2 operational analysis, Rotor residual magnetism, Field boost (12-20 kw) – Generac Power Systems 8 kW LP User Manual

Page 37: Part 2

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Page 35

AC GENERATORS

sEctioN 2.2

oPEratioNal aNalYsis

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.

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.

DIODE

PIN 5

PIN 1

BASE

TRANSISTOR

FIELD

BOOST

RESISTOR

FIELD

BOOST

DIODE

STARTER
CONTACTOR

TO
STARTER

+12 VDC

FIELD

BOOST

TO

ROTOR

CRANk
RELAY k1

CIRCUIT BOARD

56

4

13

Figure 2. Field Boost Circuit Schematic

Part 2

STATOR

EXCITATION

WINDING

VOLTAGE

REGULATOR

FIELD BOOST FROM

CONTROL LOGIC

CIRCUIT BOARD

STATOR

POWER

WINDING

STATOR

POWER

WINDING

MAGNETIC

FIELD

MAGNETIC

FIELD

MLB = MAIN LINE CIRCUIT BREAKER

ROTOR

SENSING

TO LOAD

MLB

ENGINE -
DIRECT
DRIVE

12-20 kW Units

Figure 1. Operating Diagram of AC Generator

CAPACITOR

STATOR

EXCITATION

WINDING

STATOR

POWER

WINDING

STATOR

POWER

WINDING

MAGNETIC

FIELD

MAGNETIC

FIELD

MLB = MAIN LINE
CIRCUIT BREAKER

ROTOR

TO LOAD

MLB

ENGINE -
DIRECT
DRIVE

8/10 kW Units

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