Guardian 4721, Repair Manual

Page: 62 / 115

GENERAL
Information in this section is provided to familiarize
the reader with the various components that make up
the DC control system on prepackaged units having a
liquid cooled engine. These components may be
divided into two (2) broad categories as follows:
• Components in the generator control console.
• Engine mounted components.
CONTROL CONSOLE COMPONENTS
LOCATION AND DESCRIPTION:
The control console includes (a) three terminal boards,
(b) a sensing transformer, and (c) a control board.
Figure 1. Control Board
CONTROL WIRE TERMINAL BOARD:
The terminal board provides convenient attachment
points for control system wiring that must be intercon-
nected between the prepackaged transfer switch and
generator during system installation. This wiring must
be installed and interconnected between the control
console terminal board and an identically marked ter-
minal board in the prepackaged transfer switch. The
following terminals are identified:
A. N1 and N2 (Utility 1 and Utility 2)
1. These terminals deliver “Utility” power source
line-to-line voltage from the prepackaged trans-
fer switch and to the primary coil of a sensing
transformer.
2. Dropout of the “Utility” sensing voltage below a
preset value will result in generator startup due
to control board action.
3. This line-to-line “Utility” voltage is also used by
the control board to operate a battery “trickle
charge” circuit. That circuit helps maintain battery
state of charge when the engine is not running.
B. T1 and T2 (Load 1 and Load 2):
1. This line-to-line power is taken from transfer
switch “Load” Terminal Lugs T1 and T2.
2. This “Load” voltage power is used by the control
board to operate a “7-day exercise” clock or
timer circuit. This circuit starts and exercises the
generator once every seven days, on a day and
at a time of day selected by the installer.
C. Terminals 23 and 194:
1. The control board delivers a +12 volt DC signal
to a transfer relay coil in the transfer switch, via
Terminal and Wire 194. The 12 volt DC circuit is
completed through the transfer relay coil and
back to the control board, via Wire 23.
2. The control board normally holds the Terminal
23 circuit open to ground and the transfer relay
is de-energized.
3. During a “Utility” power source outage, control
board action will crank and start the generator.
Following generator startup, control board
action will complete the Terminal 23 circuit to
ground. The transfer relay will then energize to
initiate transfer of “Load” circuits to the
“Standby” power source.
4. When “Utility” source voltage is restored, control
board action will again open the Terminal 23 circuit
to ground. The transfer relay will then de-energize
to initiate retransfer back to the “Utility” source.
Figure 2. Terminal Board TS3
TRANSFORMER T1:
The line-to-line voltage from the “Utility 1/Utility 2” ter-
minals is delivered to the primary coil of this step-
down transformer. A voltage of about 14 volts AC is
induced into the transformer’s secondary coil and is
delivered to the control board as sensing voltage.
The reduced secondary coil output from the trans-
former is used by the control board not only as “Utility”
source sensing voltage, but also to operate a battery
trickle charge circuit. The latter helps maintain the bat-
tery at a high state of charge during non-operating
periods.
CONTROL BOARD:
This solid state circuit board controls all standby elec-
tric system operations, including engine cranking,
startup, running, automatic transfer and shutdown.
Other operations controlled by the circuit board
include the following:
UTILITY 1 UTILITY 2
LO
AD 1
LO
AD 2
23
194
PART 4
Page 58
DC CONTROL
LIQUID COOLED
ENGINE UNITS
SECTION 4.1
DESCRIPTION AND COMPONENTS