Ac Shunt Relay
adjustment sets the range of operating values. Usually,
the operating adjustment is the only one required.
An ac shunt relay is illustrated in figure 2-24. The
basic function of the relay is to make or break an
electrical control circuit when the relay coil is
Power relays, also known as contractors, use a
energized. To do this, voltage is applied to the
relatively small amount of electrical power to control
operating coil, 2 (connected across the line), which
the switching of a large amount of power. The relay
attracts the armature, 3. When the armature is pulled
permits you to control power at other locations in the
down, it closes the main contacts, 4.
equipment, and the heavy power cables need be run
The pull-in and dropout current values may be
only through the power relay contacts.
adjusted. In figure 2-25 the various adjustment points
Only lightweight control wires are connected
of the ac shunt-type relay are indicated. The spring, A,
from the control switches to the relay coil. Safety is
and the setscrew, E, control the pickup and dropout
also an important reason for using power relays, since
values. Before the relay is adjusted, the screw, F,
high-power circuits can be switched remotely without
should be set to clear the armature when the armature
danger to the operator.
is in the closed position. The pull-in value can be
raised by increasing the spring tension or by
MAINTENANCE OF RELAYS
increasing the armature gap.
Relays are some of the most dependable
electromechanical devices in use; but like any other
mechanical or electrical device, they occasionally
wear out or become inoperative.
The series-type relays are operated by circuit
current flowing through the coil or coils. This feature
Relay contact surfaces must be kept clean and in
makes it possible to use the relay as a field failure relay
good operating condition. Contact clearances or gap
or for any application where the relay operation is in
settings must be maintained according to the relay's
response to changes in circuit current flow.
Under normal operating conditions, most relay
There are two adjustments on the two-coil relay.
contacts spark slightly; this will cause some minor
One adjustment sets the difference between the
burning and pitting of the contacts.
opening and closing current values. The other
The buildup of film on the contact surfaces of a
relay is another cause of relay trouble. Carbon
buildup, which is caused by the burning of a grease
film or other substance (during arcing), also can be
troublesome. Carbon forms rings on the contact
Figure 2-24.--Ac shunt relay.
Figure 2-25.-Adjustment points of an ac shunt relay.