120 ELEMENTS OF ELECTRICAL ENGINEERING. 



The two motors in series are of course started with resistance 

 in circuit as shown in Fig. 88, and this resistance is slowly cut 

 out in steps as the speed increases. When the speed has reached, 

 say, 10 miles per hour and the resistance is all cut out, the con- 

 nections are quickly changed * to those shown in Fig. 89. That 

 is, the motors are quickly connected in parallel and the starting 

 resistance is connected in series with them. The speed then 

 continues to rise, and as it rises the starting resistance is again 

 slowly cut out. 



The use of two series f motors in this way is called the " series- 

 parallel system of control." All of the successive connections 

 described above are made in the proper order by turning a crank 

 which actuates a spindle upon which metal sectors are mounted 

 as shown in Fig. 90. This figure shows a controller spindle 

 with eleven sectors (some of which are hidden behind the spin- 

 dle) and eleven contact-fingers which touch the sectors when 

 the spindle is turned. The small cylinder at the upper right 

 hand in the figure is arranged to make the necessary changes of 

 connections of the armature terminals for reversing the direc- 

 tion of driving of the motors, by turning the small lever at the 

 top. The large oval structure is an electromagnet, one pole piece 

 of which is spread out in the region behind the controller spin- 

 dle, and the other pole piece is hinged to the core of the elec- 

 tromagnet, no that it may be swung back, as shown in the figure, 



* This change of connections is actually carried out in four distinct steps made in 

 rapid succession as follows : () Some resistance is reconnected in series with the 

 two motors. () One motor, No. 2, Fig. 88, is then short-circuited, and this short-cir- 

 cuit constitutes a connection from motor No. I to the rail, (r) The rail terminal of 

 motor No. 2 is then disconnected, (d] This terminal of motor No. 2 is then connected 

 to the rheostat terminal of motor No. I, giving the connection shown in Fig. 89. 

 During operation (b] motor No. 2 cannot build up as a generator, inasmuch as it is 

 rpnning in the wrong direction. 



f Sometimes four series motors are used on one car. With such a four-motor 

 equipment it is customary to connect the motors as two units, each unit consisting of 

 two motors permanently connected in parallel. These two units are then treated as 

 two single motors, and operated by the series-parallel controllers exactly as explained 

 above ; that is, at starting the two units are in series and at full speed the two units 

 are in parallel. 



