Various devices are used to change the connections in the circuit. The most common are the
contactors, interlocks, magnet valves, and relays.
a. Contactors are switches used for opening and closing circuits. In circuits where the current is
low, the contactors are magnetically operated and are energized by current from the battery. The magnet
brings a movable contact into touch with a fixed contact in the main circuit; when they are not being
used, a spring pulls the movable contact away. When a contactor is used in a traction motor circuit and
has to handle heavier current, the contact tips must be pressed together tightly. Operated by compressed
air, they are called pneumatic contactors although they use magnetic valves. When the operating circuit
is closed and current flows through the coil, it moves the core by magnetic attraction and this movement
opens the air valve.
b. Interlocks are auxiliary switches designed to control the connecting or breaking of circuits.
Operated by another switch or perhaps by a contactor or valve, interlocks are called either "In"
interlocks, closed when the main switch is closed, or "Out" interlocks, closed when the main switch is
c. A magnet valve consists of a magnet coil and a core acting upon an air valve. The coil is in
the low-voltage control circuit and connected through interlocks, automatic controls, or manual controls
in the same manner as the coil on a contactor. It usually returns to its de-energized position by spring
action. The valve and coil are used in such applications as shutter control, compressor synchronization,
and sanding, as well as in reversers and power switches as a built-in portion of the equipment.
d. A relay, a small magnetic electric switch consisting of a coil and several small contacts,
changes connections in one part of a circuit in response to changes taking place elsewhere in the circuit.
The relay coil may be in either the high-voltage or low-voltage circuits of the locomotive. Operating in
much the same way as contactors, relays are not required to carry heavy current and are generally much
smaller. They may be adjusted to open and close at various voltages to respond to operating conditions.
When the contacts of a relay close, they close the circuit to the coil of the contactor or magnet valve
controlled by the relay. More than one contactor in a magnet valve can be controlled by a relay by
building the relay with several sets of contacts.