ABB HU Instruction Leaflet page 4

41-347.12C
N
N
BALANCED AIR GAPS
D U (DU)
IFFERENTIAL NIT
The differential unit of the HU relay consists of two
air-gap restraint transformers, three full-wave rectifi-
ers, saturating operating-transformer, and a dc polar
unit.
The HU-1 relay, in addition to the above compo-
nents, has a third air-gap restraint transformer, an a
fourth full-wave rectifier.
Each of the restraint transformers and the operating
transformer are provided with taps to compensate for
mismatch of line current transformers. These taps
are incorporated in the relay in such a manner that
changing a tap on a restraint transformer automati-
cally changes the same tap on the operating trans-
former.
H -R
ARMONIC ESTRAINT
The harmonic restraint unit of the HU and HU-1
relays consists of an air-gap operating transformer, a
second harmonic block filter, a fundamental block-
second harmonic pass filter, two full-wave rectifiers,
indicating instantaneous trip unit, varistor, and a dc
polar unit.
Taps are also incorporated in this unit to compensate
for mismatch of the line current transformers. Chang-
ing a tap on the restraint transformer of the differen-
tial unit also changes the tap of this unit.
4
Permanent
Magnet
S
Armature
S
Moving Contact
Figure 5. Polar Unit Permanent Magnet Flux Paths
U (HRU)
NIT
Shunt
N
N
UNBALANCED AIR GAPS
P U
OLAR NIT
The polar unit consists of a rectangular shaped mag-
netic frame, an electromagnet, a permanent magnet,
and an armature. The poles of the crescent shaped
permanent magnet, bridge the magnet frame. The
magnetic frame consists of three pieces joined in the
rear with two brass rods and silver solder. These
non-magnetic joints represent air-gaps, which are
bridged by two adjustable magnetic shunts. The
windings are wound around a magnetic core. The
armature is fastened to this core and is free to move
in the front air-gap. The moving contact is connected
to the free end of a leaf spring, which, in turn, is fas-
tened to the armature.
I C
NDICATING ONTACTOR
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 front spring, in addition to holding the target, pro-
vides restraint for the armature and thus controls the
S
Additional
Flux Path
S
N
Sub 5
183A062
S U (ICS)
WITCH NIT