p. Wheel Slip Relays.
(2) The ground relay resistor is connected
across the high voltage line, and since the
(1) This locomotive is equipped with wheel
midpoint of the resistor is grounded by the
slip relays. The relays are shown on the
ground relay coil, the maximum voltage to
locomotive wiring diagram (fig. 44) as
ground that can exist under normal
WS1 and WS2, and are connected in a
conditions is but one-half of the line
bridge circuit. One relay is used for each
voltage. If a ground occurs, a voltage will
pair of motors. It is connected so as to
appear across the ground relay coil. If
compare the IR drop across the series
this voltage is 12 volts or greater, the
fields of the two motors. If a pair of
relay will pick up and function as
wheels slip, the motor whose wheels are
mentioned earlier. The magnitude of the
slipping will draw less current. The IR
voltage that appears across the ground
drop across its series field will be less
relay coil is proportional to the distance of
than across the other; the bridge will thus
the ground from center potential and
become unbalanced, and the relay will
inversely proportional to the. resistance
pick up when the unbalanced voltage
of the ground.
equals the pickup voltage of the relay.
n. Ground Relay Cutout Breaker.
Refer to figure 44 for the relay pickup
voltage.
(1) This is a three-pole breaker with two poles
(2) When a wheel slip relay picks up, it
that close in the ON position and one pole
energizes a wheel slip indicating light.
that closes in the OFF position. The
When the wheels stop slipping, the wheel
breaker is thrown to the OFF position to
slip relay will drop out, and the indicating
cut out the ground relay. This disconnects
lamp will be de-energized.
the ground relay coil and lights the ground
indicating light to show that the ground
relay is cut out.
Caution
(2) If only one ground exists, opening the
Due caution should be exercised not
ground relay cutout breaker will open the
to come in contact with the generator
ground circuit, and no damage will occur if
terminals or any part of the main
the locomotive is operated. If, however,
circuit while the engine is in
two grounds exist or a second ground
operation. The voltage employed on
occurs after the ground relay is cut out,
the generator and traction motor
current will flow between the two grounds,
circuits is sufficiently high to give a
serious damage may be done to the
very severe shock.
electrical equipment, and the ground relay
is unable to function and protect the
18. Safety Precautions
locomotive because it has been cut out of
a. Observance of the following simple precautions
the circuit. Therefore, the ground relay
may prevent serious shock or burns:
should be cut out in case of emergency
(1) Do not attempt to make adjustments on
only, and all grounds should be removed
the generator while it is in operation.
immediately.
(2) Do not operate the TRACTION MOTOR
o. Ground Relay Reset.
SWITCHES or REVERSER by hand while
(1) When the ground relay reset button is
the engine is running.
pushed, the ground relay holding coil will
(3) In general, the switch cabinet doors
be de-energized, and the ground relay will
should not be opened when the engine is
drop out, thus allowing normal operation
in operation.
of the locomotive after the ground has
been removed.
b. The electrical circuits which lead to the devices
whereby the operator controls the locomotive carry low-
(2) If the ground relay is reset and an attempt
voltage battery current.
is made to operate the locomotive without
removing the ground, the ground relay will
c. In testing equipment having pneumatic
again pick up as soon as excitation is
apparatus, use compressed air from some external
applied to the main generator.
source or pump up the reservoirs by running engine, but
be certain to shut (down engine before testing circuits.
8
