7-10 GOVERNOR CONTROL UNIT (MEP-115A).
(1) The electric governing system Is a speed
(frequency) sensing system used to maintain constant
engine speed and generator output frequency during
periods of unchanging load and when load additions
or deletions occur. The system consists of a
control unit, magnetic pickup and an electric
throttle actuating unit. The engine speed signal
is obtained from a magnetic pickup mounted in the
flywheel housing in close proximity to the flywheel
The frequency of the pickup signal is
proportional to engine speed. Figure 7-14 shows
the functional theory of operation.
(2) The control unit has four distinct cir-
cuits; pickup signal amplifier, frequency reference
oscillator, phase comparator and output circuit.
(a) The Frequency Reference Oscillator is
The frequency setting is
adjusted by applying zero to 10 volts at the
Frequency Reference Oscillator input. The internal
frequency adjust provides this voltage setting when
the control unit is in operation. The Reference
Oscillator does not maintain a constant frequency.
It deviates from its nominal frequency as the engine
speed changes during load changes. The Reference
Oscillator is forced by the Phase Comparator to
track the amplified pickup signal representing
The voltage representing speed
error is the amount of voltage required to drive
the Reference Oscillator off frequency in propor-
tion to the engine speed deviation.
(b) The Phase Comparator Circuit receives
signals from the Pickup Signal Amplifier and the
Reference Frequency Oscillator and compares the
difference in frequency. The Phase Comparator
measures the amount the engine signal is ahead
or behind the Reference Oscillator signal. Its
voltage output is used to force the Reference
Oscillator to the same frequency as the signal
from the engine.
The Phase Comparator output
is proportional to the speed error. The gain
control is used to couple the Phase Comparator
output to the Reference Oscillator.
ing the coupling, a small voltage change from
the Phase Comparator represents a large frequency
change and vice-versa.
(c) The Output Circuit allows governing by
introducing a temporary drop during a load change
for stability purposes.
It has an adjustable means
to control the magnitude and time constant of the
drop to match the dynamic characteristics of the
engine. The output current switching portion of
the circuit provides current to drive the actuator.
The output transistor is switched on and off at a
frequency of 200 Hz.
This is above the natural
frequency of the actuator. The actuator responds
to the average current from the transistor and moves
in proportion to position the engine throttle. The
output transistor is switched on and off to reduce
The following procedures are to be performed with
the Governor Control Unit in the generator set.
(1) Connect the breakout cable between
wiring harness plug P17 (1, figure 7-15) and the
(2) Using a digital voltmeter, make the
voltage readings in table 7-4 at the breakout
All readings are measured between
the terminal and ground. Terminals F, G, H and T
(1) Disconnect electrical connector P17
(1, figure 7-15).
(2) Remove 4 hex head screws (2), lock
washers (3) and nuts (4).
(3) Remove the control unit.
Install the Governor Control Unit in reverse order
of removal procedures.
e. Adjustment Procedure.
(1) Disconnect the actuator linkage rod from
the engine fuel shutoff lever.
(2) Place the generator set START-RUN-STOP
switch (located on the generator set control panel
in the RUN position.
(3) Place the generator set BATTLE SHORT
WITCH (located on the generator set control panel
in the ON (override) position.
(4) Beginning at the fully counterclockwise
position, rotate the engine fuel shutoff lever in
a clockwise direction until a slight resistance is
felt; hold the lever in this position.
This resistance is the fuel injection
pumps internal governor linkage hook
engaging the metering valve arm.
(5) Move the actuator 1 ever to the full fuel
This is the direction against the spring
resistance (fully counterclockwise).