pi + pi Hence the synchronous speed of the two motors connectedin direct concatenation is the same as that of a singlemotor having pi + pi pairs of poles. Two similar induction motors connected in cascadeshare the load with approximate equality; thus the secondmotor utilizes a considerable portion of the energy thatwould otherwise be consumed in the starting resistanceswhen operating at speeds below the synchronous speed ofthe combination. At the latter speed, however, the torqueexerted is zero, and with further increase in speed, such as 102 TRACTION AND TRANSMISSION. occasioned by running down grades, the torque becomesnegative and the cascade set operates as a generator, return-ing energy to the line. In most cases of cascade control the motors are dividedinto groups, each of which consists of a main motor and anauxiUary motor, the latter being employed during cascade 7 TROLLED. Fig. 46. operation only. In starting, each auxiliary motor is con-nected in cascade with the corresponding main motor,and the starting resistances in the secondary circuits ofthe former are cut out in successive steps. The cascadeconnection is then broken by short-circuiting the second-ary windings of the main motor through the startingresistances, which are thereafter cut out progressively asbefore. Thus the auxiliary motors are required to operateonly intermittently on a low voltage, and the full-speedpower factor of the main motors is higher than would bethe case if their load were shared with the auxiliary motorsby connecting the latter across the line. Fig. 46 showsa scheme of connections for this method of control. RAILWAY MOTOR CONTROL. 103 36. Controllers. — All types of railway motor controlmust include means for changing the direction of rotationof the motors. A series motor is reversed by interchang-ing the connections of
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