Woodward TG-10E, Installation And Operation Manual

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TG-10E/-13E/-17E Actuator Manual 04045
24 Woodward
Opposing the downward magnetic force exerted by the transducer (12) is a
resultant spring force from the load spring (24). The load spring (24) sits on top
of the case in which the transducer (12) is located, and exerts a constant upward
force on the magnet (22) and its attached flapper (11). The restoring spring (25),
weaker than the load spring (24), exerts a downward force on the magnet (22)
and its attached flapper (11). The amount of downward force exerted by the
restoring spring (25) depends upon the position of the restoring lever (13).
When the power piston (6) which is attached to the terminal lever (4) moves up,
the terminal lever (4) and the restoring lever (13) also move up, decreasing load
on the restoring spring (25). When the power piston (6) moves down, the
terminal lever (4) and the restoring lever (13) move down, increasing load on the
restoring spring (25).
With a constant current, the resultant spring force and the constantly opposing
magnetic force always balance, and the pilot-valve plunger is “centered.” The
control land of the pilot-valve plunger exactly covers the control port in the pilot-
valve bushing, and no oil flows to or from the power piston.
Operation of the TG Actuator
General Information
Refer to Figure 5-1 to better understand the operation of the TG actuator.
The description that follows discusses the TG actuator operation. It must be
remembered that the actuator only responds to a change of dc voltage coming to
the transducer. The load or speed sensing must come from an electronic control.
The description that follows is based on load changes. The integrating control
senses the load change and proportionally varies the dc voltage going to the
actuator. (Changes in the system control speed setting produce the same
actuator movements as do changes in load on the turbine.)
Decrease in Load or Speed Setting
Assume that the turbine is running on-speed under steady-state conditions. The
control voltage to the actuator is therefore constant. The pilot-valve plunger (20)
is centered over the control port of the pilot-valve bushing (7) and the control
land stops the flow of the pressure oil through the bushing control port. There is
no movement of the power piston (6) and no movement of the actuator terminal
shaft (2).
A decrease in load (or speed setting) causes a decrease in control voltage to the
transducer (12). This in turn causes a decrease in the magnetic force tending to
raise the flapper (11). For the reasons explained above, the pilot-valve plunger
(20) always follows the flapper and the pilot-valve plunger moves upward, above
center, uncovering the pilot-valve bushing (7) port. Oil escapes from under the
power piston (6), causing the power piston to move downward and the terminal
shaft (2) to rotate in the decrease-steam direction. As the terminal shaft rotates in
the decrease-steam direction, it causes the restoring lever (13) to move down,
increasing the restoring spring force (25). The terminal shaft rotates until the
increase in restoring spring (20) force equals the decrease in downward
magnetic force in the transducer (12). Then the pilot-valve plunger (20) is re-
centered.