Hobart 6824-1, Operation And Maintenance Manual, Page: 35 / 214

b. Line-drop current transformers
The three line-drop current transformers (9), in conjunction with burden resistors (8), detect
the magnitude and power factor of current flowing from generator to load. They feed a signal
to the voltage regulator which interprets the signal and alters the exciter field current as
required to maintain a constant predetermined voltage at the load
(see Voltage Regulator
Manual No. TM-232).
c. Overload current transformers
Three overload current transformers (3), in conjunction with burden resistors (1), monitor the
output load current in each of the three output phases, and supply a reduced value current
signal to the overload module
(7).
(3) Overload module
The overload module (7) is a solid-state device designed to interpret a signal from transformers
(3)
and to send a signal to the protective monitor module
(4, Fig. 8)
when an overload condition
exists in any generator output phase. A pull-apart electrical connector is mounted on the
overload module to provide quick-disconnect facilities for all wiring to the module. The overload
module is equipped with a hermeticallysealed, reed-type relay. Relay contacts are normally
open. The solidstate circuitry is designed to close relay contacts when output current in ANY
phase reaches 125% of normal rated output capacity. The closed relay sends a signal to the
protective monitor. This signal “gates” the overload SCR
(silicone-controlled rectifier)
in the
protective monitor and interrupts the load contactor holding circuit, allowing the load contactor to
open.
The following is a list of overload module characteristics:
NOTE: The overload protective system will function when any phase carries 123% to 127% of rated
load.
•
At 125% load the module will function in 4 minutes.
•
At 150% load the module will function in 16 seconds.
•
At 200% load the module will function in 4 seconds.
NOTE: All times are plus or minus 25% and are nonadjustable.
(4) Rectifier
A diode-bridge rectifier (5) receives 400-Hz AC from phase C of the generator output an
converts it to a pulsating, direct current for energization of the load contactor holding coil only.
This DC coil-holding circuit is controlled indirectly be controlling the 400-Hz AC to the rectifier.
The ground circuit for the rectifier’s AC supply must pass through the relay contacts in the
protective monitor module to ground cable N. Therefore, any time a protective device functions
to open the protective monitor relay, the rectifier’s AC circuit is opened. No DC is then available
for the load contactor holding coil, hence, the load contactor opens.
(5)
Terminal boards
Three terminal boards (2) provide connection facilities for small leads.
OM-499
May 31/94 Revised 1-1
Page 21