PGEV and PGE Locomotive Governors. Manual - part 9

Manual 36703

PGEV and PGE Locomotive Governors

Woodward

Operation with Load Decrease

Under the same conditions as stated above, a decrease in electrical load will
reduce engine load and cause the engine to decrease fuel and, in the process,
lower the right end of the floating lever. This moves the pilot-valve plunger below
center and directs pressure oil through the lower control port in the bushing to the
increase excitation side of the vane servo. With an increase in load, the engine
will underspeed and the governor will act to increase fuel. This increase in field
excitation current and engine fuel will continue until the power piston and floating
lever have returned to their original positions. This re-centers the pilot-valve
plunger and stops the servomotor. Consequently the electrical load is increased
sufficiently to again balance engine-power output with the engine on-speed.

Operation with Speed Setting Increase

Advancing the throttle to a higher step causes the piston to move downward.
This lowers the left end of the load-control floating lever which displaces the load
control pilot valve plunger below center. Pressure oil is directed to the increase-
excitation side of the vane servo. The governor acts to increase fuel to
compensate for both the increase in speed setting and the simultaneous increase
in electrical load. As the power piston moves upward, it raises the right end of the
floating lever to return the pilot-valve plunger to its centered position. This stops
the servomotor as the power piston reaches its new higher position
corresponding to the increased speed setting. At this point, the electrical load has
been sufficiently increased to balance the increase in engine power output.

Operation with Speed Setting Decrease

Moving the throttle to a lower speed setting causes the speed setting piston to
move upward. This raises the left end of the load-control floating lever and lifts
the pilot valve plunger above center. Pressure oil is directed to the decrease
excitation side of the vane servo. The governor acts to decrease fuel to
compensate both for the decrease in speed setting and the simultaneous
decrease in electrical load. As the power piston moves downward, it lowers the
right end of the floating lever to return the pilot valve plunger to its centered
position. This stops the servomotor as the power piston reaches its new lower
position corresponding to the decreased speed setting. At this point, the electrical
load has been sufficiently decreased to balance the decrease in engine power
output.

Load Control Balancing

The rate of vane servo movement (timing) must be controlled to effect a
controlled rate of load application and to provide stability of the overall system.
Several methods are commonly used to provide a balanced action and are
identical in that they restrict the flow of oil to and from the vane servo and thus
determine its rate of movement.

In some governors, the oil flow is restricted by the number, size and position of a
group of orifice holes (restricted porting) in the load control bushing, which are
opened and closed by the movement of the pilot valve plunger. With this
arrangement, a progressively increasing (or decreasing) rate of movement
occurs depending on the degree of movement of the plunger. These rates will not
necessarily be the same in both directions.

PGEV and PGE Locomotive Governors

Manual 36703

Woodward

Governors used with remote servos may have a separate timing valve assembly
consisting of two adjustable ball check valves in series within a common housing
The assembly may be externally mounted on the governor or remotely located
and connected into one of the lines between the governor and servo. The valves
are individually adjustable to provide the desired maximum rate of movement
over the full travel of the servomotor in either the increase or decrease excitation
direction.

Governors with an integral-vane servo may use a similar arrangement to the
timing valve assembly, except that the ball valves are individually housed and
internally installed in the top of the governor column

Minimum or Maximum Field Start Adjustment

The load control system in the governor may be set up for either "Minimum or
Maximum" field start.

Minimum Field Start

—builds up engine load slowly, providing a smooth take-up

of slack in the train. The load control pilot valve is mechanically set above center
with the throttle in IDLE position. Field excitation is retarded due to the retarded
position of the pilot valve plunger. The vane servo rheostat remains in the
minimum excitation position until the throttle is moved in the increase speed
direction. This lowers the load control pilot valve to the re-center position and
beyond to increase excitation.

Maximum Field Start

—enables the engine load to build up immediately, for

rapid accelerations. The load control pilot valve is mechanically set below center
with the throttle in IDLE position. Field excitation is advanced due to the
advanced position of the pilot valve plunger. The vane servo rheostat remains in
the maximum excitation position until the throttle is moved in the increase speed
direction to raise the load control pilot valve.

Load Control Override (Optional)

Under certain conditions of locomotive operation (transition, maximum-field start
and wheel slip), it is sometimes desirable or necessary to override the normal
action of the governor load control mechanism to cause a reduction in generator
excitation current when it would normally respond by increasing excitation
current.

The load control override mechanism in the governor consists of an overriding
solenoid (ORS), a two-position overriding control valve, and an overriding piston
within a cylinder which surrounds the upper end of the load control pilot-valve
plunger. See Figure 3-1.

Energizing the ORS pushes the overriding valve plunger down, closing the drain
to Sump and allowing pressure oil to flow into the overriding cylinder. The
overriding piston moves upward, contacting the spring collar on the stem of the
pilot-valve plunger and lifting the plunger above its centered position. The slot in
the link connecting the pilot-valve plunger to the floating lever permits the plunger
to rise independently of the lever. This directs pressure oil to the decrease-
excitation side of the vane servo, thus reducing generator output. When the ORS
is de-energized, the overriding-valve plunger moves upward, closing the
pressure port and allowing the oil to drain from the overriding cylinder. This
restores normal load-control system operation.

Manual 36703

PGEV and PGE Locomotive Governors

Woodward

Transition

—A condition where the electrical circuits between the generator and

traction motors are automatically changed, as road speed changes, to provide
more efficient transmission of electrical power. Overriding is used in this
circumstance to protect the switchgear from arcing which would occur during
transition if high current existed in the traction motor circuits.

Modified Maximum Field Start

—A variation used in some applications where

the load control mechanism is arranged for maximum field start but it is desirable
to hold the vane servo rheostat in the minimum excitation position at idle speed.
Normal operation is restored when the throttle is advanced to the first step,
allowing the vane servo to increase excitation toward maximum as the train
starts.

Wheel Slip

—When rail and load conditions cause drive wheel slip, an immediate

decrease in load occurs at the traction motors and generator. The resulting
increase in engine speed would normally cause the load-control system to
respond by increasing generator output at a time when there is no demand.
Overriding is used in this circumstance in conjunction with wheel-slip relays, if the
locomotive is so equipped, to cause a reduction in generator output until wheel
slippage ceases.

Operation of the ORS is done through automatic switching devices.

Fast Unloading

—may be used in conjunction with the load control override

mechanism. It provides a quick unloading of the integral-vane servo. Fast
unloading cannot be used for remote-servo applications using external timing
valves. Two methods are available. (1) The lower end of the load-control pilot-
valve plunger has an additional land. An additional port in the bushing bypasses
the restricted port in the bushing. When the ORS energizes, the plunger un-
centers upward and oil is released through the extra port for fast unloading of the
vane servo, (2) The plunger has an additional land on its lower end. Instead of a
bypass port, the bushing has small orifice holes in one side of the bushing. A
radial slot is located in the bushing and midway between the orifice holes. As the
ORS energizes, the plunger un-centers upward releasing oil through the orifice
holes first and if a large displacement of the pilot valve takes place, oil is
released from the servo through both the orifice holes and the radial slot. Both
methods allow the vane servo to move rapidly in the direction of minimum
excitation.

Integral Vane Servo Assembly (Optional)

The integral-vane servo is used with low wattage pilot or amplifier type excitation
systems. It functions in conjunction with the load control mechanism in the
governor to automatically regulate generator output and thereby maintain a
constant engine-power output at each throttle setting.

Vane servos use either a resistor pack or a ceramic resistor. Figure 3-3a shows a
vane servo with a resistor pack. A vane servo with a ceramic resistor appears in
Figure 3-3b.

Both servos have a vane-type rotary servomotor. Drain oil flows through the
covers of both units to cool the resistor pack or the ceramic resistor. The
commutator and resistor pack or ceramic resistor are electrically insulated from
the vane servo unit.

The vane servo shaft output shaft has external serrations with one missing tooth.
The slot formed by the missing tooth mates with a ridge on the brush drive shaft
to make sure that the two shafts assemble correctly.

PGEV and PGE Locomotive Governors

Manual 36703

Woodward

Contact Woodward for details and actual limits.

Whenever the load-control pilot valve in the governor column is un-centered,
pressure oil is directed to one or the other side is the vane servo while the
opposite side is opened to drain. This causes the vane to rotate which, in turn,
rotates the contact-brush assembly about the commutator. The position of the
brushes on the commutator segments determines the circuit resistance and
thereby the generator field-excitation current.

Figure 3-3a. Integral Vane Servo (with Resistor Pack Assembly)

Figure 3-3b. Integral Vane Servo (with Ceramic Resistor)

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