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.