Generac 86640 - Manual (Page 8)

Excitation System
GENERAL:
The excitation system is shown schematically in Figure
9. During operation, unregulated AC current from the Stator
excitation winding is delivered to the Voltage Regulator via a
Thermal Protector, Wires 2 and 6, an excitation circuit
breaker (CB4), and Wires 2A and 6. The Voltage Regulator
rectifies and regulates the current flow, which is then deliv­
ered to the Rotor windings via Wires 4 and 1, and the brushes
and slip rings. Major components of the excitation system
include the following;
D Stator Excitation (DPE) Winding,
n Thermal Protector (TP).
D Excitation Circuit Breaker (CB4).
D
AC Voltage Regulator.
D Brushes and Slip Rings,
n Rotor Assembly.
THERMAL PROTECTOR:
A Thermal Protector (tP) is physically imbedded in the
wire windings of the Stator assembly and electrically con­
nected in series with the excitation winding AC output leads
to the Voltage Regulator. The device is a normally-closed (N.
C.), temperature sensitive switch. Should Stator tempera­
tures exceed a preset level, the switch contacts will open. The
switch is self-resetting. That is, its contacts will close when
Stator temperatures decrease below a safe, preset level.
If the switch contacts open, excitation winding output to
the Regulator will terminate and the Regulator will shut down.
Generator AC output voltage will then drop to a value com­
mensurate with the Rotor’s residual magnetism (about 2 to 7
volts AC).
The thermal protector is NOT accessible and cannot be
replaced. If it has failed open, it can be bypassed by connect­
ing excitation lead No. 2 to a bypass lead No. 5. Once the
switch has been bypassed, overtemperature protection is no
longer available.
EXCITATION CIRCUIT BREAKER (CB4):
If this circuit breaker has failed open, the results will be
the same as a Thermal protector that has failed open. That
is, generator AC output will drop to about 2-7 volts AC
(residual).
AC VOLTAGE REGULATOR:
See Figure 12. Sensing leads deliver ACTUAL AC power
winding voltage signals to me Regulator via Leads 11 and 22
and Regulator terminals 5 and 6. The Regulator electronically
compares this ACTUAL signal to a preset REFERENCE
voltage, then acts to maintain an ACTUAL voltage that Is
equal to the REFERENCE voltage.
If ACTUAL voltage is less than the preset REFERENCE
voltage, the Reoulator will increase excitation current flow to
the Rotor. The Rotor's magnetic field will then strengthen and
the AC output voltage will increase.
If ACTUAL voltage is greater than preset REFERENCE
voltage. Regulator action will decrease excitation current flow
to the Rotor. The Rotor’s magnetic field strength will drop and
ACTUAL voltage will decrease.
The typical AC Voltage Regulator shown in Figure 12 has
a single amustable potentiometer, used to establish the de­
sired REFERENCE voltage.
BRUSHES AND SLIP RINGS:
The Brushes and Slip Rings (Figure 13) allow excitation
current flow to be transmitted rrom a stationary member to a
rotating member. The Slip Rings are actually a part of the
Rotor assembly. As a general rule, the positive slip ring is the
one nearest the Rotor bearing. Wire No. 4 is the positive
brush lead; Wire No. 1 is the negative baish lead.
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