Operational Analysis - Guardian 4389 Diagnostic Repair Manual

AC GENERATORS
ROTOR RESIDUAL MAGNETISM
The generator revolving field (rotor) may be
considered to be a permanent magnet. Some
'residual" magnetism is always present in the rotor.
This residual 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 (K1). 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 (K1) is energized, i.e., while the
engine is cranking.
PART 2
Figure 1. Operating Diagram of AC Generator

OPERATIONAL ANALYSIS

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 2. Field Boost Circuit Schematic
SECTION 2.2
Page 2.2-1