Fig. 27.
Diagram of Connections
for
L2 Controller
Reversing is accomplished by one-quarter revolutions to the right and left of the segments shown. It is evident that this will connect either A1 or A A1, to the trolley. And likewise connect the other armature leads.
Reversal. The reversing handle and the main controller handle are made interlocking so that the motors cannot be reversed without first throwing the controller to off position. This is to prevent damage to the motors through careless or inadvertent throwing of the reverse handle when the controller is on some of its higher points. Such a reversal would cause an enormous current to flow through the motors, and would be likely to damage them and to open all the circuit breakers and fuses in that circuit. The reason for the enormous flow of current is, of course, that the counter-electromotive force of the motors, when reversed with the car going at some speed, would materially add to the electromotive force of the trolley line, instead of opposing it as when the cars are in operation. The current flowing through the motor circuit would then be equal to (electromotive force of line + electromotive force of motors) ÷ (resistance of motors), which would result in a very large current.
Magnetic Blow-Out. On the Type K controller as well as on most other successful controllers, the flashing or arcing between contact rings and fingers, which occurs when the circuit is broken, is materially reduced by a magnet that produces what is called the magnetic blow-out to extinguish the arc. This magnet derives its current from the main circuit, and is so arranged as to create a strong magnetic field in the neighborhood of the place where the arc is formed. [Fig. 21] shows a Type K controller open with the magnetic blow-out magnet thrown back on a hinge. The coil which produces this magnet is seen in the right side of the controller. The main contact drum is in the middle, and the reversing drum at the right hand. There are in use a number of other controllers built upon these same general principles but differing in mechanical arrangement.
Controller Notches. All controllers are provided with some device which prevents the motorman from stopping the controller handle between the various points or notches, as the stopping between points might result in drawing an arc or an imperfect contact. The most common arrangement to prevent this is a notched wheel on the controller shaft, against which bears a small wheel of just the right size to enter the notches. The small wheel is held against the notched wheel by a strong spring. As the tendency of the small wheel is to seek the bottom of the notches, it is difficult to stop the controller handle anywhere between notches, and the motorman is thus given a guide which tells him without any effort on his part just where the notches are.
To prevent advancing the controller handle too rapidly and avoid the jerking of passengers, excessive currents and slipping of wheels during acceleration, several devices have been planned. On the multiple unit control systems, a limit switch is usually provided which prevents the controller advancing when the current exceeds a predetermined amount. A device to accomplish the same results on the K type of controllers is termed the Automotoneer. A cam connected with a dash pot prevents movement of the controller handle to the successive notches faster than a previously prescribed rate.
A switch is usually provided in a controller, for cutting out of service one motor or a pair of motors if defective, and allowing the car to proceed with the good motor or motors.
Fig. 28a. Car Wiring for G. E. Train Control System.
