Operational Analysis; Rotor Residual Magnetism; Field Boost (12-20 Kw); Field Boost (12-20 Kw Units) - Generac Power Systems 10 kW LP Repair Manual

AC GENERATORS

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 residual
magnetism is sufficient to induce a voltage into the
stator AC power windings that is approximately 2-12
volts AC.

Field BooSt (12-20 kW)

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.
TO LOAD
MLB = MAIN LINE
MLB
CIRCUIT BREAKER
STATOR
POWER
WINDING
MAGNETIC
FIELD
ENGINE -
DIRECT ROTOR
DRIVE
MAGNETIC
FIELD
STATOR
EXCITATION
WINDING
CAPACITOR
8/10 kW Units
Part 2
STATOR
POWER
WINDING
ENGINE -
DIRECT
DRIVE
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.
+12 VDC
DIODE
CRANk
RELAY k1
BASE
TRANSISTOR
CIRCUIT BOARD
Figure 2. Field Boost Circuit Schematic
TO LOAD
MLB
MLB = MAIN LINE CIRCUIT BREAKER
STATOR STATOR
POWER POWER
WINDING WINDING
MAGNETIC
FIELD
ROTOR
MAGNETIC
FIELD
STATOR
EXCITATION
WINDING
12-20 kW Units
sEctioN 2.2
13
PIN 5
TO
56
STARTER
FIELD
BOOST
RESISTOR
PIN 1
STARTER
FIELD
4
CONTACTOR
BOOST
DIODE
FIELD
BOOST
TO
ROTOR
SENSING
FIELD BOOST FROM
CONTROL LOGIC
CIRCUIT BOARD
VOLTAGE
REGULATOR
Page 35