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bearings. t he shaft rides between the bottom pin
bearing and the upper pin bearing with the cylinder
rotating in an air-gap formed by the electromagnet
and the magnetic core.
The bridge is secured to the electromagnet and
frame by two mounting screws. In addition to holding
the upper pin bearing, the bridge is used for mount-
ing the adjustable stationary contact housing. The
stationary contact housing is held in position by a
spring type clamp. The spring adjuster is located on
the underside of the bridge and is attached to the
moving contact arm by spiral spring. The spring
adjuster is also held in place by a spring type clamp.
2.5. UNDERVOLTAGE UNIT
The voltage unit is an induction-cylinder unit.
Mechanically, the voltage unit is composed like the
directional unit, of four components: A die cast alumi-
num frame, an electromagnet, a moving element
assembly, and a molded bridge.
The electromagnet has two pairs of voltage coils.
Each pair of diametrically opposed coils is connected
in series. In addition one pair is in series with a paral-
lel R-C combination. These sets are in parallel as
shown in figure 3. The adjustable resistor serves not
only to shift the phase angle of the one flux with
respect to the other to produce torque, but it also pro-
vides a pick-up adjustment.
Otherwise the undervoltage unit is similar in its con-
struction to the directional unit.
2.6. TELEPHONE RELAY
The telephone relay (X) has a slow drop-out charac-
teristic. When energized, the solenoid core attracts
an iron right-angle armature bracket which in turn
opens the break contacts. In actual service, the relay
is normally energized holding the break contacts
open. (Note: the make contacts are not used.)
Drop-out delay adjustment is obtained by varying the
air-gap between the armature and the core.
2.7. INDICATING CONTACTOR SWITCH UNIT
(ICS)
The dc indicating contactor switch is a small clap-
per-type device. A magnetic armature, to which
leaf-spring mounted contacts are attached, is
attracted to the magnetic core upon energization of
the switch. When the switch closes, the moving con-
tacts bridge two stationary contacts, completing the
trip circuit. Also during this operation two fingers on
the armature deflect a spring located on the front of
the switch, which allows the operation indicator tar-
get to drop. The target is reset from the outside of the
case by a push-rod located at the bottom of the
cover.
The front spring, in addition to holding the target, pro-
vides restraint for the armature and thus controls the
pickup of the switch.
3. OPERATION
The relay is connected and applied to the system as
shown in figure 4. The directional unit closes its con-
tacts for lagging var flow into the machine., Its zero
torque line has been set at -13˚ from the R-axis. Its
primary function is to prevent operation of the relay
during external faults. The impedance unit closes its
contacts when, M as a result of reduction in excita-
tion, the impedance of the machine as viewed from
its terminals is less than a predetermined value. The
operation of both the impedance and directional units
sounds an alarm, and the additional operation of the
undervoltage unit trips the machine. As shown in fig-
ure 4, the contacts of all three units are connected in
series across a telephone type relay designated X,
which provides approximately 15 cycles time delay
on dropout before energizing the trip coil. This time
delay is to insure positive contact coordination under
all possible operating conditions. During normal con-
ditions, all contacts are open.
3.1. PRINCIPLE OF DISTANCE UNIT OPERA-
TION
The distance unit is an induction cylinder unit having
directional characteristics. Operation depends on the
phase relationship between magnetic fluxes in the
poles of the electromagnet.
One set of opposite poles, designated as the operat-
ing poles are energized by voltage V
voltage derived from the long reach compensator T
The other set of poles (polarizing) are energized by
the same voltage V
except modified by a voltage
AO
derived from the short reach compensator T
flux in the polarizing pole is so adjusted that the unit
closes its contacts whenever flux in the operating set
of poles leads the flux in the polarizing set.
Reach of the distance unit is determined by compen-
sators T
and T
as modified by autotransformer set-
A
C
I.L. 41-748.1F
modified by a
AO
.
A
: The
C
5
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