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AIR FORCE 35C2-3-445-2
NAVY NAVFAC P-8-624-34
3-65. ELECTRIC GOVERNOR CONTROL UNIT (MEP-113A).
a. General.
(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 ring gear. The frequency of the pickup signal is
proportional to engine speed. Figure 3-85 shows the functional theory of operation.
(2) The control unit has four distinct circuits: pickup signal amplifier, frequency reference oscillator,
phase comparator and output circuit.
(a)
(b)
(c)
The frequency reference oscillator is voltage controlled. 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 engine speed. The voltage representing speed error is the
amount of voltage required to drive the reference oscillator off frequency in proportion to the engine
speed deviation.
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. By increasing the coupling, a small voltage
change from the phase comparator represents a large frequency change and vice-versa.
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 power dissipation.
b. Malfunction.
The following procedures are to be performed with the governor control unit in the generator set.
(1) Connect the breakout cable (7, figure 3-86) as follows: Disconnect P17 (6) from J17(1), connect
P17A(7) to J17(1), connect J17A(7) to P17(6).
(2) Using a digital voltmeter, make the voltage readings in table 3-7 at the breakout cable plug. All
readings are measured between the terminal and ground. Terminals F, G, H and T are ground.
c. Removal.
(1) Disconnect electrical connector P17 (1, figure 3-86).
(2) Remove 4 hex head screws (2), Iockwashers (3) and nuts (4).
(3) Remove the governor control unit (5).
d. Installation
(1) Position governor control unit (5, figure 3-86).
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