Hobart 9OC24S Operation Manual page 208

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HOBART BROTHERS COMPANY
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OPERATION, MAINTENANCE, AND OVERHAUL MANUAL, TM-240A
SELF-PROPELLED GENERATOR SET, SPEC. NO. 5381A
Inside the box these pulses are converted to DC voltage proportional to
engine speed. An increase in engine speed results in an increased DC volt-
age, and conversely a decrease in engine speed results in a decrease in DC
voltage. The DC voltage converted from the pulsing signal out of the mag-
netic pickup becomes the speed signal in the control box. In operation,
the speed signal voltage is compared to the speed setting signal voltage as
,adjusted by the potentiometer (2RH), Figure 1. The difference between these
signals determines the value of the output signal voltage to the actuator.
The hydraulic actuator is a proportional type, that is, its output shaft takes
an angular position proportional to a given input signal voltage. As stated
above, the actuator is electrically control led and is hydraulically powered
by fue I oil pressure.
It has no revolving parts. (See Figure 4.) A pilot
valve controls flow of oil to (or from) a servo piston which is connected by
an internal lever to an output shaft.
Actuator power is transmitted fromthe
actuator output shaft to the engine speed control arm through a lever and ad-
justable rod mechanism. As the pi lot valve moves downward, oil is admitted
under the servo piston to push it up and increase engine speed. When the
pilot valve moves upward, oil is released from beneath the piston , allowing
it to move downward and reduce engine speed.
The actuator pilot valve is attached to a permanent magnet in a transducer.
The output signal from the control box passes through the transducer coil
and tends to push the magnet and pilot valve plunger downward. A feedback
spring and a top centering spring add to this downward force. A bottom ten -
tering spring resists the downward push. When the engine is running "on
speed 'I, these opposing forces are equal and the pilot va Ive is ten tered so
that no oil flows to, or from, the actuator servo to change the position of
the output shaft.
A change in the voltage signal from the control box re-
sults in a change in the pilot valve position. The resulting oil flow to
(or from) the servo produces a change in the output shaft to either increase
or decrease engine speed.
For each input signal voltage from the control
box, there is but one position of the output shaft at which the bottom center-
ing spring force exactly opposes the force in the transducer coil and the top
springs when the pilot va Ive is centered.
During an underspeed condition,
when engine speed is below the speed
selected by the potentiometer,
the control box output voltage to the
actuator is at maximum (as high as 8 to 10 V DC), to force the magnet
and pilot valve down and allow the actuator piston and output shaft to
move to maximum fuel position.
During an overspeed condition when
engine speed exceeds the set speed, the control box output voltage will
be zero and the actuator will move to minimum fuel position.
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