Field Boost Assembly - Briggs & Stratton 86262GS Familiarization & Troubleshooting Manual

Portable Generator Familiarization & Troubleshooting Guide
Section 2 • Generator Components & Systems

Field Boost Assembly

Some types of voltage regulators require approximately
12VAC before they turn on. Residual output voltage may not
be sufficient to produce this minimum voltage. A field boost
circuit may be included on these generator models to excite
the regulator and turn it on.The circuit uses a thermistor
device called a "PTC" (positive temperature coefficient) and
a diode.
The Field Boost assembly consists of:
• Wire #4
• Field Boost Diode
• The (PTC) thermistor device
• Wire #162
The device is connected across terminals 11 and 4 of the
Figure 2.69 — Field Boost Assembly
electronic voltage regulator (Figure 2.69).
The (PTC) and diode are enclosed in a "field boost
assembly," which is housed in the receptacle panel
(Figure 2.70).
Field boost voltage is delivered to the rotor on every
startup.This current flow "flashes the field" every time the
engine is started.
Figure 2.70 — Field Boost Assembly Schematic
"Field Boost" operation may be briefly described as follows:
• During start up, residual (AC) voltage is delivered to the
(PTC) and diode from the stator power winding.
• When the thermistor is cold, resistance is low. Current
can flow to the diode where the (AC) current is
rectified to direct current (DC) and delivered to the
rotor winding via wire # 4.This causes the rotor's
magnetic field strength to increase which causes the
power winding voltage to increase.
• The increase in power winding output voltage is then
enough to turn on the voltage regulator.
• As the thermistor warms up, its resistance increases
until the device becomes, in effect, an open circuit. Field
boost current flow to the rotor windings then ceases.
Inductive Reactance
Inductive reactance may be defined as the condition that
exists when current lags behind voltage.
Magnetic lines of force called "flux" are created around a
current-carrying conductor. Since alternating current is
constantly increasing to a maximum negative value, the
magnetic field around the conductor must also increase,
collapse to zero, then increase again in the opposite
direction (polarity) (Follow Figure 2.71).
Figure 2.71 — Inductive Reactance
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