Figure 2-24.--Ac shunt relay.

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

POWER RELAYS

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

Series-Type Relays

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

operational specifications.

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.

2-16