THE CONTINUOUS CURRENT MOTOR 181 



transition points the tractive effort on the car, and hence 

 its acceleration, is much decreased. On points 5 and 6, 

 Fig. 108, the tractive effort will evident ly be less than that 

 on point 4 because on these points only one motor is active; 

 the other has no current flowing through it. The accelera- 

 tion of a car equipped with this system is somewhat uneven. 



The Bridge System of Control. In modern railway 

 equipments a different set of connections are used in accel- 

 erating the car, which is known as the bridge control. 

 A diagram of the connections used in this scheme is shown 

 in Fig. 109. Here T stands for a trolley connection and 

 G for a ground connection. 



At first the two motors are connected in series with all 

 the resistance and there are three steps in cutting out 

 this resistance as before. When the " bridge " is put in 

 the first running point is reached. Next come the transi- 

 tion points, on the first of which the connection A is removed, 

 in the second, connection is made to the trolley and ground 

 as indicated, and in the third, the bridge connection is 

 removed. 



During this transition period each motor continues to 

 carry the same current as it did on running point 4. The 

 starting resistances are so designed that when the bridge 

 is removed no sudden change takes place in the current 

 through the two motors. Then on points 5, 6, and 7 the 

 resistance is again cut out, and in point 8 the second running 

 speed is reached. All modern railway equipments are 

 being furnished with this bridge system of control as it 

 gives a more uniform acceleration than the older method 

 shown in Fig. 109. 



Possibility of Field Weakening. If the motors are 

 equipped with commutating poles, two other running points 

 may be obtained. After point 4 has been reached a resis- 

 tance is shunted across each motor field, thus weakening 

 the field and increasing the motor speed and giving what 

 we may call running point 4a. In changing from point 4 



