Electric railways

by American School of Correspondence; James Raley Cravath

MULTIPLE-UNIT CONTROL.

A system called “multiple-unit control” or “train control” has come into use where it is desired to operate motors under a number of different cars in a train; all the motors being controlled from the head of the train or from any other point on the train where the motorman may be stationed.

There are several types of multiple-unit control. In all of them there is on each car a controller of some kind which controls the current flowing to the motors on that car. This controller is operated from a distance by means of electro-magnetic or electro-pneumatic devices controlled by circuits called pilot circuits, which circuits are connected to the motorman’s controller. All the pilot circuits of a train are connected together by means of train plugs which make the connections between the cars. The pilot circuits of each car are connected to a motorman’s controller on that car and this makes it possible to operate the train from any controller.

Sprague Multiple-Unit System. In the earliest form of multiple-unit control—which was that devised by F. J. Sprague—the motors on each car were controlled by an ordinary Type K controller, which had geared to its shaft a small pilot motor. The pilot motor was controlled by the pilot circuits connected with the motorman’s controller.

In the more recent forms of multiple-unit control, the use of main controllers having contact cylinders has been practically abandoned. The contacts are made instead by a number of electro-magnetic or electro-pneumatic contact devices sometimes called contactors.

General Electric Train Control. In the General Electric train-control system each contact for the motor circuits is made by a solenoid magnet which draws together two heavy copper contact fingers to establish the circuit. A magnetic blow-out coil in series with the contact is also provided. The contactors make contact only when energized by a small amount of current from the master or motorman’s controller. In [Fig. 28]a is a diagram of the car wiring for a motor car equipped with this system. The motorman’s controller is a drum controller, but is comparatively small since it has to handle only the small amount of current necessary to operate the solenoid magnets of the contactors. It is evident that by connecting together the pilot circuits, which are connected to the motorman’s controller, so that the pilot circuits will be continuous for the entire length of the train, any number of cars equipped with the train-control system can be operated; and similar contacts will be made by the contactors under all the cars simultaneously, by virtue of the circuits established by the master controller at any platform.

Besides controlling the contactors, the master or motorman’s controller must control an electro-magnetic reversing switch, or reverser, to change the direction of car travel.

The handle of the motorman’s controller is provided with a push button, which must be depressed while the current is turned on. Should the motorman release this push, the circuit through the controller will be opened and all the contactors will fall open. This handle is called the dead man’s handle because it is put there to provide for cutting off the current should the motorman fall dead or in a faint at his post.

The flow of the current in the control circuits, which operates the reverser and picks up the contactors on the several points may be followed in the diagram [Fig. 28]a. With the reverse handle in the forward position and the controller on the first point, current passes from the main circuit through a single-pole fused switch called the control switch and through the auxiliary blow-out coil to a finger bearing on the upper section of the master controller cylinder by which connection is established to the adjacent finger and thence to the reverse cylinder. It leaves this over wire No. 8, passing by way of the connection board and control cut-out switch to the forward operating coil of the reverser, thence through the forward blow-out coil and over wire 81, through the switch underneath contactor No. 2 and to ground G, by way of wire B 2 after passing through the fuse shown. The current through the operating coil of the reverser, having thrown this, the path is changed somewhat. The current then instead of passing from the reverser over wire 81, is conducted through wire 15, through the operating coils of contactors No. 1, 2, 3, and 11 in series, through the switch under contactor No. 12, and to ground through finger 1 of the controller. Contactors 1 and 2 are in multiple and when raised connect the trolley with the contactors controlling the resistance leads. Contactor 3 connects R to the line while contactor 11 places the two motors in series. The motors then operate with all of the resistance in circuit. When contactor 2 raises, it opens the switch immediately below it, making it impossible for the reverse to operate while current is flowing through the motors. On the second notch of the controller an additional path is opened by way of finger 3 of the controller. This path leads from finger 3 through four of the control circuit rheostat coils, through contactor No. 5 and to ground over 32. On the 3rd, 4th and 5th points contactors 6, 7 and 9 respectively are raised. The motors are then in full series. Between the 5th and 6th points all the control circuits are broken preparatory to starting the multiple connections of motors. On the 6th or the first multiple point the ground through finger 1 of the master controller is opened while a ground through finger 3 is established. The current from the reverser then, after raising contactors 1 and 2 as before, instead of passing through contactors 3 and 11, passes through the coils of 4, 12 and 13, through the switch under contactor 11 and to ground over finger 2. Contactor 12 connects motor No. 2 to R₇, while contactor 13 grounds No. 1 motor. The motors now operate in parallel and on successive notches of the controller, contactors 6, 7, 8, and 9 are raised, cutting out all of the resistance. The switches underneath contactors 11 and 12 make it impossible for 11 to raise with 12 and 13 or vice versa. The reason for this arrangement is very evident, as a direct ground for R₇ would result.

The Westinghouse Electro-Pneumatic System of Control. In this system of multiple unit or train control, the current to the motors is supplied through a set of unit switches or circuit breakers which are sometimes placed in a circular case or turret underneath the car and in other cases are ranged in a row under the car. The opening and closing of these unit switches is done with compressed air acting on a piston in an air cylinder. When the circuit is to be closed, compressed air is admitted behind the piston and forces it down against the tension of a seventy-pound spring, and the contacts are brought together. When the switch is to be opened, the air is let out of the cylinder and the spring forces the piston back. The air supply is obtained from the storage tanks of the air brake system. The valve controlling the air supply to the cylinder of each unit switch is operated by electromagnets which derive current from a seven cell, fourteen-volt, storage battery. The small master controller operated by the motorman, makes and breaks the battery connections to the magnets controlling the air valves.

Fig. 28b. Car Wiring for Westinghouse Control System.

An advantage of this over other multiple-unit systems is that by the use of battery current the control system is not disturbed by interruptions of the main supply of current. The chief advantage of this is that it makes it possible to reverse the motors and operate them as brakes in emergencies at all times.

The battery is charged from the main line through lamps as resistance, or may be charged by being connected in series with the air compressor motor.

In the accompanying diagram, [Fig. 28]b, there are two batteries shown which are charged in series with the compressor motor. By means of two double-pole, double-throw switches, first one and then the other battery is connected for charging and for service. The battery is charged in shunt with a resistance and a relay is connected in the circuit as shown, so as to open the battery circuit whenever the current through the motor stops, and thus prevent the battery discharging through the resistance.

The master controller has a double set of segments in order to decrease the length of the shaft. The handle, therefore, is moved only one-sixth of a revolution from off to full speed. The various circuits can be traced by the letters and numbers each wire bears, so that the circuits will not be gone over in detail. The first position of the master controller throws the reverser switch in the proper direction and also closes the main circuit breaker. On the second point the motors are connected in series with all resistance in circuit, and these resistances are automatically cut out one by one. On the next point of the controller the motors are in multiple and the resistances are automatically cut out in a similar manner. The automatic cutting out of resistances is accomplished by a limit switch in conjunction with operating and holding coils on the electro-pneumatic valves. This limit switch is a kind of a relay which has the current from one of the motors flowing through its coil and which acts to open a certain battery circuit which operates the electro-pneumatic valves whenever the current in the motor circuit in question exceeds the amount for which the limit switch is set. The automatic acceleration or cutting out of resistance is accomplished as follows:

Each electro-pneumatic valve has two magnet coils, one of which is an operating coil and the other a holding coil for holding the valve open after it is operated. When first the current flows through a circuit to one of the electro-pneumatic valves, it flows through the operating coil and operates the valve to close the corresponding switch or switches of the main circuit by turning the air into the cylinders. As soon as the main switch is closed, it cuts into circuit the holding coil of its corresponding electro-pneumatic valve and this coil will, with the battery current, hold the switch closed even though the circuit to the operating coil may be opened momentarily by the limit switch as each step of resistance is cut out. This prevents the switches from opening when they are once closed and allows the operating coils to open an air valve each time the current through the limit switch coil falls below the amount for which it is set. The contacts which close the holding coil circuit on each valve whenever a main switch is closed, are called interlocks and are indicated on the diagram.

Fig. 29. Diagram of Electric Heaters.

The main line circuit breaker, which is electro-pneumatically operated, will open automatically on overload and can be reset by the motorman on all the cars of a train by closing a switch located beside each controller.