Guardian 4721, Repair Manual

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ROTOR RESIDUAL MAGNETISM
The generator revolving field (rotor) may be consid-
ered to be a permanent magnet. Some ‘residual”
magnetism is always present in the rotor. This resid-
ual magnetism is sufficient to induce a voltage into
the stator AC power windings that is approximately 2-
12 volts AC.
FIELD BOOST
FIELD BOOST CIRCUIT:
When the engine is cranking, direct current flow is
delivered from a circuit board to the generator rotor
windings, via Wire 4.
The field boost system is shown schematically in
Figure 2. Manual and automatic engine cranking is
initiated by circuit board action, when that circuit
board energizes a crank relay. Battery voltage is then
delivered to field boost Wire 4 (and to the rotor), via a
field boost resistor and diode. The crank relay, field
boost resistor and diode are all located on the circuit
board.
Notice that field boost current is available only while
the crank relay is energized, i.e., while the engine is
cranking.
Field boost voltage is reduced from that of battery
voltage by the resistor action and, when read with a
DC voltmeter, will be approximately 9 or 10 volts DC.
Figure 7. The Field Boost Circuit
(from Schematic Drawing #0F5244)
4 RED
4
AR
1
1
4
0
4
0
VR
PCB
15
17 A
B
14
16
4
3
E
12
5 13 1 2 7 D 11 9 10
Figure 1. Operating Diagram of AC Generator
PART 2
Page 24
PREPACKAGED LIQUID
COOLED AC GENERATORS
SECTION 2.2
OPERATIONAL ANALYSIS