Fig. 52. Diagram of Straight Air Equipment.
Compressors. A small air compressor driven by an electric motor is frequently employed on electric cars to keep the storage reservoir of the car supplied with air. These air compressors are carried under the car or in the motorman’s cab. They are generally arranged with an automatic device which closes the motor circuit and starts the motor as soon as the air pressure falls below a certain amount; and the motor will continue in operation pumping air until the pressure rises to the amount for which the automatic device is set. The pressure carried in the storage reservoir is usually from 60 to 90 pounds per square inch, which, as a general thing, is considerably more than is required to apply the brakes hard enough to slide the wheels.
Automatic Governor for Air Compressors. Automatic governors are often installed in connection with air compressors in order that a fairly even air pressure may be maintained in the storage reservoir. In these the fall and rise of the air pressure within certain limits closes and opens the circuits to the motor. In some styles the air acting on a piston operates the circuit breaker.
The diagram shown in [Fig. 53] shows the principle of the Christensen governor, in which the air pressure is employed to make and break a secondary circuit.
When the pressure in the storage reservoir falls below a predetermined value, the hand of the air gauge makes contact with lug A. This closes the circuit through solenoid No. 1. Lug D, mechanically connected to the armature of the solenoids is pulled in contact with lug C, and this closes the circuit to the motor, and shunts the winding of solenoid No. 1. When the air pressure rises to a predetermined value the hand of the air gauge is thrown in contact with lug B. This energizes solenoid 2 by connecting it across the motor terminals. The armature is pulled to the right and the circuit to the motor is broken. When this is done it is evident that the current through the energized solenoid is broken. It is evident from the description that current passes through the solenoids only during the short periods that the armature is moving from one position to the other and the air gauge never has to break a circuit in which there is an appreciable voltage so that there is no arcing at lugs A and B.
Fig. 53.
A blow-out coil in series with the motor is provided immediately under lug C which extinguishes the arc at that point when the motor circuit is broken.
A Westinghouse air compressor is shown in [Fig. 54].
Storage Air Brakes. The storage air-brake system does not have a small independent compressor on each car, but is equipped with a large storage tank, in which air is carried under high pressure—250 to 300 pounds per square inch. This storage tank is filled at regular intervals when the car passes some point on its route at which a compressor is located. In this case the car is obliged to stop long enough to make connection to the tank of the compressor plant, and to allow the car storage tank to be filled. This operation, however, does not take long. The advantages of the system are a saving of the weight and a saving in the maintenance of a small compressor on each car. From the main storage tank under the car, air is led through a reducing valve to an auxiliary storage tank. This reducing valve allows enough air to pass through to maintain a pressure of about 50 pounds per square inch in the auxiliary storage tank. The auxiliary storage tank corresponds to the regular storage tank on a system employing compressors on each car. The method of operation after the air has entered the auxiliary storage tank is the same as with any air-brake system.