41-748P
KLF Loss-of-Field Relay
4
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 (2) mounting screws. In addition to
holding the upper pin bearing, the bridge is used for
mounting 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 a 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 of diametrically opposed coils is connected in
series. In addition one pair is in series with an adjust-
able resistor. These sets are in parallel as shown in
Figure 2, page 5. 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 (nominal 200 ms)
drop-out characateristic. When energized, the sole-
noid core attracts an iron right-angle armature
bracket which in turn opens the break contacts. In
actual service, the relay is normally energized hold-
ing the break contacts open. Drop-out delay adjust-
ment 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 clapper-
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 contacts bridge
two stationary contacts, completing the trip circuit.
Also during this operation two fingers on the arma-
ture deflect a spring located on the front of the
switch, which allows the operation indicator target 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.
3.0
OPERATION
The relay is connected and applied to the system as
shown in Figure 3, page 6. The directional unit closes
its contacts for lagging var flow into the machine. It’s
zero torque line has been set at -13
°
from the R-axis.
It’s primary function is to prevent operation of the
relay during external faults. The impedance unit
closes its contacts when, as a result of reduction in
excitation, 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 Figure 3, the contacts of all three units are con-
nected in series across a telephone type relay
designed X, which provides approximately 12.5
cycles time delay on dropout before energizing the
trip coil. This time delay is to insure contact coordina-
tion under all possible operating conditions. During
normal conditions, all contacts are open.
3.1 PRINCIPLE OF IMPEDANCE UNIT
OPERATION
The impedance unit is an induction cylinder unit hav-
ing 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
1T
modified by a
voltage derived from the long reach compensator T
A
.
The other set of poles (polarizing) are energized by
the same voltage V
1T
except modified by a voltage
derived by the short reach compensator T
C
. The flux
in the polarizing pole is adjusted so that the unit
closes its contacts whenever flux in the operating set
of poles leads the flux in the polarizing set.
The voltage V
1T
is equal to
V
AT
= V
AB
+ 0.5 V
BC
= 1.5 V
AN
(1)
As shown in Figure 3, one-half of V
BC
, voltage is
physically derived in the relay at midtap of a reactor
connected across voltage V
BC
.
